2010 - ATF Evaluation Study Final Report - LA Sewers

Technical Memorandum - FINAL 

Team 

City of Los Angeles 

Department of Public Works 

Bureau of Sanitation 

Air Treatment Facility (ATF) Review Study 

FINAL REPORT 

December 2010


Team 





FINAL REPORT 

EXECUTIVE SUMMARY 

November 2010

Table of Contents 

Executive Summary ...................................................................................................................... 3 

1.0 Introduction ........................................................................................................................ 3 

2.0 Airflow Modeling Study ....................................................................................................... 5 

3.0 Sewer Siphon Study ........................................................................................................... 7 

4.0 Drop Structures Study ...................................................................................................... 12 

5.0 Total Non-Methane Hydrocarbon Study ........................................................................... 17 

6.0 ATF Study ........................................................................................................................ 22 

6.1 ATF Locations .............................................................................................................. 22 

6.2 Air Emissions Characteristics ....................................................................................... 22 

6.3 Air Emissions Control Technologies ............................................................................. 24 

7.0 Recommendations ........................................................................................................... 25 

7.1 ATF ............................................................................................................................... 25 

7.2 Drop Structure Modifications ........................................................................................ 25 

7.3 Flow Diversion .............................................................................................................. 26 

7.4 Technology ................................................................................................................... 26 

List of Figures 

Figure 1 - Base Map of LABOS Collection System under Study .................................................. 4 

Figure 2 - Odor Areas of concern Location Map ........................................................................... 6 

Figure 3 - Detailed Sampling Location Map .................................................................................. 8 

Figure 4 - Baseline Air Pressure ................................................................................................... 9 

Figure 5 - Passive Air Duct Connection Data ............................................................................. 10 

Figure 6 - Combined NORS-NCOS Air Pressure with Fan Active .............................................. 11 

Figure 7 - Drop Structure Baseline Pressure .............................................................................. 13 

Figure 8 - Drop Structure Air Return Line Plugged Pressure ...................................................... 14 

Figure 9 - Drop Structure Pressure with Flow Management ....................................................... 15 

Figure 10 - Total NMHC VOC Sampling Result Summary.......................................................... 19 

Figure 11 – Hydrogen Sulfide Sampling Results Summary ........................................................ 20 

Figure 12 - Hydrogen Sulfide Emission Control Overview .......................................................... 21 

Figure 13 - Mission & Jesse Additional Air Pickup ...................................................................... 25 

Figure 14 - NEIS/ECIS Recommendation Summary .................................................................. 26 

List of Tables 

Table 1 - Mission & Jesse Interceptor Raw Emissions Characterization Results ....................... 23 

Air Treatment Facility Review Study 2 of 28 

Final Report - FINAL 



The City of Los Angeles has a long history of implementing proactive and innovative steps to 

control odors from its interceptor collection system including using chemical addition to control 

odors with an extensive use of magnesium hydroxide and sodium hydroxide. Recently, the City 

has been working to improve odor control in several areas identified by local community groups. 

Air treatment facilities (ATFs) with known or predicted odor “areas of concern” were included in 

the 2004 Collection System Settlement Agreement (CSSA). 

Each technical memorandum and the Draft Final Report submitted during the course of this study 

were reviewed by the Independent Odor Expert. Comments generated by the Independent Odor 

Expert were incorporated into the Final Report and the technical memoranda it includes. The 

Final Report and its recommendations are consistent with the opinion of the Independent Odor 

Expert. 

The comments of the Independent Odor Expert are presented in Appendix A. 

1.0 INTRODUCTION 

The HDR Team conducted a study of the City’s wastewater collection system in order to evaluate 

the ability of proposed ATFs to provide satisfactory odor relief to the collection system. 

The study was performed between January 2008 and August 2010. It included an analysis of the 

sewer system as a whole including both current conditions and planned modifications to the 

sewer system. It evaluated the effects that collection system structures such as drop structures and 

siphons have on the ventilation of sewer odors. Taking into account the pattern of sewer odor 

complaints and results of the drop structures and siphons field data collection, it assessed 

proposed facilities intended to mitigate sewer odor complaints. The study also analyzed total 

non-methane hydrocarbons associated with sewage and determined how to effectively remove 

them from the sewer system. 

The present document incorporates the results of the study and includes the five technical 

memoranda developed for the study: 

 

 

 

 

 

Airflow Modeling Study 

Sewer Siphons Study 

Drop Structures Study 

Total NMHC Study 

ATF Study 

Figure 1 depicts the system under study. 




Figure 1 - Base Map of LABOS Collection System under Study 




2.0 AIRFLOW MODELING STUDY 

The Airflow Modeling Technical Memorandum provides documentation of the developed airflow 

model and presents the modeling results for the City’s current interceptor system configurations 

and the two planned wastewater flow diversion scenarios. 

The benefits of the airflow modeling study include supporting the findings in other studies and 

evaluating the impact of the identified future operational changes in terms of odor areas of 

concern. However, the airflow modeling results are not intended to be the sole material for the 

evaluations of the proposed ATFs and any recommendations for ATF installation. 

For the purposes of this study, an odor “area of concern” is defined as a location in the 

wastewater collection systems where airflow bottleneck occurs in the sewer pipe headspace. Past 

experience has shown that these locations are typically the primary cause of positive differential 

air pressure buildup in large diameter gravity pipe sewer systems. Positive differential air 

pressure buildup (difference in air pressure between inside and outside the sewer) can result in the 

release of odorous air to atmosphere, thereby resulting in odor complaints. 

The scope of the airflow modeling study is to identify locations with potential for odor emissions, 

referred to as “the odor areas of concern” under current conditions and examine the impact of the 

two planned wastewater flow diversion scenarios as follows: 

1. Flow diversion from NORS back to NOS 

2. Flow diversion of 30 cubic feet per second (cfs) from NOS to COS. 

In order to identify the odor “areas of concern”, a customized computer program called OPTools 

(Odor Predictive Tools) was developed as a part of this study. The program uses empirical 

surface drag equations to estimate air flow capacities in the sewer headspace, and calculates the 

difference in airflow capacities for consecutive sewer segments to identify locations with 

potential for positive air pressure buildup. 

The results of the airflow modeling are graphically represented in Figure 2 below. 




Figure 2 - Odor Areas of Concern Location Map 

Based on the airflow modeling results, it was determined that: 

 

 

 

Under the current system configurations, nine areas of concern are predicted. These 

include five locations along the ECIS from downstream of the Mission and Jesse drop 

structure to upstream of the USC drop structure; the ECIS siphon at Jefferson Boulevard 

and La Cienega Boulevard; the NORS/ECIS Junction; the NCOS Siphon at I-405 and the 

NORS Siphon at I-405. 

With the additional flow diverted into the NOS from the NORS, the upstream end of the 

NOS siphon and the NOTF location are predicted to experience positive pressure buildup 

under the assumption that the NOTF ATF is currently not being run pending ongoing 

public involvement efforts; 

The diversion of 30 cfs from NOS to COS will not cause additional areas of concern in 

the study sewers. However, the addition of the wastewater flow in the COS may cause 

positive pressure buildup along the COS. 




3.0 SEWER SIPHON STUDY 

Previous City sampling efforts have shown that headspace air pressures in the NCOS are typically 

at or below atmospheric levels, whereas in the NORS they are highly positive. Other samplings 

conducted by the City have also resulted in the conclusion that the source of this positive air 

pressure is the air dragged in the headspace of the NORS to the siphon located under the I-405. 

Field samplings from a total of twelve maintenance holes (see Figure 3 below) were collected 

over a period of time in the summer of 2009 followed by physically connecting the NORS and 

NCOS interceptors with a 200-foot, 36-inch diameter, and corrugated HDPE duct laid aboveground 

in Culver City. Air flow and air velocity samples were taken for a period of several days 

which was identified as the passive ventilation phase. 




Figure 3 - Detailed Sampling Location Map 

Following that phase, the duct was separated to permit the installation of a fan. During this active 

duct phase, more air pressure and air velocity data were collected to measure the effect of the fan 

air withdrawal from the NORS to the NCOS with fan speed varying between 5000 and 12,500 

cfm. 

Finally, the permanent 24-inch/42-inch diameter air duct connecting the upstream ends of the 

siphons of the NOS and NCOS near the 405 Freeway was plugged to test the differential air 

pressure. 

Figure 4 depicts the baseline pressure data recorded on the NORS and NCOS during the first 

week of testing. The most notable feature of this data set is the difference between the average 

differential air pressures on the NORS and NCOS. 




Figure 4 - Baseline Air Pressure 




Figure 5 - Passive Air Duct Connection Data 

Figure 5 shows the effects of the passive 36-inch diameter duct connection explained previously 

in this report between the NORS and NCOS maintenance holes. The duct was connected on July 

20, 2009. The most notable feature of this data set is that MH 535-05-016 on the NCOS was the 

location whose air pressure was most visibly affected by the passive duct connection. 

Differential air pressure data recorded along the NCOS and NORS interceptors during the active 

ventilation phase of the duct test are presented graphically in Figure 6. 




Figure 6 - Combined NORS-NCOS Air Pressure with Fan Active 

1 

Combined NORS NCOS Air Pressure 8/18/09 ‐ 8/27/09 

With Fan Active (Installed 8/18/09) 

0.8 

0.6 

Air Pressure (in. wc) 

0.4 

0.2 

The data indicates that the most significant effect of the active ventilation exercise was measured 

in the downstream reach of the NCOS (MH 535-09-009). At this location, the average pressure 

between the passive and active ventilation phases increased by 1.51 inches of water column as a 

result of the fan operation. 

The following provides a summary of the findings of the NORS/NCOS duct connection study: 

 

 

 

 

0 

‐0.2 

‐0.4 

‐0.6 

‐0.8 

During baseline testing, the average pressure levels at all locations measured were lower 

while the NORS/ECIS ATF was turned on. 

The passive air duct connection resulted in the NORS and NCOS coming to equilibrium 

with respect to differential air pressure. The pressures in the NCOS generally increased 

and the pressures in the NORS generally decreased. 

Actively forcing air from the NORS into the NCOS resulted in an increase in pressure in 

the NCOS that is deemed to be unacceptably high. The pressures in the NORS were not 

reduced significantly as a result of active ventilation. 

The cause of the negative pressures in the NCOS was likely due to either one or both of 

two factors: 

o 

o 

NCOS US of Duct Cxn (535‐05‐016) 

NORS Culver City Park (535‐05‐021) 

NORS DS of Duct Cxn (535‐13‐007) 

NCOS DS of Duct Cxn (535‐09‐009) 

NORS US of Duct Cxn (DS of NOTF) (535‐09‐006) 

NORS ECIS Jnc (535‐09‐022) 

8/18/09 8/19/09 8/20/09 8/21/09 8/22/09 8/23/09 8/24/09 8/25/09 8/26/09 8/27/09 8/28/09 

Time 

The presence of a permanent air duct that connects the NOS to the NCOS 

Low flow conditions that were in effect along the NCOS as a result of upstream 

flow diversions into the NORS due to the then ongoing rehabilitation of the NOS 

It should be noted that, during the sampling effort, a diversion was in effect that redirected all the 

NOS flow to the NORS. The project team determined that additional sampling would be needed 

when the diversion to the NORS is removed. 




4.0 DROP STRUCTURES STUDY 

Drop structures in sewer collection systems are known to be the locations of odor problems as a 

result of increased air pressures caused by the rapid vertical drop of sewage within the structures. 

The increased air pressures often result in release of the odorous air to the outside with 

subsequent odor complaints. 

The purpose of the drop structure study was to evaluate the impact of drop structures on the sewer 

differential air pressures (difference in air pressure between inside and outside the sewer), under 

current conditions and planned modifications. This study focused on measuring and examining 

differential air pressures at the four existing sewer drop structures in the City's wastewater 

collection system: 

 

 

 

Division 

Humboldt 

Mission and Jesse 

23 rd and San Pedro 

A baseline condition was established, which consisted of measuring the sewer differential air 

pressures along a route starting at the Division drop structure and ending at a maintenance hole in 

the Culver City Park as depicted in Figure 7. 




Figure 7 - Drop Structure Baseline Pressure 




This was followed by the physical plugging of the return air lines at each drop structure in order 

to measure the impact on the differential air pressures at the drop structure and downstream 

sewer. The purpose was to examine the impact of each return line upon differential air pressure 

at the structure and in the system. The results of that test are summarized in Figure 8. 

Figure 8 - Drop Structure Air Return Line Plugged Pressure 




Finally, flow management (diversion/redistribution resulting in less flow into Humboldt and more 

flow into Mission & Jesse) was introduced to the system and more differential air pressure data 

was collected to valuate the effect of increased/decrease flow on the differential air pressures at 

the structures. Results are shown on Figure 9. 

Figure 9 - Drop Structure Pressure with Flow Management 




During theses three different phases, Air Treatment Facilities (ATF) at Mission & Jesse and 23 rd 

& San Pedro were turned on and off several times in order to study their impact on the air 

pressure. 

The following provides a summary of site-specific pressure responses to the various 

manipulations performed during the study. 

 

 

 

Plugging the air return lines generally resulted in increasing pressures along the entire 

NEIS/ECIS tunnel alignment. There was some evidence of slight decrease in pressure 

immediately upstream of the drop structure. 

Allowing less flow through the Humboldt drop structure into the NEIS and more flow 

being directed into the ECIS via the Mission and Jesse Drop Structure (by removing the 

stop log at the two locations), generally resulted in decreasing pressures on the NOS. 

Those locations where the pressures didn’t decrease remained at atmospheric levels. 

The ATF’s generally reduce pressures along the entire length of the NEIS/ECIS tunnel. 

Noticeable pressure reductions occur when the ATF’s are turned on. Noticeable pressure 

increases occur when the ATF’s are turned off. 




5.0 TOTAL NON-METHANE HYDROCARBON STUDY 

In recent history, the City of Los Angeles (City) has encountered increasing public concern to 

sewer odors, and has been working to improve odor control in several areas identified by local 

community groups. Formal, permanent air treatment facilities (ATFs) for seven areas with known 

or predicted odor “areas of concern” were including in a 2004 Collection System Settlement 

Agreement (CSSA) that the City entered into with the United States Environmental Protection 

Agency (USEPA), Santa Monica Baykeeper, and others. The CSSA included several 

requirements including the installation and operation of interim and permanent air treatment 

facilities. Permanent ATFs are in the final stages of construction at two of the seven targeted 

areas. The location of the five remaining proposed ATFs need to be reevaluated based on the 

effect of wastewater flow diversions on interceptor headspace air pressure; reported performance 

issues and continued odor complaints at several of the locations. Accordingly, the City has 

embarked on a study to determine the number and location of the new proposed ATFs, assess the 

performance of the interim odor control facilities, and decide on the ATF technologies. This 

Technical Memorandum (TM) summarizes the monitoring results for parameters of interest 

conducted at the five proposed ATF locations where interim odor control facilities are currently 

operating at four of the locations. No control is provided at23rd and San Pedro, the fifth location. 

In some cases, existing interceptor odor control facilities and ATFs have air permit limits for total 

non-methane hydrocarbons (TNMHC) in their exhaust. A single distinct exhaust limit value is 

listed in each site’s air permit. The various sites have different individual permit limit values. 

Those permit values vary from 18 to 36 parts per million by volume in air (ppm v ). All of these 

existing ATFs are required to meet a hydrogen sulfide stack exhaust limit of 1 ppm v , with the 

exception of the Decotah Street facility which has an exhaust H 2 S permit limit of 0.6 ppm v . 

Given that future air pollution permits similar to the existing air permit limits for TNMHC and 

hydrogen sulfide, the City wants to have a greater understanding of the composition of the sewer 

headspace air at the five interim odor control facility locations so that permanent ATF systems 

can be designed to meet the likely future air permit requirements. Parameters of interest include: 

TNMHC, speciated volatile organic compounds (VOCs), hydrogen sulfide, and total reduced 

sulfur compounds. The interim air permits for the existing operating odor control facilities and 

ATFs require analysis of VOCs by South Coast Air Quality Management District (SCAQMD) 

test methods, including the use of SCAQMD Method 25.3 to analyze for TNMHC. 

A sampling/analysis program was conducted in April 2010 at the following five interim odor 

control facility locations where raw untreated sewer air and the exhaust from the odor control 

facilities were assessed. 

 

 

 

 

 

 

North East Interceptor Sewer (NEIS) – Richmond (Richmond) 

North Outfall Relief Sewer (NORS) –ECIS (NORS-ECIS) 

NEIS – Humboldt (Humboldt) 

East Central Interceptor Sewer (ECIS) – Mission & Jesse (Mission & Jesse) 

ECIS - 23 rd Street & San Pedro (23 rd & San Pedro) 

Note that the interim ATF at 23 rd and San Pedro was physically removed well before our 

sampling program started. As a result, the raw sewer air space was sampled and analyzed 

for all the same parameters of interest as the other locations. 

Air samples were analyzed for the following parameters: 

 

Total non-methane hydrocarbons (TNMHC) 




Speciated volatile organic compounds (VOCs) 

Hydrogen Sulfide (H 2 S) by Jerome Meter 

Continuous H 2 S by OdaLog unit 

Speciated organic reduced sulfur compounds 

The TNMHC, speciated VOCs and total reduced sulfur compound analyses were conducted by an 

independent laboratory. The speciated VOC and speciated organic reduced sulfur compound 

analyses were conducted in order to provide an understanding of the compounds that makeup the 

TNMHC air sample results. Hydrogen sulfide, in addition to the grab sample evaluation with a 

Jerome Meter, was also monitored in the sewer at each location using a continuous monitor with 

data-logging capabilities (OdaLog). 

The City has shown interest in using a hand-held VOC measuring device as an alternative to 

running Method 25.3 to determine TNMHC. A photoionization detector (PID) was used to 

analyze samples and the results were compared to TNMHC results. 

Following is a summary of findings from the sampling/analytical effort: 

At all locations, the odor control facility inlet (raw sewer air) and exhaust Method 25.3 

results were below the air permit TNMHC emission limits of 18 to 36 ppm v , except at 

Humboldt where the inlet and exhaust concentrations were 23.0 and 18.4 ppmC, 

respectively. This is graphically displayed in Figure 10. It is believed that the higher 

TNMHC and elevated hydrogen sulfide activated carbon exhaust concentrations are due 

to spent carbon at the Humboldt odor control facility. It is expected that if the carbon 

was still effective the exhaust TNMHC and hydrogen sulfide results would be notably 

reduced. The figure below, Total Non-Methane Hydrocarbon VOS Sampling Result 

Summary illustrates inlet and exhaust TNMHC concentrations relative to the permit limit 

range of 18 to 36 ppm v . 




Figure 10 - Total NMHC VOC Sampling Result Summary 

 

 

 

The percentage of speciated VOCs which accounted for Method 25.3 VOCs ranged from 

approximately 32% to approximately 100% except for locations where apparent sampling 

interferences occurred at the Humboldt inlet/exhaust, and at Mission & Jesse. 

There was limited correlation between PID readings and TNMHC results produced by 

Method 25.3 results. Given that a PID’s reading is dependent on the compounds present, 

the City should consider using an alternative hand-held field measurement monitor if a 

substitute for Method 25.3 is desired. 

The short term grab sampling of the odor control facility inlet or raw sewer air hydrogen 

sulfide concentrations ranged from approximately 2 ppm v (at Richmond) to >50 ppm v (at 

Mission & Jesse and 23 rd & San Pedro). Figure 11 below, Hydrogen Sulfide Sampling 

Results Summary, illustrates the odor control facility inlet and exhaust hydrogen sulfide 

concentrations and the average OdaLog readings. 




Figure 11 – Hydrogen Sulfide Sampling Results Summary 

 

The carbon adsorbers at Richmond and NORS-ECIS were providing hydrogen sulfide 

removals of > 99%. The carbon at Humboldt appeared to be exhausted, with an exhaust 

hydrogen sulfide concentration of 11.7 ppm v . 

An overview of hydrogen sulfide control technologies as a function of inlet concentration in 

general, and the values measured in this study are presented in Figure 12. Carbon is generally 

cost-effective at inlet concentrations less than 10 ppm v . Biofilters are cost-effective at inlet 

concentrations up to 50 ppm v and greater. At higher concentrations however, the biofilter media 

degradation accelerates. Bio-trickling filters are cost-effective at inlet concentrations of 20 ppm v 

and higher, which indicates some overlap for the two biological systems. Figure 12, Hydrogen 

Sulfide Emission Control Overview, can be used as a basis to evaluate control technologies in the 

next phase of this project. 




Figure 12 - Hydrogen Sulfide Emission Control Overview 




6.0 ATF STUDY 

The ATF Study was conducted after all field tests were performed and the associated Technical 

Memorandum described the results of the analysis of the data collected for the ATF Review 

Study. It includes 

 

A summary of the field test conducted for the study, including the siphons and drop 

structures 

 

 

 

 

 

The results of the airflow modeling for current conditions and future scenarios 

Recommendations for an ATF location, drop structure modifications and flow diversion 

based on the results of observations and analysis following the field data collection 

Characterization data from five of the seven locations identified in the 2004 Collection 

System Settlement Agreement 

Various available technologies for controlling nuisance related odors and air emissions 

that are associated with wastewater collection systems 

Recommended technology for the ATF at Mission and Jesse. 

6.1 ATF Locations 

The following are the general conclusions that may be drawn from analyzing pressure data 

recorded during the drop structure field study with the various system modifications described in 

the technical memorandum. 

 

The existing interim air scrubbers generally reduce pressures along the entire length of 

the NEIS/ECIS tunnel. This appears to be especially the case with the Mission and Jesse 

interim scrubber. Measurable pressure reductions occur in the NEIS/ECIS tunnel when 

this interim scrubber is turned on. Measurable pressure increases occurred in the 

NEIS/ECIS tunnel system when the Mission and Jesse interim scrubber was turned off. 

 

 

 

The Humboldt interim scrubber only appears to affect pressures in its immediate vicinity 

Plugging the air return line plugs generally resulted in increasing pressures along the 

entire NEIS/ECIS tunnel alignment 

The stop logs removal decreasing the flow at Humboldt and increasing it at Mission & 

Jesse generally resulted in decreasing pressures on the NOS. Those locations where the 

pressures didn’t decrease remained at atmospheric levels. 

6.2 Air Emissions Characteristics 

To obtain emissions characterization data from five of the seven locations identified in the 2004 

Collection System Settlement Agreement (CSSA) that the City entered into with the USEPA, 

Santa Monica Baykeeper, and others, a monitoring plan was developed. The remaining two 

locations were not included in this characterization plan since the City determined to move ahead 




with constructing ATFs at these sites. Those ATFs have been constructed, have passed their 

performance tests, and are about to be commissioned. The monitoring plan included sampling at: 

North East Interceptor Sewer (NEIS) – Richmond 

North Outfall Relief Sewer (NORS) – NORS – ECIS 

NEIS – Humboldt 

East Central Interceptor Sewer (ECIS) – Mission & Jesse 

ECIS – 23 rd Street & San Pedro 

The samples collected at these five locations were analyzed for the following parameters: 

Hydrogen sulfide (H 2 S) 

TNMHC 



The following analytical methods were used to determine the concentrations of the various 

specific compounds and classes of compounds: 

TNMHC - SCAQMD Method 25.3 

Speciated volatile organic compounds - EPA Method TO – 15 and 12 Modified 

Speciated organic reduced sulfur compounds - ASTM 5504 

These methods were carefully selected to identify those compounds that together made up the 

TNMHC value, and to ensure that the two approaches reached a similar endpoint. 

For more details on the sample collection, the analytical procedures, and the analytical results the 

reader is referred to the Total Non-Methane Hydrocarbon Monitoring Results Technical 

Memorandum (TM) submitted in draft to the City in June 2010. 

The analytical results for the samples collected from the manhole at Mission and Jesse where the 

interceptor odorous air is extracted and forwarded to the interim odor control system (consisting 

of activated carbon) are presented in Table 1. Table 1 includes those analytical results that are 

directly related to permit limits. For information on speciated organic compounds and reduced 

sulfur compounds refer to the TNMHC TM. 

Table 1 - Mission & Jesse Interceptor Raw Emissions Characterization Results 

 

Parameter 

TNMHC – Methods 25.3 (ppmC) 1 14.0 

Non-VOCs (ppmC) 1 0.47 

Adjusted Method 25.3(ppmC) 1 13.53 

Hydrogen Sulfide (ppm) 

Grab Sample >50 2 

OdaLog Unit 4 

• Minimum 

• Average 

• Maximum 

1 

ppm C – parts per million as carbon. 

2 

The monitoring device, Jerome Meter, has a upper H 2 S limit of 50 ppm. 

3 

The monitoring device, OdaLog, has a lower detection limit of 0.25 ppm. 

4 

The OdaLog unit was placed in the interceptor maintenance hole. 

Mission & Jesse Analytical Results 

6.3 Air Emissions Control Technologies 

Many different technologies are available for controlling nuisance related odors and air emissions 

that are associated with wastewater collection systems. Each technology has a niche when it is 

most suitable to provide the level of control required to reduce off-site odor impacts. The degree 

of control required at any one site is dependent on several factors, including, but not limited to: 

physical setting; metrological conditions, topography of the land surrounding the odor control 

site; proximity of the nearest neighbors and/or nearest sensitive receptors; permit requirements 

established by South Coast Air Quality Management District (SCAQMD); and unobstructed view 

of the air treatment facility (ATF). These factors will need to be considered when evaluating the 

many different control technologies. The challenge is identifying the most cost effective, viable 

control technology that will achieve the goal of the City of Los Angeles (City) and comply with 

the limits established in the air permit for each site. 

The most commonly applied air control technologies used today that were suitable to this study 

include the following: 

 

Wet scrubbing 

 

 

Biotrickling 

Activated Carbon 

 

Combinations 

For a more detailed description of the technology, please refer to the ATF TM that is part of this 

study. 




7.0 RECOMMENDATIONS 

7.1 ATF 

No forced air treatment system is recommended for Division, Humboldt, Richmond and 23 rd & 

San Pedro. Since the Mission & Jesse drop structure appeared to be the primary conduit through 

which the NEIS/ECIS tunnel relieved its air pressure back into the NOS upstream, a permanent 

ATF is therefore recommended for this location. 

7.2 Drop Structure Modifications 

The pressures in the NOS upstream approach sewer were reduced slightly as a result of plugging 

the air return line at all the drop structures. There is evidence that installation of a flow regulation 

device (“dampler”) in the air return line of the drop structure, such as an adjustable damper may 

be beneficial. However, it is recommended to complete the drop structure model testing before 

finalizing the damper concept. Such a device would give BOS the flexibility to adjust the amount 

of air that is allowed to flow through the air return line under current or future operational 

conditions. 

Additional modifications to the Mission and Jesse Drop Structure could include installing two 

curtains or similar flexible devices. One curtain would be installed at the downstream end of the 

plunge pool; the other curtain would be installed on the NEIS just upstream of the junction with 

the Mission and Jesse drop structure as shown in Figure 13. However, these decisions should be 

delayed pending the outcome of the hydraulic model study. 

An additional air pickup should also be designed and constructed for the recommended Mission 

and Jesse ATF. This air pickup would be located along the NEIS tunnel just upstream of the 

recommended NEIS curtain. This additional air pickup is shown in Figure 13. 

Figure 13 - Mission & Jesse Additional Air Pickup 




7.3 Flow Diversion 

The system air pressures appeared to react favorably to less flow through the Humboldt drop 

structure into the NEIS and more flow being directed into the ECIS via the Mission and Jesse 

Drop Structure. This was accomplished by removing the stop log at the two locations, which 

allowed the approach flows in the NOS to split naturally between that sewer and the NEIS. It is 

therefore recommended that the stop logs be configured at Humboldt in such a manner that a 

minimum amount of flow is directed into the NEIS and to be configured at Mission & Jesse in 

such a manner that a maximum amount of flow is directed into the ECIS. This would likely 

result in less air being dragged into the NEIS and therefore, lower air pressures in the NEIS/ECIS 

tunnel system. 

An overall graphic summary of the recommendations for the NEIS/ECIS drop structure locations 

between the Division Drop Structure and the ECIS Jefferson Siphon is presented in Figure 14 

below. 

Figure 14 - NEIS/ECIS Recommendation Summary 

These above recommendations must be revisited following the completion of the pending 

physical model testing. Additionally the effects of the recommendations should be verified 

with a full scale fan test field study aimed at verifying the effect of the combination of the 

adjustable air plugging devices and the recommended ATF at Mission and Jesse. 

7.4 Technology 

Based on the monitoring results from the TNMHC study, the discussions on the technologies, and 

the comparison tables of advantages and disadvantages for each of the individual control 




technologies, the recommended technology for the Mission and Jesse location is a two stage, 

combination system consisting of: 

 

First Stage - Biotrickling filter 

 

 

 

 

Second Stage - Activated Carbon 

The biotrickling filter will remove 99% of the influent H2S and limited other reduced 

sulfur compounds, and the carbon will: remove any residual H2S down to the low 

parts per billion (ppb) concentrations (Well below the projected permit limit of 1.0 

ppm.); remove any residual reduced sulfur and odorous compounds; and reduce the 

levels of VOCs. This will comply with the permit conditions and keep nuisance odors 

from impacting the surrounding communities. 

This recommended full air treatment facility is necessary in order to treat the elevated 

levels of influent H 2 S concentrations, the modest concentrations of VOCs and the 

remaining odorous organic and inorganic compounds. Implementing this two-stage, 

combination, air emissions control system will minimize any risk facing the City with 

respect to permit compliance and being a good neighbor. 

As presented in Section 5- Determine Air Flow, the actual air flow to be treated by the 

ATF is yet to be established. In the interim it is projected that a 20,000 cfm flow rate 

would be necessary at this recommended Mission and Jesse ATF. The ATF attributes 

will depend on the vendor providing the ATF units and the design criteria developed 

by the design engineer in coordination with the City staff. 




APPENDIX A 

Air Treatment Facility (ATF) Review 

Draft Final Report 

Comments by the Independent Odor Expert – Dirk Apgar 

October 2010 




Air Treatment Facility (ATF) Review 

Draft Final Report 

Comments by the Independent Odor Expert – Dirk Apgar 

October 2010 

Contents 

Introduction ..................................................................................................................................... 1 

Airflow Modeling Study ................................................................................................................. 2 

Sewer Siphon Duct Connection Study ............................................................................................ 4 

Drop Structure Study ...................................................................................................................... 7 

Total Non-Methane Hydrocarbon Study ........................................................................................ 9 

Air Treatment Facility Study ........................................................................................................ 12 

Recommendations of the ATF Review Study Draft Final Report ................................................ 15 

General Remarks and Conclusions of the Independent Odor Expert ........................................... 16

Introduction 

This paper is intended to provide a constructive critique of the City of Los Angeles’ Air 

Treatment Facility Technical Draft Final Report (Report). The Report summarizes the technical 

memoranda produced during the City’s Air Treatment Facility (ATF) Review Study, which was 

conducted to develop an understanding of the causes of odor problems originating from the 

City’s wastewater conveyance system and reach conclusions on methods of controlling those 

nuisance odor problems. 

Under the original Collection System Settlement Agreement (CSSA), the City had planned to 

construct seven new ATFs to help control odors from the wastewater conveyance system. Of the 

seven, two have almost been completed (Jefferson & La Cienega and 6000 Jefferson), but design 

and construction of the remaining five was placed on hold pending the outcome of the ATF 

Review. As part of the review, the City reevaluated the location and necessity of the five 

remaining proposed ATFs based on the effect of changing wastewater flows through its 

interceptors and the associated effect on sewer headspace air pressure, measured performance of 

the interim carbon scrubber based ATFs, and continued odor complaints. Final recommendations 

on the location and technologies to be used in a single ATF at the Mission and Jesse site were 

reported as a result of the ATF Review. The determination of the size of the ATF will be made 

during the design process for the new facility utilizing information the City will obtain through 

testing of a drop structure scale model similar to the structure at Mission and Jesse. 

The City has undergone a significant level of work in an attempt to develop an understanding of 

the airflow and air pressurization within its wastewater conveyance system. It is rare that this 

level of effort is seen within the United States or, for that matter, worldwide. While the City and 

its consulting engineering firms have many capable technical experts, it is important to 

understand that estimating the causes and effects of the airflow dynamics in complex sewer 

systems and the resulting likelihood of odor emissions and impacts can only be done within a 

broad range of accuracy. It is possible that while odor impacts may be diminished, communities 

may continue to experience odors even with the activation of the two ATFs currently nearing 

completion, modification of wastewater flow patterns, and the implementation of the 

recommended ATF at Mission and Jesse. As the ATFs at Jefferson & La Cienega and 6000 

Jefferson are brought up to their design airflow capacities, neighboring citizens should be able to 

assess their ability to control odor emissions and impacts in their community. 

Below is a summary of the major components of the ATF Review Study. Each has been 

discussed previously in comments by the Independent Odor Expert with input by 

Community Liaison, The City Project. 

1

Airflow Modeling Study 

The Airflow Modeling Study (Modeling Study) was conducted by the City in an attempt to better 

understand how flowing wastewater influences the movement of the free air in the sewer system 

and better predict where odor emissions may occur. The model created in this instance relied on 

major assumptions regarding the City’s existing sewer system. As with all models, the 

conclusions of the study can only be as good as the assumptions upon which it is based. The 

usefulness of the Modeling Study results should not be summarily discounted, however, they 

should be viewed as being within a broad range of accuracy and should not be viewed as precise 

or as the only information that should be used to locate and size ATFs for odor control. 

The movement of air above wastewater in a sewer is largely dependent on the viscous drag of the 

liquid on the gas. Viscous drag is the force exerted on the air by the moving water due to friction. 

Air movement is also influenced by the ambient air pressure and movement outside the sewer, 

heating or cooling of the air, and obstructions within the sewer. As moving air meets an 

obstruction it can be compressed, resulting in increased sewer air pressure. If the air pressure 

internal to the sewer is higher than that of the surrounding atmosphere, there is the potential for 

foul air to escape and cause nuisance odor impacts in the community. 

The stated scope of the Modeling Study was to create a model that could identify odor “hot 

spots” under several potential wastewater flow scenarios. These hot spots are not points of actual 

or even modeled odor impacts, but locations in the sewer system where the model indicates 

airflow is likely to be restricted. According to the Study, past experience has shown that airflow 

restrictions are the primary cause of high air pressure in the sewer. Since we know high sewer 

pressure (in comparison to lower atmospheric pressure) can cause foul air to escape, being able 

to predict possible locations of sewer airflow obstruction/high air pressure, is one way to predict 

the location of odor impacts in the community. 

To estimate where these hot spots could occur, the City’s engineering team used a pair of 

computer models to understand wastewater flow conditions and developed a third model to 

estimate how flowing wastewater would cause air to move within the sewer and where that 

movement may become restricted. It was explicitly stated that the airflow model is not intended 

to model air differential pressure, but only to identify hot spots as defined above. 

Modeling Methodology 

The City used commercially available computer software to model and analyze wastewater flows 

under various conditions. They developed an additional computer program that used the output 

of the wastewater flow predicting software to estimate where the characteristics of the sewers 

and the flowing wastewater could result in a location where the capacity of the system to carry 

air would become restricted. In that case, a “hot spot” was identified. 

The Modeling Study explains that the airflow model was developed assuming only that the 

movement of the wastewater is responsible for air movement within the sewer. The airflow 

model calculates the airflow capacity within a segment of the sewer based on the wastewater 

velocity, the depth of wastewater, and the geometry of the pipe. Based on that information, the 

airflow velocity is estimated as a fraction of the wastewater velocity. That fraction varies with 

depth of the wastewater and the diameter of the sewer. The volumetric airflow is calculated by 

2

multiplying the air velocity by the cross section of the free air space. The airflow is similarly 

calculated for the next downstream sewer segment. If the first segment’s airflow is greater than 

the second segment’s, it is assumed that the potential for compression of the air exists. If the 

difference in airflows in the two segments is greater than 2,000 cubic feet per minute (cfm) a hot 

spot is identified. 

Modeling Assumptions and Accuracy 

The City’s model is based on the assumption that flowing wastewater is the only influence 

causing air to move within the sewer. This phenomenon is well documented and could provide a 

reasonable first estimate of airflow. However, while the drag of flowing wastewater has a 

significant influence on sewer airflow, air pressure up and downstream of the sewer segment 

modeled can also have a profound effect on air movement as can the effects of air buoyancy and 

friction between the moving air and the sewer wall. A stated limitation of the model is that it 

does not account for back pressure effects from siphons, drop structures, slope reductions or 

junction structures. The effect of air treatment facilities on airflow is also ignored by the model. 

All models predict reality with some degree of error and the absolute accuracy of the City’s 

model is difficult to gage. Field data was collected in an attempt to determine if the model was 

predicting those areas where pressurization of the sewer air was occurring. Air pressure was 

monitored at six locations where hot spots were predicted by the model. Data from one of the 

locations was discounted due to the strong effect of the air treatment facility at the Hyperion 

wastewater treatment plant. Data from the remaining five locations did show good correlation 

between the modeled and real-world conditions. However, only locations identified by the model 

to have a build-up of pressure were sampled. It would have been helpful if locations that the 

modeled airflows indicated lower or neutral pressure were also monitored. This would have 

provided additional insight to the model’s accuracy. 

Model Analysis and Hot Spot Identification 

Analysis to identify hot spots and estimate the airflow within certain sewer sections was 

performed using three different wastewater flow scenarios. These represented the current and 

possible future sewer configurations that would divert flows from the North Outfall Relief Sewer 

(NORS) to the North Outfall Sewer (NOS) and from the NOS to the Central Outfall Sewer 

(COS). For each of these three scenarios, the airflow model was used to identify hot spots based 

on a 48-hour period using typical weekday and weekend wastewater flows. 

The Study states that the airflow rate within the sewer is one of the design criteria for any future 

air treatment facility. This is a reasonable statement. Knowledge of the sewer airflow is 

necessary to determine the rate of air extraction by an ATF to lower the pressure in the sewer to 

prevent odor emissions. Current airflow rates were estimated at several locations that appear to 

be associated with known odor problems. Airflows at the upstream ends of the North Central 

Outfall sewer (NCOS) and NORS siphons near the I-405 freeway and five drop structures were 

modeled. The Study states that actual airflow rates can differ from the model-estimated rates due 

to the disregard of ATF ventilation and airline effects. The Study reports the modeled airflows 

for eleven locations near the NCOS, NORS, and the drop structures for each of the three 

wastewater flow scenarios. 

3

Some of the model-identified hot spots were dismissed due to known actual conditions. Those 

locations that were dismissed were those known or assumed to be affected by ATFs and 

therefore not sources of odor problems. Under the current wastewater conveyance system 

configuration, nine hot spot locations identified by the model were considered valid. Six of these 

hot spots were located on the East Central Interceptor Sewer (ECIS), one was on the 

NORS/ECIS Junction, and the remaining two were on the NCOS and NORS siphons at I-405. 

When modeling the two potential future sewer configurations, three additional hot spots were 

identified on the ECIS, one at the NOS siphon and one immediately downstream of the North 

Outfall Treatment Facility (NOTF) on the NOS. 

General Remarks and on the Airflow Modeling Study 

The City’s engineering team has produced a model to estimate airflows and hot spot sites of 

potentially high pressure within the sewer system. The model estimates the airflow based on the 

assumption that air movement is only due to the viscous drag on the air by moving wastewater 

and ignores several other significant parameters. However, given the state of the art of sewer 

airflow modeling, it is not an unreasonable way to gain insight into the potential for air pressure 

increases, potential locations of odorous emissions, and to understand the magnitude of the air 

volumes moving within the sewer. The estimate of the volumetric airflow is important to 

quantify so that air treatment facilities can be sized to neutralize the potential for high air 

pressure at the model identified hot spots. 

Given the current state of the art of modeling airflow within sewers, it is important to corroborate 

modeled information with real world data and observations. The City’s team has conducted some 

field data collection that indicates their model is capable of identifying points of air 

pressurization within the sewer. These locations are compared with odor complaint locations in 

the ATF Study (discussed later in this paper) to further verify the validity of the model. 

From the information in the Modeling Study, one can conclude that, as a first estimate, the 

airflow model provides a reasonable estimate within a broad range of accuracy. The model was 

used in the ATF Study along with field data and observations as an aid to determining the need, 

location, and size of ATFs. 

Sewer Siphon Duct Connection Study 

The stated purpose of the Sewer Siphon Duct Connection Study (Siphon Duct Study) was to 

examine the effect that existing interceptor siphons located under the I-405 freeway have on 

airflow dynamics in the interceptor collection system. The interceptor collection system 

discussed in the Siphon Duct Study is made up of the North Outfall Relief Sewer (NORS), East 

Central Interceptor Sewer, North Central Outfall Sewer (NCOS) and the North Outfall Sewer 

(NOS). 

The focus of the Siphon Duct Study was to examine the potential benefit of adding a duct to 

connect the air spaces of the NORS and NCOS. Because the NORS airspace has been shown to 

have consistently higher than atmospheric pressure and the NCOS lower than atmospheric 

pressure, it was hoped that the two could be connected, allowing air to flow from the high to low 

pressure sewers and decrease the odorous emissions from the NORS. Earlier investigation on this 

topic was described in the City’s NORS Siphon Air Line Feasibility Study. After that work was 

4

produced, additional investigatory tasks were accomplished and documented in the Sewer Duct 

Study that reports on the findings and conclusions based on an analysis of the data collected. 

In addition to examining the effects of joining the NORS and NCOS air spaces, the Siphon Duct 

Study also reports on the affects of blocking an existing permanent air duct that joins the NCOS 

and the NOS. This was done in an attempt to understand why the NCOS is at a consistently low 

pressure. 

Rational and Methodology of Siphon Study 

The City and its consulting engineers formulated the Siphon Study to perform air pressure 

measurements in the as-built (no NORS/NCOS air duct connection) to establish a baseline data 

set. The air duct would then be installed and air allowed to flow freely between the NORS and 

NCOS while additional air pressure data and airflow measurements through the duct were 

recorded. After the free airflow data was collected a fan would be place in the air duct forcing air 

from the NORS into the NCOS to determine if that scenario would provide positive results with 

respect to controlling odors. In order to gain an understanding of the affect the existing air 

treatment facilities’ extraction of air from the sewers had on air pressure and flow within the 

sewer system under these various scenarios they would be cycled on and off during the data 

gathering period. 

Baseline Data Collection Phase 

The baseline air pressure measurements were taken over approximately three week’s time 

beginning in early June of 2009. During that time the NORS was consistently pressurized 

between approximately 0 to 0.4 inches water column with an average of approximately 0.2 

inches water column. There were almost daily excursions of the NORS pressure below 0 inches 

water column, but for the most part it was positively pressurized with respect to the atmosphere. 

The NCOS baseline pressure measurements were taken simultaneously with the measurements in 

the NORS. The NCOS data showed that that sewer’s air space was consistently lower in pressure 

than the surrounding atmosphere with an average pressure of approximately -0.39 inches of 

water column. 

The pressure of 1 inch of water column is literally the pressure exerted by water one inch below 

its surface. This may seem slight but the ventilation of air requires very slight pressure 

differences. Pressure in heating and air conditioning systems are typically measured in these 

units. Significant air volumes can be moved with differential pressures in the 0 to 1 inch water 

column range and the pressures measured in the NORS and NCOS could reasonably be assumed 

to have the capability to transfer large volumes of air between the two sewers. This justified the 

further investigation of the efficacy of allowing that to occur with the air duct connection. 

Further baseline data collection was taken while the NORS/ECIS ATF system was turned off so 

that its 10,000 cfm capacity was not being extracted from just upstream of the NORS/ECIS 

junction. During that time the data revealed that the pressure in the two sewers was higher than it 

was when the AFT was turned on as one would expect. The pressure measurement data taken 

from the NCOS showed that the ATF’s influence could be detected miles from the point at which 

5

the air was extracted from near the NORS/ECIS junction. This data could prove useful in the 

locating and sizing of future AFT facilities. 

Passive Air Duct Connection Data Collection Phase – Description and Critique 

A 36-inch duct was used to connect maintenance holes on the NORS and NCOS. This allowed 

the higher pressure in the NORS to be relieved while causing air in the range of 3,000 to 3,500 

cfm to flow into the NCOS and raising the pressure in that sewer. The increase in pressure in the 

NCOS was significant, rising from an average of -0.39 to +0.03 inches water column, while the 

pressure in the NORS decreased by only 0.06 inches wc from 0.20 to 0.14 inches wc. The 

decrease in pressure in the NORS could result in fewer odor emissions but the increasing 

pressure in the NCOS could result in odor emissions from that sewer. 

During the passive air duct connection pressure monitoring, the NORS/ECIS and Jefferson ATFs 

were cycled on and off. When the scrubbers (rated at 10,000 cfm each) were extracting air from 

the sewers, the airflow from the NORS into the NCOS was from 3,000 to 3,500 cfm as 

mentioned above. With those interim ATFs turned off, the airflow dropped to approximately half 

those values and the pressure increased by 0.1 and 0.17 in the NORS and NCOS respectively. 

This would indicate that the effect of the air extraction by the ATFs has a more profound effect 

on the NCOS than on the NORS in the vicinity of the duct connection. 

Active Air Duct Connection (Forced Ventilation) Data Collection Phase – Description 

and Critique 

During the active air duct data collection phase, an axial flow fan was placed in the 36 inch 

diameter duct to extract air from the NORS and force it into the NCOS. The apparent intention of 

this phase was to determine if mechanically extracting air would relieve more pressure from the 

NORS than during the passive ventilation phase and what affect would be seen in the NCOS. 

The fan was operated to achieve target airflows of 5,000, 7,500, 10,000 and 12,500 cfm. 

As a result of the active ventilation described above, three significant and related issues became 

evident. First, and most importantly, the average pressure in the NCOS became very high at 1.5 

inches wc above that found in the passive ventilation test. Second, the pressure in the NCOS 

fluctuated extremely between the positive and negative pressure values indicating that very 

unpredictable odorous emissions could come from the NCOS with the forced ventilation option. 

Third, there was only a minor benefit to the pressure condition in the NORS with the average 

pressure decreasing by only about -0.03 inches wc in that sewer. 

The fact that the active ventilation led to only a small reduction in the NORS average pressure 

while the NCOS became so highly and erratically pressurized leads to the conclusion that the 

active air duct connection is not a viable option for controlling odors in the NORS/NCOS 

system. 

Air Duct Isolation between NOS and NCOS – Description and Critique 

With the information obtained from the investigation and data gathering described above, the 

reason for the consistent negative pressure in the NCOS still remained unclear. The City’s team 

determined that additional sampling would be needed to determine the cause. It was decided to 

6

lock the airflow in the permanent air duct that connects the NCOS to the NOS near their 

respective siphons under the I-405 freeway. It was reasoned that this would allow the team to 

determine if the low pressures in the NCOS were influenced by the NOS. 

The permanent air duct was blocked and pressure monitors were used to record pressure data for 

a period of approximately two weeks. Pressure in the NCOS was observed to not change 

appreciably while the pressure in the NOS markedly increased. This would indicate that the 

NCOS was not influenced by the NOS but that the opposite was true. The NCOS influenced the 

NOS pressures, keeping them lower as a result of the air duct being open. 

One additional conclusion was noted in the Siphon Duct Study. That was that the low flow 

conditions in the NCOS at the time the data was being collected were the cause of the negative 

pressures in that sewer. It would be reasonable to assume that the lack of high pressure would 

result from the low flow conditions but there are likely additional factors that lead to negative 

pressure in the NCOS. These lower than atmospheric pressures could be caused by wind passing 

over maintenance holes causing air to flow out of openings in their lids, temperature differences 

between the sewer air and the surrounding atmosphere or sewer conditions upstream of the 

pressure data collection area that may influence the NCOS near the I-405 freeway siphon. 

General Remarks on the Siphon Duct Study 

Understanding the dynamics of airflow and pressure distribution is necessary to effectively 

control wastewater conveyance system odors. Good real-world data helps engineers locate, and 

size air treatment facilities for appropriate foul air extraction and treatment to minimize odor 

emissions from sewers and adequately treat the air prior to its discharge to the neighborhood. 

The Siphon Duct Study provides some of the fundamental information that will aid with the final 

design of odorous air management and control systems. 

Complete understanding of the airflow characteristics in a sewer system is never achievable but 

it is possible to move forward with good designs based on limited knowledge. Information 

regarding the way wastewater flow influences airflow and pressure distributions in the sewers 

was not included in the Siphon Duct Study. That topic was covered in the Airflow Modeling 

Study (discussed above). 

In a separate NORS Siphon Air Line Feasibility Study, the City concluded that installing a 

permanent duct between the NORS and NCOS could provide partial relief of the high pressure in 

the NORS and that an attractive aspect of this alternative is its simplicity and relatively low cost. 

That study indicates that it would cost approximately $200,000 to install the permanent duct. The 

work in the Sewer Siphon Duct Connection Study would support this conclusion. 

Drop Structure Study 

The purpose of the Drop Structure Study was to develop a better understanding of how 

wastewater flowing through drop structures influences the pressure of the free air space in the 

structure and connecting sewers. The Drop Structure Study also examined the affect turning on 

and off various interim air treatment facilities had on the sewer air pressure. 

7

A drop structure is necessary when the elevation of a sewer must change abruptly. This can be 

required at the confluence of two sewers when one is higher than the other. Air above the 

flowing wastewater is dragged in the approach sewer and with the flow falling into the drop 

structure. This action can cause high pressures to develop if the moving air is obstructed from 

traveling downstream from the drop structure. To help alleviate this problem an air return line 

connecting the bottom and top of the drop structure is often included in the design. The City had 

observed that drop structures with these air return lines often had high pressures in their 

approach sewers and that at least one drop structure without a return line operated with an 

approach sewer having air pressure lower than atmospheric. Those sewers with high pressures 

could cause odors to escape and impact the local community while the low air pressure sewer 

would be unlikely to be offensive. 

Due to the observed air pressures in and around the drop structures, the City performed a series 

of tests to measure the effects of (1) blocking the air return lines, (2), modifying wastewater 

flows, and (3) cycling the relevant interim carbon scrubber ATFs off and on to measure the 

effects on air pressures upstream and downstream of five drop structures. 1 

The Drop Structure Study’s additional stated objective was to identify potential changes to the 

planned odor control strategies and methods to control odorous air releases in the proximity of 

the drop structures. 

Investigation Procedures 

The City’s team conducted a series of experiments in the field that first involved measuring the 

air pressure in the sewers as they are currently built and operated to obtain baseline data. Data 

was gathered in the North Outfall Sewer (NOS) and Eagle Rock Interceptor Sewer (ERIS) 

approaches to the drop structures, in the drop structures, and in the North Outfall Relief Sewer 

(NORS), East Central Interceptor Sewers (ECIS) and North East Interceptor Sewer (NEIS) on 

the downstream side of the drop structures. 

The air return lines were then blocked with inflatable plugs to prevent airflow from the bottom of 

the drop structures from flowing back to the top of the drop structures. As this was done, 

additional air pressure data was collected. Then, wastewater flows through the drop structures 

were manipulated by removing barriers known as stop logs within diversion structures in the 

sewers. These diversion structures, as the name implies, are used to divert flow from one sewer 

to another. Finally, while the air return lines were blocked and the flows manipulated in the drop 

structures, the interim ATFs at the Humboldt and Mission and Jesse locations were cycled on 

and off. Air pressure data was collected to determine if the air they extracted from the sewer 

would influence the air pressures in the drop structures as well as the upstream and downstream 

sewers. 

1 The drop structures tested were located at Division, Humboldt, Mission & Jesse, and 23 rd San 

Pedro and the University of Southern California (USC). 

8

General Remarks on the Drop Structure Study 

The City of Los Angeles has gone to considerable effort to understand how sewer air pressure is 

influenced by drop structure design, wastewater flow, and operation of interim ATFs. While 

there is good information that can be derived from the data collected, there is no one obvious 

conclusion from the information obtained that will allow a clear decision to be made regarding 

the solution to the problem of air emissions from the sewers and the odor impacts that result. 

This is not to say that the information is not valuable; the information should reasonably lead to 

better informed engineering decisions. What can be gleaned from the data is that a number of 

changes that each contribute a little to the solution could be made to lower the probability of 

odor emissions. 

The most significant conclusion that can be drawn from examining the Drop Structure Study is 

that it will likely be necessary to use more appropriately sized and located ATFs to remove air 

from the sewers and thus reduce the high pressures that result in uncontrolled odor emissions. As 

long as such high pressures exist in the sewers, uncontrolled emissions are virtually certain. 

Because none of the methods investigated in the Drop Structure Study (including blocking air 

return lines and manipulating wastewater flows), could dependably achieve a substantial and 

consistent reduction in air pressure, the only real alternative is to remove more sewer air with 

ATFs. 

The information obtained in the Drop Structure Study was used to help initially size and locate 

ATFs. Final ATF locations and sizes should be determined after scale drop structure model 

testing in the City’s hydraulics laboratory and through a formal pre-design and design process. 

Total Non-Methane Hydrocarbon Study 

The City is required under the terms of the CSSA to install and operate interim and permanent 

ATFs for the control of odors from the sewers. These ATFs must have permits issued by the 

South Coast Air Quality Management District (SCAQMD) that will allow the City to build and 

operate the facilities. Permits of this type place limits on ATF pollutant emissions that could 

have nuisance or human health effects. As part of its ATF Review, the City monitored total nonmethane 

hydrocarbons (TNMHC) and odorous compounds in the sewer air and produced a 

technical memorandum summarizing its finding. 

Some organic 2 pollutants that may be emitted from the ATFs can react in the atmosphere 

forming ozone (O 3 ), which is one of the first pollutants regulated under the Clean Air Act. The 

concentration of O 3 in the City’s air is higher than the regulatory limit. As a result, the City is 

considered a non-attainment area for that pollutant by the United States Environmental 

Protection Agency (US EPA). The SCAQMD limits emissions of these organic pollutants as a 

way to limit the production and concentration of O 3 . The existing interim ATFs are required to 

meet SCAQMD TNMHC and hydrogen sulfide (H 2 S) emission concentration limits described in 

their permits. Future permanent ATFs will certainly have limits on these pollutants as well. 

2 Substance whose molecules contain one or more carbon atoms with the exception of 

carbonates, cyanides, carbides, and a few others. 

9

As part of the City’s ATF Review, the City wanted to gain knowledge about the pollutants in the 

sewer air that the new ATFs would be treating and about the ability of the interim ATFs to 

control these pollutants. This would allow the City to determine if the interim ATF technology 

(carbon bed scrubbers) could be used as part of the new ATF systems. The purpose of the work 

was the following: 

Understand the characteristics of the air pollutants in the sewer air that would be treated 

by the new ATFs; 

Determine if the concentrations of the various organic compounds in the sewer air 

summed to a similar value as that determined by the SCAQMD TNMHC testing method; 

Determine if there were organic compounds in the sewer air that should not be included 

in the calculation of the TNMHC concentration; 

Compare the measured pollutant concentrations in the interim ATF exhaust against the 

SCAQMD permit limits; 

Assess the pollutant removal efficiency of the interim ATFs; and 

Determine if a handheld device could be used to measure TNMHC concentrations 

Sampling and Analysis of Sewer Air Pollutants 

The City simultaneously extracted air samples for analysis by several laboratory methods. Two 

EPA methods, TO-12 and TO-15, were used to determine the concentration of specific organic 

compounds, SCAQMD Method 25.3 was used to quantify TNMHC concentration, and American 

Society for Testing and Materials (ASTM) Method D5504 was used to quantify sulfur compound 

concentrations in the sewer air. In addition to these methods, H 2 S data monitors were used to 

spot check and collect concentration data over extended periods. A handheld photo-ionization 

detector (PID) was also used with hopes that it could be used as an alternative to Method 25.3 for 

measuring TNMHC concentrations. 

Pollutants in the Air Stream 

The two EPA methods yielded a great deal of information about which organic compounds are 

present in the sewer air that made up the interim ATF inlet airstream and in the exhaust air 

streams. The information about the individual compounds is valuable in that it could be used in 

the evaluation of future ATF technologies for controlling their emission into the atmosphere at 

concentrations acceptable to the SCAQMD. The pollutants identified by these methods probably 

contribute only slightly to the total sewer odor given the concentrations reported. The sulfurous 

compounds identified by the ASTM method are likely the main cause of sewer odors in these air 

streams. 

The City was interested if the EPA methods would find some compounds that do not cause O 3 

production in the atmosphere, but could influence the TNMHC value determined by the 

SCAQMD method. If the EPA methods did find such compounds, that could call into question 

the permit limit on TNMHC concentration that SCAQMD uses to limit atmospheric O 3 levels. 

Ultimately, discounting the influence of those non-O 3 producing compounds did not appreciably 

change TNMHC concentration, so the SCAQMD method appears to be a reasonable way of 

finding the concentration. 

10

Using the information obtained by the EPA methods, the total carbon in the organic compounds 

was summed and compared to the TNMHC value obtained by the SCAQMD Method 25.3. The 

values determined by the two methods differed by as much as a factor of five and were not 

consistent between sites. Examination of the Method 25.3, TO-12, and TO-15 did provide 

information on which compounds most influenced the TNMHC concentrations. This information 

could be used in the selection of ATF technology if it is deemed necessary to lower the 

concentration of those most influential compounds to meet the permit limits. 

The interim ATFs were able to control the emission of H 2 S and TNMHC to their individual 

permit limits at all locations with the exception of the Humboldt interim ATF. The concentration 

of TNMHC in the exhaust stream was measured at 18.4 parts per million as carbon (ppmC). This 

exceeded the permit limit of 18 ppmC. The Technical Memorandum states that the relatively 

high exhaust concentration was likely due to the fact that the carbon was spent, which means that 

it was past the end of its useful life. This is a reasonable conclusion since the other interim ATFs 

sampled were able to adequately control the TNMHC emissions to levels below the permit 

limits. Simply installing fresh carbon should allow the Humboldt interim ATF to meet the permit 

limit. 

A handheld PID was used to sample the sewer air to determine if that type of device could be 

used to measure TNMHC concentrations rather than the more cumbersome SCAQMD method 

25.3. The handheld PID would have the advantage of providing almost instantaneous values at 

the ATF locations, whereas the SCAQMD method requires that samples be taken to a laboratory 

for analysis and it can take weeks to obtain results. Unfortunately, the information from the two 

methods compared poorly so it is not likely that use of the PID will provide its desired advantage 

unless a suitable correction factor could be used to convert the PID readings. The SCAQMD 

would have to accept such a methodology. 

General Remarks on the TNMHC Study 

The City conducted a substantial effort and developed an understanding of the types and 

concentrations of pollutants that are treated by the interim ATFs that have been installed for odor 

control. Because the SCAQMD places permit limits on the emission of TNMHC and hydrogen 

sulfide, the City monitored the inlet and outlet airstreams of the existing interim ATFs. It was 

shown that these current systems, based on carbon bed scrubbers, can be effective in meeting the 

permit requirements. 

The monitoring and analysis did show that there were some organic compounds that influence 

the TNMHC concentrations but do not contribute to O 3 production in the atmosphere, which is 

what the permit limit is intended to prevent. However, this influence was slight and will not 

likely change the requirement to limit the TNMHC emissions. 

The interim ATFs were shown to be capable of removing pollutants with sufficient efficiency to 

meet their permit requirements. This is not to say that the currently installed interim ATFs have 

provided adequate relief from odor impacts on the community, but only that the permit 

conditions can be met. In one case the permit limit for TNMHC was exceeded and was likely due 

to the need for new carbon. In a similar manner, hydrogen sulfide and other odorous compounds 

can be emitted if the carbon in these devices is not changed with adequate frequency. 

11

A handheld PID was tested as a simple means of determining TNMHC concentrations. The 

initial conclusion of the draft TNMHC Technical Memorandum was that the PID was not likely 

an adequate tool for determining permit compliance. However, the final version raises the 

possibility of using a PID with an appropriate calibration curve that would allow the instrument’s 

output to be correlated to SCAQMD Method 25.3 results, but it also recommends further work to 

determine if this is feasible and that an alternative but unnamed hand-held device may provide 

better service. 

The City was able to gain some useful information regarding the composition of the sewer air 

and the various testing methods for organic air pollutants and TNMHC concentrations that are 

available. It also verified that the interim ATF carbon bed scrubber technology was adequate for 

meeting its current permit limits. 

Air Treatment Facility Study 

The Air Treatment Facility Technical Memorandum (ATF Tech Memo) was produced to justify 

the selection of future ATF locations, technologies, and foul air treatment volumes. The ATF 

Tech Memo is based on the earlier work (and related technical memoranda) on sewer drop 

structure air pressure studies, airflow modeling, and analytical testing of non-methane 

hydrocarbons and inorganic odorous compounds such as hydrogen sulfide (H 2 S), all of which 

comprise the broader ATF Review Study and are discussed in detail above. 

Under the original CSSA, the City had planned to construct seven new ATFs to help control 

odors from the wastewater conveyance system. Of these seven, two have almost been completed, 

but design and construction of the remaining five was placed on hold pending the outcome of the 

City’s ATF Review. The City has continued to treat foul air from the sewers with interim ATFs 

based on carbon filtration while the ATF Review has been conducted. As a result of the work, 

the City is recommending proceeding with only one of the originally planned but yet un-built 

ATFs. 

Field Testing of Sewer Air Pressure 

The City has conducted what could be considered a very large scale set of tests to characterize 

the air pressure within its sewer system in its study on differential air pressure at drop structures. 

In that study the City conducted measurements on sewer air pressure to determine how it was 

influenced by the changing of wastewater flows through and around drop structures, installation 

of airflow blocking devices in and near drop structures, and the operation of existing interim 

ATFs. The pressure data collected provided some insight to how each of these influenced air 

pressure in and around the drop structures. While the City was able to draw some general 

conclusions about how each of these might influence air pressures, there was no one clear 

solution that would lower sewer air pressure to the point that odor emissions would be 

completely eliminated. This should not be viewed as an unreasonable result. Due to the complex 

network of sewer connections and the dynamic character of wastewater flow, air pressures vary 

widely and significantly within sewers. It would indeed be the rare case where any sewer 

operator could completely eliminate the potential for odor emissions. 

12

Sewer Airflow Modeling 

The City developed a model to estimate where odor emissions from the sewers might occur. The 

model used by the City to predict wastewater flows was used to estimate the velocity and depth 

of water in connecting segments of the sewers. Based on the information provided by this model, 

a second model was developed to predict where changes in the air space geometry from one 

segment to another indicated a reduction in the amount of air that could be conveyed from an 

upstream to a downstream segment. Where this occurred, the potential for odor emission was 

predicted. This was a reasonable method of arriving at a first estimate of odor emission locations. 

That said, the modeling of airflow in sewers and the points of emissions has proven to be 

difficult and, at best, only a rough estimate of each can be expected. 

The odor emission locations were overlaid on a map in the ATF Tech Memo with the location of 

odor complaints received between the months of October 2008 to September 2009. There 

appeared to be some correlation between the predicted points of emissions and the registered 

odor complaints. However, not all clusters of complaints corresponded to predicted emission 

points. Also, it is not possible to show how all real odor emissions and impacts correlate to the 

predicted emission points because not all true impacts result in complaints formally registered 

with the City. 

ATF Locations Recommendations 

Based on the results of the airflow modeling discussed above, the City developed 

recommendations on the methods of reducing air pressure within the various sewers in the 

vicinity of drop structures. Of the four ATFs originally recommended for installation at drop 

structures, only the ATF at the Mission and Jesse site is now being recommended for 

construction by the City. No mention of the originally planned ATF at NORS/ECIS is made in 

the draft ATF Tech Memo. 

In addition to the ATF at the Mission and Jesse site, modifications in the drop structures and 

adjacent sewers are being recommended. These include the use of flow diversion devices known 

as stop logs to modify the flow patterns in the sewers to minimize pressure in and around the 

drop structures. They also include the installation of airflow prevention devices that would be 

installed in the drop structure air return lines and plastic curtains in the sewers to prevent 

pressurization of the air spaces downstream of the drop structures. 

The ATF Tech memo also recommends that air pressure tests be conducted in the sewer in the 

vicinity of the newly constructed ATF treating air from the East Central Interceptor Sewer 

(ECIS) Jefferson Siphon. It is further recommended that if the operation of that ATF does not 

result in satisfactory pressure reductions along the North Outfall Relief Sewer (NORS) that the 

city should initiate a fan test along the NORS between the siphon at the Fox Hills Mall and the 

Culver City Park. The purpose of that test would be to determine how much air would need to be 

extracted from the NORS to reduce the air pressure within the sewer and thereby control odor 

emissions. 

13

Determining ATF Airflows Required to Control Odor Emissions 

The ATF Tech Memo projects that it would be necessary to extract 20,000 cubic feet per minute 

(cfm) of air at Mission and Jesse to lower sewer air pressure and control odors at that location. It 

further recommends that the City conduct airflow tests on a scale model of a sewer drop structure 

in its hydraulics laboratory to better estimate the actual airflow necessary. This should provide a 

deeper understanding of what ATF airflows would be required and result in a more efficient ATF 

design to be developed. At this writing the City is modifying a physical scale model of a drop 

structure in its hydraulics laboratory to conduct this testing. 

Identification of Air Emission Control Technologies 

The ATF Tech Memo describes commonly applied air pollution control devices used for 

controlling wastewater odors. Using the information gained in the TNMHC study, the City 

evaluated the ability of various air emission control technologies to control odors from the 

wastewater conveyance system. With the exception of biotrickling filters and carbon adsorbers, 

all of the technologies are eliminated from consideration as viable for the normally unstaffed 

wastewater conveyance facility locations such as the Mission and Jesse site. 

Recommendations of the ATF Tech Memo 

The ultimate recommendation set forth in the ATF Tech Memo is that a combination of a 

biotrickling filter and a carbon adsorber be used to control odors from the Mission and Jesse drop 

structure and surrounding sewers near that location. Information in the ATF Tech Memo 

indicates that high levels of hydrogen (H 2 S) sulfide gas can be present in the foul air from this 

part of the sewer. A biotrickling filter should be capable of removing virtually all of H 2 S from 

the foul air stream. Use of a carbon adsorber to remove most of the residual odors that are not 

removed by the biotrickling filter should provide very good control of odors that are introduced 

to the ATF. 

General Remarks on the ATF Tech Memo 

The City’s engineering team has developed recommendations to construct one additional ATF at 

Mission and Jesse. Odor control technologies and methods for estimating foul air treatment 

volumes are also recommended for that single location that should provide good odor control 

based on the City’s findings and assumptions are correct. Of the remaining four sites originally 

proposed for ATFs, it has been recommended that only wastewater flow patterns be modified 

and air flow prevention devices be placed in the sewers to prevent the build-up of high air 

pressures. 

With the information assembled by the City’s team, it is not possible to definitively state that the 

reduced number of ATFs, changes to wastewater flow patterns, and installation of airflow 

prevention devices will or will not adequately control odors from the sewers. One could not 

make such a definitive statement even if all five of the originally proposed ATFs were 

determined to be necessary by the City’s team. The state of the art of odor control from sewers is 

not yet sufficient to make such strong conclusions even with the large amount of information and 

knowledge gained in the process of the City’s ATF Review Study. However, it would be safe to 

14

say that five well designed, carefully located and adequately maintained ATFs would provide 

better odor control than a single ATF. 

The City has constructed and is near final completion of ATFs at Jefferson and La Cienega and 

6000 Jefferson. The City’s team has recommended that sewer air pressure monitoring be 

conducted after these are completed to determine air pressures along the North Outfall Relief 

Sewer. This would be a prudent course of action to determine how much the high air pressures 

have been relieved in that sewer and how far away from the ATF the air pressures remain 

acceptably low within the sewer. It would also be prudent to measure air pressures in the sewers 

connected to the new ATFs to obtain additional knowledge that could be used in the design of 

future ATFs. 

Of the seven originally proposed conveyance system ATFs, two are nearing completion and one 

is being proposed. It will be possible to determine if these are adequate to control odors that have 

been impacting the surrounding communities after they are fully operational. Typical public 

works projects in the size range of the ATF design and installation can be expected to take 

between three and five years to move from initial authorization to completion. Because these 

timelines can be considered long, the City should move with forward soon with design and 

installation of the recommended ATF at the Mission and Jesse site. With the completion of the 

two newly constructed ATFs, the City should quickly test how the airflow extracted from the 

sewers influences pressures in the conveyance system. 

Recommendations of the ATF Review Study Draft Final Report 

The ATF Review Study Draft Final Report (Report) pulls together the findings of and makes 

recommendations based upon the previously discussed airflow modeling, drop structure, siphon, 

TNMHC, and ATF technical memorandum. The most significant recommendation set forth in 

the Report is that of the seven originally planned ATFs four will not be built. These were at the 

drop structures located at Division, Humboldt, Richmond and 23 rd & San Pedro. The City’s 

justification for building only an ATF at the Mission and Jesse site is that it is the drop structure 

through which the NEIS/ECIS tunnel appeared to relieve pressure back into the NOS. 

The installation of the ATF at the Mission & Jesse site would likely not be sufficient in and of 

itself to alleviate odor emissions from the drop structure and the connecting sewers. 

Modifications to the drop structure are also being recommended to prevent air from being carried 

away from the structure by the flowing wastewater and to allow the interim and future ATF to 

efficiently capture foul air from the structure. Plastic curtains are recommended to prevent 

airflow from the bottom of the drop structure into the downstream NEIS. A damper at the top of 

the drop structure air return line will prevent the air movement induced by the water falling in 

the drop structure to pressurize the upstream NOS. A new foul air pick-up point that is being 

recommended at the bottom of the drop structure will, in conjunction with the damper, allow the 

ATF to efficiently capture air from the structure. Final decisions regarding these recommended 

modifications will be made by the City with information obtained from testing water and airflow 

in a scale model drop structure in its hydraulics laboratory. 

Changes to wastewater flow patterns in the NOS and NEIS are also being recommended. The 

City found that air pressures were lower when more of the wastewater was allowed into the NOS 

past the Division, Humboldt and Richmond drop structures and into the ECIS through the drop at 

15

Mission & Jesse. Removing stop logs in the diversion structures to accomplish this has been 

recommended. This would seem a reasonable conclusion based on what is known about the 

airflow and pressurization dynamics in and around drop structures. The recommendation has 

been made to revisit these conclusions after the physical testing of the drop structure testing in 

the City’s hydraulics laboratory. 

General Remarks and Conclusions of the Independent Odor Expert 

The City of Los Angeles Bureau of Sanitation has undergone a large study of the cause of odor 

emissions from its wastewater conveyance system. Conclusions have been drawn based on air 

pressure data gathered during physical manipulation of the system to change wastewater flow 

patterns and change potential airflow pathways in the sewers. Computer modeling based on 

wastewater flow and sewer geometry has been conducted to estimate where airflow restrictions 

could cause the air to be pressurized and emitted to the neighborhoods above the sewers. Foul air 

testing has been conducted to determine the chemical species that need to be controlled by air 

treatment facilities. Studies of this breadth and depth are indeed rare and can yield good 

information regarding the cause and effects of odor emissions and impacts. However, the state of 

the art in modeling of airflow dynamics in sewers will only allow for general conclusions to be 

drawn on results that lie within a broad range of accuracy. There is much art left in the science of 

wastewater conveyance system odor control and solutions must still be made based on inference 

by those with real world experience and knowledge based on past successful and unsuccessful 

attempts to control sewer odor emissions and impacts. 

The City’s plan forward is to complete the two ATFs currently under construction at Jefferson 

and La Cienega and at 6000 Jefferson and to construct only one of the originally recommended 

five additional ATFs. Given the information provided in the ATF study technical memorandums 

on the various studies this does not appear to be an unreasonable approach. However, as 

mentioned above, because these conclusions are based on a range of accuracy is very broad, only 

the installation and operation of these improvements will allow the neighbors and the City to 

determine if they will be successful in preventing odor impacts in the community. With the full 

scale operation of the two ATFs currently in construction, the community should soon be able to 

determine if these projects will provide relief from sewer odors. It remains to be seen if these 

efforts will be sufficient or if additional measures will be required to provide the community 

with sufficient relief from sewer odors. 

16


Team 





AIRFLOW MODELING STUDY 

FINAL 

November 2010 

Air Treatment Facility Review Study Page 1 of 27 

Airflow Modeling Study Technical Memorandum – FINAL 




1.0 Scope and Objectives ........................................................................................................ 4 

2.0 Approach ............................................................................................................................ 7 

3.0 Background ........................................................................................................................ 7 

3.1 Existing Hydraulic Model – MIKE URBAN and MIKE View ............................................ 7 

3.2 Need for Customized Airflow Model Development ......................................................... 7 

4.0 Airflow Modeling ................................................................................................................. 7 

4.1 Methodology ................................................................................................................... 7 

4.2 Model Assumptions & Limitations ................................................................................. 12 

5.0 Model Verification ............................................................................................................. 12 

5.1 Analysis of Current Conditions ..................................................................................... 12 

5.2 Field Data Collection .................................................................................................... 14 

5.3 Discussions .................................................................................................................. 16 

6.0 Analysis ............................................................................................................................ 18 

6.1 Estimated Airflow Rates ............................................................................................... 18 

6.1.1 Scenario 1: Current System Flow ............................................................................... 19 

6.1.2 Scenario 2: Flow Diversion from NORS to NOS ......................................................... 20 

6.1.3 Scenario 3: Flow Diversion of 30 cfs from NOS to COS ............................................. 21 

6.2 Area of Concern Location Identifications ...................................................................... 22 

7.0 Conclusions ...................................................................................................................... 24 


Table 1 – Implemented Air-to-Wastewater Velocity Ratios ........................................................... 8 

Table 2 – Computed Airflow Capacity Differentials for Selected Sampling Sites ....................... 13 

Table 3 - Air Differential Pressure Data ...................................................................................... 16 

Table 4 – Estimated Airflow Flow Rate Summary ....................................................................... 19 

Table 5 - Estimated Airflow Flow Rate Summary ....................................................................... 20 

Table 6 - Estimated Airflow Flow Rate Summary ....................................................................... 21 

Table 7 – Model Predicted Areas of Concern for Current System (Scenario 1) ......................... 22 

Table 8 – Additional Areas of Concern for Scenario 2 & Scenario 3 .......................................... 23 





Figure 1 - System Map .................................................................................................................. 6 

Figure 2 - OPTools Startup GUI .................................................................................................. 10 

Figure 3 - OPTools Scenario Manager ....................................................................................... 10 

Figure 4 – Selected Sampling Sites for Airflow Model Verification ............................................. 13 

Figure 5 – Photo for Site 1 .......................................................................................................... 14 

Figure 6 – Photo for Site 2 .......................................................................................................... 14 

Figure 7 - Photo for Site 3 ........................................................................................................... 14 



Figure 10 - Photo for Site 6 ......................................................................................................... 15 

Figure 11 - Air Differential Pressure Data ................................................................................... 16 

Figure 12 - Correlation between Model Results and Field Data ................................................. 17 

Figure 13 – Odor Area of Concern Location Map ....................................................................... 24 







odors with an extensive use of magnesium hydroxide. Recently, the City has been working to 

improve odor control in several areas identified by local community groups. Air treatment 

facilities (ATFs) with known or predicted odor “areas of concern” were included in the 2004 

Collection 

System Settlement Agreement (CSSA). 

The Airflow Modeling Technical Memorandum provides documentation of the developed airflow 

model and presents the modeling results for the City’s current interceptor system configurations 

and the two planned wastewater flow diversion scenarios. The benefits of the airflow modeling 

study include supporting the findings in other studies and evaluating the impact of the identified 

future operational changes in terms of odor areas of concern. However, the airflow modeling 

results are not intended to be the sole material for the evaluations of the proposed ATFs and any 

recommendations for ATF installation. Based on the airflow modeling results, it is determined 

that: 

 

 

 













1.0 SCOPE AND OBJECTIVES 

For the purposes of this technical memorandum, an odor “area of concern” is defined as a 

location in the wastewater collection systems where airflow bottleneck occurs in the sewer pipe 

headspace. Past experience has shown that these locations are typically the primary cause of 

positive differential air buildup in large diameter gravity pipe sewer systems. Positive differential 

air pressure buildup results in the release of odorous air to atmosphere, thereby resulting in odor 

complaints. 

The scope of the airflow modeling study is to identify the odor areas of concern under current 

conditions and examine the impact of the two planned wastewater flow diversion scenarios as 

follows: 

1. Flow diversion from NORS back to NOS 

2. Flow diversion of 30 cubic feet per second (cfs) from NOS to COS. 

Based on the scope, the interceptor sewers to be analyzed include ECIS, LCIS, LCSFVRS, 

NCOS, NEIS, NORS, NOS, WHIS, WLAIS, and WRS. Figure 1 shows the locations of the study 

area and the study sewers. The analyzed interceptor sewers included approximately 1,100 pipe 




segments, in which 60 % of the segments have circular cross sections and the remaining segments 

have other cross sections such as rectangular ones or semi-elliptical ones. 

In order to satisfy the requirements of the study scope, the principal objectives of this study are 

determined to be: 

1. Developing an airflow model with capabilities to identify odor areas of concern; 

2. Utilizing the developed airflow model to perform analysis for the current system 

conditions and the two aforementioned planned flow diversion scenarios. 

It should be noted that the developed airflow model is not intended to model air differential 

pressure by any means but to identify the odor areas of concern only. 




Figure 1 - System Map 




2.0 APPROACH 

In order to achieve the first study objective, i.e. developing an airflow model, a conceptual 

airflow modeling methodology was established to form the core of the airflow model. A 

customized computer program was then implemented using the modeling methodology. Lastly, 

an airflow model was developed using the data representing the desired conditions. To confirm 

the usefulness of the model for this particular system, the model results were compared with some 

field data which were collected in the study system. 

With the aim of accomplishing the second objective, the developed airflow model was prepared 

and run for the three desired scenarios. It is expected that the model results can be used to identify 

the odor areas of concern for the study system. 

3.0 BACKGROUND 

3.1 Existing Hydraulic Model – MIKE URBAN and MIKE View 

The City uses MIKE URBAN to evaluate the hydraulic capacities in the existing interceptor 

system, so as to evaluate the needs for capital improvements, and analyze the hydraulic effects of 

operational changes. MIKE URBAN is a hydraulic modeling software package that is created by 

DHI Water and Environment (DHI) and is considered as a fully dynamic model for simulating 

sanitary sewer flows. It is capable of simulating complex hydraulics in wastewater collection 

systems, including back water effects. Similar to other dynamic models, MIKE URBAN can 

produce time series flow, depth and velocity information in the simulation results. 

MIKE View is a visualization tool developed by DHI for model results from its hydraulic 

modeling software packages including MIKE URBAN. It is a freeware and the user can 

download the installation file directly from DHI’s website. This cost free option is particularly 

useful for anyone who only needs to review or use the model results for any purpose. In order to 

review the MIKE URBAN model results in MIKE View, one simply imports the MOUSE result 

file where all the time series data are stored. 

3.2 Need for Customized Airflow Model Development 

MIKE URBAN does not have built-in capabilities of modeling airflow and predicting odor areas 

of concern for wastewater collection systems. In order for the City to model these effects in the 

Los Angeles interceptor system, a customized tool needed to be developed. 

4.0 AIRFLOW MODELING 

4.1 Methodology 

In order to identify the locations in the sewer system with potential for odor complaints referred 

to herein as odor “areas of concern”, a customized computer program called OPTools (Odor 

Predictive Tools) was developed as a part of this study. The program uses empirical surface drag 

equations to estimate air flow capacities in the sewer headspace, and calculates the difference in 

airflow capacities for consecutive sewer segments to identify locations with potential for positive 

air pressure buildup. 

The hypotheses in this modeling methodology are that: 

1. A decrease in the airflow capacity from one segment to the next creates a situation for 

positive air pressure build up in the sewer; 




2. The higher the decrease in airflow capacity from one segment to the next should result in 

a higher positive pressure build up between the two segments. 

The above hypotheses are intuitive because the incoming air from the upstream segment cannot 

be conveyed through the downstream segment by means of drag forces alone and therefore 

requires a pressure buildup to assist in its conveyance. 

Airflow capacity was calculated as headspace cross sectional area multiplied by airflow velocity. 

Equation (1) 

 

The Headspace Cross Sectional Area was calculated as the difference between the pipe cross 

sectional area and the wastewater flow area, which were calculated based on the imported sewer 

network and hydraulic output information. 

In a partially full gravity sewer, Air Velocity was assumed to be induced entirely by surface drag 

forces that occurs at the air-water interface from the wastewater flow and was estimated to be a 

certain fraction of the sewer flow velocity, depending on the water depth ratios (d/D). This 

approach ignored the pressure head differential effect in the headspace and the effect of sewer 

pipe roughness and therefore some differences between modeled and actual headspace airflows 

may occur. 

Table 1 summarizes the air-to-water velocity ratios at given wastewater depth ratio ranges which 

were implemented in the OPTools for the circular cross sections, the semi-elliptical cross 

sections, and the Burns-McDonnel cross sections. These air-to-water velocity ratios were 

determined based on the previous field experience obtained during the previous projects with the 

City. During the previous projects with the City, the same ratios were successfully applied to 

estimate airflow for several gravity interceptors that are currently being ventilated, including the 

NCOS upstream of the Jefferson Siphon and two locations on the LCSFVRS; an upstream 

location at Sierra Bonita in West Hollywood and a downstream location at the Genesee Siphon. 

A constant air-to-water velocity ratio of 0.35 was applied to rectangular cross sections and other 

special cross sections, including arch shapes, elliptical shapes and pipes with known sediment 

deposits. This is deemed to be a reasonable assumption given a limited amount of supportive field 

experience with such cross sections. 

Table 1 – Implemented Air-to-Wastewater Velocity Ratios 

OPTools 

Air/Wastewater Velocity Ratios for 

Circular, Semi-Elliptical, and Burns-McDonnel Cross Sections 

Depth Ratio (d/D) 

Air/Wastewater Velocity Ratio 

Less than 0.1 0.15 

Between 0.1 and 0.2 0.25 

Between 0.2 and 0.48 0.35 

Between 0.48 and 0.75 0.6 

Between 0.75 and 0.85 0.35 

Larger than 0.85 0.15 




The change in airflow capacity is termed “Airflow Capacity Differential” (ACD) given by: 

Equation (2) 

 

 

As shown in Equation 2, Airflow Capacity Differential at any given point in the modeled 

interceptor system is computed as the difference between the cumulative incoming airflow 

capacity and the airflow capacity in the downstream pipe. It should be noted that all the model 

nodes were assumed to be fully sealed and therefore there is no term accounting for the airflow 

exchange with the external atmosphere in the Equation 2. 

A decrease in airflow flow capacity or a positive ACD value indicates that the downstream pipe 

has insufficient capacity to convey the incoming airflow, resulting in positive pressure buildup. 

Conversely, a negative ACD value implies a potential negative pressure location. 

With the introduction of the term “Airflow Capacity Differential” or “ACD”, the study 

hypotheses can be restated as follows: 

 

 

A positive Airflow Capacity Differential or ACD results in positive air pressure in the 

sewer; 

The higher the Airflow Capacity Differential or ACD, the higher the positive pressure 

buildup potential will be. 

Given the approximate nature of the calculations, computed Airflow Capacity Differential less 

than 2,000 cubic feet per minute (cfm) were not taken as being significant with regards to 

potential for positive pressure buildup. Therefore a threshold value of 2,000 cfm was used for 

identifying odor areas of concern. With this approach, all the pipes with computed Airflow 

Capacity Differential of more than 2,000 cfm were identified as potential odor area of concern 

locations. 

Airflow Capacity Differential was calculated at the upstream ends of the sewer pipe segments. 

The computer program developed to perform the calculations, OPTools, received input data from 

MIKE URBAN and MIKE View. Two sets of data sources were required to perform the 

calculations. These were the sewer network data set and the hydraulic output data set. The sewer 

network data set contains attributes specifically for the sewer pipes, including pipe identifiers, 

upstream and downstream node identifiers, pipe geometry data, etc. This information was 

imported from the MIKE URBAN models. The hydraulic output data set included wastewater 

depths, wastewater velocities, and wastewater flows. This information was imported from the 

MIKE View models. 

A graphical user interface (GUI) was developed which allows the user to load input data, run case 

scenarios, and view, export or print the outputs. Optionally, the output can be linked with 

Geographic Information System (GIS) to present the results graphically. Figure 2 and Figure 3 

depict the GUIs of OPTools. In the back end, OPTools uses Microsoft Access 2003 as its 

database engine to store the data. 

A user guide that comes with the OPTools installation has been created and will be made 

available for step-by-step instructions. 




Figure 2 - OPTools Startup GUI 

Figure 3 - OPTools Scenario Manager 







4.2 Model Assumptions & Limitations 

The following assumptions and limitations are inherent in the above-described modeling 

methodology used in this study. For each assumption, an explanation of the related model 

limitations is included. 

1. The computed airflow capacities are estimates. This could lead to some differences 

between modeled and actual headspace airflows. 

2. The model does not account for any active or passive ATF or passive air line ventilation 

effects. Therefore, the number of resultant area of concern locations may be conservative; 

3. In identifying the areas of concern, the approach does not account for any backpressurization 

effects from headspace airflow constrictions such as siphons, drop 

structures, diameter reductions, slope reductions, or junction structures. The model will 

predict the locations in the interceptor system where airflow obstructions are most likely 

to occur (i.e., the locations of the source of positive differential air pressure buildup). 

In order to identify the zones of influence of the ATFs and the back pressure effects from the 

downstream headspace airflow obstructions, some field pressure testing can be performed to 

compensate for the model limitations in identifying odor areas of concern. 

5.0 MODEL VERIFICATION 

5.1 Analysis of Current Conditions 

In order to develop confidence in the ability of the model to identify odor areas of concern, the 

OPTools program was run to identify locations of significant positive Airflow Capacity 

Differential in the system. According to the hypothesis of the aforementioned airflow modeling 

methodology, we should measure a corresponding positive sewer air pressure at these locations. 

Moreover, we expect there to be a positive correlation between the Air Capacity Differential and 

the measured sewer air pressure. 

The OPTools run used the sewer network and hydraulic information, which were extracted from 

the City MIKE URBAN export file and the MIKE View model representing the current system 

configurations. A 24-hour model run in 15-minute time steps, representing a typical weekday 

condition, was performed for the following sewers: 

 

 

 

 

 

 

 

ECIS 

LCIS 

LCSFVRS 

NCOS 

NEIS 

NORS 

NOS 




WHIS 

WLAIS 

WRS. 

Based on the model results, six locations, including two on NOS, two on WHIS, one on WLAIS, 

and one on WRS, were identified to have positive Airflow Capacity Differentials though their 

computed Airflow Capacity Differentials are distinctively different. These locations were 

subsequently chosen to be the sampling sites to verify the hypothesis made in the modeling 

methodology. Figure 4 shows the locations of the locations where air pressure sampling was 

performed. The computed Airflow Capacity Differentials and pertinent information for these sites 

are summarized in Table 2. 

Figure 4 – Selected Sampling Sites for Airflow Model Verification 

Table 2 – Computed Airflow Capacity Differentials for Selected Sampling Sites 

Site No. Model Pipe ID Model 

Upstream 

Node ID 

Interceptor 

Name 

Computed 

Average 1 

ACD (cfm) 

Computed 

Maximum 2 

ACD (cfm) 

Computed 

Minimum 3 

ACD (cfm) 

1 5630121556208053A 56301215 NOS 736 1193 134 




2 5340714553407144A 53407145 WLAIS 560 869 133 

3 5340800653408024A 53408006 WRS 1440 1903 551 

4 5170217851702191A 51702178 WHIS 902 1045 577 

5 5180824451808167A 51808244 WHIS 864 1288 417 

6 5360903753609200A 53609037 NOS 782 1037 234 

Notes: 

1. The average ACD was based on all the ACD values calculated at 15-minute increment 

over the entire 24-hour model run 

2. The maximum ACD was calculated to be the highest ACD value over the entire 24-hour 

model run; 

3. The minimum ACD was calculated to be the lowest ACD value over the entire 24-hour 

model run; 

5.2 Field Data Collection 

Air differential pressures at the six locations were collected for two-weekday periods from 3/10 to 

3/12, 2010 at two-minute intervals. It should be noted that the data loggers were installed at six 

alternative maintenance holes, each of which are located immediately upstream of the originally 

selected maintenance holes due to accessibility reasons. This adjustment should not have a 

significant effect on the quality of the data collected for the purposes of this model verification 

effort. 

The data loggers used in the field are the SmartReader Plus 4 LPD, manufactured by ACR 

Systems. The operating ranges of these units are between 2.0 inch of water column and -2.0 

inches of water column according to the manufacturer’s specifications. Photos showing the 

vicinity of each sampling site are presented in Figures 5 to 10. 

Figure 5 – Photo for Site 1 

(Pershing Dr. North of Rees St.) 

Figure 6 – Photo for Site 2 

(Venice Blvd. and Overland Ave.) 

Figure 7 - Photo for Site 3 Figure 8 - Photo for Site 6 




(Jasmine Ave. South of Regent St.) 

(Norton Ave South of 8 th St.) 

Figure 9 - Photo for Site 5 

(Venice Blvd. West of San Vicente Blvd.) 

Figure 10 - Photo for Site 6 

(Bronson Ave. South of 41 st St.) 

The collected air different pressure results were summarized in Figure 11 and Table 3. 




Figure 11 - Air Differential Pressure Data 

Table 3 - Air Differential Pressure Data 

Location ID Maintenance 

Hole ID 

Average Pressure 

(in.wc) 

Max Pressure 

(in.wc) 

Min Pressure 

(in.wc) 

1 56301209 -1.62 -0.24 -2.37 

2 53407074 0.019 0.13 -0.19 

3 53404122 0.096 0.29 -0.02 

4 51702134 0.014 0.12 -0.03 

5 51705210 0.064 0.17 -0.04 

6 53609063 0.024 0.11 -0.18 

5.3 Discussions 

Figure 11 and Table 3 show that all the sampling sites except the site No. 1 have positive average 

and maximum differential air pressure over the sampling period. It is noted that the ATF at 

Hyperion is approximately 13,000 feet downstream of the sampling location No.1 and the ATF at 

the Hyperion wastewater treatment plant is running continuously throughout the year under 

normal conditions. Additionally, it is believed that the sewers are tightly sealed in the vicinity as 

they are elsewhere in the system. Therefore, the ATF at the Hyperion wastewater treatment plant 

is believed to be the cause of the negative pressure observed at the sampling location No. 1. 




Therefore, the above results support the hypothesis that we can expect positive sewer air pressure 

in locations where positive airflow capacity differentials are calculated. 

In order to test the hypothesis that higher airflow capacity differentials correspond to higher 

positive differential air pressure, the computed airflow capacity differentials were plotted against 

the measured air differential pressure as shown in Figure 12. Because of the potential ATF 

influence on sampling site No.1, that location is not included in this figure. As shown in Figure 

12, the field differential air pressure increases with the modeled differential airflow for site 2, 6, 5 

and 3 but not for site 4. It is suspected that the outside influences such as unknown ventilation in 

the vicinity could be the cause of the relatively low pressure observed at site 4. 

In order to verify and quantify the linearity between the two variables, a trend line along with the 

computed coefficient of determination (R 2 ) is generated using a linear regression function in 

Microsoft Excel 2007. As shown in Figure 12, a R 2 value of 0.69 was computed and it is deemed 

to be a reasonable indicator of the correlation and linearity between the computed airflow 

capacity differentials and the field air differential pressure. As a result, the higher differential air 

pressure potential would be expected at locations with higher airflow capacity differentials in the 

model. 

In summary, subject to the limitations described, the developed airflow model is considered to be 

an adequate tool for broadly locating odor area of concern in the sewer systems. 

Figure 12 - Correlation between Model Results and Field Data 




6.0 ANALYSIS 

This section summarizes the analysis which was conducted using the airflow model, including the 

estimated airflow rates at several key locations and the area of concern identifications. 

Three wastewater flow scenarios were analyzed as listed below using the provided MIKE 

URBAN and the MIKE View models as the data sources. 

 

 

Scenario1: Current system configurations. The MIKE View model was calibrated based 

on the flow conditions in the summer moths of 2008. In this scenario, the NOS is dry 

from Diversion 2 downstream to just north of I-405; 

Scenario 2: Future flow diversion from NORS back to NOS at Diversion 2. The same 

dry weather wastewater flow conditions that were used in Scenario 1 were applied. Refer 

to Figure 1 for the location of Diversion 2. 

Scenario 3: Future flow diversion of 30 cubic feet per second (cfs) from NOS to COS. 

The same dry weather wastewater flow conditions as used in Scenario 1 were applied. 

Refer to Figure 1 for the location of the diversion weir. It should be noted that the flow is 

still being diverted into the NOS at Diversion 2 in this scenario. 

For each scenario, the airflow model was set up to run for a 48-hour period using typical weekday 

and weekend flows in 15-minute time intervals. 

6.1 Estimated Airflow Rates 

Airflow rate in the sewer headspace is one of the design criteria for any future ATF design. This 

section documents the estimated airflow rates at several locations listed below using the 

developed airflow model for each of the three flow scenarios. 

 

 

 

 

Upstream end of the NCOS siphon at the 405 freeway; 

Upstream end of the NORS siphon at the 405 freeway; 

Upstream end of the LA Cienega/Jefferson ATF on ECIS; 

Upstream end of the Division drop structure on ERIS (shallow approach interceptor 

sewer); 

Upstream ends of the Humboldt drop structure on both NEIS (deep tunnel) and NOS ( 

shallow approach outfall sewer); 

 

 

 

Upstream ends of the Mission and Jesse drop structure on both NEIS (deep tunnel) and 

NOS (shallow approach outfall sewer); 

Upstream ends of the 23 rd and San Pedro drop structure on both ECIS (deep tunnel) and 

NOS (shallow approach outfall sewer); 

Upstream end of the USC drop structure on ECIS (deep tunnel) 

It should be noted that the reported airflow rates in this section were estimated solely based on the 

drag-induced airflow capacities as mentioned in Section 4.1. Because the airflow capacity 

calculation disregards ventilation effects from ATFs and air lines and pressure differential effects, 

the actual airflow rates in the field can differ from the estimated airflow rates presented herein. 




6.1.1 Scenario 1: Current System Flow 

The estimated airflow rates for the current system configurations based on the provided MIKE 

View file are shown in Table 4. Note the estimated airflow rate values were rounded to the 

nearest hundred. 

Location 

Description 

US of NCOS 

Siphon at I-405 

US of NORS 


US of La 

Cienega/Jefferson 

ATF on ECIS 

US of Division 

on ERIS (surface 

channel) 

US of Humboldt 

on NEIS (deep 

tunnel) 


on NOS (surface 

channel) 

US of M&J on 

NEIS (deep 

tunnel) 

US of M&J on 

NOS (surface 

channel) 

US of 23&SP on 

ECIS (deep 

tunnel) 


NOS (surface 

channel) 

US of USC on 

ECIS (deep 

tunnel) 

Table 4 – Estimated Airflow Flow Rate Summary 

for Current Conditions (Scenario 1) 

Model Pipe ID 

MIKE 

URBAN 

Average 

Wastewater 

Flow (cfs) 

Estimated 

Average 

Airflow 

(cfm) 

Estimated 

Maximum 

Airflow 

(cfm) 

Estimated 

Minimum 

Airflow 

(cfm) 

5590500455905005A 96.7 5,200 5,900 4,300 

5590500655905007A 318.6 10,500 14,400 8,300 

5350321353502178A 105.1 11,800 13,800 7,900 

4681215646816116A 5.4 600 900 500 

4950924349509244A 5.4 600 800 400 

4950911249509106A 22.7 1,600 1,700 1,000 

5150918751513122A 28.1 3,600 5,100 2,700 

5151312551513132A 19.5 1,000 1,100 700 

5370317753703203A 28.1 2,200 3,000 1,000 

5370319953703200A 51.4 4,100 4,400 2,700 

5370518053705183A 79.5 5,000 6,200 3,400 




6.1.2 Scenario 2: Flow Diversion from NORS to NOS 

The estimated airflow rates for the scenario of diverting the wastewater flow back into the NOS 

from the NORS are presented in Table 5. Note the estimated airflow rate values were rounded to 

the nearest hundred. It is understood that this flow diversion affects mainly the wastewater flow 

in the NORS, which is downstream of the Diversion 2 structure. Comparing Table 4 and 5, the 

wastewater flow in the NORS at I-405 (pipe segment 5590500655905007A) dropped by 

approximately 35% from 318 cfs to 203 cfs, however, the estimated average airflow rate 

decreases only by approximately 10 % from 10,500 cfm to 9,400 cfm, which is considered 

insignificant. 

Location 

Description 

US of NCOS 


US of NORS 


US of La 





channel) 


on NEIS (deep 

tunnel) 



channel) 

US of M&J on 

NEIS (deep 

tunnel) 

US of M&J on 

NOS (surface 

channel) 


ECIS (deep 

tunnel) 


NOS (surface 

channel) 

US of USC on 

ECIS (deep 

Table 5 - Estimated Airflow Flow Rate Summary 

for Flow Diversion from NORS to NOS (Scenario 2) 

Model Pipe ID 

MIKE 

URBAN 

Average 

Wastewater 

Flow (cfs) 

Estimated 

Average 

Airflow 

(cfm) 

Estimated 

Maximum 

Airflow 

(cfm) 

Estimated 

Minimum 

Airflow 

(cfm) 

5590500455905005A 103.4 7,200 7,400 6,700 

5590500655905007A 203.5 9,400 14,200 7,200 

5350321353502178A 129.8 12,400 14,100 8,400 

4681215646816116A 5.3 600 900 500 

4950924349509244A 5.3 400 800 200 

4950911249509106A 50.9 1,700 2,800 1,600 

5150918751513122A 56.2 4,900 5,300 3,100 

5151312551513132A 23.4 1,000 1,100 600 

5370317753703203A 56.2 3,600 5,200 1,400 

5370319953703200A 50.6 4,000 4,400 2,500 

5370518053705183A 106.9 6,000 7,500 4,800 




tunnel) 

6.1.3 Scenario 3: Flow Diversion of 30 cfs from NOS to COS 

The estimated airflow rates for the scenario of diverting 30 cfs wastewater flow into the COS 

from the NOS are summarized in Table 6. Note the estimated airflow rate values were rounded to 

the nearest hundred. 

This flow diversion mainly reduces the wastewater flow in the NOS and the downstream NCOS. 

Because the diverted flow is marginal, the estimated airflow rate into the NCOS siphon is very 

similar to the one computed for the Scenario 2. 

Location 

Description 

US of NCOS 


US of NORS 


US of La 





channel) 


on NEIS (deep 

tunnel) 



channel) 

US of M&J on 

NEIS (deep 

tunnel) 

US of M&J on 

NOS (surface 

channel) 


ECIS (deep 

tunnel) 


NOS (surface 

channel) 

US of USC on 

ECIS (deep 

tunnel) 

Table 6 - Estimated Airflow Flow Rate Summary 

for Flow Diversion of 30cfs from NOS to COS (Scenario 3) 

Model Pipe ID 

MIKE 

URBAN 

Average 

Wastewater 

Flow (cfs) 

Estimated 

Average 

Airflow 

(cfm) 

Estimated 

Maximum 

Airflow 

(cfm) 

Estimated 

Minimum 

Airflow 

(cfm) 

5590500455905005A 86.4 7,100 7,300 6,600 

5590500655905007A 201.1 9,400 14,000 7,400 

5350321353502178A 127.4 12,300 14,100 8,400 

4681215646816116A 5.3 600 900 500 

4950924349509244A 5.3 400 900 200 

4950911249509106A 49.0 1,700 1,700 1,600 

5150918751513122A 54.3 4,800 5,300 3,100 

5151312551513132A 22.9 1,000 1,100 600 

5370317753703203A 54.3 3,500 5,600 1,300 

5370319953703200A 50.1 4,000 4,400 2,500 

5370518053705183A 104.5 5,900 7,500 4,700 




6.2 Area of Concern Location Identifications 

This section will discuss the model results related to the area of concern location predictions. 

As discussed in Section 4.1, all the pipes with a computed airflow capacity differential above 

2,000 cfm will be identified as the areas of concern by the airflow model. However, some of the 

areas of concern identified by the airflow model were filtered out manually based on some known 

system information. This primarily includes any pipes identified as area of concern candidates 

where ATF effects are known to be significant. This adjustment will avoid overly-conservative 

predictions. 

The following locations were identified to be odor area of concern candidates by the airflow 

model but were filtered out due to known ATF effects on the pipes. It should be noted that these 

locations can be potential odor areas of concern without the current ATFs near these locations as 

mentioned below in operation. 

 

 

 

 

Candidates on the LCSFVRS were disregarded due to the negative pressures 

currently induced on this pipe by the Genesee and Sierra Bonita ATFs; 

Candidates on the pipe segment upstream of the NCOS siphon on Rodeo Dr. just 

west of La Cienega were disregarded due to the temporary ATF that is 

withdrawing air from that pipe segment; 

Pipes that are within 2,000 ft of the ATF at the Hyperion WWTP were removed 

from the area of concern list. The pressure data collected during model 

verification at maintenance hole Site No. 1, which is approximately 13,000 

upstream of the WWTP, showed that location’s headspace to be under negative 

pressure; 

Pipes that are within 500 feet of the interim ATF at Rodeo Road and Martin 

Luther King Jr. Boulevard are removed from the area of concern list. It is 

assumed that this ATF keeps negative pressure in the upstream NOS and south 

branch NOS pipes. 

The area of concern locations for the current system (Scenario 1) are presented in Table 7. Note 

the reported airflow capacity differential values were rounded to the nearest hundred. In total, 

there are 9 locations where the airflow model predicts potential positive pressure buildup. As 

discussed in Section 4.1, airflow capacity differential is caused by insufficient downstream pipe 

airflow capacities. The potential causes of the insufficient downstream pipe capacities include 

diameter reduction, slope reduction, increase of wastewater flow, sudden headspace and velocity 

reduction due to back water effects, and/or siphons with undersized air jumper lines. 

Area of 

concern 

ID 

HS-1 

HS2 

Table 7 – Model Predicted Areas of concern for Current System (Scenario 1) 

Location Descriptions 

Immediately upstream of 

the 23 rd & San Pedro Drop 

Structure on ECIS 

Model Pipe ID/ 

Upstream Node ID 

5370317753703203A 

53703177 

5370830353703177A 

53708303 

Computed 

Average 

ACD (cfm) 

Computed 

Maximum 

ACD (cfm) 

Computed 

Minimum 

ACD (cfm) 

2,600 3,400 1,700 

2,000 7,400 1,200 




HS-3 

HS-4 

HS-5 

HS-6 

Downstream of the 23 rd & 

San Pedro Drop Structure 

on ECIS at 31 st St. and 

Trinity St. 

Upstream of the USC 

drop structure on ECIS 

on Exposition Blvd. just 

east of Figueroa St. 

Downstream of the 

Mission & Jesse Drop 

Structure on ECIS on 

Santa Fe Ave. just north 

of 8 th St. 

ECIS Siphon at Jefferson 

Blvd. and La Cienega 

Blvd. 

5370617853705181A 

53706178 

5370518753705180A 

53705187 

5151313553801134A 

51513135 

5350217853502178_ 

DummyA 

53502178 

2,400 4,900 1,500 

4,100 5,900 3,100 

3,400 4,200 2,500 

3,200 3,800 2,400 

HS-7 NORS/ECIS Junction 5350902253509011A 

53509022 

HS-8 NCOS Siphon at I-405 55905005_55905005 

_Dummy_1 

55905005 

HS-9 NORS Siphon at I-405 55905007_Dummy55 

909301 

55905007_Dummy 

7,100 11,800 4,400 

2,500 3,100 2,000 

5,000 7,000 2,800 

Based on the model results for the Scenario 2, diverting flow from NORS back to NOS, all the 

areas of concern as predicted for the current system are still identified to be area of concern 

locations. In addition, a few more areas of concern are identified due to this flow diversion. The 

additional areas of concern are summarized in Table 8. Note the reported airflow differential 

values were rounded to the nearest hundred. As expected, the NOS siphon at I-405 is identified as 

a area of concern by the model. The NOTF location is included as one of the areas of concern 

under the assumption that the NOTF ATF is currently not being run pending ongoing public 

involvement efforts. It is believed that the two additional areas of concern on ECIS are caused by 

the increased wastewater flow in the ECIS. 

Area of 

concern 

ID 

HS-10 

HS-11 

Table 8 – Additional Areas of concern for Scenario 2 & Scenario 3 

Location Descriptions 

Downstream of the 

USC Drop Structure 

on ECIS on 

Exposition Blvd. just 

west of Arlington Ave. 

Immediately 

downstream of the 

NOTF on NOS 

Model Pipe ID/ 

Upstream Node ID 

5360621753606219A 

53606217 

5350902053509017A 

53509020 

Computed 

Average 

ACD (cfm) 

Computed 

Maximum 

ACD (cfm) 

2,000 3,400 ‐800 

Computed 

Minimum 

ACD (cfm) 

3,300 3,800 2,000 




HS-12 NOS Siphon at I-405 56008065_56008150 

_Dummy 

56008065 

HS-13 

Upstream of USC 

Drop Structure on 

ECIS on Grand Ave. 

just south of 35 th St. 

5370518153705188A 

53705181 

4,400 5,500 2,900 

2,100 3,100 ‐1,500 

For the Scenario 3, in general, the airflow model predicts similar results as the Scenario 2. All the 

areas of concern identified for the Scenario 2 are still identified to be potential areas of concern 

points in the system by the model. 

A map showing the locations of the thirteen areas of concern identified by the airflow model is 

shown in Figure 13. 

Figure 13 – Odor Areas of Concern Location Map 

7.0 CONCLUSIONS 

An airflow model was developed as a tool to compute airflow capacities in the sewer pipes and 

identify potential odor areas of concern for the wastewater collection systems. The developed 

airflow model leverages the existing City-owned hydraulic modeling software packages, i.e. 




MIKE URBAN and MIKE View, to provide data input for the airflow modeling. The tool can be 

used to evaluate the system responses in terms of the drag-induced airflow capacities and odor 

areas of concern to operational changes of the wastewater flow provided the corresponding MIKE 

URBAN and MIKE View models are available. 

In order to increase confidence in the model results, differential air pressure data was collected at 

several locations. With the collected field data, it has been shown that the developed airflow 

model is an adequate tool for modeling the airflow in this particular system. Three modeling 

scenarios were analyzed using the developed airflow model, including: 

 

 

Current system configurations under typical dry weather flow conditions. 

Future flow diversion from NORS back to NOS under the same flow conditions; 

Future flow diversion of 30 cfs from NOS to COS under the same flow conditions. 

For each scenario, the airflow model was set up to run for a 48-hour period, including the typical 

weekday and weekend wastewater flow data provided in the corresponding MIKE View file, in 

15-minute time step. The conclusions based on the airflow modeling results are summarized as 

follows: 

 

 

 













It should be noted that the model results presented herein are not viewed with absolute confidence 

but rather an aid to help understand the system airflow dynamics and can be used to identify 

where odor emission might occur. 




Appendix A 

Acronyms and Abbreviations 




ACRONYMS AND ABBREVIATIONS 

ACD 

ATF 

COS 

CSSA 

ECIS 

LCIS 

LCSFVRS 

NCOS 

NEIS 

NOS 

NOTF 

NORS 

WHIS 

WLAIS 

WRS 

Airflwo Capacity Differential 

Air Treatment Facility 

Central Outfall Sewer 

Collection System Settlement Agreement 

East Central Interceptor Sewer 

La Cienega Interceptor Sewer 

La Cienega San Fernando Relief Sewer 

North Central Outfall Sewer 

North East Interceptor Sewer 

North Outfall Sewer 

North Outfall Treatment Facility 

North Outfall Replacement Sewer 

Wilshire-Hollywood Interceptor Sewer 

West Los Angeles Interceptor Sewer 

Westwood Relief Sewer 




Technical Memorandum – FINAL 

Team 





SEWER SIPHONS TECHNICAL MEMORANDUM 

DUCT CONNECTION STUDY 

FINAL 



Sewer Siphons Technical Memorandum and Duct Connection Study – FINAL 




1.0 Introduction and Purpose ................................................................................................... 5 

2.0 Sampling Setup .................................................................................................................. 5 

2.1 Locations Sampled ......................................................................................................... 6 

2.2 Schedule ....................................................................................................................... 10 

2.3 Roles and Responsibilities ........................................................................................... 12 

3.0 Field Sampling ................................................................................................................. 13 

3.1 Baseline Data Collection .............................................................................................. 13 

3.1.1 Data Logger Installation ........................................................................................ 15 

3.1.2 Baseline Data Download ....................................................................................... 16 

3.2 Ductwork Installation .................................................................................................... 16 

3.2.1 Passive Ventilation Phase ..................................................................................... 17 

3.2.2 Active Ventilation Phase ........................................................................................ 20 

4.0 Test Results ..................................................................................................................... 22 

4.1 Baseline Phase Air Pressure Data ............................................................................... 22 

4.2 Passive Duct Phase Air Pressure Data ........................................................................ 27 

4.3 Active Duct Phase Air Pressure Data ........................................................................... 33 

4.4 Findings ........................................................................................................................ 37 

5.0 Air Duct Isolation between NOS and NCOS .................................................................... 38 

5.1 Sampling & Air Duct Isolation ....................................................................................... 38 

5.2 Locations to be Sampled .............................................................................................. 39 

5.3 Schedule ....................................................................................................................... 42 


5.5 Test Results .................................................................................................................. 44 

6.0 Additional Siphon Sampling ............................................................................................. 47 

7.0 Conclusions ...................................................................................................................... 51 


Table 1 Air Phase Sampling Parameter ....................................................................................... 6 

Table 2 - Sampling Location and Descriptions ............................................................................. 6 

Table 3 - Air Duct Connection Locations .................................................................................... 10 

Table 4 - Overall Sampling Schedule ......................................................................................... 11 

Table 5 - Detailed Ventilation Schedule ...................................................................................... 11 

Table 6 - Grab Sample Pressure Data ........................................................................................ 14 

Table 7 - Summary of Baseline Air Pressure Data Recorded in NCOS and NORS, 6/2/09 to 

6/9/09 .......................................................................................................................................... 23 

Table 8 – Summary of Baseline Air Pressure Data Recorded in NCOS and NORS, 6/9/09 to 

6/24/09 ........................................................................................................................................ 24 

Table 9 - Summary of Air Pressure Data Recorded in NCOS and NORS, 7/20/09 to 7/24/09, 

Passive Ventilation Phase .......................................................................................................... 27 

Table 10 - Comparison of Data Summary for Pressure Recorded from 7/20/09 to 7/22/09 ....... 30 






Active Ventilation Phase ............................................................................................................. 35 

Table 14 - Summary of Air Pressure Data Recorded in NCOS, NORS and NOS, 8/10/09 to 

8/18/09, Passive Ventilation Phase ............................................................................................ 36 





8/24/09, Passive Ventilation Phase ............................................................................................ 36 

Table 16 - Air Pressure Sampling Locations ............................................................................... 39 

Table 17 - Overall Sampling Schedule ....................................................................................... 42 

Table 18 - Summary of Air Pressure Data Recorded During Duct Isolation Study ..................... 44 

Table 19 – Sampling Locations and Description ......................................................................... 48 

Table 20 – Culver City/Baldwin Hills and San Fernando Valley Air Pressure Data Summary .... 50 


Figure 1 - Detailed Sampling Location Map .................................................................................. 9 

Figure 2 - Maintenance Hole 535-09-022 ................................................................................... 15 






Figure 8 - Duct Connection ......................................................................................................... 16 

Figure 9 - NORS Maintenance hole of Duct Connection ............................................................ 16 

Figure 10 - NCOS Maintenance hole of Duct Connection .......................................................... 16 

Figure 11 - Air Velocity Measurement ......................................................................................... 16 

Figure 12 - Air Velocity Measurements in Duct ........................................................................... 18 

Figure 13 - Maintenance hole 560-12-145 .................................................................................. 19 




Figure 17 - “Special Manhole” ..................................................................................................... 19 


Figure 19 - Fan Installation ......................................................................................................... 20 

Figure 20 – Fan Prep .................................................................................................................. 20 

Figure 21 - Fan Installation Complete ......................................................................................... 20 

Figure 22 – Fan Control Panel .................................................................................................... 20 

Figure 23 - Sound Abatement ..................................................................................................... 21 

Figure 24 -- Installation Complete ............................................................................................... 21 

Figure 25 - Fan Test Complete ................................................................................................... 21 

Figure 26 - Baseline Air Pressure ............................................................................................... 22 

Figure 27 - Detail 24 hours Baseline ........................................................................................... 23 

Figure 28 - NORS NCOS Air Pressure with NORS/ECIS ATF Shut Off ..................................... 24 

Figure 29 - Air Pressure (Absolute Value) Difference between NORS and NCOS .................... 25 

Figure 30 - 24-hour Period Combined Data on 6/19/09 .............................................................. 26 

Figure 31 - Passive Air Duct Connection Data ........................................................................... 27 

Figure 32 - Passive Duct Air Pressure Data ............................................................................... 28 

Figure 33 - Average Passive Air Velocity .................................................................................... 29 

Figure 34 - Average Passive Air Flow ......................................................................................... 29 

Figure 35 - NORS NCOS Passive Duct Test Air Pressure Data................................................. 31 

Figure 36 - NORS NCOS Air Pressure with Passive Duct Connection....................................... 32 

Figure 37 - Air Velocity Measurements in Duct ........................................................................... 33 

Figure 38 - Air Velocity Measurements in Duct (2) ..................................................................... 34 

Figure 39 - Combined NORS-NCOS Air Pressure with Fan Active ............................................ 35 

Figure 40 - Combined Air Pressure 8/18 to 8/27 ........................................................................ 36 




Figure 41 - Schematic of 24-inch/42-inch Air Duct and Special MH Flap Installation ................. 38 

Figure 42 - Air Duct Isolation with plywood and rubber flap ........................................................ 39 

Figure 43 - Sampling Location Map for Air Duct Isolation ........................................................... 40 

Figure 44 - Maintenance Hole 559-05-005 ................................................................................. 41 



Figure 47 - Maintenance Hole Special MH ................................................................................. 41 



Figure 50 - Maintenance Hole 560-08-056, Downstream of NOS Siphon .................................. 42 

Figure 51 - NOS/NCOS Duct Isolation Air Pressure Data .......................................................... 44 

Figure 52 - Typical Baseline Air Pressure Data .......................................................................... 45 

Figure 53 – Pressure Effects of ATF Shutoffs ............................................................................ 46 

Figure 54 - NORS Modeled Wastewater Flow Comparison........................................................ 47 

Figure 55 - Post Flow Diversion NORS Air Pressure .................................................................. 48 

Figure 56 - Post Flow Diversion NOS Air Pressure in Baldwin Hills ........................................... 49 

Figure 57 - NOS Air Pressure US of Radford Siphon ................................................................. 49 









facilities (ATFs) with known or predicted odor “hot spots” were included in the 2004 Collection 


The NORS/NCOS Duct Connection Test Technical Memorandum presents the results of the 

HDR Team regarding the effect that the existing interceptor siphon located under the 405 

Freeway has on airflow dynamics in the interceptor collection system. In order to accomplish the 

task, field samplings from several maintenance holes were collected over a period of time 

followed by physically connecting the NORS and NCOS interceptors with a 200-foot, 36-inch 

diameter, and corrugated HDPE duct laid above-ground in Culver City. Air flow and air velocity 

samples were taken for a period of several days which was identified as the passive ventilation 

phase. Following that phase, the duct was separated to permit the installation of a fan. During 

this active ventilation phase, more air pressure and air velocity data were collected to measure the 

effect of the fan air withdrawal from the NORS discharge to the NCOS with fan speed varying 

between 5000 and 12,500 cfm. Finally, the permanent air duct that connects the headspace of the 

upstream ends of the siphons of the NOS and NCOS near the 405 Freeway was plugged to test 

the differential air pressure between the two interceptors. This air duct is comprised of two 

sections: A 24-inch high by 57-inch wide rectangular section that connects the NOS to a “Special 

Manhole” and a 42-inch diameter round section that connects the NCOS to the “Special 

Manhole”. The “Special Manhole”, as it is called out in the NCOS as-built drawings, is the 

transition point between these two sections of the permanent air duct. 

The results of these tests and findings are described in this document. 

1.0 INTRODUCTION AND PURPOSE 

Previous City sampling efforts have shown that headspace air pressures in the NCOS are typically 

at or below atmospheric levels, whereas in the NORS they are highly positive. Previous sampling 

efforts conducted by the City have also resulted in the conclusion that the source of this positive 

air pressure is the air dragged in the headspace of the NORS to the siphon located under the 405 

Freeway. The air that is carried in the open headspace of the NORS to the siphon cannot pass 

through the siphon and positive differential air pressure builds from there to points upstream 

along the NORS alignment. Because the NORS and NCOS alignments are essentially parallel to 

one another, the possibility exists of physically connecting the headspaces of these two sewers 

(between nearby maintenance holes on each) with a short run of flexible duct in order to test the 

effectiveness of depressurizing the NORS through such a connection. The purpose of this study 

is to test the feasibility and effectiveness of relieving this positive air pressure from the NORS 

into the NCOS. 

2.0 SAMPLING SETUP 

Data collection instruments that were used during the sampling effort are shown in Table 1. The 

air pressure sampling consisted of continuous data collection at two-minute intervals during four 

separate periods at the locations shown in Table 2. Using two-minute intervals, the differential air 

pressure data loggers have the capacity to log data for approximately two weeks apiece. 




The instruments were installed from the surface, and entry into maintenance holes or structures 

was not required. The instrument installations were temporary and generally took place by 

suspension from a maintenance hole rim, step or lid. 

The quantity of air transferred through the flexible duct from the NORS to the NCOS, both in 

passive and active ventilation modes, was determined by the use of a hot-wire anemometer. A 

sampling port was cut into the flexible duct connecting the two sewers through which air velocity 

readings were taken across the cross section of the duct. This was done both in the vertical and 

horizontal directions in the cross section and is further described in the Field Sampling section of 

this memo. 

Table 1 Air Phase Sampling Parameter 

Parameter Field Data Logger Purpose 

Headspace Air Pressure ACR Smart Reader Plus 4 

To locate and quantify areas of positive 

pressure and allow for qualitative 

verification of computer airflow model 

To subsequently calculate the amount of air 

Air Velocity in Flexible Duct Hot-wire anemometer that passes between the NORS and NCOS 

under both passive and active ventilation 

conditions 

2.1 Locations Sampled 

In general, continuous differential air pressure sampling locations were selected based on the 

following criteria: 

 

 

 

 

Locations along the NORS upstream of the NORS Siphon 

Locations where previous air pressure testing had indicated that a backpressure 

relationship, due to a positive differential air pressure buildup, exists that is likely related 

to the NORS Siphon 

Locations upstream on the NCOS and NORS from proposed air duct connections 

Locations downstream on the NCOS and NORS from proposed air duct connections 

Detailed information on field sampling locations is presented in Table 2. A map of the locations 

sampled is presented in Figure 1. 

Table 2 - Sampling Location and Descriptions 

Session MH Number Sewer Field Notes 

1 535-09-022 NORS This maintenance hole is the first maintenance hole 

upstream of the air duct connection on NORS and is 

located at the NORS/ECIS carbon ATF site. The 

maintenance hole has an inner plate and the data logger in 

the protective case was placed on top of it. On August 5, 

2009, in order to prevent the tubing connected to the 

negative terminal from being pinched off by the ribs under 

the maintenance hole cover, the tubing was shortened. 

1 535-13-007 NORS This maintenance hole is the first maintenance hole 

downstream of the air duct connection on NORS and is 

located in the dirt shoulder behind the sound wall on 

Stocker Street. The maintenance hole has an inner plate 





and the data logger in the protective case was placed on 

top of it. 

1 535-05-016 NCOS This is an alternate location due to access difficulty with 

the originally selected maintenance hole and is the second 

maintenance hole located upstream of the duct connection 

maintenance hole of the NCOS interceptor. This 

maintenance hole is adjacent to the skate park on the 

Jefferson Boulevard. The data logger was secured to the 

underside of the maintenance hole cover through a 

pickhole with a rope, bolts and a p-trap assembly. On the 

field trip to reset the logger on August 18, 2009, it was 

found that the barbed hose connection was disconnected 

from the p-trap; this was likely caused by heat and/or 

moisture and caused the glue to melt. The p-trap assembly 

was replaced with a threaded one and the bottom of the 

new p-trap was sealed with duck tape without the water 

filled u shaped PVC pipe since no rainfall was expected. 

1 535-09-009 NCOS This maintenance hole is downstream of the air duct 

connection on the NCOS and is on the south side of the 

Physical Education Complex at West LA College. The 

maintenance hole does not have an inner plate and a p-trap 

assembly was installed originally in order to secure the 

logger. It was found that the bottom part of the p-trap was 

disconnected on a couple of occasions when crews went to 

download data and reset the logger. Therefore, on July 23, 

2009, the p-trap was removed and instead, the tubing 

connected to the negative terminal with brass hose 

connection was secured directly on the maintenance hole 

cover through the pickhole. It was found on a couple of 

occasions on August 5 and 18, 2009, that the maintenance 

hole cover was tarred but the vent of the hose connection 

and the tubing were cleaned on site. 

1 535-05-021 NORS This is the first maintenance hole upstream of 

NORS/ECIS Junction. The maintenance hole is located in 

Culver City Park. The maintenance hole has an inner plate 

and the data logger in the protective case was placed on 

top of it. On August 5, 2009, in order to prevent the tubing 

connected to the negative terminal from being pinched off 

by the ribs under the maintenance hole cover, the tubing 

was shortened. 

1 535-09-006 NORS This maintenance hole is the first one upstream of the 

NORS/ECIS Junction and downstream of the North 

Outfall Treatment Facility (NOTF). It is within the Turner 

Construction property boundary on Jefferson Boulevard. 

The maintenance hole has an inner plate and the data 

logger in the protective case was placed on top of it. 

2 560-12-145 NCOS This maintenance hole is located on the downstream end 

of the NCOS siphon and is on the east side of Sepulveda 

Boulevard between Howard Hughes Parkway and Center 

Drive. This maintenance hole has no inner plate. In order 

to properly install the p-trap assembly on the maintenance 

hole cover, the City replaced the existing maintenance 

hole cover, which had one pickhole, with one that has two 





pickholes. 

2 559-05-005 NCOS This maintenance hole is located on the upstream end of 

NCOS siphon and is in the southeast corner of the 

intersection of Bristol Parkway and Green Valley Circle. 

There is no inner plate in place and the data logger with a 

p-trap assembly was installed as done in previously 

similar setups. 

2 535-06-132 NORS This maintenance hole is located at the intersection of Ivy 

Way and Perham Drive. The maintenance hole has an 

inner plate and the data logger in the protective case was 

placed on top of it. 

2 535-09-017 NOS This maintenance hole is located in a commercial parking 

lot where the NOS connects with NOTF. The maintenance 

hole has no inner plate and has only one pickhole. The 

data logger in the protective case was secured through a 

rope. The tubing connected to the negative terminal with 

brass tee assembly was secured on the maintenance hole 

cover through the a pickhole. 

2 “Special MH” 42” Air 

Vent Line 

This is designated as a “Special MH” in the NCOS asbuilt 

drawing set. The maintenance hole is located on a 

dedicated air ventilation duct that connects the upstream 

end of the NOS siphon and the upstream end of NCOS 

siphon. The maintenance hole serves as a transition 

between a 24-inch diameter section of the air duct that 

connects to the NOS and a 42-inch diameter section that 

connects to the NCOS. The maintenance hole has an inner 

plate and the data logger in the protective case was placed 

on top of it. 

2 535-06-090 NCOS This maintenance hole is located just downstream of the 

Rodeo and La Cienega NCOS siphon on the east side of 

Jefferson Boulevard. This maintenance hole has no inner 

plate and the data logger p-trap assembly was installed 

here as in similarly configured locations. 




Figure 1 - Detailed Sampling Location Map 




The air pressure effects of a 36-inch diameter air duct connection were measured at the same 

maintenance holes in which air pressure data had been taken during the baseline sampling period. 

The duct was connected to the rims of the two maintenance holes shown in Table 3. Maintenance 

hole 535-09-011 (NORS) was connected to maintenance hole 535-09-008 (NCOS). This location 

is just east of the intersection of Freshman Dr. and Sophomore Dr. just north of West LA College 

in Culver City. This location was chosen due to the proximity of the two sewers to one another as 

well as the low probability of traffic disruptions during the testing. Approximately 200 linear feet 

separate maintenance holes 535-09-011 on the NORS and 535-09-008 on the NCOS. Prior to 

commencing the sampling effort, two field inspections were performed by City and the HDR 

team staff in order to assess and confirm the feasibility of performing the duct connections at the 

West LA College location. 

Table 3 - Air Duct Connection Locations 

Sampling Locations 

Location MH Number Notes 

Freshman Dr. at Sophomore Dr.; just north of 

West LA College 

535-09-011 

535-09-008 

NORS maintenance hole 

NCOS maintenance hole 

(Approx. linear distance = 200 

feet) 

2.2 Schedule 

There were two distinct, baseline data gathering sessions carried out for this sampling effort. The 

locations and sessions during which differential air pressure data were measured at each location 

are listed in Table 2 

Baseline Session 1 lasted approximately one week. One day was needed to install all the data 

loggers and another day to remove all the data loggers at the end of Session 2. 

Downloading of data from Session 1 and programming of the data loggers for Session 2 was 

performed in the field approximately seven days after the instruments were installed. An overall 

sampling schedule is presented in Table 4. A detailed ventilation schedule, covering both the 

passive and active ventilation tests, is presented in Table 5. 




Task 

Table 4 - Overall Sampling Schedule 

Session 1 

Baseline Air Pressure 

Start Date 

06/02/09 

End Date 

06/09/09 

Session 2 

Air Duct Effect Testing 

Start Date 

07/20/09 

Install Air Pressure Data Loggers Day 1 Day 2 n/a n/a 

All Data Loggers installed and 

logging data 

Day 2 Day 7 n/a n/a 

Download Baseline Air Pressure 

Data in Field; Reprogram Day 7 Day 7 n/a n/a 

Instruments for Session 2 

Install 36-inch diameter flexible 

duct to maintenance holes n/a n/a *Day 1 n/a 

referenced in Table 3 

Retrieve Instruments and 

disassemble conduits 

n/a n/a n/a Day 4 

End Date 

8/24/09 

Table 5 - Detailed Ventilation Schedule 

Task 

Connect ductwork to NORS & 

NCOS maintenance holes 

Disconnect ductwork from 

NORS & NCOS maintenance 

holes 

Connect 10,000 cfm fan to 36- 

inch duct; begin air withdrawal 

Shut off fan; disassemble and 

remove ductwork, fan and 

generator 

Passive Ventilation 

Start Date 

7/20/09 

9:00 am 

Day 1 

n/a 

n/a 

End Date 

7/27/09 

Active Ventilation 

Start Date 

8/19/09 

End Date 

8/24/09 

n/a n/a n/a 

3:00 pm 

Day 2 

n/a 

n/a 

9:00 am 

Day 3 

n/a n/a n/a 

n/a 

n/a 

3:00 pm 

Day 4 




2.3 Roles and Responsibilities 

The sampling activities were a joint effort between the HDR/Malcolm Pirnie team, its sub (GK & 

Associates) and City BOS staff. The roles and responsibilities for each of the three included the 

following: 

HDR/Malcolm Pirnie 

 

 

 

 

 

Prepare all instruments for data logging 

Installation of all data logging instruments 

Data downloads from all instruments 

Retrieval of all data instruments 

Data compilation, organization and analysis 

Documentation of results 

GK & Associates 

 

 

 

 

 

Support HDR/MP representatives in accessing, closing and properly sealing all 

maintenance holes accessed during this sampling effort (see 2009 Field Ops Workshop 

presentation for details) 

Arrange for procurement and fabrication (where necessary on appurtenant equipment) of 

10,000 cfm fan, portable generator and minimum 200-linear feet of 36-inch diameter 

flexible ductwork 

Connect approximately 200-linear feet of 36-inch flexible ductwork to maintenance holes 

shown in Table 3 according to schedule outlined in Table 5 

Operation of hot-wire anemometer during both passive and active ventilation efforts; 

record subsequent air velocity readings in daily field log. 

Set up 10,000 cfm fan, portable generator and approximately 200-linear feet of 36-inch 

flexible ductwork to maintenance holes shown in Table 3 according to schedule outlined 

in Table 5. 

Operate fan and portable generator according to schedule outlined in Table 5. 

 

Remove fan, generator, ductwork and all appurtenant equipment following completion of 

testing. 

City BOS Staff 

 

Accompany HDR/MP staff in field during instrument installation/removal and fan 

operation during normal working hours. 




3.0 FIELD SAMPLING 

This section describes the field sampling that was conducted to collect air pressure data. The 

sampling was performed according to the schedule as described in Table 4 and Table 5. During 

Baseline Session 1, six maintenance holes were used to install data loggers to collect air pressure 

data. After reviewing the passive ventilation results, an additional six maintenance holes were 

proposed for Baseline Session 2 to further assess the extent of the passive and active ventilation. 

The descriptions of each sampling maintenance hole are included in Table 2. Figure 1 provides a 

map showing the locations of the maintenance holes where the data loggers and the ductwork 

were installed. 

3.1 Baseline Data Collection 

Three weeks’ worth of baseline differential air pressure data were collected from June 2 to June 

24, 2009 in the six maintenance holes listed previously in Table 2. Grab sample pressure data 

were also collected during each field trip using a hand-held manometer. This grab sample data is 

summarized in Table 6. Detailed information on the continuous differential air pressure data 

collected is presented in Section 4.0 (Test Results). 




Table 6 - Grab Sample Pressure Data 

Maintenance 

Hole 

Information 

Data Collected on 06/02/09 

(Tuesday) 


(Tuesday) 


(Thursday) 


(Tuesday) 


(Monday) 

MH Number 

Differential 

Pressure 

(inch of water 

column) 

Time of 

Pressure 

Measurem 

ent 

Differential 

Pressure 


column) 

Time of 

Pressure 

Measurem 

ent 

Differential 

Pressure 


column) 

Time of 

Pressure 

Measurem 

ent 

Differential 

Pressure 


column) 

Time of 

Pressure 

Measurem 

ent 

Differential 

Pressure 


column) 

Time of 

Pressure 

Measurem 

ent 

NORS ECIS Jnc 

535‐09‐022 

0.28 10:10 AM 0.35 9:36 AM 0.25 9:30 AM 0.19 10:42 AM 0.04 10:15 AM 

NORS DS of 

Duct Cxn 

(535‐13‐007) 

0.28 10:30 AM 0.38 10:18 AM 0.28 10:00 AM 0.26 10:26 AM 0.16 9:00 AM 

NCOS US of 

Duct Cxn 

(535‐05‐016) 

‐0.16 8:50 AM ‐0.12 8:28 AM ‐0.3 9:40 AM ‐0.31 10:54 AM 0.05 8:10 AM 

NCOS DS of 

Duct Cxn 

(535‐09‐009) 

‐0.4 10:57 AM ‐0.3 10:34 AM ‐0.4 10:05 AM ‐0.28 10:10 AM ‐0.08 9:40 AM 

NORS Culver 

City Park 

(535‐05‐021) 

0.35 9:31 AM 0.35 8:53 AM 0.3 8:48 AM 0.19 11:24 AM 0.13 8:35 AM 

NORS US of 

Duct Cxn 

(535‐09‐006) 

0.28 9:52 AM 0.33 9:58 AM 0.26 9:20 AM 0.29 9:00 AM 0.1 8:00 AM 




3.1.1 Data Logger Installation 

Six data loggers were initially installed to record baseline differential air pressure data on June 2, 

2009 at the sampling locations indicated above. City staff was present at the time to provide 

guidance on the exact sampling locations that were proposed. Photographs taken at each sampling 

location are presented in Figure 2 through Figure 7. 

Figure 2 - Maintenance Hole 535-09-022 Figure 3 - Maintenance Hole 535-13-007 






3.1.2 Baseline Data Download 

The Baseline Session 1 pressure data was downloaded on site in the field and the data loggers 

were reset and reinstalled at the same locations on June 9, 2009. The Baseline Session 2 pressure 

data was downloaded on June 25, 2009 and all the data loggers were temporarily removed. 

3.2 Ductwork Installation 

The duct connection from the NORS to the NCOS was completely constructed on July 20, 2009. 

Figure 8 through Figure 11 present pictures of the duct connection in the field. The connection 

between the two interceptors was performed with a 36-inch diameter, corrugated HDPE duct laid 

above-ground. 

Figure 8 - Duct Connection 

Figure 9 - NORS Maintenance hole of Duct 

Connection 

(taken from west to east) 

(taken from east to west) 

Figure 10 - NCOS Maintenance hole of 

Duct Connection 

Figure 11 - Air Velocity Measurement 

(taken from west to east) 

(taken from east to west) 

On July 20, 2009, the GK field crew began the duct installation just before 8:00 am. The fence 

was removed and the lower part of the fence was cut on one side and then unscrewed to allow for 

the duct to pass through. The backhoe was used to move each segment of the duct into place. 




Once everything was lined up, the segments on the NORS side were joined together. Joint 

lubricant was applied to the gaskets, and then a truck was backed up to one side of the duct and 

used as an anchor, while the backhoe applied pressure on the other side to push the segments 

together. 

The city crew arrived to assist in opening the NCOS maintenance hole. With the city’s 

supervision, a backhoe was employed to apply the force necessary to lift the cover off. An inner 

plate underneath the maintenance hole cover was found to be severely corroded and had to be 

broken into pieces to be removed. After the cover was removed, the 90 degree duct fitting was 

mounted over the hole and anchored into the concrete surrounding it. The backhoe was used to 

level out the dirt leading from the maintenance hole to the fence. The remaining duct segments 

were joined together, and then connected to the 90 degree fitting. Air/water tight foam was 

sprayed around the base of the 90 degree fitting to form a seal around the maintenance hole. 

Once the 90-degree fitting was connected to the duct, air could clearly be felt being drawn into 

the open end of the duct into the NCOS. Measurements were taken to cut the last segment before 

joining it to the NORS maintenance hole. Once the segment was cut, the other 90 degree fitting 

was prepared to be bolted over the NORS maintenance hole. The cover was removed; the 90 

degree bend was connected to the duct and then anchored over the NORS maintenance hole into 

the asphalt. The foam was sprayed around the connection to seal it. Sandbags were setup to help 

reinforce various spots along the duct. The backhoe scooped dirt up against the base of the duct 

on the NCOS end to make it more stable. Following completion of the duct connection to both 

maintenance holes, the site was cleaned up; a temporary piece of fence was put up above the duct 

to prevent access on to private property near the NCOS maintenance hole and all remaining 

equipment was loaded onto the vehicles. 

In order to record the pressure data after the duct connection was completed, all six data loggers 

were reset and reinstalled at the original locations on July 14, 2009. The pressure data was 

downloaded on July 23 and 27, 2009. 

3.2.1 Passive Ventilation Phase 

The first test performed following the installation of the 36-inch diameter duct was to measure the 

transfer of air from the NORS to NCOS through the duct connection while simultaneously 

logging differential air pressure in the maintenance holes listed in Table 2. The air transfer for 

this portion of the test was due to the difference in differential air pressure between the NORS 

and the NCOS. 

Air Velocity Measurement 

In order to assess the airflow through the duct connection, air velocity grab samples through the 

duct were taken every hour from July 21 through July 24, 2009 by GK Associates. During each 

hourly sampling effort, a total of twelve readings were taken in the duct cross section: six 

readings across the horizontal direction and six readings across the vertical direction. A graphic 

depiction of the cross section of the air duct, and the locations in it where air velocity readings 

were taken, is presented in Figure 12. The air velocity readings taken horizontally and vertically 

across the cross section of the duct did not vary significantly during the test; therefore the 

subsequent airflow calculations were taken to be accurate and valid. 




Figure 12 - Air Velocity Measurements in Duct 

Additional Air Pressure Sampling Locations 

After reviewing the air pressure data collected at the six original sampling locations and the air 

velocity data collected in the duct, an additional six air pressure sampling locations were added to 

further assess the extent of the passive and active ventilation. Before installing the data loggers at 

these additional locations, reconnaissance was scheduled for August 5, 2009 to assess the 

accessibility and feasibility of the proposed maintenance holes. After reconnaissance, the six data 

additional loggers were installed on August 10, 2009. All sampling locations are shown in Figure 

1 and listed in Table 1. The photos for the additional locations are shown in Figure 13 through 

Figure 18. 




Figure 13 - Maintenance hole 560-12-145 Figure 14 - Maintenance hole 559-05-005 

Figure 15 - Maintenance hole 535-06-132 Figure 16 - Maintenance hole 535-09-017 

Figure 17 - “Special Manhole” Figure 18 - Maintenance hole 535-09-017 




3.2.2 Active Ventilation Phase 

Following the passive ventilation test, the duct was separated to permit a fan to be inserted in-line 

between the two duct sections (Figure 19 through Figure 22). During the next 4 days, air pressure 

data and air velocity data were recorded to measure the effect of the fan air withdrawal from the 

NORS to the NCOS. The fan speed was varied in order to achieve planned withdrawal rates of 

5,000, 7,500, 10,000 and 12,500 cfm. 

Figure 19 - Fan Installation 

Figure 20 – Fan Prep 

Figure 21 - Fan Installation Complete 

Figure 22 – Fan Control Panel 




Figure 23 - Sound Abatement 

Figure 24 -- Installation Complete 

Figure 25 - Fan Test Complete 




4.0 TEST RESULTS 

4.1 Baseline Phase Air Pressure Data 

This section provides a summary of baseline air pressure data recorded on the NORS and NCOS 

prior to connecting the 36-inch diameter duct at West LA College. 

Figure 26 - Baseline Air Pressure 

The baseline pressure data recorded on the NORS and NCOS during the first week of testing is 

presented in Figure 26. The most notable feature of this data set is the difference between the 

average differential air pressures on the NORS and NCOS. As shown in Figure 26, the average 

pressure of the locations measured on the NORS was approximately 0.2 inches w.c., whereas the 

average pressure of the locations measured on the NCOS was approximately -0.39 inches of w.c. 

This represents an average pressure difference between the two interceptors of approximately 

0.59 inches w.c. These results confirm air pressure testing results conducted by BOS 

immediately prior to this round of testing during which it was discovered that pressures on the 

NCOS were well below atmospheric. This data is summarized numerically in Table 7. 




Table 7 - Summary of Baseline Air Pressure Data Recorded in NCOS and NORS, 6/2/09 to 6/9/09 

Maintenance Hole Baseline session 1 6/2 - 6/9 

Description MAX MIN AVG STD DEV 

NCOS US of Duct Cxn (535-05-016) 0.05 -0.57 -0.38 0.08 

NCOS DS of Duct Cxn (535-09-009) -0.01 -0.69 -0.41 0.08 

NORS Culver City Park (535-05-021) 0.11 -0.07 0.01 0.03 

NORS DS of Duct Cxn (535-13-007) 0.84 -0.09 0.22 0.09 

NORS US of Duct Cxn (DS of NOTF) (535-09-006) 0.72 -0.02 0.19 0.09 

NORS ECIS Jnc (535-09-022) n/a n/a n/a n/a 

The data in Table 7 show that air pressures along the NORS reached levels as high at 0.84 inches 

w.c. and that pressures on the NCOS were as low as -0.69 inches w.c. Note that no usable data 

was recorded at the NORS ECIS junction location due to a pinched-off tube on the data logger 

used in that maintenance hole. 

Under normal operating conditions, diurnal air pressure trends in Los Angeles interceptors are 

generally believed to be a function of diurnal wastewater flow trends, therefore the general 

similarities in diurnal trends that are observed between the two interceptors are not surprising. 

Even more interesting to observe, however, are the similarities in minute trends along the two 

interceptors. For example, significant variations in pressure activity were recorded on June 3 rd in 

both interceptors. Numerous examples of this can be seen in the data chart presented in Figure 

27. Other, similar occurrences will be described later in this section. 

Figure 27 - Detail 24 hours Baseline 

1 

0.8 

0.6 

Combined NORS NCOS Baseline Air Pressure Data 6/3/09 

Prior to Passive Duct Connection 







0.4 

0.2 

0 

‐0.2 

‐0.4 

‐0.6 

‐0.8 

12:00 AM 6:00 AM 12:00 PM 6:00 PM 12:00 AM 

Time 

A second baseline pressure session was conducted between 6/9/09 and 6/24/09. During this 

approximate 2-week period the 10,000 cfm interim NORS/ECIS ATF was purposely shut off in 




order to observe the differential air pressure effects, if any, on the NORS and NCOS. Figure 28 

presents a chart of the pressure data recorded on the NORS and NCOS during this period. 

Figure 28 - NORS NCOS Air Pressure with NORS/ECIS ATF Shut Off 


1 

0.8 

0.6 

0.4 

0.2 

0 

‐0.2 

Combined NORS NCOS Air Pressure Data 

Baseline Session 2 





‐0.4 

‐0.6 

‐0.8 

6/8/09 6/10/09 6/12/09 6/14/09 6/16/09 6/18/09 6/20/09 6/22/09 6/24/09 6/26/09 

Time 

The most notable feature of this data set is the overall pressure differences between the period 

when the NORS/ECIS ATF was shut off (6/9 to 6/15) and when it was turned back on (6/15 to 

6/24). The average pressure levels at all locations measured were lower while the ATF was 

turned on. The differences in average pressure for these two ATF operation conditions are shown 

in Table 8. It is interesting to note that this effect was measured on both the NORS and the 

NCOS even though the air for this ATF is being drawn off of a point just upstream of the 

NORS/ECIS junction. This indicates that the ATF has an effect on the NCOS through a common 

headspace upstream of the NORS/ECIS junction. 

Table 8 – Summary of Baseline Air Pressure Data Recorded in NCOS and NORS, 6/9/09 to 6/24/09 

Maintenance Hole NORS/ECIS Shut OFF 6/9 - 6/15 NORS/ECIS Turned ON 6/15 - 6/24 OFF-to-ON 

Description MAX MIN AVG STD DEV MAX MIN AVG STD DEV ΔAVG. 

NCOS US of Duct Cxn 

(535-05-016) -0.05 -0.46 -0.34 0.07 0.32 -0.57 -0.41 0.08 -0.07 

NCOS DS of Duct Cxn 

(535-09-009) n/a n/a n/a n/a n/a n/a n/a n/a n/a 

NORS Culver City Park 

(535-05-021) 0.1 -0.07 0.01 0.02 0.03 -0.08 -0.01 0.03 -0.02 

NORS DS of Duct Cxn 

(535-13-007) 0.44 0.00 0.25 0.09 0.42 0.03 0.17 0.07 -0.08 

NORS US of Duct Cxn 

(DS of NOTF) 

(535-09-006) 0.37 -0.2 0.18 0.09 0.35 -0.06 0.09 0.07 -0.09 

NORS ECIS Jnc 

(535-09-022) 




Figure 29 - Air Pressure (Absolute Value) Difference between NORS and NCOS 

0.8 

Difference Btwn Average NORS & NCOS Air Pressure 

Baseline Session 2 

0.7 

Air Pressure Difference (in. wc) 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

0 

6/8/09 6/10/09 6/12/09 6/14/09 6/16/09 6/18/09 6/20/09 6/22/09 6/24/09 6/26/09 

Time 

The chart presented in Figure 29 shows the absolute value difference between the average 

measured air pressures on the NORS and the average measured air pressures on the ECIS for the 

second baseline data gathering phase which lasted from 6/9/09 to 6/24/09. The maximum values 

are approximately 0.65 inches w.c.; the minimum values are approximately 0.4 inches w.c. and 

the average values are approximately 0.55 inches w.c. 




Figure 30 - 24-hour Period Combined Data on 6/19/09 

0.6 

0.4 

Combined NORS NCOS Baseline Data, Session 2 

6/19/09 






0.2 

‐0 

‐0.2 

‐0.4 

‐0.6 

12:00 AM 6:00 AM 12:00 PM 6:00 PM 12:00 AM 

Time 

The chart in Figure 30 shows the combined data for the 24-hour period on 6/19/09. The most 

notable feature of this data set is the severe upward spike in air pressure that was measured on the 

NCOS between approximately 9:45 am and 10:45 am. The air pressure changes approximately 

0.6 inches w.c. during that time period; from -0.3 to 0.3 inches w.c. It is interesting that a similar, 

albeit less severe, effect was measure on the NORS during the same time interval. One possible 

explanation for this phenomenon is that the 40,000 cfm chemical ATF which had been ventilating 

the NOS south of West LA College was either temporarily turned off or its air pathway to the 

NCOS via the 42-inch diameter ventilation pipe that connects the NOS and the NCOS was 

temporarily blocked. 




4.2 Passive Duct Phase Air Pressure Data 

Figure 31 shows the effects of the passive 36-inch diameter duct connection explained previously 

in this report between the NORS and NCOS maintenance holes. 

Figure 31 - Passive Air Duct Connection Data 

The duct was connected on July 20, 2009. The most notable feature of this data set is that MH 

535-05-016 on the NCOS (depicted by the red line on the chart in Figure 31) was the location 

whose air pressure was most visibly affected by the passive duct connection. The average 

pressure increased from -0.15 inches of water column to approximately 0.0 inches of water 

column following the duct connection between the NORS and NCOS. The pressure data 

recorded during this initial four-day period following the duct installation is summarized in Table 

9. 

Table 9 - Summary of Air Pressure Data Recorded in NCOS and NORS, 7/20/09 to 7/24/09, Passive 

Ventilation Phase 

Maintenance Hole Passive Duct Cxn 7/20 - 7/24 


NCOS US of Duct Cxn (535-05-016) 0.25 -0.21 0.01 0.07 

NCOS DS of Duct Cxn (535-09-009) 0.11 0.08 0.10 0.01 

NORS Culver City Park (535-05-021) 

NORS DS of Duct Cxn (535-13-007) 0.48 0 0.19 0.08 

NORS US of Duct Cxn (DS of NOTF) (535-09-006) 0.49 0.02 0.19 0.08 

NORS ECIS Jnc (535-09-022) 0.45 0 0.15 0.08 




The data presented in Table 9 clearly show that the highest pressures were recorded in the NORS 

during this time period. The average pressures were approximately 0.19 inches w.c. on the 

NORS. 

Figure 32 - Passive Duct Air Pressure Data 

Figure 32 shows pressure data over a three day period during the passive duct connection phase 

of the test. Also the time interval during which the NORS/ECIS and Jefferson Siphon ATFs were 

shut down is shown. The data presented suggest that the shutdown of the ATFs had a measurable 

effect on both the NORS and the NCOS; in particular the pressures on both interceptors increased 

by approximately 0.125 inches of water column during this time interval when compared to 

similar pressures recorded the day before the ATF was shut down. 

Figure 33 and Figure 34 show the velocity readings and corresponding airflows, respectively, 

recorded in the 36-inch diameter duct during the passive air test. The average velocity and 

airflow during the passive test was approximately 450 fpm and 3,300 cfm, respectively, with the 

exception of 0930 to 1800 hours on July 22 nd . The NORS/ECIS and Jefferson Siphon ATFs, 

were temporarily shut-down during this time period. The 10,000 cfm NORS/ECIS and Jefferson 

Siphon ATFs are approximately 0.25 mile and 1.0 mile, respectively, upstream of the two 

maintenance holes that were connected by the above-ground duct. The average velocity and 

corresponding airflow through the 36-inch diameter duct decreased to approximately 225 fpm and 

1,500 cfm, respectively, during this time interval. 




Figure 33 - Average Passive Air Velocity 

Average Passive Air Velocity Through NORS/NCOS Duct 

600 

500 

400 

Velocity (ft/min) 

300 

200 

100 

NORS/ECIS and 

Jefferson Scrubbers 

Shut-off Period 

0 

7/21/2009 7/22/2009 7/23/2009 7/24/2009 7/25/2009 

Figure 34 - Average Passive Air Flow 

4000 

Average Passive Air Flow Through NORS/NCOS Duct 

3500 

3000 

Air Flow (cfm) 

2500 

2000 

1500 

1000 

500 

NORS/ECIS and 

Jefferson Scrubbers 

Shut-off Period 

0 

7/21/2009 7/22/2009 7/23/2009 7/24/2009 7/25/2009 




A summary of the pressure data taken between 0930 and 1800 hours on July 20 th , July 21 st and 

July 22 nd is presented in Table 10. 

Table 10 - Comparison of Data Summary for Pressure Recorded from 7/20/09 to 7/22/09 

Two Days Prior to 

7/20 & 7/21 (0930 to 1800 hrs.) 

ATFs Shut Down MAX MIN AVG STD DEV 


NCOS DS of Duct Cxn (535-09-009) 0.11 0.09 0.09 0.05 

NORS Culver City Park (535-05-021) n/a n/a n/a n/a 

NORS DS of Duct Cxn (535-13-007) 0.37 0.08 0.17 0.05 

NORS US of Duct Cxn (DS of NOTF) (535-09- 

006) 0.37 0.1 0.18 0.05 

NORS ECIS Jnc (535-09-022) 0.35 0.03 0.13 0.08 

7/22 (0930 to 1800 hrs.) 

During ATFs Shut Down MAX MIN AVG STD DEV ΔAVG. 

NCOS US of Duct Cxn (535-05-016) 0.24 -0.07 0.11 0.05 0.17 

NCOS DS of Duct Cxn (535-09-009) 0.11 0.1 0.10 0.00 0.01 

NORS Culver City Park (535-05-021) n/a n/a n/a n/a n/a 

NORS DS of Duct Cxn (535-13-007) 0.47 0.08 0.27 0.07 0.10 

NORS US of Duct Cxn (DS of NOTF) (535-09- 

006) 0.47 0.09 0.28 0.07 0.10 

NORS ECIS Jnc (535-09-022) 0.45 0.03 0.23 0.08 0.10 

The data presented in Table 10.suggest that the 10,000 cfm NORS/ECIS ATF and the 10,000 cfm 

Jefferson siphon ATF have a more pronounced effect on the pressures in the upstream reach of 

the NCOS than on the NORS. This is made apparent by comparing the change in average 

pressures along the NORS to that on the NCOS upstream of the 36-inch diameter duct connection 

(535-05-016). During the ATFs shut-down period (0930 to 1800 hours on July 22 nd ), the average 

pressure along the NORS increased by 0.10 inches of water column as compared to the same time 

period during the previous two days (0930 to 1800 hours on July 20 th and July 21 st ); however the 

average pressure in the NCOS at maintenance hole 535-05-016 increased by 0.17 inches of water 

column during the same three-day time period. 




Figure 35 - NORS NCOS Passive Duct Test Air Pressure Data 

Figure 35 is a continuation of the pressure data set presented previously in Figure 32. It shows 

the effects of the passive 36-inch diameter duct connection explained previously in this report 

between the NORS and NCOS maintenance holes. 

As shown in Table 11, the highest average air pressures were recorded in the NORS, where the 

values were between 0.07 and 0.12 inches of water column. As previously seen during other 

testing sessions, the lowest average air pressures were in the NCOS, where the average values 

recorded were between -0.03 and 0.03 inches water column. As was shown during the initial 

stage of the passive test the most stark pressure changes occurred in the NCOS. Because the 

pressures in the NORS decreased (albeit slightly) and the pressures in the NCOS increased 

significantly during the passive phase, this suggests that the combined system had come to some 

equilibrium as a possible result of the passive duct connection. This equilibrium phenomenon is 

more clearly shown when one compares the data presented in Figure 36 to that presented 

previously in Figure 26. 

Table 11 - Summary of Air Pressure Data Recorded in NCOS and NORS, 7/24/09 to 

7/27/09, Passive Ventilation Phase 

Maintenance Hole Passive Duct Cxn 7/24 - 7/27 



NCOS DS of Duct Cxn (535-09-009) 0.29 -0.09 0.03 0.06 

NORS Culver City Park (535-05-021) n/a n/a n/a n/a 

NORS DS of Duct Cxn (535-13-007) 0.36 0 0.12 0.06 

NORS US of Duct Cxn (DS of NOTF) (535-09-006) 0.35 0 0.11 0.06 

NORS ECIS Jnc (535-09-022) 0.32 -0.05 0.07 0.06 




The chart presented previously in Figure 26 showed that the average air pressures measured in the 

NORS during the first week of June, 2009 were approximately 0.2 inches of water column during 

the initial week of baseline pressure data recording. The pressures measured in the NCOS during 

that same time period, were approximately -0.39 inches of water column. The data presented in 

Figure 36 show that by the last week of July, 2009, after the 36-inch diameter passive duct had 

been installed for approximately 1 week, the average air pressure in the NORS had been reduced 

to 0.14 inches of water column; representing a change of -0.06 inches of water column. The 

average air pressures in the NCOS had increased to 0.03 inches of water column; representing a 

change of +0.42 inches of water column. 

Figure 36 - NORS NCOS Air Pressure with Passive Duct Connection 

A numerical summary of the air pressure data shown in Figure 36 is presented in Table 12. 

Included in this data table is a column showing the change in average pressure at each location 

between the baseline pressure data phase in June, 2009 (shown in Table 7) and the end of the 

passive duct connection pressure recording phase at the end of July, 2009. The data in Table 12 

also show that the most significant changes in average pressure were recorded on the NCOS. The 

average increases in pressure were between 0.39 and 0.46 inches of water column along that 

sewer. The NORS pressure changes were much smaller by comparison. The average pressure in 

the first maintenance hole downstream of the duct connection (MH 535-09-006), was reduced by 

-0.08 inches of water column; however this still resulted in an average differential air pressure of 

0.14 inches of water column at that location. Very little change in effect was recorded at the 

Culver City Park maintenance hole (MH 535-05-021) and the maintenance hole upstream of the 

duct connection (MH 535-09-006); however it is interesting to note that two of these locations did 

experience a slight decrease in pressure, whereas the Culver City Park maintenance hole 

experienced a slight increase in pressure This difference in reactions along the NORS is likely 




due to the fact that the NORS reach on which the Culver City Park maintenance hole is situated 

was essentially carrying no flow at the time of this test. 


Passive Ventilation Phase 

Maintenance Hole Passive Duct Cxn 7/27 - 8/3 BL to Pass 

Description MAX MIN AVG STD DEV ΔAVG. 

NCOS US of Duct Cxn (535-05-016) 0.09 -0.09 0.01 0.02 0.39 

NCOS DS of Duct Cxn (535-09-009) 0.24 -0.07 0.05 0.06 0.46 

NORS Culver City Park (535-05-021) 0.22 0.02 0.04 0.01 0.03 

NORS DS of Duct Cxn (535-13-007) 0.33 0.02 0.14 0.06 -0.08 

NORS US of Duct Cxn (DS of NOTF) 

(535-09-006) 0.31 0.02 0.13 0.06 -0.06 

NORS ECIS Jnc (535-09-022) 0.29 -0.07 0.09 0.06 n/a 

4.3 Active Duct Phase Air Pressure Data 

The air velocity data recorded in the 36-inch diameter duct during the active ventilation phase of 

this study is presented graphically in Figure 37 (which references the chart in Figure 38). The 

data indicate that the targeted airflows were met for each portion of the test. 

On the afternoon of August 20 th , the team had planned to increase the airflow from the NORS to 

the NCOS to the maximum allowed by the fan, 15,000 cfm; however the anemometer that was 

being used in the field indicated that only approximately 8,400 cfm was being moved through the 

36-inch diameter duct. 

Figure 37 - Air Velocity Measurements in Duct 




The following day, August 22 nd , a replacement anemometer was used to measure velocity through 

the duct. The fan was still running at the same speed as it had been the previous day. The 

replacement instrument indicated, as shown graphically in Figure 38, that the target airflow was 

being met. It was surmised by field personnel that the first anemometer had likely been 

inadvertently exposed to moisture which had gathered at the bottom of the 36-inch diameter duct, 

thus causing it to fail to report accurate velocity readings on the afternoon of August 21 st . 

Figure 38 - Air Velocity Measurements in Duct (2) 




Differential air pressure data recorded along the NCOS and NORS interceptors during the active 

ventilation phase of the duct test are presented graphically in Figure 39. A summary of the 

numerical data for the same locations is presented in Table 13. 

Figure 39 - Combined NORS-NCOS Air Pressure with Fan Active 

1 



0.8 

0.6 


0.4 

0.2 

0 

‐0.2 

‐0.4 

‐0.6 

‐0.8 

Table 13 - Summary of Air Pressure Data Recorded in NCOS and NORS, 8/18/09 to 

8/24/09, Active Ventilation Phase 

Maintenance Hole 






NORS ECIS Jnc (535‐09‐022) 

8/18/09 8/19/09 8/20/09 8/21/09 8/22/09 8/23/09 8/24/09 8/25/09 8/26/09 8/27/09 8/28/09 

Fan Test 

8/18 - 8/24 

Passive 

to Active 

Baseline to 

Active 

STD 

DEV ΔAVG. ΔAVG. 

Description MAX MIN AVG 

NCOS US of Duct Cxn (535-05-016) 0.09 0.02 0.05 0.01 0.04 0.43 

NCOS DS of Duct Cxn (535-09-009) 2.16 -0.61 1.57 0.85 1.51 1.97 

NORS Culver City Park (535-05-021) 0.19 -0.03 0.07 0.04 0.04 0.08 

NORS DS of Duct Cxn (535-13-007) 0.34 -0.12 0.10 0.07 -0.04 -0.10 

NORS US of Duct Cxn (DS of NOTF) 

(535-09-006) 0.4 -0.1 0.10 0.08 -0.03 -0.02 

NORS ECIS Jnc (535-09-022) 0.39 -0.14 0.06 0.08 -0.03 0.06 

Time 

The data presented in Figure 39 and Table 13 indicates that the most significant effect of the 

active ventilation exercise was measured in the downstream reach of the NCOS (MH 535-09- 

009). At this location, the average pressure between the passive and active ventilation phases 

increased by 1.51 inches of water column as a result of the fan operation. Except for the Culver 

City Park maintenance hole (535-05-021), the average pressures on the NORS were only changed 

by approximately -0.03 inches of water column. The average pressure at the Culver City Park 

maintenance hole was increased by 0.04 inches of water column. 




Following the analysis of air pressure data recorded during the June/July baseline period and the 

initial passive ventilation test, the decision was made to conduct pressure testing in four 

additional maintenance holes between August 18 th and August 27 th . The results of this additional 

testing are presented graphically in Figure 40 and summarized in Table 14 and Table 15. 

Figure 40 - Combined Air Pressure 8/18 to 8/27 

1 

0.8 

0.6 



NORS Ivy Way (535‐06‐132) 

NOS Warehouse (535‐09‐017) 

NCOS Green Valley (at siphon) 

NOS/NCOS Vent Line near NOS ("Special MH") 


0.4 

0.2 

0 

‐0.2 

‐0.4 

‐0.6 

‐0.8 

8/9/09 8/11/09 8/13/09 8/15/09 8/17/09 8/19/09 8/21/09 8/23/09 8/25/09 



Maintenance Hole Passive Ventilation Test 8/10 - 8/18 


NORS Ivy Way (535-06-132) 0.37 0.00 0.14 0.07 

NCOS Green Valley (at siphon) 0.25 -0.14 0.03 0.04 

NOS Warehouse (535-09-017) 1.97 -1.35 0.37 0.31 

NOS/NCOS Vent Line near NOS ("Special MH") 0.08 -0.49 -0.27 0.13 

Time 



Maintenance Hole Active Ventilation Test 8/18 - 8/24 

Pass to 

Fan 

Description MAX MIN AVG STD DEV ΔAVG. 

NORS Ivy Way (535-06-132) 0.33 -0.04 0.09 0.07 -0.04 

NCOS Green Valley (at siphon) 2.18 -2.25 0.23 1.10 0.20 

NOS Warehouse (535-09-017) 0.65 0.02 0.39 0.22 0.02 

NOS/NCOS Vent Line near NOS 

("Special MH") -0.04 -0.39 -0.21 0.07 0.06 




As with the data presented previously in Figure 39 and Table 13, the data presented in Figure 40 

and Tables 14 and 15 indicate that the most significant pressure effect of the active ventilation 

between the NORS and the NCOS was measured in the segment of the NCOS downstream of the 

duct connection. At the NCOS Green Valley maintenance hole (559-05-005) the average 

pressure increased 0.2 inches of water column between the active and passive duct ventilation 

portions of the test. This maintenance hole is located at the head house of the siphon on the 

NCOS at Centinela. This is not as high as the pressure increase measured further upstream on the 

NCOS at MH 535-09-009; however this can likely be explained by the presence of air jumpers at 

the NCOS siphon and the 42-inch diameter air ventilation duct that connects the NCOS siphon to 

the NOS siphon. 

4.4 Findings 

The following provides a summary of the findings of the NORS/NCOS duct connection study: 

 

 

 

 

During baseline testing, the average pressure levels at all locations measured were lower 

while the NORS/ECIS ATF was turned on. This effect was measured on both the NORS 

and the NCOS even though the air for this ATF is being drawn off of a point just 

upstream of the NORS/ECIS junction. 

The passive air duct connection resulted in the NORS and NCOS coming to equilibrium 

with respect to differential air pressure. The pressures in the NCOS generally increased 

and the pressures in the NORS generally decreased; however the increase in the NCOS 

was significantly greater than the decrease along the NORS. 

Actively forcing air from the NORS into the NCOS resulted in an increase in pressure in 

the NCOS that is deemed to be unacceptably high. The pressures in the NORS were not 

reduced significantly as a result of active ventilation. 

The cause of the negative pressures in the NCOS was likely due to either one or both of 

two factors: 

• The presence of a permanent air duct that connects the NOS to the NCOS 

• Low flow conditions that were in effect along the NCOS as a result of upstream 

flow diversions into the NORS due to the then ongoing rehabilitation of the NOS 

Based on the analysis of the results presented in this report, the project team determined that 

additional sampling would be needed in order to determine the most likely cause of negative 

pressures in the NCOS. Specifically, the team decided that the possible air pressure effects due 

to the permanent air duct that connects the NOS to the NCOS would need to be investigated. The 

procedures, results and conclusions of that investigation are presented in Section 5.0 of this 

report. 




5.0 AIR DUCT ISOLATION BETWEEN NOS AND NCOS 

The upstream ends of the siphons of the NOS and NCOS near the 405 Freeway are connected via 

a permanent 24-inch/42-inch diameter air duct which was part of the construction of the NCOS. 

The purpose of this sampling is to test the differential air pressure results of plugging this 24- 

inch/42-inch diameter duct; thereby isolating the NOS from the NCOS. Recent sampling efforts 

have shown that differential air pressures in the NCOS are typically at or below atmospheric 

levels, whereas in the past they have been highly positive. By plugging this air duct and 

simultaneously testing the air pressures along the NOS and the NCOS, air pressure conditions 

along the NOS will be either ruled out or confirmed as the likely source of the negative air 

pressures currently existing in the NCOS. 

5.1 Sampling & Air Duct Isolation 

The air pressure sampling consisted of continuous data collection at two-minute intervals over an 

approximate two-week period at the locations shown in Table 1. Using two-minute intervals, 

ACR SmartReader Plus 4 data loggers have the capacity to log data for approximately two weeks. 

The data logging instruments were installed from the surface, and entry into maintenance holes or 

structures were not required for their installation. Installations was temporary and generally took 

place by suspension from a maintenance hole rim, step or lid without damage to any liners or 

protective coatings. 

A crew from GK & Associates was deployed in the field to isolate the 24-inch high by 57-inch 

wide rectangular section of the air duct. Note that the plug installation involved a confined space 

entry into the access structure. A schematic showing this air duct connection, the flap, and the 

special maintenance hole is presented in Figure 41. In the field, the air duct was isolated using 

plywood and rubber flap and sealed with expanding foam as shown in Figure 42. 

Figure 41 - Schematic of 24-inch/42-inch Air Duct and Special MH Flap Installation 

“Special MH” 

24-inch high by 57-inch 

wide rectangular duct 

to NOS 

42-inch diameter 

air duct to NCOS 

24-inch plywood 

and rubber flap 




Figure 42 - Air Duct Isolation with plywood and rubber flap 

5.2 Locations to be Sampled 

In general, continuous differential air pressure sampling locations were selected based on the 

following criteria: 

 

 

 

Locations along the NCOS previously sampled both before and during the recently completed 

passive and active air duct connection test between the NCOS and NORS 

Locations along the NOS previously sampled during the recently completed active air duct 

connection test between the NCOS and NORS 

A location on the NOS approximately halfway between those which were previously sampled 

during the recently completed active air duct connection test between the NCOS and NORS 

Table 16 presents the differential air pressure sampling locations. Each sampling location 

represents a maintenance holes. 

Table 16 - Air Pressure Sampling Locations 

Air Pressure Sampling Locations 

Location MH Number Sewer 

MH at upstream end of NCOS siphon; Green Valley Circle 559-05-005 NCOS 

MH at skate park; Jefferson Blvd. 535-05-016 NCOS 

MH in front of West LA College gymnasium 535-09-009 NCOS 

MH along air duct that connects the NCOS to the NOS “Special MH” NOS 

Culver City; MH approximately near intersection of Bernardo 535-13-002 NOS 

and Eveward. 

MH at upstream end of NOS siphon; Fox Hills Dr. 559-05-008 NOS 

MH at downstream end of NOS siphon; near Arizona Cir. 560-08-056 NOS 

A map showing the sampling locations is presented in Figure 43. 




Figure 43 - Sampling Location Map for Air Duct Isolation 




The photos of the seven sampling maintenance holes are shown in Figure 44 to Figure 50. 


Figure 46 - Maintenance Hole 535-09-009 

Figure 47 - Maintenance Hole Special MH 





Figure 50 - Maintenance Hole 560-08-056, Downstream of NOS Siphon 

The air pressure effects of plugging the air duct connection between the NOS and the NCOS was 

measured on both sewers simultaneously. The duct can be accessed via the so-called “special 

MH” referenced in Table 16. This “special MH” is located on an unpaved parcel in the northwest 

corner of the intersection of Fox Hills Drive and Green Valley Circle. 

5.3 Schedule 

There were two distinct sessions envisioned for this sampling effort. Session 1 involved 

gathering baseline differential air pressure data at the locations listed in Table 16. Session 2 was 

similar to Session 1, except that the air pressure effects of the plugged air duct connection 

described above was measured at the same maintenance holes in which the baseline air pressure 

data had been taken during Session 1. Table 17 summarizes these two sessions. 


Session 1 

Baseline Air Pressure 

Session 2 

Air Duct Effect Testing 

Task 

Install Air Pressure Data 

Loggers 

All data loggers installed 

and logging data 

Plug air duct connecting 

the NOS to the NCOS 

All data loggers installed 

and logging data 

Retrieve data logging 

instruments and 

disassemble duct plug 

Start Date 

10/26/09 

End Date 

11/2/09 

Start Date 

11/2/09 



End Date 

11/9/09 

n/a n/a Day 8 Day 14 

n/a n/a Day 8 Day 14 

n/a n/a n/a Day1 4 

Session 1 lasted approximately one week. Approximately one day was needed to install all the 

data loggers. Approximately one day was needed to remove all the data loggers at the end of 

Session 2. 




The data download was performed immediately after the instruments were removed following the 

completion of Session 2. The flap was removed on or immediately after the day that Session 2 

was completed. 


The sampling activities were a joint effort between the HDR/Malcolm Pirnie team, its sub (GK & 

Associates) and City BOS staff. The roles and responsibilities for each of the three include the 

following: 


Preparation of all instruments for data logging 

 

 

 





GK & Associates (or designated sub-contractor) 

Support of HDR/MP representatives in accessing, closing and properly sealing all 



 

 

Arranging for procurement of inflatable sewer plug capable of plugging the 24-inch diameter 

air duct that connects the NOS to the NCOS; also arranging for procurement of air 

compressor; generator and all confined space entry equipment necessary to perform ingress 

and egress into the “Special MH” and inserting and inflating the plug in the field. Note that 

the “Special MH” constitutes the point along the layout at which the air duct diameter 

changes from 24-inches to 42-inches. For the purposes of this test it would be preferable for 

the 24-inch diameter section of the duct to be plugged. This is due to the fact that a 

differential air pressure logger was placed in the “Special MH” which was intended to record 

the pressure results with and without the headspace pressure effects of the NOS. 

Performing confined space entry into “special MH”; inserting and inflating, and subsequently 

deflating and removing, 24-inch diameter air duct plug according to plug manufacturer’s 

recommended procedures and the schedule. The static air pressure inside the plug should be 

monitored by GK field crew personnel (or their designated sub-contractor) once per day 

during Session 2 to ensure that the plug does not deflate prematurely during this time period. 

The daily pressure reading should be recorded and submitted to HDR at the end of Session 2. 

These pressure readings can be performed using a dedicated, liquid filled type static air 

pressure gage that would be installed following plug inflation. The gauge can remain in place 

during the entire Session 2 period. 


 

Accompany HDR/MP staff in field during instrument and plug installation/removal 

during normal working hours. 




5.5 Test Results 

A chart showing a graphical representation of the pressure data recorded at all locations during 

the entire two-week test is presented in Figure 51. A statistical summary of this data is presented 

in Table 18. A chart showing typical baseline pressure data is presented in Figure 52. 

Figure 51 - NOS/NCOS Duct Isolation Air Pressure Data 

Table 18 - Summary of Air Pressure Data Recorded During Duct Isolation Study 

Locations Baseline Duct Plugged ATFs OFF 

NOS - Bernardo at 

Eveward 

NOS Siphon Inlet 

(Alt.) 

NOS-Siphon 

Outlet (Alt.) 

NOS/NCOS Vent 

Line near NOS 

("Special MH") 

NCOS DS of Duct 

Cxn (535-09-009) 

Max Min Avg Max Min Avg Δ Max Min Avg Δ 

Avg. 

Avg. 

0.14 -0.42 -0.04 0.14 -0.12 -0.01 0.03 0.08 -0.13 -0.01 0.03 

0.15 -0.42 -0.02 0.16 -0.08 0.01 0.03 0.11 -0.09 0.02 0.04 

0.03 -0.11 0.00 0.03 -0.01 0.01 0.01 0.02 -0.01 0.01 0.01 

0.25 -1.50 -0.27 0.16 -0.98 -0.36 - 

0.09 

-0.06 -0.49 -0.30 -0.02 -0.49 -0.33 - 

0.03 

0.03 -0.92 -0.39 -0.12 

-0.03 -0.38 -0.23 -0.07 




Figure 52 - Typical Baseline Air Pressure Data 

The most noticeable pressure change effect took place at the “Special MH” located on the 

NOS/NCOS vent line following the installation of the duct isolation device on the vent line. The 

average air pressure at that location was reduced by approximately 0.1 inches of water column. 

The other noticeable pressure effect was recorded on the two maintenance holes along the NOS 

upstream of the siphon, namely the Bernardo at Eveward maintenance hole and the NOS Siphon 

Inlet maintenance hole. At those two locations, the minimum pressures were increased by 

approximately 0.3 inches of water column. This was likely related to the air draw effect from the 

negative headspace air pressures along the NCOS. This NCOS air draw effect was cut-off from 

the NOS once the NOS/NCOS vent line was blocked off on 11/1/09. 

A graphical representation of the pressure effects of the planned 24-hour NORS/ECIS and NOTF 

ATF shut-off is presented in Figure 53. 

The most noteworthy recorded effect of the ATF shut-off occurred on the NCOS at MH 535-09- 

009. This is the first maintenance hole downstream of the 36-inch flexible duct connection that 

was installed as part of the NORS/NCOS test described previously in this report. The average 

pressure at that location increased by 0.11 inches of water column following the planned ATF 

shut-down. This is similar to the pressure responses to NORS/ECIS ATF shut-down events 

during the passive and active duct ventilation study described previously in this report. 




Figure 53 – Pressure Effects of ATF Shutoffs 

The main conclusion drawn from the duct isolation study is that the negative pressures previously 

recorded along the NCOS were not a function of any pressure activity along the NOS. Isolating 

the permanent air duct that connects those two interceptors at their respective siphons in Culver 

City did not result in a significant reduction in air pressure along the NCOS. It did, however 

result in a significant increase in the minimum pressures along the NOS. 




6.0 ADDITIONAL SIPHON SAMPLING 

In August, 2010 additional air pressure sampling was performed in the following two general 

areas: 

 

 

Culver City/Baldwin Hills area on the NORS and NOS 

San Fernando Valley upstream of the Radford Siphon on the NOS 

The purpose of the sampling in the Culver City/Baldwin Hill area was to perform an analysis of 

air pressures in the NORS as a follow-up subsequent to flow diversions away from the NORS to 

the recently rehabilitated NOS. Prior to the flow diversion to the NOS, the average dry weather 

flows (ADWF) in the NORS were approximately 325 cfs. Following the diversion of flows to the 

NOS, the ADWF in the NORS was reduced to 200 cfs. These flow values were provided to the 

consultant team by BOS from the MIKE Urban hydraulic model that is maintained by BOS staff. 

These flow differences are depicted graphically in Figure 54. 

Figure 54 - NORS Modeled Wastewater Flow Comparison 

Wastewater Flow (cfs) 

450 

400 

350 

300 

250 

200 

150 

100 

NORS Modeled Wastewater Flow Comparison 

Avg = 

325 

Avg 

= 

200 

2009 (Pre NOS 

Activation) 

2010 (Post NOS 

Activation) 

50 

0 

12:00 AM 

4:00 AM 

8:00 AM 

12:00 PM 

4:00 PM 

8:00 PM 

12:00 AM 

4:00 AM 

Time 

Table 19 provides information on the locations of pressure sampling, both in the Culver 

City/Baldwin Hill area and in the San Fernando Valley. 




Table 19 – Sampling Locations and Description 

MH Number Sewer Field Notes 

Culver City/Baldwin Hill Area 

535-05-021 NORS Culver City Park 

535-09-006 NORS US of above-ground duct connection 

535-09-022 NORS NORS/ECIS scrubber 

535-13-007 NORS DS of above-ground duct connection 

559-05-006 NORS Bristol N. of Hannum 

535-02-155 NOS Rodeo E. of Kalisman 

535-03-156 NOS Rodeo W. of Cochran 

San Fernando Valley 

442-08-091 NOS 2 nd MH US of Radford Siphon 

442-08-090 NOS 3 rd MH US of Radford Siphon 

Findings 

The air pressure data recorded during this round of sampling is presented graphically in Figures 

55, 56 and 57. The data is summarized in Table 20. 

Figure 55 - Post Flow Diversion NORS Air Pressure 

0.5 

Post Flow Diversion NORS Air Pressure 

0.4 

0.3 

Air Pressure (inches WC) 

0.2 

0.1 

0 

‐0.1 

‐0.2 

‐0.3 

‐0.4 

NORS Culver City Park (MH 535‐05‐021) 


NORS ECIS Scrubber 


NORS at Bristol N of Hannum 

8/16/10 8/18/10 8/20/10 8/22/10 8/24/10 8/26/10 




Figure 56 - Post Flow Diversion NOS Air Pressure in Baldwin Hills 

Post Flow Diversion NOS Air Pressure in Baldwin Hills 

0.5 

0.4 

0.3 


0.2 

0.1 

0 

‐0.1 

‐0.2 

‐0.3 

‐0.4 

NOS Rodeo E. of Kalisman 

NOS Rodeo W. of Cochran 

8/16/10 8/18/10 8/20/10 8/22/10 8/24/10 8/26/10 

Figure 57 - NOS Air Pressure US of Radford Siphon 

NOS Air Pressure US of Radford Siphon 

0.5 

0.4 


0.3 

0.2 

0.1 

0 

‐0.1 

‐0.2 

‐0.3 

‐0.4 

NOS 3rd MH US of Radford Siphon 

NOS 2nd MH US of Radford Siphon 

8/16/10 8/18/10 8/20/10 8/22/10 8/24/10 8/26/10 




Table 20 – Culver City/Baldwin Hills and San Fernando Valley Air Pressure Data Summary 

MH Number Sewer Air Pressure (inches WC) 

Culver 

City/Baldwin 

Hills Area 

MAX MIN AVG STD DEV Pre to Post 

Flow Diversion 

ΔAVG. 

535-05-021 NORS 0.29 -0.17 0.06 0.07 0.05 

535-09-006 NORS 0.27 -0.08 0.05 0.06 -0.14 

535-09-022 NORS 0.3 -0.1 0.05 0.07 n/a 

535-13-007 NORS 0.42 -0.3 -0.01 0.16 -0.23 

559-05-006 NORS 0.34 -0.12 0.07 0.08 n/a 

535-02-155 NOS 0.22 -0.36 -0.01 0.06 n/a 

535-03-156 NOS 0.2 -0.41 -0.02 0.02 n/a 

San Fernando 

Valley 

442-08-091 NOS 0.11 -0.01 0.05 0.02 n/a 

442-08-090 NOS 0.18 -0.09 0.00 0.01 n/a 

As shown in Table 20, significant changes in the recorded average pressures along the NORS 

were recorded between the pre-diversion period in June 2009, when the NORS was operating 

near capacity, and the post-diversion period in August 2010, by which time over 30% of the 

NORS flows had been diverted to the NOS. There was a slight, albeit negligible increase in 

average pressure at maintenance hole 535-05-021 (in Culver City Park); however significant 

reductions in pressure were achieved at maintenance hole 535-09-006 and at maintenance hole 

535-13-007. Reductions in average pressures at these two locations were -0.14 and -0.23 inches 

of water column, respectively. These pressure reductions were likely due to the increase in 

available headspace in the NORS that resulted from the reduction in flows in that interceptor. 

Average pressures were also quite low in the NOS in the vicinity of Baldwin Hills. Pressure were 

near atmospheric at MH 535-03-156, on Rodeo just west of Cochran during the entire testing 

period, with one exception early in the testing period. Average pressures were also near 

atmospheric on the NOS at MH 535-02-155 on Rodeo east of Kalisman; however it is unclear 

why the fluctuations at this location were significantly greater than those recorded west of 

Cochran. 

The pressures recorded just upstream of the Radford Siphon on the NOS in the San Fernando 

Valley were very near atmospheric during the entire one week testing period. This is not 

surprising given the presence of an actively ventilated carbon scrubber at the upstream end of the 

siphon. These results indicate that the scrubber is maintaining acceptable pressure levels in the 

NOS in this area; thereby not allowing positive air pressure to build in the NOS upstream of the 

siphon. 





The purpose of the NOS/NCOS duct isolation study was to determine whether the negative 

pressures recorded along the NCOS were being caused by pressure effects along the NOS by 

isolating the permanent in-ground duct that connects the NOS to the NCOS. The following 

provides a summary of the additional findings of the NORS/NCOS duct connection study 

subsequent to the completion of the NOS/NCOS duct isolation study: 

 

 

 

 

 

 

Isolating the permanent air duct that connects those two interceptors at their respective 

siphons in Culver City did not result in a significant increase in air pressure along the NCOS. 

It did, however result in a significant increase in the minimum pressures along the NOS. 

The negative pressures previously recorded along the NCOS did not appear to be a function 

of any pressure activity along the NOS. 

The negative pressure conditions along the NCOS appeared to have been influencing the 

pressures along the NOS between the siphon and the NOTF via the permanent duct 

connection between the two interceptors, thus potentially keeping the pressures along the 

NOS lower than they normally would be when flow is reintroduced back to the NOS. 

Both ATF ventilation activity and the presence of the NOS/NCOS permanent air duct were 

ruled out as causes of the negative pressures recorded along the NCOS; therefore the most 

likely cause of these low pressure conditions was the low flow conditions that were in effect 

along the NCOS during that time. 

Flow diversions away from the NORS to the NOS appear to have resulted in significant 

pressure reductions on the NORS 

The Radford Siphon carbon scrubber appears to be maintaining air pressure levels very near 

atmospheric in the immediate upstream reaches of the NOS in the San Fernando Valley. 





Team 





DIFFERENTIAL AIR PRESSURE STUDY 

AT DROP STRUCTURES 

TECHNICAL MEMORANDUM 



Drop Structures and Differential Air Pressure Study Technical Memorandum – FINAL 




1.0 Introduction and Overview ................................................................................................. 4 

1.1 Purpose .......................................................................................................................... 5 

1.2 Expected Behavior ......................................................................................................... 5 

2.0 Sampling and Testing Setup .............................................................................................. 6 

2.1 Data Collection Locations ............................................................................................... 6 

2.2 Schedule ....................................................................................................................... 11 


3.0 Field Activities .................................................................................................................. 16 

3.1 Data Logger Installation ................................................................................................ 16 

3.2 Plug Installations .......................................................................................................... 19 

3.3 Flow Management ........................................................................................................ 21 

4.0 Collected Data and Results .............................................................................................. 22 

4.1 Division Drop Structure ................................................................................................. 22 

4.2 Humboldt Drop Structure .............................................................................................. 27 

4.3 Mission and Jesse Drop Structure ................................................................................ 31 

4.4 23 rd and San Pedro Drop Structure .............................................................................. 36 

4.5 USC Drop Structure ...................................................................................................... 40 

4.6 Systemwide Downstream ............................................................................................. 44 

4.7 Findings ........................................................................................................................ 48 

4.7.1 General Observations ........................................................................................... 48 

4.7.2 Division Drop Structure & Vicinity .......................................................................... 48 

4.7.3 Humboldt Drop Structure & Vicinity ....................................................................... 49 

4.7.4 Mission and Jesse Drop Structure & Vicinity ......................................................... 49 

4.7.5 23 rd and San Pedro Drop Structure & Vicinity ....................................................... 50 

4.7.6 USC Drop Structure & Vicinity ............................................................................... 50 

4.7.7 Systemwide Downstream Interceptors .................................................................. 50 

5.0 Conclusions ...................................................................................................................... 52 


Table 1 - Air Pressure Sampling Locations ................................................................................. 10 

Table 2 - Overall Sampling Schedule ......................................................................................... 12 

Table 3 – Differential Air Pressure Data (inches of water) at the Division Drop Structure .......... 23 

Table 4 - Humboldt Drop Structure Recorded Field Data ........................................................... 28 

Table 5 - Mission and Jesse Drop Structure Recorded Field Data ............................................. 32 

Table 6 - 23rd and San Pedro Drop Structure Recorded Field Data .......................................... 37 

Table 7 - USC Drop Structure Recorded Field Data ................................................................... 41 

Table 8 - Systemwide Downstream Recorded Field Data .......................................................... 45 


Figure 1 – Location of the Air Pressure Data Loggers for the Drop Structures Sampling ............. 8 

Figure 2 - Location of the Air Pressure Data Loggers Downstream ............................................. 9 

Figure 3 – Timeline of Pressure Data Collection at Drop Structures and Sewer Conditions ...... 11 

Figure 4 - Schematic of Data Logger Installation at Division Drop Structure .............................. 13 

Figure 5 - Schematic of Data Logger Installation at Humboldt Drop Structure ........................... 14 

Figure 6 - Schematic of Data Logger Installation at Mission and Jesse Drop Structure ............. 14 

Figure 7 - Schematic of Data Logger Installation at 23rd and San Pedro Drop Structure .......... 15 




Figure 8 – Data Logger Installation Kit ........................................................................................ 17 

Figure 9 - Manhole 49514241 ..................................................................................................... 17 

Figure 10 - Data Logger Installed ............................................................................................... 17 

Figure 11 - Manhole 49509106 ................................................................................................... 17 

Figure 12 - Installing Data Logger ............................................................................................... 17 

Figure 13 – Manhole 51509154 .................................................................................................. 17 



Figure 16 - Manhole 53503213 .................................................................................................. 18 


Figure 18 - Manhole 53703199 Installed .................................................................................... 18 

Figure 19 - Manhole 53706178 .................................................................................................. 18 

Figure 20 - Plug Delivery ............................................................................................................ 19 

Figure 21 - Plug Discharge ......................................................................................................... 19 

Figure 22 - Plug Installation with Crane ...................................................................................... 19 

Figure 23 - Plug Insertion in M/H ................................................................................................ 19 

Figure 24 - Adjustment to Plug ................................................................................................... 20 

Figure 25 - Plug Installation Complete ........................................................................................ 20 

Figure 26 - View of Plug in Air Return Line ................................................................................. 20 

Figure 27 - Installation of Data Loggers ...................................................................................... 20 

Figure 28 - Removal of the Stop Log .......................................................................................... 21 

Figure 29 - Stop Log Location ..................................................................................................... 21 

Figure 30 - Division Drop Structure Upstream Average Pressure .............................................. 24 

Figure 31 - Division Drop Structure Downstream Average Pressure .......................................... 25 

Figure 32 - Division Drop Structure Average Pressure ............................................................... 25 

Figure 33 - Humboldt Drop Structure Upstream Average Pressure............................................ 28 

Figure 34 - Humboldt Drop Structure Downstream Average Pressure ....................................... 29 

Figure 35 - Humboldt Drop Structure Average Pressure ............................................................ 29 

Figure 36 - Mission and Jesse Drop Structure Upstream Average Pressure ............................. 32 

Figure 37 - Mission and Jesse Drop Structure Downstream Average Pressure ......................... 33 

Figure 38 - Mission and Jesse Drop Structure Average Pressure .............................................. 33 

Figure 39 - 23rd and San Pedro Drop Structure Upstream Average Pressure ........................... 37 

Figure 40 - 23rd and San Pedro Drop Structure Downstream Average Pressure ...................... 38 

Figure 41 - 23rd and San Pedro Drop Structure Average Pressure ........................................... 38 

Figure 42 - USC Drop Structure Upstream Average Pressure ................................................... 41 

Figure 43 - USC Drop Structure Downstream Average Pressure ............................................... 42 

Figure 44 - USC Drop Structure Average Pressure .................................................................... 42 

Figure 45 - Systemwide Downstream Jefferson Siphon Upstream Average Pressure ............... 45 

Figure 46 - Systemwide Downstream NORS/ECIS Junction (ECIS) Average Pressure ............ 46 

Figure 47 - Systemwide Downstream NORS/ECIS Junction (NORS) Average Pressure .......... 46 







odors with an extensive use of magnesium hydroxide and sodium hydroxide. Recently, the City 

has been working to improve odor control in several areas identified by local community groups. 

Air treatment facilities (ATFs) with known or predicted odor “hot spots” were included in the 

2004 Collection System Settlement Agreement (CSSA). 

The present Technical Memorandum evaluates the impact of drop structures on the sewer 

differential air pressures (difference in air pressure between inside and outside the sewer and the 

ambient air), under current conditions and planned modifications. This study focused on 

measuring and examining differential air pressures at the four existing sewer drop structures in 

the City's wastewater collection system: 

 

 

 

Division 

Humboldt 




pressures along a route starting at the Division drop structure on the Northeast Interceptor Sewer 

(NEIS) and ending at a maintenance hole in Culver City Park on the North Outfall Relief Sewer 

(NORS). This was followed by the physical plugging of the return air lines at each drop structure 

in order to measure the impact on the differential air pressures at the drop structures, on the North 

Outfall Sewer (NOS) approach pipelines and downstream deep tunnels. The purpose was to 

examine the effectiveness of each return line in air recirculation at the structure. Finally, flow 

management (diversion/redistribution) was introduced to the system and more differential air 

pressure data was collected to valuate the effect of increased flow on the differential air pressures 

at the structures. During theses three different phases, Air Treatment Facilities (ATF) at Mission 

& Jesse and Humboldt were turned on and off several times in order to study their impact on the 

air pressure. 

It was determined that: 

Plugging the air return lines in the drop structures generally resulted in increasing 

pressures along the entire NEIS/ECIS tunnel alignment 

 

 

 

Plugging the air return lines generally resulted in maintaining atmospheric pressure levels 

in the NOS approach sewers, with the exception of the approach to the Mission and Jesse 

drop structure, where pressures initially increased along the NOS, then gradually 

decreased over time 

Adjusting stop logs generally resulted in decreasing pressures on the NOS. Those 

locations where the pressures didn’t decrease remained at atmospheric levels. 

The ATF’s generally reduce pressures along the entire length of the NEIS/ECIS tunnel. 

Noticeable pressure reductions occur when the ATF’s are turned on. Noticeable pressure 

increases occur when the ATF’s are turned off. 

1.0 INTRODUCTION AND OVERVIEW 

A drop structure is a manmade structure to pass water or wastewater to a lower elevation while 

controlling the energy and velocity of the flow. The purpose of the air return line in a drop 




structure is to attenuate the pumping effect of the flow going down the drop shaft. Drop 

structures in sewer collection systems are potential locations of odor problems as a result of 

increased air pressures caused by the rapid vertical drop of sewage within the structures and the 

backpressurization caused by interconnection to the receiving tunnel. The increased air pressures 

often result in release of the odorous air to the neighborhood with subsequent odor complaints. 

1.1 Purpose 

The purpose of this TM is to better understand the sewer differential air pressures and 

fluctuations at the drop structures, and examine the impact of the following measures upon the 

differential air pressures: 

1. Blocking of the return air lines at each drop structure 

2. Increasing / decreasing the wastewater flow at each drop structure 

3. Operating (cycling off and on) the existing interim carbon scrubbers 

The broader objective is to identify any potential changes to the planned odor control strategies 

and methods to control odorous air release in the proximity of the drop structures. 

After collecting baseline measurements, return airlines within the drop structures were plugged in 

order to evaluate the effect that they have on air pressures within the structures and subsequently 

in the NORS, East Central Interceptor Sewer (ECIS) and the approach sewers such as the NOS 

and Eagle Rock Interceptor Sewer (ERIS). Several operation scenarios were also performed 

including diverting flows at the Humboldt and the Mission and Jesse drop structures and 

adjusting on/off status of the carbon scrubber at Humboldt and Mission and Jesse drop structures 

and the damper at the Humboldt drop structure, which together will facilitate understanding of the 

air movement as well as the zone of influence near each drop structure. 

1.2 Expected Behavior 

Previous studies performed on the interceptor system in Central Los Angeles have resulted in 

observations which showed that differences in observed positive differential air pressure 

phenomena in a given sewer over an extended time period could be directly correlated to 

wastewater flow volumes. For example, during times when flows are higher, differential air 

pressure tends to be higher and vice versa. This has been shown to be true not only with respect 

to diurnal flow patterns, but also before and during periods when flows have been diverted from 

one interceptor to another. 

In addition, previous differential air pressure readings taken on the approach sewer that carries 

flow into the USC Drop Structure on the ECIS have shown that the approach pipe headspace is 

under almost constant negative pressure. Unlike all other NEIS and ECIS drop structures, the 

USC Drop Structure has no air return line connecting the receiving chamber to the top of the 

vertical drop shaft; therefore there would be no open conduit for positive differential air pressure 

to travel into the shallow, upstream approach pipe. 

It was therefore theorized that the positive air pressure and subsequent odor complaints that occur 

primarily along the NOS could be better understood by conducting a series of tests that would 

involve the following system manipulations: 

 

 

Plugging the air return lines in place at the Division, Humboldt, Mission & Jesse and 

23rd & San Pedro drop structures along the NEIS and ECIS 

Adjusting stop logs in place at the Humboldt and the Mission & Jesse drop structures on 

the NEIS and ECIS, respectively to allow more flows to enter the drop structure at 





In conjunction with these two primary manipulations, various ON/OFF tests were to be staged 

with the interim carbon scrubbers at the Humboldt and Mission & Jesse drop structures. 

Given the previously described system pressure observations, the expected results at the start of 

the drop structure study were as follows: 

 

 

Plugging the air return lines would reduce the pressures along the shallow, approach 

sewers at each drop structure location 

Plugging the air return lines would result in an increase in differential air pressure along 

the NEIS and ECIS alignments 

Adjusting stop logs at Humboldt and Mission and Jesse would result in less air dragged 

into the NEIS at Humboldt and more air dragged into the ECIS at Mission and Jesse. 

The results of the drop structure test, compared to the expected results, were as follows: 

 

 

 

Plugging the air return lines did significantly reduce the differential air pressures on the 

NOS approach sewer at Mission &Jesse. Other NOS pressures at Humboldt and 23rd & 

San Pedro remained near atmospheric levels throughout the entire duration of the study. 

The pressure at the Division drop structure was significantly reduced as a result of 

plugging the air return line at that location. 

Plugging the air return lines did result in significant differential air pressure increases 

along the entire NEIS/ECIS alignment. 

Stop log adjustments at Humboldt and Mission and Jesse, in conjunction with all air 

return lines being plugged, appeared to result in an overall reduction in the amount of air 

dragged into the NEIS and ECIS. This was evidenced by the difference in air pressures 

between the Mission and Jesse and Humboldt drop structures when comparing the 

plugged condition versus the combined plugged and stop log adjustment condition. 

2.0 SAMPLING AND TESTING SETUP 

The air pressure data collection consisted of continuous pressure recording at two-minute 

intervals over two one-week periods at the locations shown in Table 1. ACR SmartReader Plus 4 

data loggers were used, which at a two-minute interval setting, have the capacity to log data for 

approximately two weeks. The data loggers were visited every two weeks to download the 

collected data, after which they were replaced with fresh data loggers in the maintenance holes 

for additional data collection. 

The data logging instruments were installed from the surface, which meant that entry into 

maintenance holes or structures was not required for their installation. Installations were 

temporary and generally took place by suspending the instruments from a maintenance hole rim, 

step or lid without damage to any maintenance hole liners or protective coatings. 

2.1 Data Collection Locations 

In general, the locations of the differential air pressure data loggers were selected as follows: 

 

 

 

Within each drop structure, locations in the drop shafts immediately above the 

connections with the air return lines; 

First manhole immediately upstream of each drop structure in the NEIS/ECIS tunnel and 

on the approach sewers; 

First manhole immediately downstream of each drop structure in the NEIS/ECIS tunnel; 




Manholes upstream and downstream of the USC drop structure on ECIS; 

Additional manholes near each drop structure if needed; 

One manhole upstream of Jefferson siphon on ECIS 

Manholes upstream of NORS/ECIS junction; 

Figure 1 and Figure 2 show the location of the differential air pressure data loggers relative to the 

drop structures and in downstream sewers. Table 1 provides the location details for each 

differential air pressure data logger. Each data logger was installed at maintenance holes 

referenced in Table 1. Figure 3 shows the pressure data collection timeline for the different sewer 

conditions tested (baseline, return line plugs, scrubbers on/off, and flow management). Figures 4, 

5 and 6 show the location of the differential air pressure data logger within each drop structure. 




Figure 1 – Location of the Air Pressure Data Loggers for the Drop Structures Sampling 




Figure 2 - Location of the Air Pressure Data Loggers Downstream 




Table 1 - Air Pressure Sampling Locations 

Group Manhole ID Sewer Description Street 1 Street 2 

Inner 

Cover 

46812156 ERIS 

First manhole upstream of 

Division drop structure CYPRESS AVE CAZADOR ST NO 

46816116 ERIS/NEIS Division drop structure 

SAN 

FERNANDO RD 

R/W 

NW/O 

DIVISION ST YES 

Division 

46713313 NEIS 

First manhole downstream of 

Division drop structure 

(1400 BLK)SAN 

FERNANDO RD 

NW/O ARVIA 

ST 

YES 

49505144 NEIS 


Humboldt drop structure on 

NEIS 

SAN 

FERNANDO RD 

N/O FIGUEROA 

ST (Home 

Depot) 

YES 

49509108 NOS/NEIS 


Humboldt drop structure on 

NOS 

W/O SAN 

FERNANDO RD 

N/O 

HUMBOLDT ST YES 

49509106 NEIS Humboldt drop structure 

W/O SAN 

FERNANDO RD 

N/O 

HUMBOLDT ST YES 

Humboldt 

49514241 NEIS 

Manhole midway between 

Humboldt and Mission/Jesse MAIN ST AVE 21 YES 

51505288 NEIS 


Mission/Jesse drop structure 

on NEIS MISSION RD KEARNEY ST YES 

51509154 NOS 


Mission/Jesse drop structure 

on NOS MISSION RD 6TH ST NO 

51513137 NEIS Mission/Jesse drop structure 

W/O MISSION 

RD N/O JESSE ST YES 

Mission and Jesse 51513136 ECIS 


Mission/Jesse drop structure SANTA FE AVE N/O 7TH PL YES 

23rd and San 

Pedro 

USC 

Systemwide 

Downstream 

Effects 

53703177 ECIS 

53702211 NOS 


23rd/San Pedro drop 

structure on ECIS 

Second manhole upstream of 


(1100 BLK) 

22ND ST 

E/O GRIFFITH 

AVE 

YES 

structure on NOS TRINITY ST S/O 23RD ST Unknown 



SAN PEDRO W/O SAN 

structure on NOS 

ST ALLEY PEDRO ST YES 

53703199 NOS/ECIS 


SAN PEDRO 

53703200 ECIS structure 

ST R/W 23RD ST YES 



W/O TRINITY 

53706178 ECIS structure 

31ST ST ST 

YES 


53705181 ECIS USC drop structure GRAND AVE S/O 35TH ST YES 

EXPOSITION E/O MENLO 

53705184 ECIS USC drop structure 

BLVD N/RDWY AVE 

YES 

First manhole downstream of EXPOSITION 

53607171 ECIS USC drop structure 

BLVD N/R LA SALLE ST YES 

Upstream of Jefferson JEFFERSON W/O 

53503213 ECIS Siphon 

BLVD 

COCHRAN YES 

Upstream of NORS/ECIS LA CIENEGA 

53506116 ECIS Junction on ECIS 

BLVD ALADDIN ST YES 

Upstream of NORS/ECIS JEFFERSON CULVER CITY 

53505021 NORS Junction on NORS 

BLVD R/W PARK 

YES 




2.2 Schedule 

There were three distinct sessions for the sampling effort. Session1 involved collecting baseline 

differential air pressure data with the scrubber turned on or off at the locations listed in Table 1. 

Session 2 was similar to Session 1, except that the return airline at each drop structure was 

plugged in an attempt to evaluate the air ventilation effectiveness through these existing return 

airlines. Session 3 consisted of monitoring the air pressures under several flow diversion 

scenarios along with the installation of a curtain at 23 rd and San Pedro. The purpose of the curtain 

was to isolate the headspace between the NOS approach sewer at 23 rd and San Pedro and the 

ECIS tunnel. 

Session 1 lasted four weeks (from 2/16/2010 to 3/16/2010) and included delays due to rain and 

contractor contract preparation. Session 2 started on 3/16/2010 with the installation of the fist 

plug at Division. Sampling of air pressure with plugs installed lasted from 3/16/2010 to 4/1/2010. 

Session 3, consisting of flow management, started on 4/2/2010 and lasted till 5/4/2010. The 

curtain was installed on 4/29/2010 and removed on 5/7/2010. Figure 3graphically depicts the 

timeline of the sampling sessions including the scrubbers’ on/off conditions at both Humboldt 

and Mission and Jesse. 

Figure 3 – Timeline of Pressure Data Collection at Drop Structures and Sewer Conditions 




Table 2 below shows the overall data collection schedule. 

Event 


Event Period 

Session 1 

Baseline Scenarios 

(Scrubber on/off) 

Start Date 

2/16/2010 

End Date 


Session 2 

Plug Test Scenarios 

Start Date 


End Date 


Session 3 

Flow Management 

/Curtain Test Scenarios 

Start Date 


Install Air Pressure Data 2/16 to 2/17 n/a n/a n/a n/a n/a n/a 

Loggers 

All data loggers installed and n/a Day 1 Day 29 n/a n/a n/a n/a 

logging data for Session 1 

Plug return airlines/Session 2 3/16, 3/19, 3/24, 3/30 n/a n/a Day 29 Day 45 n/a n/a 

begins 

Reset data loggers 3/2, 3/3 and 3/23, n/a n/a n/a n/a n/a n/a 

3/24 

All data loggers reset and n/a n/a n/a Day 29 Day 45 n/a n/a 

logging data/Session 2 

continues 

Adjust stop logs at Humboldt 4/2 and 4/13 n/a n/a n/a n/a Day 46 Day 57 

and M&J/Session 3 begins 

Reset data loggers 4/2, 4/3 and 4/20, 

4/21 

n/a n/a n/a n/a n/a n/a 

Install curtain at 23 rd and San 

Pedro 

All data loggers reset and 

logging data/Session 3 

continues 

Retrieve data loggers/Remove 

plugs/Remove curtain/Back to 

normal operations 

End Date 


Day 75 


4/29 

n/a n/a n/a n/a n/a Day 46 Day 80 

Day 77 to Day 85 

(5/7 to 5/12) 


Figure 4 thru Figure 7 below graphically show the drop structures and locations within each 

where pressure monitors were installed. An existing access maintenance hole at the Division 

Drop Structure is not depicted in Figure 4. 




Figure 4 - Schematic of Data Logger Installation at Division Drop Structure 




Figure 5 - Schematic of Data Logger Installation at Humboldt Drop Structure 

Figure 6 - Schematic of Data Logger Installation at Mission and Jesse Drop Structure 




Figure 7 - Schematic of Data Logger Installation at 23rd and San Pedro Drop Structure 


The sampling activities were a joint effort between the HDR/Malcolm Pirnie team, its 

subcontractor (E2) and City BOS staff. The roles and responsibilities for each of the three 

included the following: 


Prepare all instruments for data logging 

E2 

 

 

 

 

 

 





Documentation of results 

Support HDR/MP representatives in accessing, closing and properly sealing all 







Accompany HDR/MP staff in field during instrument installation/removal and fan 

operation during normal working hours. 

Adjust scrubber/damper status 

City Designated Contractor 

Procure inflatable plug capable of plugging the 42-inch diameter return airline at the 

Division drop structure 

 

 

 

 


Mission and Jessie drop structure 

Procure inflatable plug capable of plugging the 78-inch diameter return airline at the 23 rd 

and San Pedro drop structure 


Humboldt drop structure 

Perform confined space entry into the four drop structures; insert and inflate, and 

subsequently deflate and remove each of the four return air line plugs described 

previously in this section according to plug manufacturer’s recommended procedures and 

the schedule. 

Adjust stop logs at Humboldt and Mission and Jesse diversion structures during phase 3 

flow management test 

 

If necessary, install and remove the air curtain at 23rd and San Pedro drop structure 

3.0 FIELD ACTIVITIES 

This section describes the field activities that were performed to collect the differential air 

pressure data. The sampling was performed according to the schedule as described in Table 2. 

3.1 Data Logger Installation 

Twenty two data loggers were installed to record baseline differential air pressure data on 

February 16 and 17, 2010 at the data collection locations indicated in Table 1above. City staff 

were present at the time to provide guidance on the exact sampling locations that were proposed. 

Photographs taken at various data logger locations are presented in Figure 8 through Figure 19. 

Due to the length of the data collection period, the recorded data in the data loggers was 

periodically downloaded and then reset. In general, the data loggers were reset every two weeks 

to accommodate a two minute sampling sequence. 




Figure 8 – Data Logger Installation Kit Figure 9 - Manhole 49514241 

Figure 10 - Data Logger Installed Figure 11 - Manhole 49509106 

Figure 12 - Installing Data Logger 

Figure 13 – Manhole 51509154 




Figure 14 - Manhole 51513136 Figure 15 - Manhole 51513137 

Figure 16 - Manhole 53503213 Figure 17 - Manhole 53703199 

Figure 18 - Manhole 53703199 Installed 

Figure 19 - Manhole 53706178 




3.2 Plug Installations 

Following the baseline session, plugs were installed in the air return lines of the four drop 

structures (Division 3/16/10, Humboldt 3/19/10, Mission and Jesse 3/24/10 and 23 rd and San 

Pedro 3/30/10). The plugs were installed in order to evaluate the effect that they have on the 

differential air pressures within the structures and subsequently in the NORS and ECIS sewers 

themselves. The plugs were installed sequentially, starting at the Division drop structure and data 

was collected for 3 days before plugging the next one. City contractor Murray Company installed 

the plugs supported by City staff. 

Figure 20 - Plug Delivery 

Figure 21 - Plug Discharge 

Figure 22 - Plug Installation with Crane 

Figure 23 - Plug Insertion in M/H 




Figure 24 - Adjustment to Plug 

Figure 25 - Plug Installation Complete 

Figure 26 - View of Plug in Air Return Line 

Figure 27 - Installation of Data Loggers 




3.3 Flow Management 

Following the plug installation, the third session of the field differential pressure data collection 

was under way. Stop logs at two drop structures (Mission and Jesse, and 23 rd and San Pedro) 

were removed. During the first week of May, two stop logs were reinserted into the upstream 

NOS approach diversion at Humboldt. 

Stop log removals at Humboldt and Mission and Jesse resulted in a reduction of flow to the NEIS 

via the Humboldt drop structure and additional flow to the NEIS via the Mission and Jesse drop 

structure. 

The removal at the 23 rd and San Pedro drop structures brought additional flow to the NOS. 

Figure 28and Figure 29 show the removal of the stop log at Mission and Jesse. 

Figure 28 - Removal of the Stop Log 

Figure 29 - Stop Log Location 




4.0 COLLECTED DATA AND RESULTS 

This section provides the results of the field data collection. Data is compiled by drop structures 

and spans the length of the data collection period covering all three sessions of baseline, plugs 

and flow management. 

The large amount of data recorded over three months by each of the 22 data loggers collecting 

pressure readings every two minutes were processed to calculate the minimum, average, and 

maximum values. To calculate the average value, five time intervals were established first to 

divide a 24 hour period of a day. For each time interval, an average value was calculated using 

all 2-minute readings in that time period. Finally, the average value for that day was calculated as 

the average of the time intervals. The results are provided as tables and figures in this section. 

Each logger was essentially dormant during the approximate time period between March 3 rd and 

March 8 th , 2010. This is reflected in the data sets by the flat line at the atmospheric pressure level 

in each chart; however this time period was disregarded when calculating statistical summaries 

for each data set. 

4.1 Division Drop Structure 

Three data loggers were installed in the locations listed below. 

 

 

 

MH 46812156, 270 feet US of Division drop structure on ERIS 

MH 46713313, 2,900 feet DS of Division drop structure on NEIS 

MH 46816116, Division drop structure on NEIS 




(Note that in the figure above, US means “Upstream” and DS means “Downstream”) 

Table 3 – Differential Air Pressure Data (inches of water) at the Division Drop Structure 

Division Drop Structure Ups tream Downstream Drop Structure 

ERIS at MH 

46812156 

NEIS at MH 

46713313 

ERIS /NEIS at MH 

46816116 

Sewer Condition 

Min Avg Max Min Avg Max Min Avg Max 

Baseline 

Start 


End 

3/16/2010 0.2 0.23 0.28 -0.21 0.17 0.51 -0.35 -0.11 0.06 

Plug 

Installed 


Removed 

5/6/2010 0.22 0.24 0.29 -1.14 0.17 0.68 -2.36 -1.19 -0.1 

Stop Logs 

Removed 


Reinstalled 

5/4/2010 n/a n/a n/a -1.14 0.18 0.68 -2.3 -1.08 -0.11 

Note: The logger at manhole 46812156 was removed and stopped recording on 

3/24 due to extreme low pressure observed exceeding ACR logger limit 




The following three figures summarize the differential air pressure recorded at the maintenance 

holes at Division. 

Figure 30 - Division Drop Structure Upstream Average Pressure 




Figure 31 - Division Drop Structure Downstream Average Pressure 

Figure 32 - Division Drop Structure Average Pressure 




Plug Effect: The positive pressures recorded at the ERIS manhole during the baseline testing 

period were likely the result of air being forced up into the drop structure air return line from the 

NEIS tunnel, which was also under positive pressure. The plugged air return duct at the Division 

drop structure resulted in an extremely negative pressure at the upstream ERIS manhole. During 

field data download activities approximately two weeks after the air return line plug insertion at 

the Division drop structure, field crew members noted that the pressures at this location were well 

below the minimum -2.45 inches w.c. that the ACR data loggers are able to record. This was 

likely due to the fact that the approach pipe from the ERIS to the drop structure had been denied 

its primary source of headspace air when the plug was inserted in the drop structure air return 

line. The drag induced by the flowing wastewater on the drop structure approach pipe headspace 

therefore drew a vacuum in the upstream manhole. Crew member observations of airflow into 

this manhole confirmed that the pressures here were extremely negative. The plugged air return 

duct at the Division drop structure generally resulted in an increasing pressure trend on the NEIS 

just downstream of the drop structure. Once the plug was removed, the pressure on the NEIS MH 

at this location was immediately reduced to near baseline levels. The plugged air return duct at 

the Division drop structure clearly resulted in a severe pressure reduction at the drop structure. 

Once the plug was removed, the pressure at this location immediately increased to near baseline 

levels. 

Stop Log Removal: The pressure on the NEIS MH at this location showed a slight reduction 

trend during the stop log removal period. The pressure on the NEIS MH at this location remained 

fairly constant during the stop log removal period, during which time flows were directed away 

from the Humboldt drop structure and towards the Mission and Jesse drop structure. This was 

likely due to the fact that the NOS flow at the Humboldt drop structure was diverted away from 

the NEIS due to the stop log removal, thereby resulting in less air being dragged into the drop 

structure and into the NEIS. 

Scrubbers on/off: The scrubber at the Humboldt drop structure has a slight pressure reduction 

effect on the Division drop structure and a slight effect on the NEIS downstream of the Division 

drop structure. Several instances shown on the chart show direct, albeit slight, pressure increase 

effects when the scrubber was turned off and pressure decrease effects when the scrubber was 

turned back on. 




4.2 Humboldt Drop Structure 

Four data loggers were installed at the locations shown in the map below. 

MH 49505144, 2,800 feet US of Humboldt drop structure on NEIS 

MH 49509108, 60 feet US of Humboldt drop structure on NOS 

MH 49514241, 4,800 feet DS of Humboldt drop structure on NEIS 

MH 49509106, Humboldt drop structure on NEIS 





Table 4 - Humboldt Drop Structure Recorded Field Data 

Humboldt Drop Structure Upstream Downstream 


Min Avg Max Min Avg Max Min Avg Max Min Avg Max 

Baseline 

Start End 

2/16/2010 3/19/2010 

Scrubber Off 2/16/2010 3/4/2010 0.01 0.42 0.77 -0.50 -0.16 0.04 0.05 0.50 1.00 -0.27 0.05 1.48 

Scrubber On 3/4/2010 3/10/2010 0.27 0.41 0.59 -0.19 -0.09 0.04 0.24 0.41 0.76 -0.04 0.02 0.10 

Scrubber Off 3/10/2010 3/19/2010 -0.04 0.20 0.57 -0.23 -0.11 0.02 0.22 0.52 1.07 -2.42 -0.14 2.30 

Plug 

Installed 


Removed 


Scrubber On 3/19/2010 3/22/2010 -0.03 0.13 0.33 -0.27 -0.14 -0.01 0.24 0.61 0.92 -2.42 -1.92 0.05 

Scrubber Off 3/22/2010 4/2/2010 0.11 0.57 1.07 -0.29 -0.05 0.09 0.49 1.03 1.66 -2.42 -0.80 0.06 

Stop Logs 

Removed 


Reinstalled 


NEIS at MH 

49505144 

NOS at MH 

49509108 

NEIS at MH 

49514241 

Drop Structure 

NEIS/NOS at MH 

49509106 

Scrubber On 4/2/2010 4/7/2010 0.26 0.55 1.27 -0.18 0.00 0.09 0.75 1.14 1.88 -2.43 -1.84 0.34 

Scrubber Off 4/7/2010 4/9/2010 0.41 0.70 1.08 0.01 0.03 0.05 1.02 1.57 2.09 -2.41 -1.84 0.09 

Scrubber On 4/9/2010 4/13/2010 0.17 0.44 1.15 -0.11 0.01 0.05 0.73 1.15 1.80 -2.42 -2.23 0.08 

Scrubber Off 4/13/2010 4/19/2010 0.29 0.60 1.11 -0.01 0.03 0.05 0.80 1.29 2.01 -2.42 -2.28 0.04 

Scrubber On 4/19/2010 4/28/2010 0.08 0.64 1.12 -0.13 0.02 0.21 0.58 1.17 1.96 -2.43 -2.34 -1.67 

Scrubber Off 4/28/2010 5/4/2010 0.25 0.66 1.02 -0.01 0.02 0.06 0.83 1.11 1.58 -2.41 -2.27 -0.65 

The following three figures summarize the differential air pressure recorded at Humboldt. 

Figure 33 - Humboldt Drop Structure Upstream Average Pressure 




Figure 34 - Humboldt Drop Structure Downstream Average Pressure 

Figure 35 - Humboldt Drop Structure Average Pressure 




Plug Effect: The plugged air return ducts generally resulted in an increasing pressure trend in 

both the upstream and downstream maintenance holes on the NEIS. This was likely due to the 

fact that the interceptor system could not relieve its buildup of air pressure through the plugged 

drop structure air return lines and that more air was likely being dragged into the tunnel system 

from the Division and Humboldt drop structures. The pressure increase recorded at the NOS, 

however, was very minimal. This indicates that the momentum of flowing wastewater into the 

drop structure would not allow a significant air back pressurization into the NOS. Once the plug 

was removed, the pressure on the NEIS MH upstream of this location was immediately reduced 

to near baseline levels. The pressure at the drop structure remained at very low levels following 

plug removal. It is unclear why the pressure at the drop structure did not immediately increase 

back to near the baseline atmospheric level following plug removal at the air return line; however 

it could be that this was delayed and began to occur after the instrument was removed. The 

instrument removal time is depicted in the chart by the straight vertical line that terminates at the 

atmospheric pressure level just after May 12 th . 

Stop Log Removal: During the baseline pressure test phase, four stop logs were in place on the 

NOS side of the upstream diversion into the Humboldt drop structure. The pressure fluctuations 

in the NOS manhole just upstream of the Humboldt drop structure generally flattened after the 

stop logs were removed at the drop structure, but remained near atmospheric levels. This further 

indicates that the wastewater flow momentum into the drop structure would not allow a back 

pressurization effect to occur in the NOS. Pressures remained very near atmospheric levels from 

the day the stop logs were removed until the end of the test period. The pressure trends on the 

NEIS MH upstream of the drop structure remained fairly constant during the stop log removal 

period; however once the stop logs were re-installed an immediate reduction in pressure was 

recorded. A minimal downward trend in pressure was recorded at the maintenance hole 

downstream of the Humboldt drop structure during the stop log removal period. Pressures 

remained well below atmospheric levels at the drop structure itself. This indicates that from an 

air pressure perspective, the volume of flow entering the drop structure during the stop log 

removal period was optimal. The negative pressures recorded at the drop structure could have 

been lower than the minimum -2.42 recorded as this is very near the lower limit of the pressure 

data logging instruments. 

Scrubbers on/off: The scrubber at the Humboldt drop structure clearly has a slight pressure 

reduction effect on the NEIS MH upstream of that location; on the NEIS MH downstream of that 

location and at the drop structure itself. Several instances shown on the data charts show direct, 

albeit slight, pressure increase effects when the scrubber was turned off and slight pressure 

decrease effects when the scrubber was turned back on. 




4.3 Mission and Jesse Drop Structure 

Four data loggers were installed at the locations shown in the map below. 

MH 51505288, 5,000 feet US of Mission & Jesse drop structure on NEIS 

MH 515009154, 800 feet US of Mission & Jesse drop structure on NOS 

MH 51513136, 1,600 feet DS of Mission & Jesse drop structure on ECIS 

MH 51513137, Mission & Jesse drop structure on ECIS 

(Note that in the figure above, US means “Upstream” and DS means “Downstream”. Also note 

that the NEIS and NOS are very close together and almost parallel making it difficult to see that 

those two sewer lines in the figure.) 




Table 5 - Mission and Jesse Drop Structure Recorded Field Data 

Mission and Jesse Drop Structure Upstream Downstream Drop Structure 


Min Avg Max Min Avg Max Min Avg Max Min Avg Max 

Start End 

Baseline 

2/16/2010 3/24/2010 

Scrubber Offtart2/16/201 3/4/2010 0.00 0.30 0.70 -0.40 0.27 0.57 -0.02 0.31 0.60 0.05 0.29 0.53 

Scrubber On 3/4/2010 3/10/2010 -0.15 0.30 0.58 0.94 1.07 1.23 0.18 0.33 0.57 0.20 0.30 0.47 

Scrubber Off 3/10/2010 End3/24/2010-0.54 0.10 0.56 0.79 1.59 2.17 0.08 0.32 0.70 0.09 0.19 0.43 

Plug 

Installed 


Removed 



Scrubber Off 3/26/2010 4/2/2010 0.05 0.38 0.79 1.36 1.89 2.16 0.42 0.83 1.42 0.09 0.34 0.67 

Stop Logs 

Removed 


Reinstalled 


NEIS @ MH 

51505288 

NOS @ MH 

51509154 

ECIS @ MH 

51513136 

NEIS @ MH 

51513137 


Scrubber Off 4/7/2010 4/9/2010 0.26 0.75 1.13 1.18 1.44 1.61 1.04 1.54 2.14 0.09 0.46 0.89 

Scrubber On 4/9/2010 4/13/2010 -0.02 0.47 0.91 -0.68 1.23 1.72 0.53 1.45 2.09 0.07 0.27 0.60 

Scrubber Off 4/13/2010 4/19/2010 0.19 0.64 1.17 0.85 1.19 1.54 1.16 1.80 2.28 -0.10 0.22 0.66 

Scrubber On 4/19/2010 4/28/2010 -0.02 0.39 1.16 0.82 1.39 2.10 1.12 1.92 2.29 -0.10 0.07 0.52 

Scrubber Off 4/28/2010 5/4/2010 0.09 0.39 0.79 1.57 1.77 2.08 2.01 2.20 2.32 -0.11 0.07 0.41 

The three figures below summarize the air pressure recorded at the MHs at Mission & Jesse. 

Figure 36 - Mission and Jesse Drop Structure Upstream Average Pressure 




Figure 37 - Mission and Jesse Drop Structure Downstream Average Pressure 

Figure 38 - Mission and Jesse Drop Structure Average Pressure 




Plug Effect: The plugged air return duct at the Mission & Jesse drop structure generally resulted 

in an increasing pressure trend in the upstream maintenance hole on the NEIS; on the ECIS 

maintenance hole downstream of Mission & Jesse and in the drop structure itself. This was likely 

due to the fact that the interceptor system was not able to relieve air pressure buildup through the 

plugged NOS approach pipes at the drop structures. As plugs were inserted in the Division and 

Humboldt drop structures along the NEIS upstream of Mission and Jesse between 3/16/10 and 

3/19/10, the pressures in the NOS approach sewer upstream of Mission and Jesse began to 

increase significantly. This pressure increase was likely exacerbated by the presence of an 

approximate 30% diameter constriction along the NOS in the first maintenance hole upstream of 

the one in which the pressure testing was occurring. As the air from the NEIS/ECIS tunnel 

backpressurization effect filled the upstream NOS, the diameter reduction caused a constriction in 

the available headspace in the upstream NOS, resulting in higher than normal pressures. 

However the plug insertion at the Mission and Jesse drop structure on 3/23/10 appears to have 

caused a significant decreasing pressure trend along the same upstream NOS reach between the 

tested maintenance hole and the Mission and Jesse drop structure. This trend, which resulted in 

pressures being reduced from approximately 2.2 inches of water column down to approximately 

0.8 inches of water column, lasted for over four weeks. This gradual pressure reduction was 

likely due to the fact that the source of positive pressure-causing backpressurization air from the 

NEIS/ECIS tunnel in the NOS was denied by the presence of the plug at Mission & Jesse. Once 

the plugs at all drop structures were removed, the pressures on the NEIS MH upstream of this 

location and at the drop structure were immediately reduced to near baseline levels. Following 

plug removal, the pressures at the downstream maintenance hole on the ECIS and at the upstream 

NOS approach sewer maintenance hole were reduced as well. 

Stop Log Removal: The pressure trends on the NEIS MH upstream of the drop structure showed 

a general decreasing trend during the stop log removal period. This was likely due to the fact that 

less air was being dragged into the NEIS from the NOS approach at Humboldt. The pressure 

trends at the ECIS maintenance hole downstream of Mission and Jesse showed a general 

increasing trend during the stop log removal period. This was likely due to the fact that air was 

being dragged into the ECIS at Mission and Jesse through the NOS approach sewer during the 

stop log removal period, something that normally does not occur when the Mission and Jesse stop 

logs are in place. The pressure trends at the drop structure itself, showed a general decreasing 

trend during the stop log removal period. More air was being dragged into the Mission and Jesse 

drop structure through the NOS, causing lower pressures at the top of the drop structure. The 

data presented in Figure 37 provides further evidence that more air appeared to be dragged into 

the system at Mission and Jesse than is the case when the stop logs are in place. The pressures 

recorded at that ECIS maintenance hole, MH 515-13-136, could have actually been greater than 

those recorded. The maximum values recorded during the stop log removal period were very 

near the maximum value that the instruments are able to record. 

Scrubbers on/off: The scrubber at the Mission & Jesse drop structure clearly has a pressure 

reduction effect on the NEIS MH upstream of that location; the downstream ECIS manhole and at 

the drop structure itself. The recorded air pressure data show direct air pressure increases when 

the scrubber was turned off and pressure reduction effects when the scrubber was turned back on. 

The pressure on the NOS was reduced significantly during the approximately 40-day period that 

the plugs were in place and the stop logs were removed. The only exception to this trend was 

during a 10-to-15 day period between April 19th and May 2nd during which time the pressure 

rose in the NOS. This could have been a reaction to the absence of stop logs at the Mission and 

Jesse drop structure. In the immediate aftermath of stop log removal at Mission and Jesse, 

enough air was eventually dragged into the drop structure at that location to cause an increase in 




air pressure in the NOS approach sewer over a ten day period. It is theorized that not enough 

wastewater flow momentum was being generated in the NOS approach to physically impede this 

eventual air back pressure effect, hence the 10-day lag period between the removal of the Mission 

and Jesse stop logs and the increase in air pressure that appeared to begin on April 20th. 

A pressure reduction phenomenon was recorded on March 17th that did not appear to be caused 

by any planned manipulation to the drop structures or interceptor flow control system. In 

discussing this phenomenon with BOS staff, it was learned that a damper was fully closed on the 

Humboldt ATF air duct header during a routine inspection on that day. This could have directly 

caused the recorded reduction in pressure. 




4.4 23 rd and San Pedro Drop Structure 

Five data loggers were installed at the locations shown in the map below. 

MH 53703177, 1,700 feet US of 23rd & San Pedro drop structure on ECIS 

MH 53702211, 700 feet US of 23rd & San Pedro drop structure on NOS 

MH 53703199, 100 feet US of 23rd & San Pedro drop structure on NOS 

MH 53706178, 2,800 feet DS of 23rd & San Pedro drop structure on ECIS 

MH 53703200, 23rd & San Pedro drop structure on ECIS 

(Note that in the figure above, US means Upstream and DS means Downstream) 




Baseline 

Plug 

Table 6 - 23rd and San Pedro Drop Structure Recorded Field Data 

23rd and San Pedro Drop Structure 

Sewer Condition Min Avg Max Min Avg Max Min Avg Max Min Avg Max 

Start End 

2/16/2010 3/30/2010 -0.10 0.25 0.70 -1.22 0.10 1.74 0.25 0.58 0.95 -0.25 0.03 0.36 

Installed 


Installed 


ECIS @ MH 

53703177 

Upstream 

NOS @ MH 

53703199 

Downstream 

ECIS @ MH 

53706178 


ECIS @ MH 

53703200 

Removed 

4/30/2010 0.15 0.49 1.11 -0.22 0.05 0.12 0.01 0.68 1.50 -0.70 -0.14 0.12 

Removed 

5/7/2010 0.02 0.23 0.70 -0.10 0.04 0.10 0.25 0.48 0.93 -2.23 -0.99 0.13 

Curtain 

Note: The raw data for the logger at manhole 53706178 (31st & Trinity) from 2/16 to 3/24 

was not used due to erroneous data observed and obstruction during retrieval on 3/2 and 3/3 

The three figures below summarize the air pressure recorded at the MHs at 23 rd & San Pedro. 

Figure 39 - 23rd and San Pedro Drop Structure Upstream Average Pressure 




Figure 40 - 23rd and San Pedro Drop Structure Downstream Average Pressure 

Figure 41 - 23rd and San Pedro Drop Structure Average Pressure 




Plug Effect: The plug inserted at the 23 rd and San Pedro drop structure generally resulted in 

increasing pressures along the ECIS and a reduction in pressure at the drop structure itself. The 

plugged air return ducts on the three upstream drop structures generally resulted in an increasing, 

albeit slight, pressure trend at the upstream ECIS maintenance hole and a slight decreasing 

pressure trend at the drop structure By the time the last plug was inserted (at the 23rd and San 

Pedro drop) the pressures at the upstream location had increased by approximately 0.5 inches w.c. 

Once the last plug was removed, the pressures at the upstream ECIS maintenance hole and at the 

drop structure were immediately reduced to near baseline levels. 

A car was parked over the maintenance hole on the ECIS downstream of the 23rd and San Pedro 

drop structure for several days during the early part of the plug test. As a result it is difficult to 

ascertain the overall pressure trend at this location as a result of the insertion of all three plugs in 

air return lines at the three drop structures; however a significant increase in air pressure was 

recorded at this location following the installation of the air plug at 23rd and San Pedro. 

Stop Log Removal: The pressure on the ECIS at this location showed a slight reduction trend at 

the upstream ECIS maintenance hole; and at the drop structure during the stop log removal 

period. A significant reduction trend at the downstream ECIS maintenance hole was recorded 

while the stop logs were removed. 

Scrubbers on/off: The scrubber at the Mission & Jesse drop structure has a pressure reduction 

effect on the ECIS maintenance holes upstream and downstream of the 23rd and San Pedro drop 

structures. Several instances shown on the data charts show direct, albeit slight, pressure 

increase effects when the scrubber was turned off and pressure decrease effects when the scrubber 

was turned back on. Similar, albeit very slight, pressure effects were recorded at the 23rd and 

San Pedro drop structure. 

Curtain Installation: As shown in Figures 39 and 40, increases in air pressure were recorded at 

the maintenance holes just upstream and downstream of the 23rd and San Pedro drop structure 

during the curtain installation period while the plug was still in place at the drop structure. This 

was likely due to the fact that the source of ECIS pressure relief was sealed off. 




4.5 USC Drop Structure 

Three data loggers were installed at the locations shown in the map below. 

MH 53705181, 2,800 feet US of USC drop structure on ECIS 

MH 53607171, 5,300 feet DS of USC drop structure on ECIS 

MH 53705184, USC drop structure on ECIS 





Baseline 

Plugs 

Table 7 - USC Drop Structure Recorded Field Data 

USC Drop Structure Upstream Downstream 

Stop Logs 



Start End 

2/16/2010 3/16/2010 0.01 0.32 0.67 -0.44 0.17 0.78 -2.28 -1.82 0.04 

Installed Removed 

3/16/2010 4/2/2010 0.01 0.39 0.93 -0.84 0.35 0.90 -2.29 -1.79 -1.09 

Removed 


ECIS @ MH 

53705181 

ECIS MH @ 

53607171 


ECIS @ MH 

53705184 

Reinstalled 

5/4/2010 0.06 0.49 1.15 -1.01 0.18 0.93 -2.24 -1.78 -1.07 

The three figures below summarize the air pressure recorded at the MHs at the USC drop 

structure. 

Figure 42 - USC Drop Structure Upstream Average Pressure 




Figure 43 - USC Drop Structure Downstream Average Pressure 

Figure 44 - USC Drop Structure Average Pressure 




Plug Effect: The plugged air return ducts on the upstream Division, Humboldt, Mission and 

Jesse and 23 rd and San Pedro drop structures generally resulted in an increasing pressure trend at 

the USC drop structure and at the two maintenance holes on the ECIS upstream and downstream 

of the USC drop structure. Once the last plug was removed, the pressures at these three 

locations were immediately reduced to near baseline levels. This was likely due to the fact that 

the interceptor system could no longer relieve its pressure buildup through the NOS approach 

sewers into the upstream drop structures. 

Stop Log Removal: The pressure on the ECIS upstream and downstream of the USC drop 

structure showed a significant reduction trend during the stop log removal period. A slight 

pressure reduction was recorded at the drop structure itself during the stop log removal period. 

One possible explanation for this is that the NOS approach sewer at Humboldt could be a more 

significant contributor of air drag into the NEIS/ECIS system when NOS flows are being directed 

to the Humboldt drop structure, compared to the amount of air dragged by the NOS into the 

NEIS/ECIS system when flows are directed to the Mission and Jesse drop structure. Once the 

stop logs were removed at Humboldt, flow was able to pass through the NOS away from the drop 

structure unimpeded, thereby reducing the amount of air dragged into the NEIS at Humboldt and 

potentially reducing the overall system pressurization increase trend that had begun following 

introduction of the air line return plugs at the Division, Humboldt and Mission and Jesse drop 

structures. 

Scrubbers on/off: The scrubber at the Mission & Jesse drop structure clearly has a pressure 

reduction effect on the ECIS upstream and downstream of the USC drop structure. Several 

instances shown on the pressure charts show direct pressure increase effects when the scrubber 

was turned off and pressure decrease effects when the scrubber was turned back on. As with 

other systems changes, slight pressure reduction effects were recorded at the drop structure itself 

when the Mission & Jesse ATF was turned on. 




4.6 Systemwide Downstream 

Three data loggers were installed at the locations shown in the map below. 

MH 53503213, 2,400 feet US of Jefferson Siphon on ECIS 

MH 53506116, 5,300 feet US of NORS/ECIS junction on ECIS 

MH 53505021, 8,000 feet US of NORS/ECIS junction on NORS 




Systemwide Downstream 

Table 8 - Systemwide Downstream Recorded Field Data 

Downstream 

ECIS @ MH 

53503213 

Downstream 

ECIS @ MH 

53506116 

Downstream 

NORS @ MH 

53505021 



Baseline 

Start End 

2/16/2010 3/16/2010 -0.42 -0.08 0.33 -0.56 0.11 0.45 -0.04 0.09 0.48 

Plugs 

Installed Removed 

3/16/2010 4/2/2010 -0.44 -0.16 0.10 -0.37 0.08 0.48 0.00 0.08 0.22 

Stop Logs 

Removed Reinstalled 

4/2/2010 5/4/2010 -0.47 -0.25 0.16 -2.35 0.08 0.73 -0.40 0.06 0.55 

The three figures below summarize the air pressure recorded at the MHs downstream of the drop 

structures. 

Figure 45 - Systemwide Downstream Jefferson Siphon Upstream Average Pressure 




Figure 46 - Systemwide Downstream NORS/ECIS Junction (ECIS) Average Pressure 

Figure 47 - Systemwide Downstream NORS/ECIS Junction (NORS) Average Pressure 




Plug Effect: The plugged air return ducts on the four upstream drop structures at Division, 

Humboldt, Mission and Jesse and 23 rd and San Pedro generally resulted in a slight increasing 

pressure trend at the maintenance hole upstream of the ECIS Jefferson Siphon, on the ECIS 

between the siphon and the NORS/ECIS junction and on the NORS upstream of the NORS/ECIS 

junction. This was likely due to the fact that the interceptor system could not relieve air pressure 

buildup through the NOS approach sewers at the upstream drop structures. Once the last plug 

was removed, the pressures at the downstream system locations were immediately reduced to 

near baseline levels. 

Stop Log Removal: The pressures on the ECIS from just upstream of the Jefferson Siphon to the 

NORS/ECIS junction showed a general reduction trend during the stop log removal period. As 

with other locations along the studied interceptor system, one possible explanation for this is that 

the NOS approach sewer at Humboldt could be a more significant contributor of air drag into the 

NEIS/ECIS system when NOS flows are being directed to the Humboldt drop structure, 

compared to the amount of air dragged by the NOS into the NEIS/ECIS system when flows are 

directed to the Mission and Jesse drop structure.. Once the stop log was removed at Humboldt, 

flow was able to pass through the NOS away from the drop structure unimpeded; thereby 

reducing the amount of air dragged into the NEIS at Humboldt and therefore reduced the overall 

interceptor system pressure. As explained previously, more air appeared to be dragged into the 

system at Mission and Jesse than is the case when the stop logs are in place, as evidenced by the 

data recorded at ECIS MH 515-13-136 (see Figure 37). 

Scrubbers on/off: The scrubber at the Mission & Jesse drop structure has a slight pressure 

reduction effect on the two ECIS maintenance holes. Several instances shown on the chart show 

direct pressure increase effects when the scrubber was turned off and pressure decrease effects 

when the scrubber was turned back on. No effects due to NEIS/ECIS drop structure ATF activity 

are discernable on the NORS upstream of the NORS/ECIS junction. The NORS/ECIS junction 

ATF had been off between mid-February and mid-April due to maintenance issues. The scrubber 

was turned back on April 12th. An initial reduction in air pressure was recorded when the 

scrubber was turned back on. 




4.7 Findings 

The following conclusions are drawn from analyzing pressure data recorded during the drop 

structure field study from February 2010 to May 2010. The General Observations apply to all 

drop structures, and their immediate upstream and downstream vicinities, that were studied. 

Explanations for location-specific phenomena that are not believed to have occurred as a result of 

events described in the General Observations section are provided in each location-specific 

section. 

4.7.1 General Observations 

The plugged air lines at the drop structures generally resulted in a gradual buildup of air pressure 

in the NEIS/ECIS system that began when the plugs were inserted at the drop structures and 

continued until the stop logs were removed at the Humboldt and Mission and Jesse NOS 

diversions. This is likely primarily due to the fact that the plugs did not allow air pressure 

buildup in the tunnel system to be relieved through the NOS approach sewer segments. 

During the stop log removal period of the study, the air return plugs were still in place at the drop 

structures. During the stop log removal phase, pressures in the NEIS/ECIS system were 

generally, gradually reduced. This was likely due to the fact that NOS flow at the Humboldt drop 

structure was diverted away from the NEIS due to the stop log removal, thereby resulting in less 

air being dragged into the drop structure and into the NEIS. The exception to this appeared to be 

at ECIS MH 515-13-136, which is immediately downstream of the Mission and Jesse drop 

structure. Pressures generally increased at this location during the stop log removal period. This 

is likely due to more air being dragged into the ECIS at Mission and Jesse than is the case when 

the stop logs are in place. 

The ATF’s generally reduce pressures by 0.1 to 0.25 inches of water column along the entire 

length of the NEIS/ECIS tunnel. Discernable pressure reductions occur when the ATF’s are 

turned on. Discernable pressure increases occur when the ATF’s are turned off. This indicates 

that the ATF’s are removing some air from the NEIS/ECIS system. 

The following provides summary descriptions of the pressure observations at each drop structure 

at their immediate upstream and downstream vicinities 

4.7.2 Division Drop Structure & Vicinity 

 

 

Division Drop Structure: The most favorable pressure conditions (i.e., lowest air 

pressures) were achieved at this location, as well as the maintenance holes immediately 

upstream (on the ERIS) and downstream (on the NEIS), when the plugs were in place in 

all four drop structure air return lines. Also, removing the stop logs at Humboldt and 

Mission and Jesse NOS diversions did not cause significant air pressure increases at this 

location. This was likely due to the fact that the NOS flow at the Humboldt drop 

structure typically drags a significant amount of air into the NEIS. During the stop log 

removal period flows were being diverted away from the NEIS at Humboldt. This 

resulted in less air being into the NEIS than is normally the case when the Humboldt stop 

logs are in place. 

ERIS: Once the plug was in place at the Division Drop Structure, the air pressures on the 

ERIS just upstream of the drop structure became too negative for the ACR instruments to 

properly record. This was likely due to the fact that the ERIS approach to the Division 

drop structure had been denied positive backpressurization from the NEIS when the plug 

was inserted in the drop structure air return line. The drag induced by the flowing 




wastewater on the approach pipe headspace therefore drew a vacuum in the upstream 

manhole. This indicates that they were at -2.5 inches w.c. maximum. 

NEIS: The lowest air pressures were recorded on the NEIS just downstream of the drop 

structure when the plugs were in place in all four drop structure air return lines. This was 

likely due to the fact that this segment of the NEIS is the furthest away from several 

potential sources of positive air pressurization (i.e. other drop structures and siphons) 

along the NEIS/ECIS alignment. Also, diverting flows away from the Humboldt drop 

structure resulted in a decreasing air pressure trend at this location. As explained 

previously, this was likely due to the fact that a net reduction in air drag into the NEIS 

occurred when the NOS flows were diverted away from the Humboldt drop structure. 

4.7.3 Humboldt Drop Structure & Vicinity 

 

 

 

 

The most favorable pressure conditions (i.e., lowest air pressures) were achieved at this 

location when the plugs were in place in all four drop structure air return lines. Also, 

removing the stop logs at the Humboldt and Mission and Jesse NOS diversions did not 

cause significant air pressure increases at this location. 

NEIS: The pressures on the 2 NEIS maintenance holes immediately upstream and 

downstream of this location increased as a result of the presence of plugs in the four drop 

structure air return lines. 

NOS: The air pressure on the NOS at the Humboldt diversion was generally well below 

or right at atmospheric during the entire duration of the field study. The most favorable 

(i.e., lowest) pressure conditions were recorded while the stop logs were removed and the 

plugs were in place at the drop structure air return lines. 

Noticeable reductions in air pressure were recorded on the NOS, NEIS and at the 

Humboldt drop structure when the Humboldt ATF was in operation, although the 

pressures in the NEIS tunnel in the vicinity of Humboldt remained positive. 

4.7.4 Mission and Jesse Drop Structure & Vicinity 

 

 

 

Mission and Jesse Drop Structure: The lowest air pressure conditions were achieved at 

this location when the plugs were in place in all four drop structure air return lines and 

the NOS stop logs had been removed at both this drop structure and the Humboldt 

location. 

NEIS/ECIS: The pressures on the two maintenance holes immediately upstream and 


structure air return lines. The pressure at the ECIS maintenance hole immediately 

downstream of this location also increased as a result of stop log removals at Humboldt 

and Mission and Jesse. The pressure at the NEIS maintenance hole immediately 

upstream of this location generally decreased as a result of stop log removals at 

Humboldt and Mission and Jesse. 

NOS: The air pressure on the NOS at the Mission and Jesse diversion decreased 

significantly during the plug insertion and stop log removal period of this study. The 

exception to this was during an approximate 15 day period toward the end of this testing 

period during which the pressures exhibited an increasing trend. This was likely due to 

more air being dragged into system at this location than is normally the case with the stop 

logs in place; however it also indicates that this is a primary source of pressure relief for 

the tunnel system. 




Noticeable reductions in air pressure were recorded on the NEIS, ECIS and at the 

Mission and Jesse drop structure when the Mission and Jesse ATF was in operation. 

4.7.5 23 rd and San Pedro Drop Structure & Vicinity 

 

 

 

 

 

23rd and San Pedro Drop Structure: The lowest air pressure conditions were achieved at 

this location when the plugs were in place in all four drop structure air return lines and 

the NOS stop logs had been removed at both the Mission and Jesse and the Humboldt 

locations. The pressure in the drop structure was approximately atmospheric during the 

baseline period. The pressures went below atmospheric shortly after the plug was 

inserted in the air return line at this drop structure. 

ECIS: The pressures on the 2 ECIS maintenance holes immediately upstream and 


structure air return lines. The pressures at these maintenance holes generally decreased 

during the stop log removal test. 

NOS: The air pressure on the NOS at the 23rd and San Pedro diversion decreased 

slightly during the plug insertion and stop log removal period of this study. The 

pressures were generally near atmospheric for the majority of the entire drop structure 

testing period. 

The curtain installation at 23 rd and San Pedro did not result in a significant pressure 

change at the 23 rd and San Pedro drop structure. A slight, albeit measurable, pressure 

reduction trend was recorded on the maintenance holes upstream and downstream of the 

drop structure on the ECIS during the curtain installation period. This indicates that the 

curtain might have isolated a small source of air into the ECIS through the drop structure. 

Noticeable reductions in air pressure were recorded on the ECIS and at the 23rd and San 

Pedro drop structure when the Mission and Jesse ATF was in operation. 

4.7.6 USC Drop Structure & Vicinity 

 

 

 

USC Drop Structure: The air pressures at the drop structure did not appear to be 

significantly affected by any system-wide manipulations to the ATFs, drop structure air 

return lines or interceptor stop logs. This is likely due to the fact that there is no air 

return line on this drop structure which, if there was one in place, would provide a source 

of back pressurization from the ECIS tunnel. 

ECIS: Compared to upstream locations along the ECIS and NEIS, there were very little 

changes to overall pressures immediately upstream or downstream of the USC drop 

structure on the ECIS as a result of the planned system-wide manipulations. 

Noticeable reductions in air pressure were recorded on the ECIS immediately upstream 

and downstream of the USC drop structure when the Mission and Jesse ATF was in 

operation. 

4.7.7 Systemwide Downstream Interceptors 

 

 

The lowest air pressure conditions were achieved during the stop log removal period. 

Upstream of Jefferson Siphon (ECIS): The pressures at this location were at or below 

atmospheric levels for the majority of the study period. The air pressures at this location 

slightly increased during the period that the air return line plugs were in place at the four 

drop structures. During the time period that the plugs were in place and the stop logs 




were removed from Humboldt and Mission and Jesse the air pressure generally decreased 

at this location and remained below atmospheric. There also appears to have been a 

direct correlation between the interim scrubbers being turned off and on and increases 

and decreases, respectively, in pressure levels at this location. 

NORS/ECIS Junction Upstream (ECIS): The air pressures generally increased when the 

plugs and stop logs were in place. Once the stop logs were removed the pressures 

exhibited a general downward trend. Following plug removal and stop log re-insertion 

the pressures went to near atmospheric. 

NORS/ECIS Junction Upstream (NORS): The pressures in the NORS maintenance hole 

immediately upstream of the NORS/ECIS junction increased slightly during the time in 

which the plugs were in place in the four drop structure air return lines. The pressures at 

these maintenance holes generally decreased and fluctuated more significantly in the 

negative direction during the stop log removal test. During this time period, on April 

12 th , the NORS/ECIS scrubber was brought back online, which could have resulted in the 

fluctuating pressure conditions. Following plug removal and stop log re-insertion the 

pressures went below atmospheric with some slight spikes above atmospheric levels; 

however these pressure conditions were generally lower than those recorded during the 

baseline condition when the scrubber was out of service. 

Reductions in air pressure were recorded at each of these three locations when the 

Mission and Jesse ATF were in operation. The most notable example was at the ECIS 

location upstream of the NORS/ECIS junction. 





The following provides a summary of the conclusions drawn from the Findings presented in this 

report: 

 

 

 

 

 

The plugs inserted in the air return lines in the drop structures generally resulted in 

increasing pressures along the entire NEIS/ECIS tunnel alignment. The plugs also 

generally resulted in a reduction in pressure along the NOS approach sewer into the 

Mission and Jesse Drop Structure. The NOS diversions into the Humboldt and 23 rd and 

San Pedro drop structures didn’t display pressure reductions, but remained near 

atmospheric levels during the plug insertion test period. 

The stop logs removal generally resulted in decreasing pressures on the NOS along that 

segment that discharges into the Mission and Jesse drop structure. One anomaly was 

recorded during the stop log removal test along this segment. The pressure increased 

over an approximate four-day period about half-way through the stop log test. This 

increase phenomenon was not observed at other locations. As with the air return line 

plug insertions at drop structures, those locations where the pressures didn’t decrease 

remained at atmospheric levels during this part of the test period, such as the NOS 

approach segments at Humboldt and 23 rd and San Pedro. When the stop logs are in place 

at Humboldt and Mission and Jesse, NEIS/ECIS tunnel pressure primarily appears to be 

relieved through the Mission and Jesse NOS approach sewer. 

The ATF’s generally reduce pressures by 0.1 to 0.25 inches of water column along the 

entire length of the NEIS/ECIS tunnel. Discernable pressure reductions occur when the 

ATF’s are turned on. Discernable pressure increases occur when the ATF’s are turned 

off. 

The curtain installation at the NOS diversion structure just upstream of the 23 rd and San 

Pedro drop structure resulted in increases in air pressure on the ECIS tunnel at the 

maintenance holes just upstream and downstream of the drop structure while the plug 

was still in place at the drop structure. This was likely due to the fact that the source of 

ECIS air pressure relief was sealed off in the vicinity due to the presence of both the plug 

and the curtain. 

In general the drop structures appeared to have the following effects on the air pressures 

in the NEIS/ECIS and the NOS approach sewers during plug insertion and flow 

management operations: 

o 

o 

o 

o 

Air is drawn into the NEIS/ECIS tunnel system from the NOS approach sewers 

through the drop structures, causing positive pressurization of the NEIS/ECIS 

tunnel system 

The air return lines in the drop structures tend to have a pressure reduction effect 

on the NEIS/ECIS tunnel system; likely due to the drop structure air return lines 

acting as conduits for backpressure relief from the NEIS/ECIS tunnel system 

The Humboldt drop structure appears to be a more significant contributor of air 

into the NEIS/ECIS system than does the Mission and Jesse drop structure. Air 

pressures increased only slightly on the NOS approach to Humboldt, yet even 

then remained near or below atmospheric, during the entire duration of the drop 

structure testing period 

The Mission and Jesse drop structure appears to serve as the primary relief 

pressure relief point for the NEIS/ECIS system. When the stop logs are in place 




o 

at this location, the vertical drop shaft and the air return line appear to serve as a 

conduit for air back-pressurization on the NOS upstream of the drop structure. 

Plugging the air return line caused a gradual reduction in pressure along that 

reach of the NOS; however, once the stop logs were removed and wastewater 

flow was allowed to enter unimpeded into the Mission and Jesse drop, the 

pressures gradually increased along the NOS approach into Mission and Jesse. 

This was likely due to an increased air backpressure effect in the ECIS due to the 

upstream flow diversions. 

The pressures in the Humboldt and 23 rd and San Pedro NOS approach pipes were 

very near atmospheric before and during the air line plug test. 





Team 





TOTAL NON-METHANE HYDROCARBON 

MONITORING RESULTS TECHNICAL 

MEMORANDUM 

FINAL 



Total Non-Methane Hydrocarbon Monitoring Results Technical Memorandum – FINAL 




1.0 Introduction and Purpose ................................................................................................... 9 

2.0 Air Permits ........................................................................................................................ 12 

3.0 Sampling Setup ................................................................................................................ 17 

3.1 Sample Locations and Schedule .................................................................................. 17 

3.2 Sampling Equipment, Measurement Parameters and Analytical Methods ................... 18 

3.2.1 Sampling Equipment and Measurement Parameters ............................................ 18 

3.2.2 Analytical Methods ................................................................................................ 20 

3.2.3 ATF Inlet Air Velocity Measurements and Air Flow Rates ..................................... 23 

4.0 Collected Data and Sampling Results .............................................................................. 25 

4.1 Richmond ..................................................................................................................... 25 

4.2 NORS-ECIS .................................................................................................................. 32 

4.3 Humboldt ...................................................................................................................... 38 

4.4 Mission and Jesse ........................................................................................................ 49 

4.5 23 rd and San Pedro ....................................................................................................... 55 

4.6 Summary ...................................................................................................................... 60 

5.0 Comparison of Current and Historical Data ..................................................................... 65 


Table 1 - ATF TNMHC Emission Limits ...................................................................................... 12 

Table 2 - Summary of the Permits .............................................................................................. 14 

Table 3 -Sampled Locations and Dates ...................................................................................... 17 

Table 4 - TO-15 Compound List ................................................................................................. 21 

Table 5 - TO-12 Modified Compound List ................................................................................... 22 

Table 6 - ASTM 5504 Measured Compounds ............................................................................ 23 

Table 7 - Richmond - TNMHC, VOC and H2S Sample/Analytical Results ................................. 27 

Table 8 - Richmond TO-15 Sample/Analytical Results ............................................................... 28 

Table 9 - Richmond TO-12 Modified Results .............................................................................. 29 

Table 10 - NORS-ECIS TNMHC, VOC and H2S Sample/Analytical Results.............................. 33 

Table 11 - NORS-ECIS TO-15 Sample/Analytical Results ......................................................... 34 

Table 12 - NORS-ECIS TO-12 Modified Results ........................................................................ 35 

Table 13 - Humboldt TNMHC, VOC and H 2 S Sample/Analytical Results ................................... 44 

Table 14 - Humboldt TO-15 Sample/Analytical Results .............................................................. 45 

Table 15 - Humboldt Inlet ATF Reduced Sulfur Compounds ...................................................... 45 

Table 16 - Humboldt TO-12 Modified Results ............................................................................. 46 

Table 17 - Mission & Jesse TNMHC, VOC and H2S Sample/Analytical Results ........................ 50 

Table 18 - Mission & Jesse TO-15 Sample/Analytical Results ................................................... 51 

Table 19 - Mission & Jesse ATF Inlet Reduced Sulfur Compounds ........................................... 51 

Table 20 - Mission & Jesse TO-12 Modified Results .................................................................. 52 

Table 21 - 23rd & San Pedro TNMHC, VOC and H 2 S Sample/Analytical Results ...................... 56 

Table 22 - 23rd & San Pedro TO-15 Sample/Analytical Results ................................................ 57 

Table 23 - 23rd & San Pedro TO-12 Modified Results ............................................................... 58 

Table 24 - Total NMHC, VOC and H 2 S Sample/Analytical Results Summary ............................ 61 

Table 25 - Comparison of 2010 and 2009 Total VOC Data (ppb v ) ............................................. 66 

Table 26 - Comparison of 2010 and 2009 Reduced Sulfur Data (ppm v ) .................................... 66 





Figure 1 - Interim ATF and Monitoring Location Map ................................................................. 18 

Figure 2 - Sampling Train Schematic .......................................................................................... 19 

Figure 3 – Air Sampling Train ..................................................................................................... 19 

Figure 4 - Richmond ATF (1) ...................................................................................................... 30 



Figure 7 - NORS ECIS ATF (1) ................................................................................................... 36 



Figure 10 - Humboldt ATF (1) ..................................................................................................... 47 




Figure 14 - Mission and Jesse ATF (1) ....................................................................................... 53 



Figure 17 - 23rd and San Pedro (1) ............................................................................................ 59 

Figure 18 - 23rd and San Pedro (2) ............................................................................................ 59 

Figure 19 - TNMHC VOC Results Summary .............................................................................. 62 

Figure 20 - Hydrogen Sulfide Sampling Result Summary .......................................................... 63 

Figure 21 - Hydrogen Sulfide Emission Control Overview .......................................................... 64 





In recent history, the City of Los Angeles (City) has encountered increasing public concern to 

sewer odors, and has been working to improve odor control in several areas identified by local 

community groups. Formal, permanent air treatment facilities (ATFs) for seven areas with known 

or predicted odor “hot spots” were including in a 2004 Collection System Settlement Agreement 

(CSSA) that the City entered into with the United States Environmental Protection Agency 

(USEPA), Santa Monica Baykeeper, City of El Segundo, and others. The CSSA included several 

requirements including the installation and operation of interim and permanent air treatment 

facilities. Permanent ATFs are in the final stages of construction at two of the seven targeted 

areas. The location of the five remaining proposed ATFs need to be reevaluated based on the 

effect of wastewater flow diversions on interceptor headspace air pressure; reported performance 

issues and continued odor complaints at several of the locations. Accordingly, the City has 

embarked on a study to determine the number and location of the new proposed ATFs, assess the 

performance of the interim odor control facilities, and decide on the ATF technologies. This 

Technical Memorandum (TM) summarizes the monitoring results for parameters of interest 

conducted at the five proposed ATF locations where interim odor control facilities are currently 

operating at four of the locations and no control is provided a the fifth location. 

In some cases, existing interceptor odor control facilities and ATFs have air permit limits for total 

non-methane hydrocarbons (TNMHC) in their exhaust. A single distinct exhaust limit value is 

listed in each site’s air permit. The various sites have different individual permit values. Those 

permit values vary from 18 to 36 parts per million by volume in air (ppm v ). All of these existing 

ATFs are required to meet a hydrogen sulfide stack exhaust limit of 1 ppm v , which with the 

exception of the Decotah Street facility has an exhaust H 2 S permit limit of 0.6 ppm v . Given the 

existing air permit limits for TNMHC and hydrogen sulfide, the City wants to have a greater 

understanding of the composition of the sewer headspace air at the five interim odor control 

facility locations so that permanent ATF systems can be designed to meet future air permit 

requirements. Parameters of interest include: TNMHC, speciated volatile organic compounds 

(VOCs), hydrogen sulfide, and total reduced sulfur compounds. The interim air permits for the 

existing operating odor control facilities and ATFs require analysis of VOCs by South Coast Air 

Quality Management District (SCAQMD) test methods, including the use of SCAQMD Method 

25.3 to analyze for TNMHC. 

A sampling/analysis program was conducted in April 2010 at the following five interim odor 

control facility locations where raw untreated sewer air and the exhaust from the odor control 

facilities were assessed. 

 

 

 

 

 






Note that the interim ATF at 23 rd and San Pedro was physically removed well before our 

sampling program started. As a result, the raw sewer air space was sampled and analyzed 

for all the same parameters of interest. 

Air samples were analyzed for the following parameters: 

 

Total non-methane hydrocarbons (TNMHC) 





Hydrogen Sulfide (H 2 S) by Jerome Meter 

Continuous H 2 S by OdaLog unit 


The TNMHC, speciated VOCs and total reduced sulfur compound analyses were conducted by an 

independent laboratory. The speciated VOC and speciated organic reduced sulfur compound 

analyses were conducted in order to provide an understanding of the compounds that makeup the 

TNMHC air sample results. Hydrogen sulfide, in addition to the grab sample evaluation with a 

Jerome Meter, was also monitored in the sewer at each location using a continuous monitor with 

data-logging capabilities (OdaLog). 

While reviewing the analytical results in subsequent paragraphs, please keep in mind that the 

SCAQMD Method 25.3 results will be used to determine compliance with the permit limits. The 

speciated results were developed to gain a better understanding of the general compounds that 

make up the TNMHC value resulting from Method 25.3 and are not used to directly demonstrate 

compliance with any permit limits. Due to the variable nature of sewer gases and the cocktail of 

compounds present at all different concentrations, a direct equal correlation between Method 25.3 

and speciated analyses results is not anticipated nor expected. 

The City has shown interest in using a hand-held VOC measuring device as an alternative to 

running Method 25.3 to determine TNMHC. A photoionization detector (PID) was used to 

analyze samples and the results were compared to TNMHC results. 

Following is a summary of findings from the sampling/analytical effort: 

At all locations, the odor control facility inlet (raw sewer air) and exhaust Method 25.3 

results were below the air permit TNMHC emission limits of 18 to 36 ppm v , except at 

Humboldt where the inlet and exhaust concentrations were 23.0 and 18.4 ppmC, 

respectively. This is graphically displayed in Figure ES-1. It is believed that the higher 

TNMHC and elevated hydrogen sulfide activated carbon exhaust concentrations are due 

to spent carbon at the Humboldt odor control facility. It is expected that if the carbon 

was still effective the exhaust TNMHC and hydrogen sulfide results would be notably 

reduced. The figure below, Total Non-Methane Hydrocarbon VOS Sampling Result 

Summary illustrates inlet and exhaust TNMHC concentrations relative to the permit limit 

range of 18 to 36 ppm v . 




Figure ES-1 

 

 

 


approximately 32% to approximately 100% except for locations where apparent sampling 

interference occurred at the Humboldt inlet/exhaust, and at Mission Jesse. The speciated 

compounds were determined by EPA Method TO-15, EPA Method TO-12 Modified, and 

by a reduced sulfur scan (ASTM 5504) and were expressed as parts per million by 

volume by the laboratory. The Method 25.3 results are expressed as ppm as Carbon 

(ppmC). In order to compare the speciated results to the Method 25.3 results, the 

speciated results were converted to ppmC by multiplying by the volumetric concentration 

by the number of carbon atoms in each compound, per EPA guidance. After converting 

all the speciated results to ppmC, these compounds were totaled and were compared to 

the Method 25.3 results, by calculating a quotient, (ppmC-Speciated)/(ppmC- Method 

25.3). 

There was limited correlation between PID readings and TNMHC results produced by 

Method 25.3 results. Given that a PID’s reading is dependent on the compounds present, 

the City should consider using an alternative hand-held field measurement monitor if a 

substitute for Method 25.3 is desired. 

The short term grab odor control facility inlet or raw sewer air hydrogen sulfide 

concentrations ranged from approximately 2 ppm v (at Richmond) to >50 ppm v (at 

Mission & Jesse and 23 rd & San Pedro). Figure ES-2, Hydrogen Sulfide Sampling 

Results Summary, illustrates the odor control facility inlet and exhaust hydrogen sulfide 

concentrations and the average OdaLog readings. 




Figure ES-2 

 


removals of > 99%. The carbon at Humboldt appeared to be exhausted, with a unit 

exhaust hydrogen sulfide concentration of 11.7 ppm v . 

An overview of hydrogen sulfide control technologies as a function of inlet concentration in 

general, and the values measured in this study are presented in Figure ES-3. Carbon is generally 


concentrations up to 50 ppm v and greater. At higher concentrations however, the biofilter media 

degradation accelerates. Bio-trickling filters are cost-effective at inlet concentrations of 20 ppm v 

and higher, which indicates some overlap for the two biological systems. The Figure below, 

Hydrogen Sulfide Emission Control Overview, can be used as a basis to evaluate control 

technologies in the next phase of this project. 




Figure ES-3 




1.0 INTRODUCTION AND PURPOSE 

Management of a large wastewater collection system presents many challenges and the control of 

odors is an increasingly demanding challenge for most owners. The City of Los Angeles (City) 

has an extensive collection system that is over 9700 km (6,000 miles) in length with several areas 

that present unique odor control challenges. In addition, the City has a long history of 

implementing proactive and innovative steps to control odors from its interceptor collection 

system including a long and successful history of using chemical addition to proactively control 

odors. Unfortunately, chemical addition alone has been unable to eliminate the release of odors 

and total non-methane hydrocarbon (TNMHC) from the interceptors. In recent history, the City 

has encountered increasing public sensitivity to sewer odors, and they have been working to 


facilities (ATFs) for seven areas with known or predicted odor “hot spots” were including in a 

2004 Collection System Settlement Agreement (CSSA) that the City entered into with the United 

States Environmental Protection Agency (USEPA), Santa Monica Baykeeper, City of El 

Segundo, and others. The CSSA includes several requirements related to operations, 

maintenance, capital improvement, and odor control. A key component included the 

implementation and operation of interim and permanent air treatment facilities, in addition to the 

existing odor control systems and methods currently operating. 

Although two out of seven of the proposed permanent ATFs were located in areas that were 

clearly identified as current and proposed future interceptor hot spots, five of the proposed ATFs 

locations needed to be reevaluated based on planned future interceptors expansion and 

wastewater flow diversions. Accordingly, the City has embarked on a study to determine the 

number of and location of the new proposed ATFs, assess the performance of the interim odor 

control facilities, and decide on the ATF technologies. This Technical Memorandum (TM) will 

examine the parameters of concern that the ATFs will have to control based on monitoring the 

existing five interim odor control facility locations. 

The City has an extensive network of several major interceptors. Those interceptors consist of 

shallow and deep pipelines, including drop structures that convey the wastewater and the airspace 

of the various interceptors. The interceptors also include inverted siphons that were constructed 

to get around major structures such as interstate highways, rivers and railroad/utility structures. 

All of these features not only complicate the location of the ATFs, but they also contribute to the 

production of odors and the stripping of odors and other volatile organic compounds. 

Odors released from the interceptor collection system have been the trigger for odor complaints 

and have served to get the attention of the community and the local air regulators, the South Coast 

Air Quality Management District (SCAQMD). Existing interceptor odor control facilities and 

ATFs are required to meet hydrogen sulfide, and in some cases TNMHC limits in their exhaust. 

Although the City is ultimately concerned about odors, they must also address TNMHC 

emissions whose limits are based on human health affects and/or the total accumulation of 

volatile organic compounds (VOCs) in the atmosphere. The City and the surrounding air basin 

are in a non-attainment area for Federal (USEPA) ozone ambient air quality standards and must 

reduce the levels of ambient ozone by restricting ozone precursors like VOCs. Current odor 

control facility and ATF permits have a single district exhaust TNMHC and H 2 S limit value in 

each site’s air permit. The district exhaust limit is different from site to site. The permit values 

range from 18 to 36 ppm v for TNMHC and from 0.6 to 1.0 ppm for H 2 S. The five locations 

identified in the CSSA where monitoring will be conducted are: 

 

 









Interim odor control systems consisting of fans and carbon adsorption units were initially 

installed at all five of these locations. The interim odor control system installed at 23 rd & San 

Pedro has since been removed in response to neighborhood odor complaints. The carbon media 

was being consumed quite rapidly by the high concentrations of hydrogen sulfide (H 2 S) and other 

odorous and non-odorous compounds. It was determined that it was less of an off-site odor 

problem when the interim carbon system was shut completely off. After the system was shut off 

and the odor complaints dropped noticeably, the City decided to remove the system from this site. 

Nevertheless, this remains one of the original recommended ATF locations that will be monitored 

during this task of the project. 

The City provided several permits that were associated with odor control systems throughout 

their interceptor network. Among the permits provided by the City were those for the four 

interim odor control systems located at the proposed ATF sites. Each of the permits was issued 

by the SCAQMD, the regional air permitting authority, to construct/operate the interim odor 

control facilities. The permits each have a maximum allowable hydrogen sulfide emission 

concentration of 1 part per million by volume (ppm v ) and a single district TNMHC limit that 

varies from 18 to 36 ppm v . 

Given the permit limits for hydrogen sulfide and TNMHC for the existing odor control facilities 

and ATFs, and the anticipated permit limits for future proposed ATFs, the City wants to have a 

greater understanding of the composition of the sewer headspace air at the five interim odor 

control facility locations so that permanent ATF systems, if placed at the same locations, can be 

designed to meet any future air permit requirements. Parameters of interest include: hydrogen 

sulfide, total reduced sulfur compounds, and volatile organic compounds (VOCs). The interim 

permits require analysis of VOCs by SCAQMD test methods, including SCAQMD Method 25.3. 

Method 25.3 reports the total VOC content of the air as carbon, expressed as ppmC. 

A sampling/analysis program was conducted in April 2010 at the five interim odor control facility 

locations noted above where raw untreated sewer air and the exhaust from the activated carbon 

vessels were assessed. 

The purpose and objectives of this TM include: 

 

 

 

 

 

Monitoring the five locations previously identified in the CSSA where initially interim 

odor control facilities have been installed. 

Understanding what the untreated air characteristics are at the five locations. 

Monitoring and collecting samples at the inlet and exhaust from the five existing interim 

odor control facilities. 

Analyzing the samples collected for the following parameters: 

o Hydrogen Sulfide (H 2 S) 

o Total non-methane hydrocarbons (TNMHC) 

o Speciated volatile organic compounds (VOCs) 

o Speciated organic reduced sulfur compounds 

Evaluating whether the result from the general sum of the speciated compounds is 

consistent with the results from the total non-methane hydrocarbon testing (TNMHC). 




Determining whether there are compounds identified by the speciated analysis that are 

not technically considered VOCs and should therefore is subtracted from the TNMHC 

values. 

Comparing the measured values from this task against similar monitoring conducted by 

the City at these and/or other odor control facilities and ATF locations. 

Comparing the measured values from this task for the interim odor control facilities 

controlled exhaust against the air permit limits at the same site, and in general against air 

permit limits for other odor control facilities and ATFs. 

Assessing the removal efficiency of the existing interim odor control facilities relative to 

H 2 S and TNMHCs. 




2.0 AIR PERMITS 

Air permits for interim air pollution control systems were issued by SCAQMD for the following 

locations: 

 

 

 

Richmond 

NORS-ECIS 

Humboldt 

Mission & Jesse 

Permits to construct were also issued for future, permanent odor control systems. The following 

summarizes permit limits for the interim and permanent odor control systems: 

 

 

The maximum hydrogen sulfide emission concentration is 1 part per million by volume 

(ppm v ) for the interim and permanent systems 

The maximum TNMHC emission limits are summarized in Table 1 for the indicated 

facilities. No TNMHC limits were set for other facilities. No justification or reason was 

provided by SCAQMD for the difference in TNMHC permit limits for the different sites. 

Reviewing the actual permits it would appear that different permit writers may have been 

involved pursuant to the inconsistencies in the language of the permit. However, it is 

more than likely that a consistent TNMHC load has been designated for each site and the 

limits reflect the allowable concentrations given the anticipated air flows. 

Table 1 - ATF TNMHC Emission Limits 

ATF Location 

TNMHC Limit 

(ppm v ) 

North Central Outfall Sewer (NCOS) 30 

NORS-ECIS 30 

La Cienega – San Fernando Valley Sewer 36 

ECIS Inverted Siphon 18 

 

 

A source test was required within 180 days of initial operation to test for the following 

parameters using SCAQMD procedures: 

o Total sulfur as hydrogen sulfide 

o TNMHC 

o Speciated organics 

o Air flow rate, moisture, temperature 

There was consistency in the requirement to speciate organic species. However, there was 

some inconsistency in the stated methods recommended to be used to conduct these 

analyses. For instance, in some cases the requirement to speciate the organic compounds 

is stated as “typical sewer gas organic species using approved methods.” In other 

instances speciated is stated in the terms, “speciated using Methods 8015/8021 and 8260, 

or other approved methods.” Methods 8015, 8021 and 8260 were designed for liquid and 

solid waste matrices. While these methods can be used to analyze for VOC in air 

samples, EPA Method TO-15 was written for ambient air samples with lower 

concentration VOCs and would be the more appropriate method to use. 




The requirement to measure total organics also varied. In some instances the 

measurement of TNMHC was stated, in others, total organic compounds was stated. 

A permit had been issued for an odor control system for the 23 rd and San Pedro location, but this 

unit was removed in 2008 and relocated to the North Outfall Treatment Facility (NOTF) located 

at 10127 Jefferson Blvd, Culver City, CA. 

Table 2 is a summary of the permits. It is divided into two sections: the upper section (2A) lists 

the permits for the interim odor control facilities; and the lower section (2B) lists permits for the 

permanent ATFs. 




Table 2 - Summary of the Permits 

Table 2A 

Air Permit Summary For City of Los Angeles, Dept. of Public Works, Bureau of Sanitation, Interceptor Collection System, Interim Air Treatment Facilities 

Carbon 

Air Treatment Facility Location Air Flow Rate Treatment 

Operating 

H2S Limit 

Operating 

TNMHC 

Limit 

Carbon 

Change- 

Out H2S 

Threshold 

Change- 

Out 

TNMHC 

Threshold 

Source Test in Accordance w/ 

SCAQMD Test Procedures 

Within 180 days of Iniitial 

Operation 

(cfm) (ppmv) (ppmv) (ppmv) (ppmv) 

NEIS - Richmond 886 N Mission Rd, Los Angeles, CA 90023 10,000 

NORS ECIS 10100 Jefferson Blvd., AFT #804, Culver City, CA 90232 10,000 

NEIS - Humboldt 303 1/2 N. San Fernando, Los Angeles, CA 90031 10,000 

ECIS - Mission & Jesse 

633 S. Mission Road, Los Angeles, CA 90023 

10,000 

Weekly 

Monitoring 

During 

Operation 

Permit Type 

Mist Eliminator, One carbon 

adsorber, one fan 1.0 na 0.8 na In/Out H2S Construct/Operate 6/28/2005 

Total Sulfur as H2S 

TNMHC 

"Typical Sewer Gas Organic 

Species" (speciated organics) 

Air flow rate, moisture, 

temperature 

Demister, One carbon 



"Total Organic Compounds" 

Speciated using: Methods 

8015/8021 and 8260, or other 

SCAQMD method 


temperature 




TNMHC 




temperature 






8015/8021 and 8260, or other 

SCAQMD method 


temperature 

Permit 

Date 




Air Permit Summary For City of Los Angeles, Dept. of Public Works, Bureau of Sanitation, Interceptor Collection System, Additional Air Treatment Facilities 

Carbon 

Operating 

TNMHC 

Limit 

Carbon 

Change- 

Out H2S 

Threshold 

Change- 

Out 

TNMHC 

Threshold 



w/iin 180 days of Iniitial 

Operation 

Name Location Air Flow Rate Treatment 

Operating 

H2S Limit 


NCOS (Interim) 

NCOS (Future) 

5900 1/2 W Jefferson Blvd., Traffic Island, Los Angeles, CA 

90016 10,000 

Oil/Grease Trap, three (3) 

biotrickling filters (parallel), 

mist eliminator, two (2) 

6000 Jefferson Blvd, Los Angeles, CA 90016 (North Central 

Outfall Sewer (NCOS Inverted Siphon and air line along 

Jefferson Blvd. Near Holdrege Ave.) 

carbon trains (two series 

vessels in parallel), two fans 

12,000 (one standby) 1.0 30 0.9 27 

NORS-ECIS (Future) 9940 Jefferson Blvd., Culver City, CA 90232 12,000 Demister, Oil/Grease Trap, 

three (3) biotrickling filters 

(parallel), two (2) carbon 

trains (two series vessels in 

parallel), two fans (one 

standby) 

Weekly 

Monitoring 

During 

Operation 

Permit Type 


adsorber, one fan 1.0 na 0.8 na In/Out Construct/Operate 7/2/2004 




8015/8021 and 8260, or other 

SCAQMD method 


temperature 

Permit 

Date 

Bio-Trickiling Filter In/Out, 

Carbon Train outlet H2S, TNMHC Construct 7/17/2007 



Speciated organic compounds 

using SCAQMD method 


temperature 

1.0 30 0.9 27 In/Out H2S, TNMHC (or 

otherwise appv'd 

frequency) 



Construct 1/11/2007 

"Speciated organic compounds" 


temperature 

North Outfall Treatment Facility 

(NOTF) 10127 Jefferson Blvd., Culver City, CA 90232 


10,000 adsorber, one fan 1.0 na 0.8 na TO-15 H2S Construct/Operate 8/13/2008 

La Cienega-San Fernando Valley 

Relief Sewer 

Table 2B 

1403 N. Gardner St., Los Angeles, CA 90046 10,000 Mist eliminator, one carbon 

adsorber 

1.0 36 0.9 34 In/Out H2S, TNMHC Construct 7/12/2007 


Total organics 

Speciated organics 


temperature 




Table 2B (con't) 

Air Permit Summary For City of Los Angeles, Dept. of Public Works, Bureau of Sanitation, Interceptor Collection System, Additional Air Treatment Facilities 

Operating 

TNMHC 

Limit 

Carbon 

Change- 

Out H2S 

Threshold 

Carbon 

Change- 

Out TNMHC 

Threshold 

Name Location Air Flow Rate Treatment 

Operating 

H2S Limit 


3410 S La Cienega, Los Angeles, CA 90016 20,000 Oil/Grease Trap 1.0 18 0.9 16 

ESIC Inverted Siphon - La Cienega / 

San Fernando Vally Interceptor 

Sewer 

Five (5) biotrickling filters 

(parallel), mist eliminator 



w/iin 180 days of Iniitial 

Operation 

In/Out 


Weekly 

Monitoring 

During 

Operation 

Permit Type 

Permit 

Date 

H2S, TNMHC Construct 1/11/2007 

ESIC Inverted Siphon - La Cienega / 

San Fernando Vally Interceptor 

Sewer 

3410 S La Cienega, Los Angeles, CA 90016 

10,000 

Ballona Creek Pumping Plant 5550 Inglewod Blvd., PP 654, Los Angeles, CA 90030 5,000 

ng 4250 N. Radford, Studio City, CA 91604 5,000 

Dacotah Pumping Station 1164 Dacotah St., Los Angeles, CA 90023 3,000 

Various Locations in SCAQMD 5,000 

Various Locations in SCAQMD 5,000 

Notes: 

na = not applicable 

ng = not given in permit 

Four (4) parallel carbon trains 

(two vessels in series per 

train), two fans (one standby) 

Total organics compounds 



temperature, bio-trickling filter 

solution flow rate and pH, diff. 

pressure 


adsorber, one fan 1 na 0.8 na In/Out H2S Construct/Operate 7/2/2004 


Total organics compounds 


using EPA Methods 8015/8021 

and 8260, or other approved 

SCAQMD methods 


temperature 

Mist Eliminator, one carbon 



TNMHC 




temperature 

Prefilters, carbon adsorber, 

one fan 1.0 na 0.8 na na H2S Construct/Operate 2/23/2005 

Mist eliminator,carbon 



TNMHC 




temperature 

Pre-filters, carbon adsorber, 

one fan 1.0 na 0.8 na na na Construct/Operate 2/23/2005 

Pre-filters, carbon adsorber, 

one fan 0.6 na 0.5 na na H2S Operate 11/5/2004 




3.0 SAMPLING SETUP 

The sampling was conducted per the Total Non-Methane Hydrocarbon VOC and H 2 S Draft 

Sampling Plan dated April 2010, and as described in this document. 

3.1 Sample Locations and Schedule 

The locations sampled and sample dates are listed in Table 3. Also included in the table is the 

location as called-out on the sewer air treatment systems map prepared by the City of Los 

Angeles, Sanitation Department of Public Works. This map appears in Appendix A. Figure 1 

shows the sample locations 

As is the case in most collection systems, it was expected that hydrogen sulfide emissions in the 

City of Los Angeles collection system follows a diurnal cycle, and it was assumed that the 

mechanisms that drives the volatilization of hydrogen sulfide also applies to the release of VOCs. 

In order to determine the time of day to conduct the sampling, a hydrogen sulfide measurement/ 

device with data-logging ability was suspended in the headspace of the inlet manhole at each 

ATF location before the sampling began. OdaLog units were used and are described below in the 

hydrogen sulfide measurement section below. The sample times determined from the recorded 

hydrogen sulfide concentrations are summarized in Table 3. 

Location - Description 

NEIS – Richmond. Interim carbon 

scrubber 

NORS ECIS – Interim carbon 

scrubber 

NEIS – Humboldt. Interim carbon 

scrubber 

ECIS – Mission & Jesse. Interim 

carbon scrubber 

ECIS – 23 rd & San Pedro. Interim 

carbon scrubber removed. 

Table 3 -Sampled Locations and Dates 

Interceptor Air Treatment 

System Map Location No. 

Sample Date- 

Time 

2 4/22/10 - 

12:30 

7 4/22/10 – 

18:45 

1 4/23/10 – 

7:30 

3 4/23/10 – 

12:00 

4 4/23/10 – 

14:30 




Figure 1 - Interim ATF and Monitoring Location Map 

3.2 Sampling Equipment, Measurement Parameters and 

Analytical Methods 

3.2.1 Sampling Equipment and Measurement Parameters 

Air samples were collected for analysis from a continuous air stream that was pulled from the 

ATF inlet, ATF exhaust, or ATF inlet manhole using an air pump. A generator was used to run 

the pump. The ATF inlet manhole was sampled at Mission & Jesse because the carbon of its 

ATF was being replaced during the sampling; and as noted above the ATF at 23 rd and San Pedro 

had been removed. For air samples collected from the ATF inlet, a threaded fitting was installed 

in one of the inlet duct sample ports, ½” (inside diameter) Teflon tubing was connected via a 

Swagelock fitting to this threaded fitting and the other end of the tubing was connected to the 

suction side of the air pump. Tee fittings were installed in the ½” Teflon tubing and a line from 

each Tee was run to two Summa canisters, which were used to collect samples for the TNMHC 

samples and volatile organic compound (VOC) analyses as discussed below. The air volume 

pulled by the air pump was approximately 300 times greater than the air volume collected in the 

Summa canisters which enabled the simultaneous filling of both Summa canisters. Air samples 

were also collected from the air pump discharge for additional analyses. Figure 2 is a schematic 

of the sampling train. Figure 3 is a photograph of one of the sampling trains. 




Figure 2 - Sampling Train Schematic 

Figure 3 – Air Sampling Train 




For air samples collected from the ATF exhaust, ½” Teflon tubing was connected to the threaded 

pressure port located on the exhaust stack or exhaust plenum downstream of the carbon bed and 

samples were collected as described above for the ATF inlet. Samples were collected from the 

ATFs’ inlet and exhaust simultaneously. 

At Mission & Jesse and 23 rd & San Pedro, air samples were collected by placing approximately 

20 lineal feet of the ½” Teflon sample tubing down into the manhole which connected to the 

sample train setup as described for the ATF inlet and exhaust 

3.2.2 Analytical Methods 

The analytical methods used to characterize the raw untreated sewer air and the ATF exhaust air 

were based upon SCAQMD permits for the ATFs and the City’s interest in understanding the 

composition of these air streams. The permits had two common emission limits as noted above: 

TNMHC and hydrogen sulfide. 

The permits require TNMHC is to be measured by SCAQMD Method 25.3 which oxidizes 

volatile organic compounds (VOCs) to carbon dioxide, reduces the carbon dioxide to methane, 

and measures the methane with a flame ionization detector. The method uses a gas 

chromatographic system to separate methane and ethane from the analysis because these 

compounds are recognized as not contributing to photochemical reactions. The air streams were 

analyzed by Method 25.3, and three additional methods in order to understand the organic 

compounds present in the subject air streams before their conversion to methane: EPA Method 

TO-15, EPA TO-12 Modified and ASTM 5504. These methods are described below. 

The City has received approval to use a photo-ionization detector (PID) device as an alternative 

method to measure the VOCs in the ATF exhaust air streams. PIDs have varying sensitivities to 

different compounds. Given this varying sensitivity, the use of a PID as an alternative to Method 

25.3 was evaluated as part of this study. 

The following paragraphs describe the test methods used: 

Total Non-Methane Hydrocarbon Compounds: Samples were collected/analyzed using 

SCAQMD Method 25.3 by an independent outside laboratory, Atmospheric Analysis & 

Consulting (AAC), Ventura, CA. Per this method, the air sample was passed through a 

condensate trap chilled in ice and then collected in a 6-liter evacuated stainless steel sample 

canister (Summa canister). The purpose of the condensate trap is to remove carbon dioxide and 

moisture which may contribute to a positive interference. The condensate was analyzed for total 

carbon and inorganic carbon. Total organic carbon (TOC) is determined by taking the difference 

of these values and is expressed as parts per million as carbon (ppmC). The air collected in the 

Summa canister was passed through a gas chromatographic column which separated carbon 

monoxide, carbon dioxide, methane, ethane and ethylene from the other organic compounds 

present, referred to as non-methane, non-ethane organic compounds (NMNEOC). The remaining 

compounds were then oxidized to carbon dioxide, reduced to methane and analyzed by a flame 

ionization detector (FID). The results were reported as ppmC. Samples were collected over 1- 

hour through the use of a flow controller that was provided with the Summa canister. See 

Appendix B for the analytical reports from AAC. 

EPA Method TO-15: Samples were collected in a 6-liter evacuated sample Summa canister and 

were analyzed by gas chromatography/mass spectrometry (GC/MS) to identify the compounds 

present by the laboratory, AAC. Prior to analysis, the sample was first concentrated 

cryogenically. Water was then removed using a hydrophobic drying system before being 

analyzed by the GC/MS system. Table 4 lists the compounds identified by this analytical 

method. Samples were collected over 1-hour using a flow controller at the same time samples 




were collected for analysis by SCAQMD 25.3. See Appendix B for the analytical reports from 

AAC. 

Chlorodifluoromethane 

Propylene 

Dichlorodifluoromethane 

Chloromethane 

1,2-Dichloro-1,1,2,2 tetrafluoroethane 

Vinyl chloride 

Methanol 

1,3-Butadiene 

Bromomethane 

Chloroethane 

Dichlorofluoromethane 

Ethanol 

Vinyl Bromide 

Acetone 

Trichlorofluoromethane 

Isopropanol 

Acrylonitrile 

1,1-Dichloroethylene 

Methylene Chloride 

Allyl Chloride 

Carbon Disulfide 

1,1,2-Trichloro-1,2,2-trifluoroethane 

t-1,2-dichloroethylene 

1,1-dichloroethane 

MTBE 

Vinyl Acetate 

2-butanol (MEK) 

cis-1,2-dichloroethylene 

Hexane 

Chloroform 

Ethyl Acetate 

Tetrahydrofuran 

1,2-dichloroethane 

1,1,1-trichloroethane 

Table 4 - TO-15 Compound List 

Benzene 

Carbon Tetrachloride 

Cyclohexane 

1,2-Dichloropropane 

Bromodichloromethane 

1,4-Dioxane 

Trichloroethene 

2,2,4-trimethylpentane 

Heptane 

cis-1,3-dichloropropene 

4-methyl-2-pentanone (MIBK) 

Trans-1,3-dichloropropene 

1,1,2-trichloroethane 

Toluene 

2-Hexanone 

Dibromochloromethane 

1,2-Dibromoethane 

Tetrachloroethene 

Chlorobenzene 

Ethylbenzene 

m,p-xylenes 

Bromoform 

Styrene 

1,1,2,2-Tetrachloroethane 

o-xylene 

Ethylotoluene 

1,3,5-trimethylbenzene 

1,2,4-trimethylbenzene 

Benzyl chloride 

1,3-dichlorobenzene 



1,2,4-trichlorobenzene 

Hexachlorobutadiene 

TO-12 Modified: This method is based on the methodology developed to measure ozone 

precursors as part of the Clean Air Act Amendments (Title 40 Code of the Federal Register, Part 

58). The amendments require States to establish Photochemical Assessment Monitoring Stations 

(PAMS). As a result this method is also referred to as the PAMS protocol. Fifty seven (57) VOCs 

are measured which consist of aliphatic and aromatic hydrocarbons generally having a carbon 

number in the range C 2 through C 12 . The sample is passed through a gas chromatographic column 

and then into a flame ionization detector (FID). A retention-time multi-component calibration 

standard is used to identify the compounds and the FID response to each compound is calibrated 

using propane. Concentrations are reported as parts per billion as carbon (ppbC). Table 5 is a list 




of compounds measured by this analytical method. See Appendix B for the analytical reports 

from AAC. 

Ethylene 

Acetylene 

Ethane 

Propylene 

Propane 

Isobutane 

l-Butene 

n-Butane 

Trans-2-Butene 

Cis-2-Butene 

Isopentane 

1-Pentene 

n-Pentane 

Isoprene 

trans-2-Pentene 

cis-2-Pentene 

2,2-Dimethylbutane 

Cyclopentane 

2,3-Dimethylbutone 

2-Methylpentane 

3-Methylpentane 

1-Hexene 

n-Henane 

Methylcyclopentane 

2,4-Dimethylpentane 

Benzene 

Cyclohexane 

2-Methylhexane 

Table 5 - TO-12 Modified Compound List 

2,3-Dimethylpentane 

3-Methylhexane 

2,2,4-Trimethylpentane 

n-Heptane 

Methylcyclohexane 

2,3,4-Trimethylpentane 

Toluene 

2-Methylheptane 

3-Methylheptane 

n-Octane 

Ethylbenzene 

m/p-Xylenes 

Styrene 

o-Xylene 

Nonane 

Isopropylbenzene 

n-Propylbenzene 

m-Ethyltoluene 

p-Ethyltoluene 

1,3,5-Trimethylbenzene 

o-Ethyltoluene 


n-Decane 


m-Diethylbenzene 

p-Diethylbenzene 

n-Undecane 

n-Dodecane 

Handheld Photo-ionization Detector (PID): Air samples were collected in Tedlar bags and 

analyzed by a handheld PID, a Mini-Rae 2000, manufactured by Rae Systems. The unit was 

calibrated using isobutylene at a concentration of 100 ppm v . Measurements are expressed as 

ppm v. However they are actually ppm v equivalents as isobutylene. A PID uses an ultraviolet lamp 

to emit photons which ionizes compounds. The unit that was used had lamp with an ionization 

potential of 10.6 millivolts, which is the most commonly used. The instrument response is 

proportional to the concentration of compounds which become ionized. It is noted that only a 

fraction of the compounds present are typically ionized because the PID’s sensitivity to different 

compounds varies. The order of a PID’s sensitivity to different types of compounds, from greater 

to less sensitivity is as follows: aromatic compounds, olefins, ketones, ethers, amines, sulfur 

compounds, esters, aldehydes, alcohols, aliphatics, chlorinated aliphatics and ethane. The 

sensitivity of compounds is expressed as a correction factor (CF). When measurements of 

mixtures are made, the reading should be adjusted by the CF for each compound present. The 

overall CF for an air sample containing multiple compounds would be, 

CF (Mix) = 1/(X i /CF i + X j /CF j + X k /CF k …) 




The instrument’s reading would then be multiplied by the correction factor to yield the volumetric 

concentration equivalents as isobutylene. 

For example, with an air sample containing 5% benzene (CF=0.53) and 95% n-hexane (CF=4.3), 

the CF (Mix) would be, 

CF (Mix) = 1/(0.05/0.53 + 0.95/4.3) = 3.2 

A reading of 100 would then be multiplied by 3.2 to yield 320 ppm v equivalents as isobutylene. 

Reduced Sulfur Compounds: Air samples were collected from the ATF air inlet streams in a 

Tedlar bag and were analyzed for reduced sulfur compounds by the laboratory, AAC. The 

method used, ASTM 5504, analyzes for 20 reduced sulfur compounds by using gas 

chromatography (GC) and a sulfur chemiluminescence detector (SCD). The compounds this 

method measures are listed in Table 6. See Appendix B for the analytical reports from AAC. 

Table 6 - ASTM 5504 Measured Compounds 

Hydrogen Sulfide 

Carbonyl Sulfide 

Methyl Mercaptan 

Ethyl Mercaptan 

Dimethyl Sulfide 

Carbon Disulfide 

Isopropyl Mercaptan 

tert-Butyl Mercaptan 

n-Propyl Mercaptan 

Ethyl Methyl Sulfide 

Thiophene 

Isobutyl Mercaptan 

Diethyl Sulfide 

Butyl Mercaptan 

Demethyl Disulfide 

3-Methylthiophene 

Tetrahydrothiophene 

2-Ethylthiophene 

2,5-Dimethylthiophene 

Diethyl Disulfide 

Hydrogen Sulfide: Hydrogen sulfide was measured using two different types of field 

measurement devices. OdaLog units were suspended in the manholes that exhaust to the ATF 

units, and at the 23 rd & San Pedro manhole that discharged to the ATF before it was removed, in 

order to determine when elevated concentrations occur within the diurnal cycle, so that the 

sampling could be scheduled as discussed above. The units were left in the manholes for three 

days to record data for the daily cycle and then up to two additional days until the sampling was 

complete. These devices are intrinsically safe, utilize an electro-chemical sensor, have a lower 

detection limit of 0.25 ppm v and depending on the model, can measure up to 50, 200, or a 1,000 

ppm v . They have data-logging capabilities and were programmed to take a measurement once per 

minute. 

A low level monitor, the Jerome Meter, manufactured by Arizona Instruments, was used to 

measure hydrogen sulfide collected in Tedlar bags. The bags were analyzed immediately onsite. 

(The same bags were analyzed by the PID unit during the sampling event.) The Jerome Meter has 

a measurement range of 0.003 to 50 ppm v . 

3.2.3 ATF Inlet Air Velocity Measurements and Air Flow Rates 

Velocity measurements were made of the inlet air stream to the ATFs at Richmond, NORS-ECIS, 

Humboldt and Mission & Jesse. The measurements were made using an Airdata Multimeter 

ADM-860 and a pitot tube. The inlet duct at these locations has two sample ports 90 degrees 




apart from each other. A traverse across the duct was made using both ports. The average 

velocity was multiplied by the inlet duct cross sectional area (diameter = 24”) to determine the 

inlet air flow rate. 




4.0 COLLECTED DATA AND SAMPLING RESULTS 

While reviewing the analytical results in subsequent paragraphs, please keep in mind that the 

SCAQMD Method 25.3 results will be used to determine compliance with the permit limits. The 

speciated results were developed to gain a better understanding of the general compounds that 

make up the TNMHC value resulting from Method 25.3 and are not used to directly demonstrate 

compliance with any permit limits. Due to the variable nature of sewer gases and the cocktail of 

compounds present at all different concentrations, a direct equal correlation between Method 25.3 

and speciated analyses results is not anticipated nor expected. 

4.1 Richmond 

Method 25.3/Speciated VOCs: The Richmond VOC sample/analytical results by Method 25.3 

are 3.8 and 4.8 ppmC for the inlet and exhaust, respectively, as shown in Table 7. These values 

are relatively low as compared to the permit emission limits in Table 1 (18 to 36 ppm v ). 

However, it is noted that the exhaust TNMHC concentration exceeded the inlet concentration 

indicating the carbon is exhausted for its VOC removal capacity. Every attempt was made to 

sample the inlet and exhaust simultaneously. Accordingly, the difference in inlet and exhaust 

Method 25.3 values cannot be readily explained by highlighting the continuous odor and 

TNMHC variability in the interceptor headspace. In this situation, although the difference in 

values is merely 1.0 ppm, the activated carbon may be the cause. It is theorized that when 

activated carbon becomes saturated with long chain organic compounds, some of those 

compounds can be displaced while others can be broken releasing intermediate compounds. The 

displaced and additional intermediate compounds could account for the increase in the exhaust 

levels. Fortunately, the inlet and exhaust levels are well below any anticipated TNMHC permit 

limits. 

It is believed the reason greater concentrations were being measured in the exhaust than the inlet 

is that the carbon had reached saturation for some compounds. At this condition, compounds in 

the inlet were displacing compounds on the carbon which were then measured in the exhaust. The 

VOCs breaking through the carbon had a higher concentration than the inlet. 

The sum of the VOC speciated compounds results from the TO-15, TO-12 Modified and ASTM 

5504 were compared to the Method 25.3 results. In order to make this comparison, the TO-15 

and ASTM 5504 results, which were reported in volumetric concentrations for each compound, 

had to be converted to ppmC, the reporting basis for the Method 25.3 results. Per EPA guidance, 

volumetric concentrations are converted to ppmC by multiplying by the number of carbon atoms 

in each compound. 

This conversion was made for the TO-15 and ASTM 5504 results which were then expressed as 

total speciated VOC as ppmC (Table 7). There are eleven (11) compounds that are measured by 

both the TO-15 and TO-12 Modified methods. When this comparison was made, TO-15 

compounds that were also reported by the TO-12 Modified method were excluded from the 

summation. In addition, the VOCs which were measured and have negligible photochemical 

activity were subtracted from the Method 25.3 and the TO-15 results. Comparisons of Method 

25.3 and total speciated VOCs for the inlet and exhaust are as follows: 

 

Richmond ATF Inlet. The total speciated VOC concentration for the Richmond ATF inlet 

was 3.66 ppmC which slightly exceeded the Method 25.3 value, 3.29 ppmC. The ratio of 

speciated VOCs/Method 25.3 VOCs was 1.11 (Table 7). Given the limits of the precision 

and accuracy of the analytical methods, the speciated VOCs account for the Method 25.3 

VOCs. The required relative percent difference (RPD) for laboratory control spikes for 

Method 25.3, TO-15 and TO-12 Modified is +/- 25%. Compounds with concentrations 




above 100 ppbC, key speciated compound contributors, which contributed to the inlet VOCs 

are: propane, isobutane, n-butane, and isopentane (Table 9). The total speciated VOC 

concentrations are shown in Table 7. For a complete list of speciated compounds with 

measured concentrations refer to Tables 8 and 9. 

Richmond ATF Exhaust. The total speciated VOC concentration for the Richmond ATF 

inlet was 3.66 ppmC which slightly exceeded the Method 25.3 value, 3.29 ppmC. The ratio of 

speciated VOCs/Method 25.3 VOCs was 1.11 (Table 7). Given the limits of the precision 

and accuracy of the analytical methods, the speciated VOCs account for the Method 25.3 

VOCs. The required relative percent difference (RPD) for laboratory control spikes for 

Method 25.3, TO-15 and TO-12 Modified is +/- 25%. Compounds with concentrations 

above 100 ppbC, key speciated compound contributors, which contributed to the inlet VOCs 

are: propane, isobutane, n-butane, and isopentane (Table 9). The total speciated VOC 

concentrations are shown in Table 7. For a complete list of speciated compounds with 

measured concentrations refer to Tables 8 and 9. 

Non-VOCs. The TO-15 analytical list includes numerous compounds which do not participate in 

photochemical reactions that produce ozone. EPA excludes 51 of these compounds from its 

definition of VOCs. Method 25.3 excludes two of these compounds, methane and ethane. 

Acetone, methylene chloride and tetrachloroethylene which are included on the EPA non-VOC 

list were measured at Richmond. Acetone was detected in the ATF inlet. Methylene chlorine and 

tetrachloroethylene were below the detection limit of the inlet but were measured in the exhaust. 

The concentration of these compounds is summarized in Table 7. The adjusted Method 25.3 

results for the inlet and exhaust which accounts for the subtraction of the methylene chloride and 

acetone are given in Table 7. 

Table 8 and Table 9 summarize the TO-15 and TO-12 Modified results, respectively. TO-15 

results were reported by the laboratory in units of ppb v and were converted to units of ppbC. 

PID Results: The PID results for the ATF inlet and exhaust which were 2.04 and 0.25 ppmC, 

respectively. The ratio of the PID/Method 25.3 values for the inlet and exhaust were 0.54 and 

0.05, respectively, showing a low bias for both the inlet and exhaust air streams, but relatively 

greater correlation for the inlet air as compared to the exhaust air. (Table 7) 

Hydrogen Sulfide and Reduced Sulfur Results: 

The Jerome Meter hydrogen sulfide inlet and exhaust concentrations were 2.29 and

Measured Air Flow Rate: The fan was not operating during the sampling. Air was being moved 

through the ATF using the positive pressure of the collection system. The measured air flow rate 

at the ATF inlet was 1,120 cfm. This compares to rated system capacity of 10,000 cfm. 

Figure 4 thru Figure 6 are photographs of the Richmond ATF, and sampling and velocity 

measurement effort. 

Table 7 - Richmond - TNMHC, VOC and H2S Sample/Analytical Results 

Analytical Method Richmond Inlet Richmond Exhaust 

TNMHC 

Method 25.3, ppmC 3.80 4.80 

TNMHC Removal Efficiency By Carbon 

Adsorber 

Exhaust Conc. > Inlet Conc. 

Non VOCs 

Acetone, ppmv/ppmC 0.164/0.492

Table 8 - Richmond TO-15 Sample/Analytical Results 

Compound 

Richmond Inlet Richmond Inlet Richmond Exhaust Richmond Exhaust 

(ppbv) (ppbC) (ppbv) (ppbC) 

Methanol 27.6 27.6 40.5 40.5 

Ethanol 24.6 49.2 294 588.0 

Acetone

Table 9 - Richmond TO-12 Modified Results 

Compound 

Richmond Inlet Richmond Exhaust 

(ppbC) 

(ppbC) 

ETHYLENE 8.1 6.8 

Acetylene

Figure 4 - Richmond ATF (1) 









4.2 NORS-ECIS 

Method 25.3/Speciated VOCs: The NORS-ECIS VOC sample/analytical results by Method 25.3 

are 15.2 and 3.9 ppmC for the inlet and exhaust, respectively, as shown in Table 10. These values 

are relatively low as compared to the emission limits in Table 1 (18 to 36 ppm v ). The TNMHC 

removal efficiency of the carbon adsorber was 74%. 

Using the approach described for Richmond, the VOCs measured by method 25.3 were compared 

to the speciated VOC results: 

 

NORS-ECIS ATF Inlet: The total speciated VOC concentration for the NORS-ECIS ATF 

inlet adjusted for non-photochemically active VOCs was 8.42 ppmC which is approximately 

58% of the Method 25.3 value (Table 10). However, if the measured speciated and Method 

25.3 results are viewed with their allowable range given the +/- 25% RPD limits for these 

methods, the speciated VOCs account for the Method 25.3 VOCs. Compounds with 

concentrations above 100 ppbC, key speciated compound contributors, which contributed to 

the inlet VOCs are: ethanol, acetone, methylene chloride, chloroform (Table 11); and 

propane, isobutane, n-butane, isopentane, methylcyclohexane, toluene, 2-methylheptane, 3- 

methylheptane, n-octane, m/p-xylenes, o-xylene, nonane, n-propylbenzene, m-ethyltolune, p- 

ethyltoluene,1,3,5-trimethylbenzene, o-ethyltoluene, 1,2,4-trimethybenzene, n-decane, 1,2,3- 

trimethylbenzene, n-undecane and n-dodecane (Table 12). For a complete list of speciated 

compounds with measured concentrations refer to Tables 11 and 12. 

NORS-ECIS ATF Exhaust: The total speciated VOC concentration for the NORS-ECIS 

ATF exhaust was 3.25 ppmC, which is approximately 80% of the Method 25.3 concentration. 

As noted for the inlet, if the measured speciated and Method 25.3 results are viewed with 

their allowable range given the +/- 25% RPD limits for these methods, the speciated VOCs 

account for the Method 25.3 VOCs. Ethanol at a concentration of 1,476 ppbC had the highest 

VOC concentration in the exhaust. Other compounds above a concentration of 100 ppbC, key 

speciated compound contributors, that contributed to the exhaust VOCs are: propane, 

isobutane, and n-butane. For a complete list of speciated compounds with measured 

concentrations refer to Tables 11 and 12. 

Non-VOCs. The non-VOCs, acetone, methylene chloride and tetrachloroethylene were 

measured at NORS-ECIS. Their concentrations and the adjusted Method 25.3 results are 

summarized in Table 10. 

Table 11 and Table 12 summarize TO-15 and TO-12 Modified Results, respectively. TO-15 


PID Results. The PID results for the ATF inlet and exhaust were 8.45 and 0.40 ppmC, 

respectively. The ratio of the PID/Method 25.3 values for the inlet and exhaust were 0.56 and 

0.10, respectively, showing a low bias for both the inlet and exhaust air streams, but relatively 

greater correlation for the inlet air as compared to the exhaust air. The PID and Method 25.3 

results shown for the NORS-ECIS is similar to that shown for Richmond. Table 10) 

Hydrogen Sulfide and Reduced Sulfur Results 

The Jerome Meter hydrogen sulfide inlet and exhaust concentrations were 22.2 and 0.005 ppm, 

respectively, resulting in greater than 99% removal (Table 10). 

An OdaLog unit was placed in the NORS-ECIS ATF manhole from April 19 to 23. The average, 

maximum and minimum hydrogen sulfide concentrations were, 32, 67 and < 0.25 ppm v , as 

summarized in Table 10. The NORS-ECIS OdaLog graph appears in Appendix C. 




The only reduced sulfur compound measured in the ATF inlet in addition to hydrogen sulfide 

was methyl mercaptan at a concentration of 0.584 ppm v . 

Measured Air Flow Rate: The fan was operating during the sampling. The measured air flow 

rate at the ATF inlet was 9,190 cfm. This compares to the rated system capacity of 10,000 cfm. 

Figure 7 thru Figure 9 are photographs of the NORS-ECIS ATF and the sampling effort. 

Table 10 - NORS-ECIS TNMHC, VOC and H2S Sample/Analytical Results 

Analytical Method 

NORS ECIS Inlet 

NORS ECIS 

Exhaust 

TNMHC 

Method 25.3, ppmC 15.2 3.90 

TNMHC Removal Efficiency By Carbon Adsorber 74% 

Non VOCs 

Acetone, ppmv/ppmC 0.130/0.390 99% 

OdaLog Results 

Average Hydrogen Sulfide, ppm 32 ns 2 

Maximum Hydrogen Sulfide, ppm 67 ns 2 

Minimum Hydrogen Sulfide, ppm

Table 11 - NORS-ECIS TO-15 Sample/Analytical Results 

Compound 

NORS-ECIS 

Inlet 

NORS-ECIS 

Inlet 

NORS-ECIS 

Exhaust 

NORS-ECIS 

Exhaust 

(ppb v ) (ppbC) (ppb v ) (ppbC) 

Chlorodifluoromethane 18.4 18.4 16.2 16 

Chloromethane 43.7 43.7 42.4 42 

Methanol 44.8 44.8 53.5 54 

Chloroethane 23.9 47.8 29.0 58 

Ethanol 180 360 738 1476 

Acetone 130 390

Table 12 - NORS-ECIS TO-12 Modified Results 

Compound 

NORS-ECIS Inlet NORS-ECIS Exhaust 

(ppbC) 

(ppbC) 

Ethylene 25.3 20.2 

Acetylene

Figure 7 - NORS ECIS ATF (1) 









4.3 Humboldt 

Method 25.3/Speciated VOCs: The Humboldt VOC sample/analytical results for Method 25.3 

are 23.0 and 18.4 ppmC for the inlet and exhaust, respectively, as shown in Error! Not a valid 

bookmark self-reference.. The exhaust concentration is just above the emission limits in 

Table 1. The TO-15 and TO-12 Modified data were reviewed to determine if one or more 

compounds at Humboldt may have caused this increase. As is discussed below, the only 

measured compound that may have contributed to this higher Method 25.3 value is ethanol, 

however, due to the interference described below, the actual ethanol concentration in the inlet and 

exhaust air streams is not known. The TNMHC removal efficiency of the carbon absorber was 

20% indicating the carbon has approached its VOC adsorption capacity. The likely TNMHC 

removal efficiency with fresh carbon will vary based on the make-up of the inlet air stream, but 

can generally be expected to range from 70 to over 90% removal from typical wastewater 

environments. 

 

Humboldt ATF Inlet: The sum of the speciated VOC results from the TO-15, TO-12 

modified and ASTM 5504 were compared to the Method 25.3 results with VOCs having 

negligible photochemical activity subtracted (Table 13). The total speciated VOC 

concentration for the Humboldt ATF inlet was 115 ppmC which is approximately five times 

greater than the Method 25.3 results. 

This is believed to be the result of condensate which accumulated at the Tee where the ½” 

Teflon tubing air line connected to the ¼” Teflon branch line that ran to the Summa canister 

used for the TO-15 and TO-12 Modified analysis. It is believed that this condensate absorbed 

a disproportionate amount of ethanol from the total air stream being moved by the air pump 

which in turn was released into the air stream being directed to the TO-15 and TO-12 

Modified canister. This hypothesis is supported by the high ethanol concentration of 54,700 

ppb v in the TO-15 results for the Humboldt inlet air stream (see 




Table 14). This compares to ethanol concentrations in the Richmond inlet and exhaust; and 

the NORS-ECIS inlet and exhaust that are all considerably less than 1,000 ppb v (refer to 

Tables 8 and 11). 

In addition to the ethanol already noted, compounds with concentrations above 100 ppbC 

which contributed to the inlet VOCs are acetone, ethyl acetate, tetrachloroethylene ( 




Table 14); and propane, isobutane, n-butane, and isopentane (Table 16). 

Humboldt ATF Exhaust: The total speciated VOC concentration for the Humboldt ATF 

inlet was 66.6 ppmC which is approximately 3.7 times greater than the Method 25.3 results. 

This is believed to be the result of the same condensate accumulation mechanism as was 

described for the elevated total speciated VOC concentration in the ATF inlet TO-15 and TO- 

12 Modified samples. An ethanol concentration of 31,100 ppmC was reported for the TO-15 

results for the NORS-ECIS inlet air stream (see 




Table 14). In addition to ethanol, compounds with concentrations above 100 ppbC, key 

speciated compound contributors, which contributed to the exhaust VOCs are: acetone, 

toluene ( 




Table 14); and, propane, isobutane, and isopentane (see Table 16). For a complete list of 

speciated compounds with measured concentrations refer to Tables 14 and 16. 

Non-VOCs: The non-VOCs, acetone, methylene chloride and tetrachloroethylene were 

measured at Humboldt. Their concentrations and the adjusted Method 25.3 results are 

summarized in Error! Not a valid bookmark self-reference.. 




Table 14 and Table 16 summarize the TO-15 and TO-12 Modified results, respectively. TO-15 


PID Results: The PID results for the ATF inlet and exhaust which were 12.0 and 5.57 ppmC, 

respectively. The ratio of the PID/Method 25.3 inlet and exhaust were 0.52 and 0.30, respectively, 

showing a low bias for both the inlet and exhaust air streams, but relatively greater correlation for 

the inlet air as compared to the exhaust air, similar to the results for Richmond and NORS-ECIS. 


 

The Jerome Meter hydrogen sulfide inlet and exhaust concentrations were 19.1 and 11.7 ppm, 

respectively, resulting in a 38% removal efficiency. Such a low removal efficiency suggests 

that the carbon in this system should be replaced. Inlet and exhaust readings should be taken 

on at least one other event to verify the need to replace the carbon. 

An OdaLog unit that was placed in the Humboldt ATF manhole from April 19 to 23. The 

average, maximum and minimum H 2 S concentrations were, 6.5, 51 and < 0.25 ppm v , 

respectively, as shown in Table 13. The Humboldt OdaLog graph appears in Appendix C. 

Two reduced sulfur compounds other than hydrogen sulfide were measured and are summarized 

in Table 15. 

Measured Air Flow Rate: The fan was operating during the sampling. The measured air flow 

rate at the ATF inlet was 12,940 cfm. This compares to a rated system capacity of 10,000 cfm. 

Figure 10 thru Figure 13 are photographs of the Humboldt ATF and the sampling effort. 




Table 13 - Humboldt TNMHC, VOC and H 2 S Sample/Analytical Results 

Analytical Method Humboldt Inlet Humboldt Exhaust 

TNMHC 

Method 25.3, ppmC 23.0 18.4 

TNMHC Removal Efficiency by Carbon Adsorber 20% 

Non VOCs 

Acetone, ppmv/ppmC 0.063/0.190 0.077/0.230 

Methylene Chloride, ppmv/ppmC 0.14/0.14 0.007/0.007 

Total Non VOCs, ppmC 0.204 0.237 

Adjusted Method 25.3, ppmC 22.80 18.16 

Speciated Results as Carbon 

TO - 15, ppmC 112.9 65.07 

TO - 12 Modified, ppmC 1.83 1.87 

ASTM 5504, ppmC 0.55 ns 1 

Total Speciated, ppmC 115.3 66.9 

Ratio: Speciated VOCs/Method 25.3 VOCs 5.0 3.6 

PID Results 

PID, ppm v 12.0 5.57 

Ratio: PID/Method 25.3 0.52 0.30 

Hydrogen Sulfide Results 

Hydrogen Sulfide, ppm 19.1 11.7 

Hydrogen Sulfide Removal Efficiency By Carbon 

Adsorber 39% 


Average Hydrogen Sulfide, ppm 6.5 ns 1 

Maximum Hydrogen Sulfide, ppm 51 ns 1 


Table 14 - Humboldt TO-15 Sample/Analytical Results 

Compound 

Humboldt 

Inlet 

Humboldt 

Inlet 

Humboldt 

Exhaust 

Humboldt 

Exhaust 

(ppb v ) (ppbC) (ppb v ) (ppbC) 

Chlorodifluoromethane

Table 16 - Humboldt TO-12 Modified Results 

Compound 

Humboldt Inlet Humboldt Exhaust 

(ppbC) 

(ppbC) 

Ethylene 22.7 18.5 

Acetylene 11.5 10.3 

Ethane 3190 2810 

Propylene

Figure 10 - Humboldt ATF (1) 










4.4 Mission and Jesse 

Method 25.3/Speciated VOCs: The ATF was not in operation during the sampling of Mission & 

Jesse because its carbon was being replaced. Therefore, only the sewer air was sampled by 

placing a sample line approximately 20 feet down into the manhole which exhausts to the ATF 

inlet. The inlet sample/analytical result for Method 25.3 was 14.0 ppmC, as shown in Table 17 

which is less than the TNMHC emission limits in Table 1. 

The total speciated VOC concentration for the Mission & Jesse sewer exhaust was 24.1 ppmC 

with VOCs having negligible photochemical activity subtracted. This concentration exceeds the 

Method 25.3 results by 1.8 times (see Table 17). As described for Humboldt, this greater total 

speciated VOC concentration is attributed to condensate that accumulated at the Tee where the 

½” Teflon tubing air line connected to the ¼” Teflon branch line that ran to the Summa canister 

used for the TO-15 and TO-12 Modified analysis. This condensate absorbed a disproportionate 

amount of ethanol from the total air stream being moved by the air pump, which in turn was 

released into the air stream being directed to the TO-15 and TO-12 Modified canister. The 

ethanol concentration of this air stream was 5,480 ppb v (Table 18). The TO-15 compound with 

the next highest concentration was toluene at 837 ppb v . 

Compounds with concentrations above 100 ppbC, key speciated compound contributors, which 

contributed to the inlet VOCs are: acetone, toluene, m & p-xylene (Table 18); and propane, 

isobutene, n-butane, isopentane, toluene, o-xylene, m-ethyltoluene, p-ethyltoluene, n-decane, 

1,2,3-trimethylbenzene, n-undecane. For a complete list of speciated compounds with measured 

concentrations refer to Tables 18 and 20. 

Table 18 and Table 20 summarize the TO-15 and TO-12 Modified Results, respectively. TO-15 


Non VOCs: The non-VOCs, acetone, methylene chloride and tetrachloroethylene were measured 

at Mission & Jesse. Their concentrations and the adjusted Method 25.3 results are summarized in 

Table 17. 

PID Results: The PID results for the sewer exhaust to the ATF inlet was 13.0 ppmC. The ratio of 

the PID/Method 25.3 values of 0.9, which shows a better correlation than for the inlet at 

Richmond, NORS-ECIS and Humboldt. (Table 17) 


 

 

The Jerome Meter hydrogen sulfide sewer exhaust concentration was >50 ppm, which is 

greater than the Jerome Meter’s measurement range (Table 17). 

An OdaLog unit was placed in the Humboldt ATF manhole from April 19 to 23. The average, 

maximum and minimum hydrogen sulfide concentrations were 16.5, 87 and < 0.25 ppm v , 

respectively, as shown in Table 17. The Mission & Jesse OdaLog graph appears in Appendix 

C. 

Two reduced sulfur compounds other than hydrogen sulfide were measured and are 

summarized in Table 19. 

Measured Air Flow Rate. As noted above the ATF was not in operation during the sampling. 

As a result the velocity was measured on a subsequent visit. The calculated air flow rate at the 

ATF inlet was 5,150 cfm. This compares to rated system capacity of 10,000 cfm. 

Figure 14 thru Figure 16 are photographs of the Mission & Jesse ATF and sampling effort. 




Table 17 - Mission & Jesse TNMHC, VOC and H2S Sample/Analytical Results 


Mission & Jesse Inlet 

TNMHC 

Method 25.3, ppmC 14.0 

Non VOCs 

Acetone, ppm v /ppmC 0.154/0.462 

Methylene Chloride, ppm v /ppmC 0.008/0.008 

Total Non VOCs, ppmC 0.162/0.47 

Adjusted Method 25.3, ppmC 13.53 


TO - 15, ppmC 12.35 

TO - 12 Modified, ppmC 11.40 

ASTM 5504, ppmC 0.95 

Total Speciated VOCs, ppmC 24.7 

Ratio: Speciated VOCs/Method 25.3 1.8 

PID Results 

PID, ppm v 13.0 

Ratio: PID/Method 25.3 0.9 


Hydrogen Sulfide, ppm >50 


Average Hydrogen Sulfide, ppm 16.5 

Maximum Hydrogen Sulfide, ppm 87 


Table 18 - Mission & Jesse TO-15 Sample/Analytical Results 

Compound 

Mission & Jesse Inlet Mission & Jesse Inlet 

(ppb v ) 

(ppbC) 

Chlorodifluoromethane 1.7 2 

Chloromethane 2.2 2 

Ethanol 5480 10960 

Acetone 154 462 

Isopropyl Alcohol 15.5 47 

Methylene Chloride 8.4 8 

Carbon Disulfide 14.6 15 

2-Butanone (MEK) 9.3 37 

Hexane 4.1 25 

Chloroform 80.2 80 

Ethyl Acetate 11.4 46 

Benzene 1.8 11 

Cyclohexane 2.0 12 

Bromodichloromethane 12.9 13 

Trichloroethene 8.7 17 

2,2,4-Trimethylpentane 7.9 63 

Heptane 6.3 44 

Toluene 837 5859 

Dibromochloromethane 8.4 8 

Tetrachloroethylene 68.4 137 

Ethylbenzene 38.9 311 

m- & p-Xylenes 159 1272 

Bromoform 2.0 2 

Styrene 5.4 43 

o-Xylene 35.6 285 

4-Ethyltoluene 1.9 17 

1,3,5-Trimethylbenzene 2.5 23 


1,4-Dichlorobenzene 17.0 102 

Table 19 - Mission & Jesse ATF Inlet Reduced Sulfur Compounds 

Compound 

(ppmv) 

Methyl Mercaptan 0.826 

Dimethyl Sulfide 0.063 




Table 20 - Mission & Jesse TO-12 Modified Results 

Compound 

Mission & Jesse Inlet 

(ppbC) 

Ethylene 33.5 

Acetylene 9.8 

Ethane 4010 

Propylene 13.0 

Propane 821 

Isobutane 285 

1-Butene 15.4 

n-Butane 147 

Isopentane 195 

n-Pentane 86.1 

2,2-Dimethylbutane 6.9 


2-Methylpentane 14.3 


n-Hexane 28.9 

Methylcyclopentane 16.4 

2,4-Dimethylpentane 8.4 

Benzene 11.2 

Cyclohexane 12.9 

2-Methylhexane 20.0 



2,2,4-Trimethylpentane 72.3 

n-Heptane 45.8 

Methylcyclohexane 73.6 


Toluene 6020 

2-Methylheptane 23.3 


n-Octane 129 

Ethylbenzene 339 

m/p-Xylenes 1350 

o-Xylene 305 

Nonane 98.3 

Isopropylbenzene 3.8 

n-Propylbenzene 21.0 

m-Ethyltoluene 140 

p-Ethyltoluene 236 

1,3,5-Trimethylbenzene 38.2 

o-Ethyltoluene 14.4 

1,2,4-Trimethylbenzene 75.1 

n-Decane 106 

1,2,3-Trimethylbenzene 239 

n-Undecane 105 

n-Dodecane 98.8 




Figure 14 - Mission and Jesse ATF (1) 









4.5 23 rd and San Pedro 

Method 25.3/Speciated VOCs: The ATF was removed from this location in 2008. Therefore, 

the sewer air was sampled by placing a sample line approximately 20 feet down into the manhole 

which had exhausted to the ATF inlet. The inlet sample/analytical results for Method 25.3 was 

16.4 ppmC, as shown in Table 21 which is less than the TNMHC emission limits of Table 1. 

The total speciated VOC concentration for the 23 rd & San Pedro sewer exhaust was 11.4 ppmC 

with the VOCs having negligible photochemical activity subtracted. This concentration is 70% of 

the Method 25.3 results (see Table 21). Given the RPD limits of the methods used to measure the 

speciated VOCs and of Method 25.3, the measured speciated VOCs account for the Method 25.3 

VOCs. Compounds with concentrations above 100 ppbC, key speciated compound contributors, 

which contributed to the inlet VOCs are: acetone, methylene chloride, chloroform, 1,2- 

dichloroethane, toluene (Table 22); propane, isobutene, n-butane, isopentane, n-pentane, toluene, 

n-octane, ethylbenzene, m/p-xylene, o-xylene, nonane, n-propylbenzene, m-ethyltoluene (Table 

23). For a complete list of speciated compounds with measured concentrations refer to Tables 22 

and 23. 

Table 22 and Table 23 summarize the TO-15 and TO-12 Modified Results, respectively. TO-15 


Non VOCs: The non-VOCs, acetone, methylene chloride and tetrachloroethylene were measured 

at 23rd & San Pedro. Their concentrations and the adjusted Method 25.3 results are summarized 

in Table 21. 

PID Results: The PID results for the sewer exhaust was 18.5 ppmC. The PID/Method 25.3 ratio 

was 1.13 (Table 21). The ratio of the PID/Method 25.3 values of 1.13, which shows a slightly 

high bias for the PID relative to Method 25.3, but a better correlation than for the inlets at 

Richmond, NORS-ECIS and Humboldt. 


 

The Jerome Meter hydrogen sulfide inlet sewer exhaust concentration was >50 ppm (see 

Table 21). 

An OdaLog unit that was placed in the 23 rd & San Pedro ATF manhole from April 19 to 23. 

The average, maximum and minimum hydrogen sulfide concentrations were, 34.5, 101, and < 

0.25 ppm v , respectively, as summarized in Table 21. The 23 rd & San Pedro OdaLog graph 

appears in Appendix C. 

The only reduced sulfur compound measured in the sewer exhaust other than hydrogen 

sulfide was methyl mercaptan at a concentration of 0.617 ppm. 

Figure 17 and Figure 18 are photographs of the 23 rd & San Pedro location and sampling effort. 




Table 21 - 23rd & San Pedro TNMHC, VOC and H 2 S Sample/Analytical Results 


23rd & San Pedro 

TNMHC 

Method 25.3, ppmC 16.4 

Non VOCs 

Acetone, ppm v /ppmC 0.138/0.414 

Methylene Chloride, ppm v /ppmC 0.308/0.308 

Total Non VOCs, ppmC 0.722 

Adjusted Method 25.3, ppmC 15.7 


TO - 15, ppmC 5.01 

TO - 12 Modified, ppmC 6.64 

ASTM 5504, ppmC 0.62 

Total Speciated VOCs, ppmC 12.3 

Ratio: Speciated VOCs/Method 25.3 0.7 

PID Results 

PID, ppm v 18.5 

PID/Method 25.3 1.13 


Hydrogen Sulfide, ppm >50 


Average Hydrogen Sulfide, ppm 34.5 

Maximum Hydrogen Sulfide, ppm 101 

Minimum Hydrogen Sulfide, ppm 0 




Table 22 - 23rd & San Pedro TO-15 Sample/Analytical Results 

Compound 

San Pedro & 23rd St. San Pedro & 23rd St. 

(ppb v ) 

(ppbC) 

Chlorodifluoromethane 6.1 6 

Chloromethane 44.8 45 

Methanol 54.6 55 

Chloroethane 42.6 85 

Ethanol 1110 2220 

Acetone 138 414 

Isopropyl Alcohol 39.6 119 

Methylene Chloride 308 308 

Carbon Disulfide 20.1 20 

2-Butanone (MEK) 5.4 22 

Hexane 5.2 31 

Chloroform 173 173 

Ethyl Acetate 3.9 15 

1,2-Dichloroethane 173 346 

Bromodichloromethane 20.3 20 

Trichloroethene 10.8 22 

2,2,4-Trimethylpentane 2.6 21 

Heptane 6.2 43 

Toluene 104 728 

Dibromochloromethane 14.3 14 

Tetrachloroethylene 90.1 180 

Ethylbenzene 12.3 98 

m- & p-Xylenes 40.5 324 

Bromoform 4.3 4 

o-Xylene 17.4 139 

4-Ethyltoluene 16.0 144 



1,4-Dichlorobenzene 10.1 61 




Table 23 - 23rd & San Pedro TO-12 Modified Results 

Compound 

23 rd & San Pedro 

(ppbC) 

Ethylene 32.2 

Acetylene 26.4 

Ethane 1170 

Propylene 16.4 

Propane 322 

Isobutane 139 

n-Butane 106 

Isopentane 106 

n-Pentane 106 

Isoprene 8.0 





n-Hexane 33.4 


Benzene 9.3 

Cyclohexane 11.7 





n-Heptane 41.2 

Methylcyclohexane 79.5 

Toluene 785 



n-Octane 224 

Ethylbenzene 108 

m/p-Xylenes 329 

o-Xylene 172 

Nonane 205 

Isopropylbenzene 29.6 

n-Propylbenzene 222 

m-Ethyltoluene 527 

p-Ethyltoluene 398 


o-Ethyltoluene 254 


n-Decane 368 


m-Diethylbenzene 16.2 

p-Diethylbenzene 39.2 

n-Undecane 261 

n-Dodecane 154 




Figure 17 - 23rd and San Pedro (1) 

Figure 18 - 23rd and San Pedro (2) 




4.6 Summary 

Table 24 is a summary of the TNMHC, VOC and hydrogen sulfide sample/analytical results for 

the five locations. Following is a summary of findings from the sampling/analytical effort: 

 

 

 

 

 

 

At all locations, the ATF inlet (raw sewer air) and ATF exhaust air Method 25.3 results were 

below the air permit TNMHC emission limits of Table 1 (obtained from various interceptor 

ATF permits the City provided) except at Humboldt where the inlet and exhaust 

concentrations were 23.0 and 18.4 ppmC, respectively. This is graphically displayed in 

Figure 19. As noted below in Table 24 with regard to the exhaust hydrogen sulfide emission 

concentration, it appears that the carbon of the Humboldt ATF is used up. It is expected that 

if the carbon was still effective the exhaust Method 25.3 and hydrogen sulfide results would 

be notably reduced. Figure 19 illustrates inlet and exhaust TNMHC concentrations relative to 

the permit limit range of Table 1 (18 to 36 ppm v ). 


approximately 32% to approximately 100% except for locations where sampling 

interferences occurred and a disproportionate amount of ethanol was collected in the TO- 

15/TO-12 Modified Summa canister, i.e., at the Humboldt inlet/exhaust, and Mission & 

Jesse. The speciated compounds were determined by EPA Method TO-15, EPA Method TO- 

12 Modified, and by a reduced sulfur scan (ASTM 5504) and were expressed as parts per 

million by volume by the laboratory. The Method 25.3 results were expressed as ppm as 

Carbon (ppmC). In order to compare the speciated results to the Method 25.3 results, the 

speciated results were converted to ppmC by multiplying by the volumetric concentration by 

the number of carbon atoms in each compound, per EPA guidance. After converting all the 

speciated results to ppmC, these compounds were totaled and were compared to the Method 

25.3 results, by calculating a quotient, (ppmC-Speciated)(ppmC-Method 25.3). 

Some consistency was shown between the PID and Method 25.3 results expressed as a 

PID/Method 25.3 ratio. For the inlet at Richmond, NORS-ECIS and Humboldt, the 

PID/Method 25.3 ratio ranged from 0.52 to 0.56. There was less consistency of the 

PID/Method 25.3 ratio on the exhaust of these locations, with the ratio ranging from 0.05 to 

0.30. Additional work is needed to evaluate whether a calibration curve can be developed to 

be used by the staff to effectively monitor exhaust TNMHC concentrations instead of using 

SCAQMD Method 25.3. The City should also consider investigating whether an alternative 

hand-held device can provide improved service. 

The short term grab ATF inlet or raw sewer air hydrogen sulfide concentrations ranged from 

approximately 2 ppm v (at Richmond) to >50 ppm v (at Mission & Jesse and 23 rd & San 

Pedro). Figure 20 illustrates the ATF inlet and exhaust hydrogen sulfide concentrations and 

the average OdaLog readings. 


removals of > 99%. The carbon at Humboldt appeared to be exhausted, with an exhaust 

hydrogen sulfide concentration of 11.7 ppm v . 

Figure 21 serves as an overview of hydrogen sulfide control technologies as a function of 

inlet concentration in general, and the values measured in this study. Carbon is generally 


concentrations up to 50 ppm v and grater. At higher concentrations however, the biofilter 

media degradation may accelerate. Bio-trickling filters are cost-effective at inlet 

concentrations of 20 ppm v and higher, which indicates some overlap for the two biological 

systems. Figure 21 can be used as a basis to evaluate control technologies in the next phase 

of this project. 




Analytical Method/Parameter 

Table 24 - Total NMHC, VOC and H 2 S Sample/Analytical Results Summary 

Richmond 

Inlet 

Richmond 

Exhaust 

NORS 

ECIS Inlet 

NORS ECIS 

Exhaust 

Humboldt 

Inlet 

Humboldt 

Exhaust 

Mission & 

Jesse Inlet 

San Pedro 

& 23rd St. 

Method 25.3, ppmC 3.80 4.80 15.2 3.90 23.0 18.4 14.0 16.4 

TO - 15, ppmC 1.42 0.11 2.23 1.72 112.9 65.07 12.35 5.01 

TO - 12 Modified, ppmC 2.61 1.45 6.33 1.53 1.83 1.87 11.40 6.64 

ASTM 5504, ppmC 0.14 ns 0.58 ns 0.55 ns 0.95 0.62 

Total Speciated VOCs, ppmC 4.17 1.55 9.14 3.25 115.3 66.9 24.7 12.3 

Ratio Speciated VOCs/Method 25.3 1.10 0.32 0.60 0.83 5.0 3.6 1.8 0.7 

PID, ppm v 2.04 0.25 8.45 0.40 12.0 5.57 13.0 18.5 

PID/Method 25.3 0.54 0.05 0.56 0.10 0.52 0.30 0.9 1.13 

Hydrogen Sulfide, ppm 2.29 50 >50 

Sewer OdaLog Results 

Average Hydrogen Sulfide, ppm 9.5 nm 32 nm 6.5 nm 16.5 34.5 

Maximum Hydrogen Sulfide, ppm 26 nm 67 nm 51 nm 87 101 


Figure 19 - TNMHC VOC Results Summary 




Figure 20 - Hydrogen Sulfide Sampling Result Summary 




Figure 21 - Hydrogen Sulfide Emission Control Overview 




5.0 COMPARISON OF CURRENT AND HISTORICAL DATA 

The City conducted sampling/analyses at ATFs within their collection system in 2009 including 

the locations that were the focus of this study. The locations sampled included the scrubber inlet 

and exhaust of the ATFs. The samples were analyzed by EPA Method TO-14 which measures 

for 39 VOCs and by SCAQMD 307-91 which analyzes for 10 sulfur compounds including 

hydrogen sulfide and sulfur dioxide. (The 2009 sample results are in Appendix D). As noted in 

Section 3 of this report, the 2010 samples were analyzed for VOCs by TO-15 and for 20 reduced 

sulfur compounds by ASTM 5504. The analytical results from the 2009 sampling/analytical were 

compared to the results from the 2010 sampling effort. 

Because the TO-14 analysis measured for 39 compounds and the TO-15 analysis measured for 68 

compounds, the results of the TO-14 and TO-15 results were compared as follows: 

 

A list was made of all the compounds for which there were detectable results from the TO-14 

analytical results. A truncated list of TO-15 compounds was then made that matched the TO- 

14 list. 

The concentration of all the compounds from the TO-14 and TO-15 results were then totaled. 

These total concentrations are compared in Table 25. 

The results are summarized as follows: 

The maximum concentration was measured in 2010 at the Mission & Jesse ATF inlet of 

1,279 ppb v 

Inlet concentrations in 2009 and 2010 ranged from 2 to 421 ppb v , and 81 to 1,279 ppb v , 

respectively. 

The exhaust concentrations in 2009 and 2010 ranged from 26 to 151 ppb v and 9 to 147 ppb v , 

respectively. 

A comparison of the 2009 total sulfur results and the 2010 reduced sulfur results are summarized 

as follows: 

 

 

 

Results from the 2009 sampling effort reports the presence of carbonyl sulfide in numerous 

samples ranging in concentration from 0.40 to 0.96 ppm v , with a method detection limit of 

0.088 ppm v . Whereas no carbonyl sulfide was measured in any of the 2010 samples, where 

the sample reporting limit was 0.050 ppm v . (See Table 26) 

In addition to hydrogen sulfide, methyl mercaptan and dimethyl sulfide were the only other 

reduced sulfur compounds measured. The range of methyl mercaptan detected in 2009 and 

2010 ranged from 0.59 to 0.96 ppm v and 0.139 to 0.826 ppm v , respectively. Dimethyl sulfide 

was measured in 2010 at concentrations of 0.055 and 0.063 ppm v . 

The 2009 and 2010 results are generally similar except for the presence of carbonyl sulfide in 

the 2009 data and dimethyl sulfide in the 2010 data. 




Table 25 - Comparison of 2010 and 2009 Total VOC Data (ppb v ) 

Richmond 

Inlet 

Richmond 

Exhaust 

Comparison of 2010 and 2009 Total VOC Data (ppb v ) 

NORS 

ECIS 

Inlet 

NORS 

ECIS 

Exhaust 

Humboldt 

Inlet 

Humboldt 

Exhaust 

Mission & 

Jesse Inlet 

Mission & 

Jesse 

Exhaust 

San Pedro 

& 23rd St. 

Inlet 

2009 Data/TO-14 248 147 270 48 2 9 366 165 421 5 

2010 Data/TO-15 81 26 591 112 181 151 1279

Appendix A 







Appendix B 




Appendix C 




Appendix D 





Team 





AIR TREATMENT FACILITY 

TECHNICAL MEMORANDUM 

FINAL 



Air Treatment Facility Technical Memorandum - FINAL 


TABLE OF CONTENTS 

Executive Summary .............................................................................................................................. 3 

1. Introduction .................................................................................................................................. 4 

2. Field Testing ................................................................................................................................. 5 

3. Modeling .................................................................................................................................... 13 

4. ATF Locations Recommendation ............................................................................................... 14 

5. Determine Air Flows .................................................................................................................. 23 

6. Air Emissions Characterization .................................................................................................. 24 

7. Identify Air Emission Control Technologies.............................................................................. 28 

8. Recommendations ...................................................................................................................... 44 

LIST OF FIGURES 

Figure 1 – Base Map ............................................................................................................................. 4 

Figure 2 - Baseline Pressure ................................................................................................................. 6 

Figure 3 - Sampling with Air Return Lines Plugged .............................................................................. 7 

Figure 4 - Sampling with Stop Logs Removed ...................................................................................... 8 

Figure 5 – Odor Complaint Locations and Predicted Odor Areas of Concern .................................... 13 

Figure 6 - Additional Air Pickup ........................................................................................................... 17 

Figure 7 - NEIS/ECIS Recommendation Summary ............................................................................ 19 

Figure 8 - Detailed Recommendations Descriptions (1) ..................................................................... 20 

Figure 9 - Detailed Recommendations Descriptions (2) ..................................................................... 20 

Figure 10 - OdaLog Continuous H2S Data Logging Results .............................................................. 26 

Figure 11 - Wet Scrubbing Process Schematic .................................................................................. 32 

Figure 12 - Biotrickling Filter Process Schematic ............................................................................... 36 

Figure 13 - Dual Bed Activated Carbon Process Schematic .............................................................. 39 

LIST OF TABLES 

Table 1 - Mission & Jesse Interceptor Raw Emissions Characterization Results ............................... 25 

Table 2 - Wet Scrubbing Emissions Control: Advantages & Disadvantages ..................................... 33 

Table 3 - Biotrickling Filter Emissions Control: Advantages and Disadvantages ............................. 38 

Table 4 - Activated Carbon Emissions Control: Advantages & Disadvantages ................................ 42 

Table 5 - Preliminary ATF Attributes ................................................................................................. 45 




Executive Summary 




improve odor control in several areas identified by local community groups. Air treatment facilities 

(ATFs) with known or predicted odor “areas of concern” were included in the 2004 Collection 


The Technical Memorandum describes the results of the analysis of the data collected for the ATF 

Review Study and includes 

 

 

 

 

 

 

A summary of the field test conducted for the study, including the siphons and drop 

structures 

The results of the airflow modeling for current conditions and future scenarios 

Recommendations for an ATF location, drop structure modifications and flow diversion 

based on the results of observations and analysis following the field data collection 

Characterization data from five of the seven locations identified in the 2004 Collection 

System Settlement Agreement 

Various available technologies for controlling nuisance related odors and air emissions that 

are associated with wastewater collection systems 

Recommended technology for the ATF at Mission and Jesse. 




1. Introduction 

A study of air flow in the City's wastewater collection system was conducted between January 2008 

and August 2010. This Technical Memorandum presents the results of the study on Air Treatment 

Facilities (ATFs) and provides recommendations for locations, size and technology for ATFs in 

order to provide satisfactory odor relief to the collection system. Figure 1 below shows the system 

under study. It includes NEIS, NOS, ECIS, NORS and NCOS starting at the Division drop structure 

downstream to the 405 freeway. 

Figure 1 – Base Map 




2. Field Testing 

Large sets of data were collected between July 2009 and May 2010 to measure internal differential 

air pressure in the interceptors (difference in air pressure between inside and outside the sewer) and 

evaluate sewer headspace ventilation dynamics. These data have been used to determine the 

locations in the interceptor system where high positive air pressures cause odorous air to be released 

to the atmosphere and, therefore where odorous air should be withdrawn and treated. The data 

collection in the field included sewer siphons and drop structures. 

Field tests at drop structures were conducted to evaluate the impact of drop structures on the sewer 

differential air pressures, under current conditions and planned modifications. The effort was 

focused on measuring and examining differential air pressures at the four existing sewer drop 

structures in the City's wastewater collection system: 

 

 

 

 

Division 

Humboldt 




pressures along a route starting at the Division drop structure and ending at a maintenance hole in the 

Culver City Park. This was followed by the physical plugging of the return air lines at each drop 

structure in order to measure the impact on the differential air pressures at the drop structure and 

downstream sewer. The purpose was to examine the impact of perceived air circulation in each 

return line at the structure upon upstream and downstream differential pressures in the sewer system. 

Finally, flow management (diversion/redistribution to increase/decrease the flow at the drop 

structures) was introduced to the system and more differential air pressure data was collected to 

valuate the effect on the differential air pressures at the drop structures and sewer system. During 

theses three different phases, interim air scrubbers at Mission & Jesse and 23 rd & San Pedro were 

turned on and off several times in order to study their impact on the air pressure. 

Figures 2 to 4 show the results of the field pressure measurements under the above-described 

scenarios 




Figure 2 - Baseline Pressure 

Figure 2 shows the results of the baseline field pressure measurements of the drop structure sampling 

for the period between 2/21/10 and 2/27/10. The three values shown for each maintenance hole are 

the measured Minimum, Average, and Maximum differential air pressures over the data collection 

period. During baseline measurements, the interim air scrubbers were turned off and the air return 

line in the drop structures were open (not plugged). 




Figure 3 - Sampling with Air Return Lines Plugged 

Figure 3 shows the results of the field pressure measurements of the drop structure sampling when 

the air return lines were all plugged. The period of measurement is between 3/31/10 and 4/1/10. The 

three values shown for each maintenance hole are the measured Minimum, Average, and Maximum 

differential air pressures over the data collection period. Data shows that when the air return lines 

are all plugged, there is a measureable increase in the differential air pressure in the NEIS 

accumulating at the Mission and Jesse drop structure. 




Figure 4 - Sampling with Stop Logs Removed 

Figure 4 shows the results of the field pressure measurements for the drop structure sampling when 

the air return lines were all plugged and the stop logs removed at Humboldt and Mission & Jesse 

drops. The period of measurement is between 4/20/10 and 4/27/10. The removal of the stop logs 

resulted in diverting more flow into the Mission & Jesse drop structure. The three values shown for 

each maintenance hole are the measured Minimum, Average, and Maximum differential air 

pressures over the data collection period. Data shows that when the air return lines are all plugged 

and the stop logs removed, there is a measurable increase in air pressure in the NEIS accumulating at 

the Mission and Jesse drop structure. 

General Conclusions 

The following are the general conclusions that may be drawn from analyzing pressure data recorded 

during the drop structure field study with the various system modifications described above. 

 

The existing interim air scrubbers generally reduce pressures along the entire length of the 

NEIS/ECIS tunnel. This appears to be especially the case with the Mission and Jesse interim 




scrubber. Measurable pressure reductions occur in the NEIS/ECIS tunnel when this interim 

scrubber is turned on. Measurable pressure increases occurred in the NEIS/ECIS tunnel 

system when the Mission and Jesse interim scrubber was turned off. 

 

 

 

The Humboldt interim scrubber only appears to affect pressures in its immediate vicinity 

Plugging the air return line plugs generally resulted in increasing pressures along the entire 

NEIS/ECIS tunnel alignment 

The stop logs removal decreasing the flow at Humboldt and increasing it at Mission & Jesse 

generally resulted in decreasing pressures on the NOS. Those locations where the pressures 

didn’t decrease remained at atmospheric levels. 

The following provides a summary of site-specific pressure responses to the various manipulations 

performed during the study. For more detail on the effects of these tests, the reader is directed to the 

TM created for this project titled, “Differential Air Pressure Study at Drop Structures”. 

 

Division Drop Structure & Vicinity 

o 

o 

Measurable reductions in air pressure were recorded on the NEIS downstream of 

this drop structure when the Humboldt interim scrubber was in operation. 

Once the plug was in place at the Division Drop Structure, the air pressures on the 

ERIS just upstream of the drop structure became too negative for the pressure 

recording instruments to properly record. This meant that the differential pressures 

were less than -2.0 inches water column, which is the recording limit of the 

instruments. 

 

Humboldt Drop Structure & Vicinity 

o 

o 

Measurable reductions in air pressure were recorded on the upstream of the 

Humboldt Drop Structure on the NEIS and at the Humboldt drop structure itself 

when the Humboldt interim scrubber was in operation. A slight reduction was 

recorded on the NOS maintenance hole at Humboldt when the Humboldt interim 

scrubber was turned on. The air pressure on the NOS at the Humboldt diversion was 

generally well below or right at atmospheric during the entire duration of the field 

study. 

The most favorable pressure conditions (i.e., lowest air pressures) were achieved at 

the Humboldt Drop Structure location (not in the tunnel) when the plugs were in 

place in all four drop structure air return lines. Also, removing the stop logs at the 

Humboldt and Mission and Jesse NOS diversions did not cause significant air 

pressure increases at this location (decreasing the flow at Humboldt and increasing it 

at Mission & Jesse). 




Richmond Drop Structure 

o 

Due to the fact that there is no flow entering the Richmond Drop Structure, no 

pressure testing was performed at or near this drop structure. 

 

Mission and Jesse Drop Structure & Vicinity 

o 

o 

o 

o 

Measurable reductions in air pressure were recorded on the NEIS, ECIS and at the 

Mission and Jesse drop structure when the Mission and Jesse interim scrubber was 

in operation. 

The lowest air pressure conditions were achieved at the Mission and Jesse Drop 

Structure itself when the plugs were in place in all four drop structure air return lines 

and the NOS stop logs had been removed at both this drop structure and the 

Humboldt location (decreasing the flow at Humboldt and increasing it at Mission & 

Jesse). This resulted in more flow, and consequently more air, being directed 

through the Mission and Jesse Drop Structure than was the case during the baseline 

period. 

The pressures on the 2 maintenance holes on the NEIS immediately upstream and 

downstream of the drop structure increased as a result of the presence of plugs in the 

four drop structure air return lines. The pressure at the maintenance hole 

immediately downstream of this location also increased during the entire duration of 

the stop log removal test. The pressure at the maintenance hole immediately 

upstream of this location generally decreased during the stop log removal test 

(decreasing the flow at Humboldt and increasing it at Mission & Jesse). 

The air pressure on the NOS at the Mission and Jesse diversion increased 

significantly immediately upon insertion of the plugs in the Division and Humboldt 

drop structure air return lines. The pressure on the NOS decreased significantly 

during the Mission and Jesse plug insertion and the stop log removal period of this 

study. 

 

23 rd and San Pedro Drop Structure and Vicinity 

o 

o 

Measurable reductions in air pressure were recorded on the ECIS and at the 23 rd and 

San Pedro drop structure when the Mission and Jesse interim scrubber was in 

operation. The pressures on the NOS approach sewer at this location were generally 

near atmospheric for the majority of the entire drop structure testing period. 

The lowest air pressure conditions were achieved at this location when the plugs 

were in place in all four drop structure air return lines and the NOS stop logs had 

been removed at both the Mission and Jesse and the Humboldt locations. 




o 

The pressures on the 2 maintenance holes on ECIS immediately upstream and 

downstream of this location increased as a result of the presence of plugs in the four 

drop structure air return lines. The pressures at these maintenance holes generally 

decreased during the stop log removal test. 

 

USC Drop Structure and Vicinity 

o 

o 

Measurable reductions in air pressure were recorded on the ECIS immediately upstream 

and downstream of the USC drop structure when the Mission and Jesse interim scrubber 

was in operation. The air pressures at the drop structure did not appear to be 

significantly affected by any system-wide manipulations to the interim scrubbers, drop 

structure air return lines or interceptor stop logs. 

Compared to upstream locations along the ECIS and NEIS, there were few changes to 

overall pressures immediately upstream or downstream of the USC drop structure on the 

ECIS as a result of the stop log manipulations or air return line plug insertions. 

 

Systemwide Downstream Interceptors 

o 

o 

o 

o 

o 

When the Mission and Jesse interim scrubber was in operation, reductions in air 

pressure were recorded at each of the three maintenance holes tested in this vicinity. 

The most notable example was at the ECIS location upstream of the NORS/ECIS 

junction. 

The lowest air pressure conditions were achieved while the stop logs were removed 

(decreasing the flow at Humboldt and increasing it at Mission & Jesse) at the 

Humboldt and Mission and Jesse diversions to the NOS. 

Upstream of Jefferson Siphon (ECIS): The air pressures at the drop structure did not 

appear to be significantly affected by the air return line plug insertions at the four 

drop structures. During the time period that the plugs were in place and the stop 

logs were removed from Humboldt and Mission and Jesse the air pressure generally 

decreased at this location and remained below atmospheric. Once the plugs were 

removed and the stop logs were put back into place, the pressure began to fluctuate 

more wildly and went above atmospheric frequently. 

NORS/ECIS Junction Upstream (ECIS): The air pressures generally increased when 

the plugs and stop logs were in place. Once the stop logs were removed the 

exhibited a general downward trend. Following plug removal and stop log reinsertion 

the pressures went to near atmospheric. 

NORS/ECIS Junction Upstream (NORS): The pressures in the NORS maintenance 

hole immediately upstream of the NORS/ECIS junction increased slightly as a result 

of the presence of plugs in the four drop structure air return lines. The pressures at 




these maintenance holes generally decreased and fluctuated more significantly 

during the stop log removal test. Following plug removal and stop log re-insertion 

the pressures went below atmospheric. 




3. Modeling 

An Airflow Modeling application program was developed to model the airflow in the system under 

study. The modeling included the City’s current interceptor system configurations and the two 

planned wastewater flow diversion scenarios, thus evaluating the impact of the identified future 

operational changes by pointing to locations with potential for positive pressure. Such locations can 

result in odor issues and were termed “odor areas of concern”. The airflow modeling results were 

intended to assist in the evaluation of the proposed ATFs and any recommendations for ATF 

installation. 

Based on the airflow modeling results and under the current system configurations, nine areas of 

concern are predicted. These include five locations along the ECIS from downstream of the Mission 

and Jesse drop structure to upstream of the USC drop structure; the ECIS siphon at Jefferson 

Boulevard and La Cienega Boulevard; the NORS/ECIS Junction; the NCOS Siphon at I-405 and the 

NORS Siphon at I-405. The results of the modeling are depicted in Figure 5 below. 

Figure 5 – Odor Complaint Locations and Predicted Odor Areas of Concern 




4. ATF Locations Recommendation 

As part of the 2004 Collection System Settlement Agreement (CSSA), the ATF Review Study was 

initiated to study the airflow in the City’s wastewater collection system. The following 

recommendations are put forth based on conclusions drawn from the results of the ATF Review 

Study. The recommendations are divided among the various locations and sections of the 

NEIS/ECIS interceptor that were studied during the various phases of the overall project. Within 

each section specific recommendations on forced air treatment systems, modifications to drop 

structures and flow diversions are presented. 

 

Division Drop Structure & Vicinity 

o 

o 

This drop structure drops the flow from ERIS upstream to NEIS downstream 

ATF: The drop at the Division Drop structure does not appear to result in a significant 

air back-pressurization of the ERIS approach sewer; therefore no forced air treatment 

system is recommended for this location. 

o Drop Structure Modifications: The pressures in the ERIS upstream approach sewer 

were reduced significantly as a result of plugging the air return line at the Division Drop 

Structure. This indicates that plugging the air line at the Division drop structure is 

beneficial. However, given the unknown impact of future flow conditions, the 

permanent plugging of the air return line is not recommended. Therefore, in order to 

provide control and flexibility, there is evidence that installation of a flow regulation 

device in the air return line of this drop structure, such as an adjustable damper 

may be beneficial. However, it is recommended to complete the drop structure model 

testing before finalizing the damper concept. This would maintain negative pressures on 

the ERIS. Such a device would give BOS the flexibility to adjust the extent of plugging 

(amount of air that is allowed to flow through the air return line) under current or future 

operational conditions. 

o 

Flow Diversion: Since the ERIS is the only sewer that flows into and terminates at this 

drop structure, flow diversion recommendations are not applicable at this location. 

 

Humboldt Drop Structure & Vicinity 

o 

o 

This drop structure drops the flow from NOS upstream to NEIS downstream 

ATF: The interim scrubber at this location only appeared to significantly affect the air 

pressures in the NEIS between the Division Drop Structure and the Humboldt Drop 

Structure. Also it did not appear to significantly reduce the pressures in the NOS, which 

were near or just below atmospheric levels throughout the duration of the study; 

therefore no forced air treatment system is recommended for this location. 




o 

o 

Drop Structure Modifications: The pressures in the NOS upstream approach sewer were 

reduced slightly as a result of plugging the air return line at the Humboldt Drop 

Structure. There is evidence that installation of a flow regulation device in the air 

return line of this drop structure, such as an adjustable damper may be beneficial. 

However, it is recommended to complete the drop structure model testing before 

finalizing the damper concept. As with the Division Drop Structure, such a device 

would give BOS the flexibility to adjust the amount of air that is allowed to flow through 

the air return line under current or future operational conditions. 

Flow Diversion: The system air pressures appeared to react favorably to the stop log 

removal at this location; which allowed approach flows in the NOS to split naturally 

between that sewer and the NEIS. This resulted in less flow through the Humboldt drop 

structure into the NEIS. It is therefore recommended that the stop logs be 

configured at this location in such a manner that a minimum amount of flow is 

directed into the NEIS via Humboldt. This would likely result in less air being 

dragged into the NEIS and therefore, lower air pressures in the NEIS/ECIS tunnel 

system. 

 

Richmond Drop Structure 

o 

o 

o 

ATF: Since there is no shallow approach sewer discharging flows into this drop 

structure, the structure cannot serve as a pathway for air backpressurization into such a 

sewer; therefore no forced air treatment system is recommended for this location. 

Drop Structure Modifications: Although there is no shallow sewer discharging flows 

into this drop structure, there is evidence that installation of a flow regulation device in 

the air return line of this drop structure, such as an adjustable damper may be 

beneficial. However, it is recommended to complete the drop structure model testing 

before finalizing the damper concept. Such a device would be available for future air 

backpressurization adjustments as a contingency if a shallow approach sewer is 

connected to this drop structure in the future. 

Flow Diversion: Because there is no shallow approach sewer currently discharging 

flows into the Richmond Drop Structure, flow diversion recommendations for this 

location are not applicable at this time. 

 

Mission and Jesse Drop Structure & Vicinity 

o 

o 

This drop structure drops the flow from NOS upstream to NEIS downstream 

ATF: The interim scrubber at this location appeared to reduce the air pressures along 

the majority of the NEIS and ECIS alignment. Also, this drop structure appeared to be 

the primary conduit through which the NEIS/ECIS tunnel relieved its air pressure back 




into the NOS upstream. A permanent ATF is therefore recommended for this 

location if the structure modifications and flow adjustment do not work. 

o 

Drop Structure Modifications: During the time the air return lines at Division and 

Humboldt were plugged, the pressures in the NOS upstream approach sewer at Mission 

and Jesse increased significantly. Once the Mission and Jesse air return line was also 

plugged, the pressure along the NOS at this location began to reduce significantly. There 

is evidence that installation of a flow regulation device in the air return line of this 

drop structure, such as an adjustable damper may be beneficial. However, it is 

recommended to complete the drop structure model testing before finalizing the damper 

concept. As with the Division and Humboldt Drop Structures, such a device would give 

BOS the flexibility to adjust the amount of air that is allowed to flow through the air 

return line under current or future operational conditions and prevent air from 

backpressuring the NOS approach sewer via this location. 

 

 

Additional modifications to the Mission and Jesse Drop Structure should include installing 

two curtains or similar flexible devices. One curtain should be installed at the downstream end 

of the plunge pool; the other curtain should be installed on the NEIS just upstream of the 

junction with the Mission and Jesse drop structure as shown in Figure 6. 

An additional air pickup should also be designed and constructed for the recommended 

Mission and Jesse ATF. This air pickup would be located along the NEIS tunnel just upstream 

of the recommended NEIS curtain. This additional air pickup is shown in Figure 6. 




Figure 6 - Additional Air Pickup 

The purpose of the curtain on the NOS would be to isolate the common headspace between the drop 

structure and the ECIS tunnel. This would allow the recommended ATF at this location to fully 

capture the air that would be carried with the NOS flows into the drop structure, thereby reducing the 

amount of air dragged into the ECIS tunnel and helping to maintain reduced pressures along the 

NOS approach sewer to Mission and Jesse. 

The recommended curtain on the NEIS would serve to isolate the air dragged by the NEIS from the 

Mission and Jesse Drop Structure and ECIS headspaces. An additional air pickup point on the 

upstream side of the NEIS curtain would direct odorous air from the NEIS to the recommended ATF 

for this location. 

 

Flow Diversion: The system air pressures appeared to react favorably to the stop log removal at 

this location; which allowed approach flows in the NOS to split naturally between that sewer and 

the NEIS. This resulted in more flow being directed into the ECIS via the Mission and Jesse 

Drop Structure. It is therefore recommended that the stop logs be configured at this location 

in such a manner that a maximum amount of flow is directed into the ECIS via Mission 

and Jesse. This would likely result in more air being dragged into the drop structure and 

therefore, lower air pressures in the NOS approach sewer and the NEIS/ECIS tunnel system. 




This air would be isolated in the drop structure due to the recommended damper and curtain 

devices and captured by the recommended ATF. 

 

23 rd and San Pedro Drop Structure and Vicinity 

o 

o 

o 

o 

This drop structure drops the flow from NOS upstream to ECIS downstream 

ATF: Following the removal of the interim scrubber at this location, odor complaints 

have been significantly reduced in the near vicinity. Also, pressure testing conducted 

during the drop structure study indicated that this structure is not a significant source of 

backpressurization into the shallow NOS approach sewer; therefore no forced air 

treatment system is recommended for this location. 

Drop Structure Modifications: The pressures in the shallow NOS approach sewer were 

reduced slightly as a result of plugging the air return line at the 23 rd and San Pedro Drop 

Structure. There is evidence that installation of a flow regulation device in the air 

return line of this drop structure, such as an adjustable damper may be beneficial. 

However, it is recommended to complete the drop structure model testing before 

finalizing the damper concept. As with the Division, Humboldt and Mission and Jesse 

drop structures, such a device would give BOS the flexibility to adjust the amount of air 

that is allowed to flow through the air return line under current or future operational 

conditions. 

Flow Diversion: Flows should be allowed to split naturally between the NOS and the 

approach sewer to the 23 rd and San Pedro Drop Structure in the upstream diversion 

structure 

 

USC Drop Structure and Vicinity 

o 

No modifications or operational recommendations are necessary at this drop 

structure. 

An overall graphic summary of the recommendations for the NEIS/ECIS drop structure locations 

between the Division Drop Structure and the ECIS Jefferson Siphon is presented in Figure 7. 

Detailed descriptions of the rationale for each site recommendation are presented graphically in 

Figures 8 and 9. 




Figure 7 - NEIS/ECIS Recommendation Summary 




Figure 8 - Detailed Recommendations Descriptions (1) 

Figure 9 - Detailed Recommendations Descriptions (2) 




These above recommendations must be revisited following the completion of the pending physical 

model testing. Additionally the effects of the recommendations should be verified with a full scale 

fan test field study aimed at verifying the effect of the combination of the adjustable air plugging 

devices and the recommended ATF at Mission and Jesse. 

Since there is evidence that the source of pressure in the shallow approach sewers emanates from the 

NEIS/ECIS tunnel system, adjustable air plugging devices in the air return lines could provide a 

beneficial effect to control the back pressurization effect from the tunnel to the shallow approach 

sewers, i.e. NOS. 

Since the purpose is to reduce the backpressurization effect into the shallow approach and given the 

fact that the system is dynamic and will change in the future, the City may want to have the 

flexibility to adjust the air flow through the return air line using adjustable air plugging devices. 

There was evidence in the approach sewers at Division and Mission & Jesse of a downward trend of 

air pressure once the return lines were plugged at Division and Mission & Jesse. However given the 

fact that the plugging of the air return lines at the four drop structures increased the pressure in the 

NEIS/ECIS tunnel, it is recommended that the City conduct a full scale fan test to verify the sizing of 

an ATF at Mission and Jesse in order to reduce the amount of positive pressure in the tunnel and to 

verify the backpressurization reduction effects of adjustable air plugging devices. 

 

Systemwide Downstream Interceptors 

The City is in the process of initiating several odor control measures downstream of the 

ECIS Jefferson Siphon. These include the startup of a 20,000 cfm ATF at Jefferson and La 

Cienega and the installation of headspace isolation curtains at Diversion Structures 1, 2 and 

3 in the Baldwin Hills/Culver City area. The purpose of the ATF is to ventilate the ECIS 

both upstream and downstream of the Jefferson Siphon. The purpose of the curtains is to 

isolate the NORS headspace from the interceptors that discharge flows into it through the 

three diversion structures. In addition to this, an interim scrubber is currently ventilating the 

NORS and the ECIS at the NORS/ECIS junction. Furthermore, flows have been diverted 

away from the NORS towards the NOS in recent months following the completion of the 

NOS rehab in the Culver City area. It is generally believed that these flow diversions will 

relieve the significantly high air pressures that have been recorded in the NORS during NOS 

rehab. 

Given these numerous odor control-related measures, the following recommendations are 

put forth for the NORS: 

o 

The City should conduct differential air pressure testing along the NORS to 

determine the effects of the flow diversions, curtain installations and effect of the 20,000 

cfm ECIS Jefferson Siphon ATF. This data should be compared to pressure data taken 

on the NORS before these measures were put into place, specifically that data which was 

recorded during Spring/Summer, 2009 as part of this study. 




If the planned odor control measures do not result in satisfactory pressure reductions 

along the NORS, the City should initiate a fan test along the NORS between the siphon 

at Fox Hills Mall and Culver City Park. The fan should be configured to withdraw air 

from a maintenance hole just upstream of the siphon while differential air pressure data 

loggers are recording data in the upstream reaches of the NORS. If this proves successful at 

reducing pressures to acceptable levels, an ATF should be designed and constructed just 

upstream of the siphon. 




5. Determine Air Flows 

It is premature to estimate the effective air flow that would be necessary to capture, contain and treat 

the odors and emissions at the Mission and Jesse location. Follow-on work on a simulated drop 

structure at the City’s hydraulics laboratory will provide additional insight into the patterns of air 

flow and the flow rates for a similar, but not identical arrangement to the Mission and Jesse drop 

structure. To pinpoint the air extraction flows that will be required to achieve the desired impact at 

the Mission and Jesse location and beyond, a fan test is warranted. 

Simulating actual conditions in the field and setting up a monitoring program to document the extent 

of the impact resulting from extracting air from the interceptor will verify the flow rate requirements, 

and reduce any second guessing or speculation on what the air extraction flow rate should be. In the 

interim it is projected that a 20,000 cfm flow rate would be required at this recommended Mission 

and Jesse ATF. 




6. Air Emissions Characterization 

To obtain emissions characterization data from five of the seven locations identified in the 2004 

Collection System Settlement Agreement (CSSA) that the City entered into with the USEPA, Santa 

Monica Baykeeper, City of El Segundo, and others, a monitoring plan was developed. The 

remaining two locations were not included in this characterization plan since the City determined to 

move ahead with constructing ATFs at these sites. Those ATFs have been constructed, have passed 

their performance tests, and are about to be commissioned. The monitoring plan included sampling 

at: 

North East Interceptor Sewer (NEIS) – Richmond 

North Outfall Relief Sewer (NORS) – NORS – ECIS 

NEIS – Humboldt 

East Central Interceptor Sewer (ECIS) – Mission & Jesse 

ECIS – 23 rd Street & San Pedro 

The samples collected at these five locations were analyzed for the following parameters: 

Hydrogen sulfide (H 2 S) 

TNMHC 



Hydrogen sulfide was analyzed in the field with two devices: a hand-held H 2 S monitor (Jerome 

meter) was used to measure untreated interceptor air samples collected in Tedlar sample bags; and a 

continuous H 2 S data logging device (OdaLog) was inserted in a manhole location at each site to 

represent the untreated air emissions from the interceptors. The remaining parameters were collected 

according to strict sampling protocol requirements in various containers (Summa canisters and 

Tedlar® bags and shipped by courier to a local laboratory for analysis. Whereas most of these 

parameters were collected as grabs or over a limited short duration time frame (1-hour), the OdaLog 

continues logging H 2 S monitors was left in place for several days to record the diurnal variation of 

H 2 S concentrations in the interceptor. 

The following analytical methods were used to determine the concentrations of the various specific 

compounds and classes of compounds: 

 

 

 

TNMHC 

o SCAQMD Method 25.3 

Speciated volatile organic compounds 

o EPA Method TO – 15 

o EPA Method TO – 12 Modified 


o ASTM 5504 

These methods were carefully selected to identify those compounds that together made up the 

TNMHC value, and to ensure that the two approaches reached a similar endpoint. The speciated 

compounds were also examined to assess whether any of these particular compounds indentified 




should not be classified as a VOC based on their inability to be converted by sunlight into ozone, or 

the pronouncement by USEPA that they were not photochemically reactive and therefore 

dropped/not included on USEPA’s list of VOC’s. 

For more details on the sample collection, the analytical procedures, and the analytical results the 

reader is referred to the Total Non-Methane Hydrocarbon Monitoring Results Technical 

Memorandum (TM) submitted in draft to the City in June 2010. The final version of that TM will be 

submitted as part of the final project report. 

The analytical results for the samples collected from the manhole at Mission and Jesse where the 

interceptor odorous air is extracted and forwarded to the interim odor control system (consisting of 

activated carbon) are presented in Table 1. Table 1 includes those analytical results that are directly 

related to permit limits. For information on speciated organic compounds and reduced sulfur 

compounds refer to the TNMHC TM. [As a point of reference, on the day that the samples were 

collected at Mission and Jesse the odor control system was not operating. The activated carbon fan 

was shut off to allow for removing the spent carbon from the vessel and replacing it with virgin 

carbon.] 

The results of the OdaLog, continuous H 2 S monitoring from the interceptor airspace is presented in 

Figure 10. 

Table 1 - Mission & Jesse Interceptor Raw Emissions Characterization Results 

Parameter 

Mission & Jesse Analytical Results 

TNMHC – Methods 25.3 (ppmC) 1 14.0 

Non-VOCs (ppmC) 1 0.47 

Adjusted Method 25.3(ppmC) 1 13.53 

Hydrogen Sulfide (ppm) 

Grab Sample >50 2 

OdaLog Unit 4 

• Minimum 

Figure 10 - OdaLog Continuous H2S Data Logging Results 




The unadjusted TNMHC results and the adjusted results, 14.0 and 13.53 ppmC, respectively, are 

below the range of current percent limits for operating odor control systems of 18.0 to 36.0 ppmC. 

Accordingly, these uncontrolled results are lower than the potential discharge/exhaust levels even 

before any control system is used. Based on previous monitoring results from several years ago, the 

City was concerned that the TNMHC results here would be higher. With the presence of an air 

treatment system, it is expect that the concentrations will be even lower than these uncontrolled 

values. 

For H 2 S, the grab sample results, >50 ppm, compares well to the concentrations recorded by the 

OdaLog unit, squarely between the average concentrations of 16.5 ppm and the maximum 

concentration of 87.0 ppm. The H 2 S concentration from Mission and Jesse were the second highest 

recorded among the five locations that were monitored during this task. The concentrations recorded 

at 23 rd Street and San Pedro were higher, with an average of 34.5 ppm and a maximum value of 

greater than 100 ppm. Nevertheless, the concentrations recorded at Mission and Jesse are elevated 

enough to be of concern to the City and demonstrate a need for the selection of a consistent, 

permanent and effective H 2 S control system. 




7. Identify Air Emission Control Technologies 

Many different technologies are available for controlling nuisance related odors and air emissions 

that are associated with wastewater collection systems. Each technology has a niche when it is most 

suitable to provide the level of control required to reduce off-site odor impacts. The degree of 

control required at any one site is dependent on several factors, including, but not limited to: physical 

setting; metrological conditions, topography of the land surrounding the odor control site; proximity 

of the nearest neighbors and/or nearest sensitive receptors; permit requirements established by South 

Coast Air Quality Management District (SCAQMD); and unobstructed view of the air treatment 

facility (ATF). These factors will need to be considered when evaluating the many different control 

technologies. The challenge is identifying the most cost effective, viable control technology that will 

achieve the goal of the City of Los Angeles (City) and comply with the limits established in the air 

permit for each site. 

Existing odor control systems and ATFs that are operating at various locations across the City’s 

interceptor network systems have air permits issued by SCAQMD. All the air permits include a 

discharge/exhaust limit for hydrogen sulfide ranging from 0.6 to 1.0 part per million by volume in air 

(ppm). Some of the permits, but not all also have discharge/exhaust limits for total non-methane 

hydrocarbons (TNMHC), but the limits vary from 18 to 36 ppmC. Although the City is ultimately 

concerned about odors, they must also address TNMHC emissions whose limits are based on human 

health affects and/or the total accumulation of volatile organic compounds (VOCs) in the 

atmosphere. The City and the surrounding air basin are in a non-attainment area for Federal 

(USEPA) ozone ambient air quality standards and reduce the levels of ozone being emitted by 

restricting ozone processors like VOCs/ TNMHCs. It is fully anticipated that any air emissions 

control system and/or ATF that will be designed and constructed moving forward from today will 

continue to include H 2 S and TNMHC limits. Accordingly, the technology, or combination of 

technologies, that will be constructed at any site must have the ability to remove both constituents to 

comply with the permit limits. The existing permit to operate the interim odor control system at 

Mission & Jesse has a discharge/exhaust H 2 S limit of 1.0 ppm and no limit for TNMHC. 

The City’s goal for sites that require odor control systems and/or ATF’s are: 

 

 

 

 

 

Comply with all regulatory requirements 

Keep collection system nuisance odors from impacting residents, drivers and visitors. 

Capture and treat the odors to eliminate fugitive nuisance odor releases to the open 

atmosphere 

Reduce odor complaints from the surrounding neighborhoods 

Be a good neighbor. 

Understanding the goals, needs and requirements for odor and emissions control at a site helps to 

establish the initial framework for the identification of viable control alternatives. Characterization 

of the emissions at the interceptor area of concern area(s) is the final critical piece of background 

information necessary to move forward with selecting the right air emissions control technology. 




The prime role of the air control technologies is to address the moderately high concentrations of 

H 2 S in the Mission and Jesse inlet air stream. The technology must be capable of consistently 

reducing the average (16.5 ppm) and the maximum (87.0 ppm) concentration of H 2 S to less than 1 

ppm. Accordingly, a technology on its own or in combination must be capable of achieving a 99% 

reduction of inlet H 2 S concentration. At a removal efficiency of 99% for these monitored H 2 S 

concentration, the City will be able to comply with the anticipated SCAQMD 1 ppm air permit 

discharge/exhaust limit, with a safety factor. Keep in mind that the SCAQMD air permit is based on 

a one hour standard. Referring to Figure 6-1, the maximum concentration of 87.0 ppm was a one 

time, short term (less than an hour) recorded value. No sustained, high H 2 S concentrations were 

observed for the four days that the OdaLog was in place monitoring the airspace of the maintenance 

hole on the interceptor at the Mission and Jesse location. The HDR Team recommends that the City 

continue to monitor the H 2 S concentrations at the Mission and Jesse location to verify the readings 

recorded during this TNMHC study and to ensure that the technology to be constructed here is 

capable of complying with the anticipated SCAQMD discharge/exhaust limit of 1.0 ppm of H 2 S. 

The most commonly applied air control technologies used today include: 

 

 

 

 

 

 

 

Wet scrubbing 

Biofilter 

Biotrickling 

Bioscrubbing 


Ionization 

Dilutions/Entrainment 

 

Combinations 

An initial screening of these technologies can eliminate several that are not viable for the conditions 

presented above. In addition to the ability of the air control technology to remove H 2 S, odors and 

some degree of TNMHC, selection of the control system must consider the conditions under which 

the system will operate, including but not limited to: 

 

 

 

 

 

Remote location of the installation 

Unmanned operation of the site 

Situated in the community where safety is of paramount importance 

Access to utilities (water, electrical service) 

The site has no natural sight barriers to prevent the facilities from being visible from the 

street and beyond 




What is the footprint of any technology or combination being considered and is their 

enough space at the site to support it 

With the railroad so close at grade, are there any special concerns, permits or approvals 

that may be required before moving forward with construction. 

Will access to the site be restricted, and/or will there be enough space to easily operate 

and maintain the facilities 

 

Does the air treatment systems need to be placed below grade in a building structure, or 

can it be constructed at grade without impacting the nearest residents 

The following technologies have been eliminated from further consideration for the reasons 

provided: 

 

 

 

Biofilter – Biofilters are better suited for conditions where H 2 S concentrations are lower 

than 87.0 ppm. In addition, the space required to erect a biofilter based technology would 

take a larger footprint than any of the other technologies listed. 

Bioscrubbing – Bioscrubbing is a technology that has not been embraced by the 

wastewater industry partly because of the increased operation and maintenance attention 

required of the separate aerated oxidation tank (biological reactor) where the recirculated 

mixed liquor solution is deposited and treatment occurs. Bioscrubbers are better suited to 

remove organic odors since the bioreactor supports heterotrophic microorganisms. 

Ionization – Ionization is a technology that has become more visible in the last few years. 

Although performance claims have been made by the manufacturers, very limited 

documented evidence of removal efficiency performance, for H 2 S and odors, are 

available. Recent inquiries for information by a research study sponsored by the Water 

Environment Research Foundation (WERF) which reviewed several thousand articles, 

reports and gray literature, received no response. As a result, undocumented technologies 

are not viable options for further consideration. 

 

Dilution/Entrainment – The solution to pollution is dilution does not work for wastewater 

discharge permits, but for controlling H 2 S odors and reducing off-site impacts it can. 

This technology becomes non-viable when specific compound discharge limits are in 

place (like H 2 S) and the exhaust concentrations exceed them. The potential for that 

condition to occur here exists. Therefore, this technology has also been eliminated. 

The remaining four technologies will be discussed in more detail below. 

(1) Wet Scrubbing 

The leading odor control technology over the last 25 years is wet scrubbing. It has proven that it 

can be an effective method for controlling traditional odors associated with wastewater 

operations. Although it has taken many forms over the years, such as cross-flow and atomizing 

mist, the vertical, packed-tower, counter-current systems have been the most popular. Newer 

designs that address concerns with the height of the system have tried to retain the best features 




in a more compact, low-profile version. Regardless, the fundamental principles of the wet 

scrubber remain the same. Wet scrubbing consists of a physical-chemical process that removes 

odorous parameters from the inlet air stream. A chemical scrubbant solution is distributed over a 

bed of plastic inert media. The most common make-up of the chemical scrubbant solution 

besides the base component of water is caustic and sodium hypochlorite. Other chemicals have 

been used, such as chlorine dioxide or hydrogen peroxide, but they have not shared the same 

level of success. 

The overriding principles of a wet scrubber are to bring the odorous air flow in direct contact 

with the chemical scrubbant solution. In achieving this contact, the odor causing compounds are 

removed from the air stream and the treated air is discharged from the exhaust stack into the 

atmosphere. The odorous compounds are removed from the air by absorption into the scrubbant 

solution. The key odorous constituent normally associated with wastewater operations is H 2 S. 

H 2 S is very soluble in high pH solutions. By raising the pH of the scrubbant solution by adding 

caustic, the H 2 S and other soluble odor compounds are absorbed into the liquid scrubbant 

solution. To maintain a strong mass transfer driving force, sodium hypochlorite is also added to 

the scrubbant solution. The sodium hypochlorite converts the absorbed H 2 S into sulfate and 

prevents it from being re-stripped from the liquid phase either in the scrubber, or after the liquid 

is drained from the scrubber and the pH drops as it combines with the lower pH interceptor 

collection system waters. 

To enhance the contact between the odorous air stream and the scrubbant solution, plastic media 

are used. The plastic media, which have taken on many different shapes, sizes and 

configurations, are designed to provide the maximum amount of surface area in the smallest 

volume while also achieving as little pressure drop as possible. The air is forced to pass through 

a torturous path of media whose surface is coated with the scrubbant solution. It is at this 

air/liquid interface that the contact and absorption (mass transfer) of the odors occurs. 

Figure 11 illustrates a single stage vertical, packed-tower, wet chemical scrubber. The odorous 

air is fed into the air plenum at the lower part of the tower just above the sump where the 

scrubbant solution ends up after it has passed through the plastic media. The air plenum is an 

open, unobstructed area of the scrubber just below the plastic media. The air plenum serves to 

reduce the velocity of the inlet air flow, equalize the air volume and force it upwards evenly over 

the cross-section of the scrubber, thereby utilizing the full plastic media bed. 




Figure 11 - Wet Scrubbing Process Schematic 

The scrubbant solution is recirculated through the wet scrubber and is distributed over the top of 

the plastic media bed through nozzles attached to a pipe network. To freshen the recirculating 

scrubbant solution, fresh water and chemical are constantly being fed, and an equal amount of 

solution is blown down to drain. The make-up water being fed is generally held at a constant 

rate. The chemicals are added on a demand basis, usually dictated by instrumentation that 

measures the pH, ORP and/or the concentration of H 2 S or chlorine in the exhaust. The fresh 

make-up water and chemical ensure an optimal mass transfer driving force for the odorous 

compounds to be absorbed into the liquid and ensure the effectiveness and efficiency of the odor 

control technology. 

Wet scrubbers are good at optimizing the removal of H 2 S under varying diurnal concentrations. 

The physical-chemical wet scrubber design allows it to quickly respond to changes in odor 

concentrations. Wet scrubbers are able to achieve 99% removal of H 2 S inlet concentrations. 

Manufacturers of wet scrubbers will guarantee a 99% removal efficiency of inlet H 2 S 

concentrations of 10 ppm v or greater. For concentrations less than 10 ppm v , they will only 

guarantee a low discharge concentration of 0.1 ppm v . 

The wet scrubber performance history for removing other reduced sulfur compounds is not 

consistently good. Depending on the specific reduced sulfur compound and its inlet 

concentration, documented removal efficiencies can range from 50 to 90+%. In situations where 




the predominant odorous compounds are reduced sulfur compounds other than H 2 S, wet 

scrubbing may not be the optimal odor control technology selection. 

Removal of VOC compounds in wet scrubbers is poor. Generally, only those highly soluble 

VOC compounds are removed in a wet scrubber, others pass through and out the exhaust. Note 

that wet scrubbers were intended to be odor control devices and most VOCs do not contribute to 

odor conditions (the odor detection threshold limit of VOC compounds are several orders of 

magnitude greater than H 2 S and reduced sulfur compounds). 

Table 2 presents a list of advantages and disadvantages of wet scrubbing as an emissions control 

technology. 

Table 2 - Wet Scrubbing Emissions Control: Advantages & Disadvantages 

Wet Scrubber 

Advantages 

Disadvantages 

Physical-Chemical process 

Uses hazardous chemical 

Small footprint 

Safety is an issue 

Automatic chemical feed 

Mechanically intensive 

Recirculates scrubbant solution 

Poor VOC removals 

Proven track record 

Continuous blowdown 

Thousands of installations 

Scale build-up 

Familiarity of mechanical system 

Removes CO 2 (sink for caustic) 

Effective H 2 S/odor removal 

More labor intensive 

Inherent capacity for variable loads Greater degree of O&M 

Can reduce chemical in off odor season Calibration needed on probes 

Single stack discharge 

More systems to monitor 

No acclimation time required 

Must take system out of service to acid wash 

Visible odor control device 

Chlorine odor in exhaust 

Less subject to upsets 

Waste chemical in blowdown 

Easy to monitor exhaust 

Hard water causes scale 

Hazardous chemical truck deliveries through 

streets 

Despite the ability of wet scrubbing to achieve the prime role of the air control technology of 

99% removal of H 2 S, and the small footprint it would require, other features make it a less 

favorable candidate for final consideration at the Mission and Jesse location, for example: 

 

 

Wet scrubbing is the most mechanically intensive technology, incorporating a fan, 

recirculating pump, and multiple chemical metering pumps, in addition to chemical 

storage tanks, and instrumentation and control systems. Each of these systems requires 

maintenance and servicing on a routine basis to ensure reliability and continued effective 

service. 

Wet scrubbing uses hazardous chemicals in sodium hypochlorite and sodium hydroxide 

to maintain operations. These hazardous chemicals delivered by tanker truck and stored 




on site in FRP tanks, represent a safety concern at a remote, unmanned location such as 

the Mission and Jesse site. Extra precautions are necessary to ensure the City staff and 

surrounding community are safe in the event of a spill, accident or premeditated act by 

outside forces. 

 

Wet scrubbing may occasionally require that the plastic media be acid washed to remove 

scale build-up, generally calcium carbonate based. The frequency of acid washing and 

the duration of this process will depend on how the system is operated. The higher the 

operating pH, the greater the chance that scaling will occur, and the more frequent that 

acid washing will be necessary. The duration will depend on the effectiveness of the 

previous acid wash and the degree of build-up since the last acid wash was performed. 

Acid washing demands that the wet scrubber be taken out of service. As a result, a 

redundant wet scrubber would be required for this technology option. 

Collection system odorous air streams normally contain elevated concentrations of CO 2 . 

CO 2, like H 2 S, consumes sodium hydroxide and can significantly increase the chemical 

demand to remove a compound that is neither an odor concern, nor a VOC. 

 

The Chemicals in the wet scrubbing systems blow-down may create a problem with 

disposal as the pH of that stream is generally greater than the allowable sewer discharge 

ordinance pH. 

Pursuant to these limitations associated with chemical safety, O&M, remote/unmanned site, 

scaling, extraneous compound chemical demand, blow-down disposal, and necessary 

redundancy requirements, wet scrubbing will not be considered further as a viable air control 

technology for the Mission and Jesse location. 

(2) Biotrickling Filter 

Treating odors using microorganisms is not a new technology. In fact, using microorganisms in 

a soil filter or a trickling filters dates back to the 1940’s, however, development of this 

technology did not emerge on the US market as a viable and marketable technology until the 

1990’s. Since then, biofilters achieved greater acceptance initially and biotrickling filters were 

slower to be recognized. Today both technologies, biofiltration and biotrickling filters are 

accepted technologies, are provided by vendors across the US, and have become the control 

technology of choice in the wastewater market area replacing the wet scrubbing alternatives 

because of their ability to achieve compliance with H 2 S and odor requirements without using 

hazardous chemicals and for a lower annual operating cost. 

Biotrickling filters consist of a containment vessel with some form of inorganic media to support 

microbial growth. That inorganic media may be a basic lava rock, plastic media or proprietary 

material. Initial research work with biotrickling filters demonstrated that standard landscape 

lava rock provided a rough and uneven surface filled with crevices and pores that supported the 

attachment and growth of a microbial slime layer that was not easily dislodged. Subsequently, 

non-uniform unit plastic media that is high-density but very light in weight when compared to 




lava rock was introduced and has proven to be extremely effective as a truly robust biotrickling 

filter media. This plastic media can be stacked in vertical stages to minimize the overall 

footprint of this technology. The media is resistant to plugging and many vendors will provide a 

guarantee that the media will last at least 10 years before needing replacement. When 

replacement is needed, the media can easily and readily be removed in one piece by lifting it out 

of the vessel and quickly inserting the new media. 

The typical biotrickling filter functions much like a conventional chemical packed-bed wet 

scrubber, with the exceptions that no hazardous chemical is used. (Refer to Figure 12). The 

odorous foul air enters the bottom of the vessel, just above the water-based solution that is used 

to recirculate over the media, and just below the supported media. The open space or plenum 

where the odorous foul air enters the biotrickling filter allows for a reduction in air flow velocity, 

a build-up of positive pressure, and even distribution of the foul air as it pushes upward and 

evenly across the full cross-sections of the media bed. Based on the positioning and 

extent/layering of this media, whether lava rock, plastic or some other proprietary blend, the 

odorous air is forced to go through a torturous path of media where it consistently comes in 

contact with the liquid film coating the outside surface area of the media. The media is kept 

moist by applying a nutrient rich water-based solution from a distribution network above the 

media. The air stream flows upward through the media and the water-based solution passes 

downward. This counter-current, air liquid flow maximizes the operation of the biotrickling 

filter. The downward flow of the water-based solution not only delivers the nutrients required by 

the microorganisms living and working in the liquid film layer on the media, but also acts to 

remove dead layers and sloughing from the media into the sump where it can be removed from 

the system, and keeps the media wet. This is important because the air passing over the media 

has a tendency to dry it out. 




Figure 12 - Biotrickling Filter Process Schematic 

Exhaust 

Media 

Inlet 

Air 

Stream 

Nutrients 

Temperature 

Control 

Blowdown 

The nutrient rich, water-based solution distributed at the top of the media can be designed as a 

once through system or it can be recirculated. When biotrickling filters are located at wastewater 

treatment plants where plant effluent is available, plentiful and can be readily used, once through 

systems are normally designed. The need for nutrient supplement is not typically necessary as 

the plant effluent will often have enough nutrients remaining. However, when plant effluent is 

not an option and potable water is all that is available, adding nutrients are critical and 

recirculating/recycling the solution is advised/recommended. 

The biotrickling filter is a biological reactor that removes H 2 S. The biomass is primarily living 

as attached growth on the media and not in the recirculated solution. Biotrickling filters rely on 

the growth of a thin biofilm on the surface of the media. The H 2 S and other compounds are 

absorbed or dissolved in the water matrix and diffuse into the biofilm, where the biological 

culture can aerobically degrade them. The biofilm must stay thin to avoid anaerobic activity, 

thereby making suitable biomass sloughing mechanisms necessary for a productive and effective 

control system. The bacteria that will be found operating in biotrickling filters with elevated 

levels of H 2 S are autotrophic. 

Microbiologists use the term autotrophic to identify this general type of bacteria that use 

inorganic compounds for their energy and carbon sources. In contrast, heterotrophic organisms 

must use organic compounds for both their energy and carbon needs. Autotrophic bacteria 

obtain their carbon from carbon dioxide gas dissolved in the water in which they live. 

Autotrophs consume inorganic compounds for energy such as sulfur based compounds 

(hydrogen sulfide [H 2 S], sulfide [HS - ]). Once inside the cell wall, biological processes act on 

these compounds to extract energy for use by bacteria. Once the energy is extracted through 




their particular metabolic process, a converted and oxidized byproduct of the compound is 

released. 

Examples of common autotrophic bacteria are Thiobacillus. They consume H 2 S gas and the 

released product is sulfuric acid (H 2 SO 4 ). The latter process is also responsible for corrosion of 

concrete and metal surfaces exposed to H 2 S gas in wastewater systems. 

Autotrophic bacteria typically have a thin cell wall to allow free movement of chemicals in and 

out of the cell. They also prefer to live as single cells in suspended culture or as thin sheets of 

cells on surfaces so that a maximum cellular surface area is exposed to collect food and 

nutrients. Therefore, autotrophic bacteria do not clump together or form thick masses of cells 

like heterotrophs. Autotrophic bacteria are also efficient nutrient scavengers and can efficiently 

recycle nutrients in dead cells around them. Because autotrophic bacteria do not get as much 

energy from the conversion of inorganic chemicals as heterotrophic bacteria get from organic 

compounds, they must convert more chemicals at a faster rate. Autotrophic bacteria can, 

therefore, quickly assimilate and convert large quantities of inorganic compounds. This latter 

ability lends to rapid conversion of H 2 S gas in sewers to sulfuric acid and destruction of the 

wastewater infrastructure. 

Because of their ability to rapidly remove inorganic compounds, biological odor control 

technologies like biotrickling filters use autotrophic organisms to remove inorganic odor 

compounds such as H 2 S, and have proven that they can consistently achieve 99+% removal of 

H 2 S with the latest synthetic plastic media. Some autotrophic organisms can also degrade some 

low molecular weight organic compounds such as carbon disulfide, methyl mercaptan, and ethyl 

mercaptan. However, in the case of organic compounds, autotrophs degrade only the functional 

sulfur groups of the compound leaving the carbon-carbon bonds untouched. 

The biotrickling filter process schematic presented in Figure 12 illustrates the ability to provide 

temperature control ahead of the biotrickling filter to maintain the system at a uniform 

temperature that optimizes the performance of the microorganisms. For the City of Los Angeles, 

this option would not necessary. 




Table 3 presents a list of the advantages and disadvantages of a biotrickling filter as an emissions 

control technology. 

Table 3 - Biotrickling Filter Emissions Control: Advantages and Disadvantages 

Biofilter 

Advantages 

Disadvantages 

Environmentally friendly 

Very poor non-H2S removal 

Green technology 

Larger footprint 

No chemicals 

Short circuiting 

Few mechanical pieces 

Requires post polishing step 

Low maintenance 

Must add nutrients 

Minimal monitoring 

Low pH blowdown 

Removes H 2 S 

Extended detention times 

Biological treatment 

Subject to upsets 

Low annual operating cost 

Biological process 

Non-hazardous media 

Need second stage for other odors/VOCs 

Minor adjustments needed 

Need CO2 in air flow/water supply 

Capable of treating variable H2S loads Short track record 

Robust microorganisms 

Requires acclimation period 

Proven media effectiveness 

Standby units must be operated 

Long detention time in media 

Less appropriate for low H2S levels 

Because the biotrickling filter would be an excellent control mechanism for H 2 S and some of the 

other reduced sulfur based compounds, but not very effective for other odorous compounds and 

VOCs, implementing this technology would require adding a second stage for residual odor 

removal and some VOC reduction. This is especially important to reduce the impact of odors on 

the surrounding community at the Mission and Jesse location. 

The sustainable operation of the biotrickling filter (no chemicals, minimal mechanical 

equipment, low operating cost in comparison to other odor control technologies, etc.) together 

with its ability to remove high and variable concentrations of H 2 S support the application of this 

technology at the Mission and Jesse location. 

(3) Activated Carbon 

The oldest of the three odor control technologies under evaluation, activated carbon, has proven 

that it can be a viable odor control alternative. Using a different removal process when 

compared to wet scrubbing and biotrickling filters, it is generally capable of providing the most 

complete removal of wastewater treatment plant odors, and is often found in a second stage 

polishing arrangement. 




Unlike wet scrubbing and biofilters, activated carbon is a dry operating system that relies on 

adsorption rather than absorption. The preferred optimal operating point for the inlet odorous air 

stream moisture content is 50% relative humidity (RH), with some activated carbons on the 

market today capable of operating up to 75% RH. Too much moisture in the odorous air stream 

is not good since the moisture then competes for sites on the activated carbon along with the 

odorous compounds that we are looking to remove. 

Activated carbon odor control is a very simple system consisting of the following elements: 

 

 

 

 

Foul air fan 

Vessel 

Activated carbon 

Exhaust stack 

The foul air fan pushes the odorous air into an open volume within the vessel, a plenum. The 

plenum reduces the velocity of the inlet air and prepares the air flow to be evenly distributed 

across the full cross-section of the activated carbon bed. The odorous air passes through the 

carbon bed at a velocity of 50 to 65 feet per minute and the odorous air compounds are adsorbed 

onto the carbon surface. The treated air is then released through an exhaust stack into the 

atmosphere. Figure 13 illustrates a typical, dual bed, activated carbon odor control system. 

Figure 13 - Dual Bed Activated Carbon Process Schematic 




Activated carbon has a finite capacity for removing various compounds from the air. Once that 

capacity is reached, the activated carbon needs to be replaced. Some activated carbons can be 

regenerated in place, but the only capacity that is restored is for H 2 S and not other reduced sulfur 

compounds or VOCs. If the odorous air flow does not have a dominant H 2 S component, the use 

of a regenerable activated carbon is not warranted and the selected carbon should be virgin 

activated carbon. 

Some compounds have less capacity for being removed by activated carbon. It is important to 

understand the characteristics of the odorous air treated in order to determine the limiting 

odorous compound so the estimated activated carbon bed life can be determined. Understanding 

the duration of the activated carbon bed life will help determine the bed replacement frequency, 

which is important in assessing the operating costs for activated carbon odor control. Hydrogen 

sulfide is commonly used to assess bed life, and replacement frequency. 

Different activated carbons are on the market today, including: 

 

 

 

Virgin 

Caustic impregnated 

Catalytic 

The type of carbon selected will depend on the characteristics of the odorous air flow. Virgin 

carbon is the oldest and most traditional carbon that would be used for volatile and odorous 

organic compounds, but has a relatively low capacity for H 2 S. Impregnated carbons applied at 

wastewater operations are usually impregnated with an alkali, such as sodium hydroxide or 

potassium hydroxide, a chemical that promotes a reaction with adsorbed acidic compounds like 

H 2 S. These impregnates enhance the activated carbon’s removal efficiencies and capacities for 

H 2 S removal. Catalytic carbons have finer pore structure, giving it a higher density and greater 

adsorption capacity for H 2 S than unimpregnated carbons. Because of its catalytic nature, most 

(more than 90%) of the H 2 S reacts to form sulfates which is water soluble and could be removed 

by washing the carbon with water. Again, this regeneration only removes H 2 S related 

constituents. From a cost perspective, virgin carbon is the least costly and catalytic is the most 

expensive. 

Two concerns have been expressed by owners of activated carbon odor control systems: bed 

fires and disposal of the used carbon. Bed fires are only a legitimate concern when impregnated 

carbon is being used. The caustic solution used to impregnate the carbon can cause an 

exothermic reaction when combined with moisture, from the ambient air for example. This can 

create high enough heat for ignition to occur if the fans are not operating. The fans would tend 

to dissipate the heat. However, when the fans are off and the activated carbon bed is not sealed 

off, allowing air to breathe through the bed, fires have been known to occur. These conditions 

do not exist in virgin and catalytic carbons, and therefore are less susceptible to bed fires. 

The proper way to dispose of activated carbon once it is spent depends on the type of carbon, the 

characteristics of the inlet odorous air, and the compounds adsorbed. The carbon should be 




tested to determine compliance with RCRA regulations. The results will determine how the 

carbon should be disposed. Impregnated carbon is likely to be considered hazardous due to the 

caustic impregnate used. Virgin and catalytic carbons can be thermally regenerated, and so may 

be removed by the supplier of the replacement carbon and not disposed of but re-used. 

To economize on the space requirements for activated carbon odor control systems, dual bed 

vessels are available (see Figure 7-3). Dual bed vessels provide two, generally three foot deep 

activated carbon beds in a single vessel. This allows for twice the flow in the same footprint as a 

single bed vessel. The space requirements for activated carbon lie in between wet scrubbing and 

biotrickling filters. 

Activated carbon odor control systems often require an in-line particulate filter before the carbon 

vessel to avoid plugging of the carbon, particularly when it is used as the prime/single odor 

control device when treating wastewater collection system air streams that contain aerosol 

particulates of oil and grease, and free moisture. Plugging created by the unwanted particulate 

matter build-up can cause uneven distribution of the air, unused sections of the activated carbon 

bed, and an increase in pressure drop. 

Pursuant to the nature of an activated carbon odor control system, no controls (other than for the 

fans) or chemicals are needed. There is no way to regulate the degree of control for activated 

carbon. Activated carbon works to remove all the compounds it is capable of when it passes 

through its bed, whether that removal is necessary or not. This can result in a shorter bed life 

and more frequent carbon replacements. 

Activated carbon is capable of removing the typical wastewater related odors, H 2 S (to levels 

greater than 99%) and other reduced sulfur compounds, as well as VOCs. Although activated 

carbon can remove reduced sulfur compounds, in some instances the removal capacity is much 

less than that associated with H 2 S. This means that these compounds could break through the 

bed before the capacity to remove H 2 S is reached. The question with activated carbon, then, is 

not whether it can effectively remove H 2 S, VOCs and other odorous compounds, as it has 

demonstrated in the past that it is still the best technology of those identified earlier at removing 

these compounds to very low levels. The potential drawback with activated carbon is its finite 

capacity for removal. If the activated carbon media needs to be replaced too frequently, the 

advantages it has over other technologies are likely to disappear. At the same time, pursuant to 

its ability to provide the best exhaust quality, applying activated carbon as a second stage and 

polishing device, because of the higher degree of control necessary for facilities and locations 

like Mission and Jesse that are stuck in the middle of the community, can be extremely positive 

and effective in demonstrating that this air control facility will be able to consistently achieve the 

SCAQMD permit limits and the odor demands of the public. 

As a prime air emissions control system, activated carbon is not suitable at the Mission and Jesse 

location. The results from the monitoring study identified average concentrations of H 2 S of 16.5 

ppm over the several days of continuous data recording. Any time that average concentrations of 

H 2 S approach and exceed 10 ppm, the application is not appropriate for activated carbon. The 




activated carbon would need to be regenerated in place and/or replaced too frequently to make 

this application cost-effective. Pursuant to the remote location of the Mission and Jesse site, 

changing the carbon frequently also creates a logistical problem for staff who need to know 

when the carbon must be changed (requiring more often system monitoring) and coordinate with 

the vendor on when it will be done. 

Table 4 presents a list of the advantages and disadvantages of an activated carbon system as an 

emissions control technology. 

Table 4 - Activated Carbon Emissions Control: Advantages & Disadvantages 


Advantages 

Disadvantages 

Simple system 

Carbon disposal 

Minimal mechanical pieces 

Finite capacity 

Proven technology 

Comparatively high pressure drop 

Effective H 2 S/odor removal 

Moisture concerns 

Dry system 

No operating flexibility 

Simple monitoring 

Potential bed fires with impregnated carbon 

Good polishing unit 

One fan per vessel 

Low maintenance 

Must monitor for bed capacity 

Moderate footprint 

Regeneration byproducts require special handling 

No acclimation required 

Replacing carbon 

Long track record 

No reserve capacity 

Reliable performance 

Two exhaust stacks per dual bed unit 

Preferred final polishing technology Always removes everything 

Combinations 

Combinations consist of multiple odor control technologies linked together to provide for more 

complete control of the inlet odorous gases. Combinations are commonly applied when the inlet 

odorous gas concentrations exceed the ability of a single control technology to achieve compliance; 

or, when one technology is not able to achieve necessary removal efficiencies of all identified 

odorous compounds; or, in the case of wet scrubbers or biotrickling filters, the exhaust gas has a 

residual odor associated with it that may or may not be representative of the feed gas. Feasible 

combinations for the technologies under consideration in this study would be: 

 

 

 

 

Wet scrubbing and activated carbon 

Biotrickling filter and activated carbon 

Biotrickling filter and wet scrubbing 

According to the discussions of the technologies above, wet scrubbing is not a technology 

that is well suited for the service in a remote location such as Mission and Jesse for the 




easons provided. Biotrickling filters will remove H 2 S, and activated carbon will remove 

H2S, VOCs and other odors, but operating as individual emissions control technologies they 

would not be able to satisfy the needs of the City nor the community. However, taking 

advantage of the best attributes of each technology 

 

 

 

 

Excellent H 2 S removal capability of biotrickling filters 

Sustainable, low energy, low O&M features of biotrickling filters 

Polishing capability of activated carbon to remove residual odors, H 2 S and VOCs 

The combination of biotrickling filters and activated carbon make for a successful, low risk 

solution to the air emissions treatment at Mission and Jesse. 




8. Recommendations 

Based on the monitoring results from the TNMHC study, the discussions on the technologies, and 

the comparison tables of advantages and disadvantages for each of the individual control 

technologies, the recommended technology for the Mission and Jesse location is a two stage, 

combination system consisting of: 

 

 

 

 

 

First Stage - Biotrickling filter 

Second Stage - Activated Carbon 

The biotrickling filter will remove 99% of the influent H2S and limited other reduced sulfur 

compounds, and the carbon will: remove any residual H2S down to the low parts per billion 

(ppb) concentrations (Well below the projected permit limit of 1.0 ppm.); remove any 

residual reduced sulfur and odorous compounds; and reduce the levels of VOCs. This will 

comply with the permit conditions and keep nuisance odors from impacting the surrounding 

communities. 

This recommended full air treatment facility is necessary in order to treat the elevated levels 

of influent H 2 S concentrations, the modest concentrations of VOCs and the remaining 

odorous organic and inorganic compounds. Implementing this two-stage, combination, air 

emissions control system will minimize any risk facing the City with respect to permit 

compliance and being a good neighbor. 

As presented in Section 5- Determine Air Flow, the actual air flow to be treated by the ATF is 

yet to be established. In the interim it is projected that a 20,000 cfm flow rate would be 

necessary at this recommended Mission and Jesse ATF. The ATF attributes will depend on 

the vendor providing the ATF units and the design criteria developed by the design engineer 

in coordination with the City staff. 

A preliminary estimate of the ATF attributes are presented in Table 5. 




Table 5 - Preliminary ATF Attributes 


Department of Public Works – Bureau of Sanitation 

Air Treatment Facility Review Study 

Preliminary ATF Attributes 

Value 

Air Flow Rate, cfm 20,000 

Number of Biotrickling Filters 

@ Contact Time of 20 seconds (27s)* (Includes one standby 

unit) 

@ Contact Time of 14 seconds (20s)* (Includes one standby 

3 

unit) 

Diameter, feet 12 

Number of Activated Carbon Vessels 

@ Single Carbon Media Bed (Includes one standby unit) 4 

@ Dual Carbon Media Bed (Includes one standby unit) 3 

Diameter, feet 12 

Number of Fans 

Single Carbon Media Bed 4 

Horsepower per Fan 30 

Dual Carbon Media Beds 3 

Horsepower per Fan 40 

 

* The contact time with the standby unit operating. 

The number of biotrickling filters was developed based on a contact time and the air 

emissions flow of 20,000 cfm. Two contact times, 14 and 20 seconds, were applied and are 

consistent with literature, experience and vendor designs. The contact times were developed 

without the standby unit in operation. Accordingly, with the standby unit in operation, which 

would represent the normal operating condition, the contact times would increase to 20 and 

27 seconds, respectively. The standby biotrickling filter unit would be operating under 

normal conditions for the because the time to reach acclimation for one of these units can be 

as long as a few weeks, and if it has to take over for another operating unit it must be ready to 

do so at any time. This is where biological units differ from physical-chemical systems such 

as wet scrubbing and dry adsorption systems like activated carbon. These systems can be 

waiting in the wings until they are needed before turning them on to provide the necessary 

service. 

The activated carbon vessels were based on an average face velocity through the media of 55 to 60 

feet per minute (fpm) and the overall flow of 20,000 cfm. This face velocity range is a standard 

design criteria in the industry and is selected to optimize the size of the vessel and the pressure drop 

across the media. Pressure drop, which equates to energy and operating cost, increases with 

increasing face velocity. Using these criteria, carbon vessels can consist of either a single or dual 

bed construction which would require four (4) and three (3) vessels, respectively. There will be a fan 

4 




for each carbon vessel. As a result, under the single bed approach, four (4) fans at 30 horsepower 

(hp) each would be necessary. Under the dual bed scenario, three (3) fans at 40 hp would be needed. 

These ATF attributes are preliminary but do provide insight into the initial system configuration of 

the Mission and Jesse ATF.

2010 - ATF Evaluation Study Final Report - LA Sewers

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