a submerged attached growth bioreactor for decentralized ...

ABSTRACT
A SUBMERGED ATTACHED GROWTH BIOREACTOR FOR
DECENTRALIZED WASTEWATER TREATMENT
P.B. Pedros * and W.K. Dobie
F.R. Mahony & Associates, Inc.
273 Weymouth Street
Rockland, MA 02370
The Amphidrome ® process is a submerged attached growth bioreactor (SAGB) that has been
used in Massachusetts, Connecticut, Rhode Island and New Jersey for decentralized wastewater
treatment systems ranging in size from 440 gpd to 150,000 gpd. The two primary advantages of
a SAGB are the small volume requirement and the elimination of downstream clarification. Five
years of data from four plants operating in Massachusetts are presented below. Each facility was
designed to treat a domestic wastewater to an effluent BOD5 ≤ 30 mg/l, TSS ≤ 30 mg/l, and total
nitrogen ≤ 10 mg/l. The process is described and the performance, including loading and
removal rates, at the four treatment plants is presented. The results indicate 1) 97% nitrification
at an organic loading of 2.5 kg-BOD5/m 3 -day, and 2) a nitrification rate of 0.427 kg-N/m 3 -day
and a denitrification rate of 0.410 kg-N/m 3 -day each at a total ammonia loading of 0.434 kg-
N/m 3 -day.
KEYWORDS
SAGB, BNR, biological filter, decentralized system
INTRODUCTION
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Due to the severe impact of eutrophication on water resources, the removal of inorganic
nutrients, nitrogen and phosphorus, from wastewater has become an increasingly important
consideration and has imposed new challenges in the design of wastewater treatment plants.
Nitrogen discharge limits for many coastal regions and tidal estuaries have become especially
stringent in recent years and biological nutrient removal (BNR) processes have been developed
to meet the challenge. One such technology is the submerged attached growth bioreactor
(SAGB), which derives its name from the fact that the media is always submerged in the process
flow. The two primary advantages of a SAGB are the small volume requirement and the
elimination of downstream clarification (Grady et al. 1999). A submerged biofilter allows for a
high biomass concentration leading to a short hydraulic retention time and, thus, a significantly
reduced reactor volume as compared to a different fixed film reactor or a suspended growth
reactor. In addition, the media in a SAGB may be fine enough to provide physical filtration for
solids separation.
During the last twenty years, different configurations of SAGBs have been conceived and
advances in the understanding of these systems have been made. SAGBs have been used to
achieve complete nitrogen removal by combining the aerobic oxidation of soluble organic matter
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(SOM) and nitrification into one operational unit and denitrification into a separate unit
operation (Andersen et al. 1995 and Holbrook et al. 1998). A pilot study of a biological aerated
filter (BAF), a specific type of SAGB, installed in series and downstream of a denitrification unit
demonstrated that a recirculation of 300% of the inflow, from the filter back to the denitrification
unit, removed organics and nitrogen to the required levels (Yoshinobu et al. 1997). Combined
removal of organics and nitrogen was demonstrated in a single BAF with a separate anoxic zone
created within the reactor (Rogalla and Bourbigot 1990). However, the use of a single-unit,
single-zone SAGB for achieving the combined removal of organics and nitrogen is an innovative
process variation.
THE SAGB PROCESS
This particular SAGB process was specifically designed for the combined oxidation of
carbonaceous matter, nitrogen removal and suspended solids removal in a single-unit single-zone
biofilter. The system includes one anoxic/equalization tank, one clear well and one SAGB. (See
figure 1.) and operates as a sequencing batch reactor in which the wastewater is cycled back and
forth through the filter. The biofilter is intermittently aerated to achieve both the aerobic
environment required for the oxidation of organics and nitrification and the anoxic environment
required for denitrification. It provides low visibility, since all tanks are typically installed
underground, compact footprint, effective nutrient removal and minimal effect from cold air
temperatures.
Figure 1 – Cross Section of the System
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The influent wastewater enters the system through the anoxic/equalization tank, which has an a a
sludge storage zone and settling zone, which serves as a primary clarifier before the SAGB.
From the anoxic/equalization tank the wastewater flows by gravity into the SAGB. The driving
force of the forward flow is the hydrostatic pressure created by the differential liquid levels
within the tanks. The reactor consists of: 1) an underdrain, 2) support gravel, 3) filter media, and
4) a backwash trough. The underdrain, located at the bottom of the reactor, provides support for
the media and even distribution of air and water into the reactor. The design allows for both the
air and water to be delivered simultaneously or separately via individual pathways to the bottom
of the reactor.
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Operation of the SAGB alternates between down-flow (forward flow) and up-flow (reverse flow)
modes. The up-flow is accomplished by pumping from the clear well back up through the filter,
following the exact same path through the reactor as it did in the forward flow cycle. However, a
check valve in the influent line of the reactor prevents the flow from returning to the
anoxic/equalization tank via that line. Instead, the flow fills the reactor until it overflows into the
return flow/backwash trough and flows back to the front of the anoxic/equalization tank by
gravity. The recycled flow, which contains nitrates, mixes with the incoming raw influent,
which contains organic carbon, and starts to flow forward again when the pump shuts off.
To achieve the required aerobic and anoxic conditions within the biofilm, process air to the
reactor is supplied intermittently via the underdrain at the bottom of the reactor and is
independent of the return flow cycles. A typical aeration sequence would be three minutes with
the process blower on and then fifteen minutes with the blower off. The cyclical forward and
reverse flow of the waste stream and the intermittent aeration of the filter achieve the required
hydraulic retention time and create the necessary aerobic and anoxic conditions to achieve the
required level of biological nitrogen removal.
PERFORMANCE RESULTS
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The results presented are from four SAGB treatment plants operating in Massachusetts. The
design criteria were based on the typical effluent limits applied to wastewater treatment plants
that discharge into the ground in Massachusetts and are listed in Table 1. The data analyzed
were from Amphidrome ® Plus systems, which included a separate, smaller filter for
denitrification to insure low nitrate levels and therefore, an effluent total nitrogen limit ≤ 10
mg/l.
Table 1. Effluent Standards for Discharge to Groundwater
Constituent Effluent Limit Regulatory Basis
BOD 30 mg/l Massachusetts DEP
TSS 30 mg/l Massachusetts DEP
Total Nitrogen 10 mg/l Massachusetts DEP
Treatment plants 1, 2, and 3 were condominium complexes and treatment plant 4 was a
combined middle school and high school. The reactor sizes, media volumes and methanol use
for each system are shown in Table 2.
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Table 2. Reactor Sizes and Media Volume
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Plant/Flow (gpd) Reactors Media Vol. Plus Reactor Media
Vol.
Methanol
1 – 36,000 2 – 9.5 ft.x10.5 ft. 900 ft. 3 1 – 4 ft. dia. 63 ft. 3 0 gpd
2 – 25,000 1 - 9.5 ft.x13 ft. 740 ft. 3 1 – 4 ft. dia. 56 ft. 3 0.5 gpd
3 – 36,000 2 – 9.5 ft.x10.5 ft. 1,300 ft. 3 1 – 4 ft. dia. 94 ft. 3 1.0 gpd
4 – 14,600 1 – 8 ft. dia. 400 ft. 3
1 – 4 ft. dia. 50 ft. 3 2.5 gpd
Process aeration of the SAGBs was based on 0.7 cfm/ft. 2 of filter area and was typically
governed by the physical requirements for an even air pattern and not the biological
requirements. Therefore, the process aeration ranged from 35 cfm to 86 cfm. The total aeration
time per day ranged from 3 to 5 hours. Each of the systems operated with recycle flows from
two to three times the daily flow.
The ammonia loading varied between 0.004 - 0.434 kg-N/m 3 -day and the organic loading from
0.11 – 3.53 kg-BOD5/m 3 -day. The effect of total organic loading (TOL) on nitrification is
illustrated in Figure 2. The results indicate that a nitrification efficiency of 97% was achieved at
a TOL of 2.5 kg-BOD5/m 3 –day, which corresponds to the 98% efficiency at 3.5 kg-bCOD/m 3 –
day reported by Rogalla et al. (1990) with a BAF designed for oxidation of organics and
nitrification. The results also indicate that near complete nitrification is possible at a TOL up to
1 kg-BOD5/m 3 –day. This is higher than the TOL limit suggested in the USEPA Nitrogen
Control Manual (1993), which suggests that to achieve 90% nitrification in a single BAF, the
TOL should not exceed 1 kg-BOD5/m 3 –day.
The effect of total ammonia loading (TAL) on reactor performance is illustrated in Figure 3.
Excluding the elevated total nitrogen (TN) effluent values, which were due to operational
problems, the results indicate that during the five-year period the effluent total nitrogen
concentration at each of the plants was below the required 10 mg/l. For the highest loading of
0.434 kg-N/m 3 –day the effluent total nitrogen was 5.4 mg/l. The ammonia loading and
nitrification rates presented in Figure 4 illustrate a nitrification rate of 0.180 kg-N/m 3 –day at an
ammonia loading of 0.187 kg-N/m 3 –day. This is in agreement with Terayama et al. (1997), who
reported a nitrification rate of 0.18 kg-N/m 3 –day at an ammonia loading of 0.19 kg-N/m 3 –day.
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Nitrification Efficiency (%)
and Rate (100 kg-N/m3-day)
100
90
80
70
60
50
40
30
20
10
0
0.0 1.0 2.0 3.0 4.0
Figure 2. TOL and Nitrification Efficiency
Effluent Total Nitrogen (mg/l)
16
14
12
10
8
6
4
2
0
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Nitrification Rate Nitrification Efficiency
Total Organic Loading (kg-BOD5/m3-day)
Plant 1 Plant 2 Plant 3 Plant 4
0.00 0.10 0.20 0.30 0.40 0.50
Figure 3. TAL and Effluent TN Concentration
Ammonia Loading (kg-N/m3-day)
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Nitrification Rate (kg-N/m3-day)
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.00 0.10 0.20 0.30 0.40 0.50
Figure 4. TAL and Nitrification Rate
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Ammonia Loading (kg-N/m3-day)
The relationship between denitrification rate and ammonia loading is illustrated in Figure 5. The
denitrification rate ranged from 0.002 kg-N/m 3 –day at a ammonia loading of 0.0171 kg-N/ m 3 –
day to 0.410 kg-N/m 3 –day at an ammonia loading of 0.434 kg-N/m 3 –day. These rates are low in
comparison with rates (0.29 to 1.6 kg-N/m 3 –day) reported in the EPA Manual for Nitrogen
Control (1993) for full-scale denitrification filters. Two operating conditions likely contributed
to the higher rates reported by the EPA: 1) the cited filters were separate unit processes dedicated
strictly to denitrification and thus had no aerobic cycle and 2) methanol was used as a
supplemental organic carbon source. In this SAGB process, the influent organic carbon was
utilized for much of the denitrification process. Although small quantities of methanol were
added, the use of sewage as the organic carbon source would tend to reduce the denitrification
rate.
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Denitrification Rate (kg-N/m3-day)
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.00 0.10 0.20 0.30 0.40 0.50
Figure 5. TAL and Denitrification Rate
CONCLUSIONS
Ammonia Loading (kg-N/m3-day)
The SAGB technology is an effective biological nitrogen removal process that offers low
visibility (all tanks are underground) and compact footprint. As with most SAGBs, it requires
less area than many other types of biological treatment processes because of the high
concentration of viable biomass within and because downstream clarification is eliminated. In
addition, the system can be constructed in concrete tanks, below grade, resulting in a small
control building and much lower construction costs. The effluent requirements of BOD5 ≤ 30
mg/l, TSS ≤ 30 mg/l were achieved at all four plants during the five year period (data not
presented). Excluding operational problems, the effluent total nitrogen limit ≤ 10 mg/l was also
achieved. At a total ammonia loading of 0.434 kg-N/m 3 -day an effluent total nitrogen of 5.4
mg/l was obtained. The nitrification rates at the four SAGB plants examined were comparable to
those reported in the literature.
REFERENCES
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Andersen, K. L., E. Bundgaard, V. R. Andersen, S. N. Hong and J. F. Heist, 1995. Nutrient
Removal in Fixed-Film Systems. Water Environment Federation, 68 th Annual
Conference & Exposition, 1, 581-590.
Grady, C. P. L. Jr., G. T. Daigger and H. C. Lim, Biological Wastewater Treatment, Second
Edition, Marcel Dekker, Inc., New York, 1999.
Holbrook, R. D., S. N. Hong, S. M. Heise and V. R. Andersen, 1998. Pilot and Full-Scale
Experience with Nutrient Removal in a Fixed Film System. Water Environment
Federation, 71 st Annual Conference & Exposition, 1, 737-774.
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Rogalla, F. and M. Bourbigot, 1990. New Developments In Complete Nitrogen Removal With
Biological Aerated Filters. Wat. Sci. Tech., 22(1-2), 273-280.
Rogalla, F., M. Payraudeau, G. Bacquet, M. Bourbigot, J. Sibony and P. Filles, 1990.
Nitrification and Phosphorus Precipitation With Biological Aerated Filters. Research
Journal, Water Pollution Control Federation, 62, 169-176.
United States Environmental Protection Agency, Office of Research and Development and
Office of Water. 1993. Manual for Nitrogen Control. EPA/625/R-93/010.
Yoshinobu, T., N. Takashi, I. Masumi and H. Morio, 1997. Feasibility Study on Nitrogen
Removal Process By Biological Aerated Filter. Water Environment Federation, 70th
Annual Conference & Exposition, 1, 641-652.
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