Integrated Pond and BNR Activated Sludge Treatment ... - eWISA

INTEGRATED POND AND BNR ACTIVATED SLUDGE
TREATMENT PROCESS TO ACHIEVE RELIABLE
NITROGEN AND PHOSPHORUS REMOVAL
Annalien Toerien
Golder Associates Africa, PO Box 6001, Halfway House, 1685. E-mail: atoerien@golder.co.za
ABSTRACT
Advanced wastewater treatment for smaller communities can be achieved without expensive plant
upgrades. Existing pond treatment systems can be integrated successfully with Biological Nutrient
Removal Activated Sludge Treatment technology. The combination of anaerobic ponds and
activated sludge can be implemented in a series or parallel configuration, with existing oxidation
ponds as a side stream treatment process or converted to Polishing Wetlands. The key process
consideration is maintenance of a favourable COD:N ratio for effective nutrient removal in the
activated sludge process.
INTRODUCTION
In many catchments stricter discharge standards are being introduced, based on:
• the sensitivity of the catchments and downstream users
• the receiving water quality limitations in already impacted river reaches and impoundments
• the access to improved technology and finance to enable the treatment of effluent to higher
standards compared to past practices
• an increase in public awareness and regulatory pressure regarding the environmental
protection.
Due to increased urbanisation and developments with associated housing and the introduction of
waterborne sanitation, sewage flows have increased substantially in many towns and cities.
Pressure on the available land and resources has also increased. The consequence is that space
is constrained in some areas and large pond systems treatment facilities may not be viable
anymore. With increased pressure for effective nutrient removal, processes that can do so
naturally, rather than chemically, is favoured.
The challenge is to incorporate existing pond infrastructure with new and advanced technology in
order to:
• minimize capital investment and operational costs
• reliably treat effluent to achieve the effluent discharge standards applicable – especially N and
P removal
• minimize the impact on land availability, surrounding communities and downstream users
BACKGROUND
Pond systems are mainly used for the treatment of domestic wastewater from smaller
communities, where the main focus is the removal of COD and TSS. Very often, the ratio of
nutrients in the wastewater is predominantly COD and nitrogen, due to the concentrated nature of
the effluent.
Anaerobic ponds are effective in the removal of carbon, while facultative and oxidation ponds
provide some polishing treatment to decrease nutrients but performance is sometimes poor,
especially in winter. Nitrogen (N) is more effectively removed than phosphorus (P). The nutrient
ratio in pond effluent is therefore changed to low carbon (COD) and an imbalance between N and
Proceedings of the 2004 Water Institute of Southern Africa (WISA) Biennial Conference 2 –6 May 2004
ISBN: 1-920-01728-3 Cape Town, South Africa
Produced by: Document Transformation Technologies Organised by Event Dynamics

P, should further nutrient removal be required.
BNR (biological nutrient removal) systems are activated sludge systems that create conditions that
enable the utilization and therefore removal of not only carbon, but also nitrogen and phosphorus.
The ratio between COD : N : P are required in a relatively narrow band to enable the biochemical
reactions to sustain the utilization and removal of both nutrients (NSP) within the required ranges.
To achieve this, pond systems and BNR activated sludge treatment processes can be combined
successfully.
PROCESS CONFIGURATION
The integration of ponds and BNR activated sludge can be either in series or in a parallel process
configuration. Two possible combinations are discussed in more detail.
Raw Sewage
Mechanical
Screening
Oxidation Ponds
Vortex
Degritters Measuring
Flume
Anaerobic Ponds
Primary
Clarifiers
Dry Sludge
Belt Press
Activated Sludge
Anaerobic
Digestion
Sludge
Fermentation
Secondary
Clarification
Figure 1. Series configuration of integrated ponds and BNR processes.
WAS
Thickeners
Chlorine
Disinfection
Series Configuration
This configuration consists of the following (refer to Figure 1):
• Preliminary treatment – screening, degritting and flow measurement;
• Primary treatment – primary clarification, with sludge fermentation for the production of volatile
fatty acids (VFA) and anaerobic pond treatment of the primary effluent and primary sludge.
Oxidation pond effluent is returned to the anaerobic ponds and utilised as an oxygen-rich
surface water layer for odour control.
• Secondary treatment – BNR activated sludge treatment with secondary clarification, sludge
return and final disinfection.
• Sludge treatment – anaerobic or aerobic sludge digestion and dewatering on either drying beds
or by mechanical means.

This configuration maximises the availability of readily biodegradable carbon in the anaerobic zone
for the uptake of Phosphorus. Nitrification and denitrification, as well as final carbon polishing is
subsequently achieved in the BNR reactor. The process configuration is a variation of the
well-known PETRO TM process, and has been successfully applied.
Parallel Configuration
The parallel configuration of ponds and the BNR activated sludge process, consists of the following
process components (refer to Figure 2):
• preliminary treatment – screening, degritting and flow measurement;
• the raw sewage feed stream is divided between the ponds and BNR reactor in parallel, to
provide a high carbon feed stream directly to the activated sludge process. This enhances the
enhanced biological P-removal;
• anaerobic pond treatment, including primary sludge digestion of the raw sewage;
• with sufficient reduction of the COD in the ponds, a nitrification trickling filter can be
incorporated on a recycle stream to modify the COD : N ratio and to improve overall nitrogen
removal. The recycle stream will also provide a NO3 -N rich surface water layer for odour
control;
• activated sludge treatment – BNR activated sludge treatment with secondary clarification,
sludge return and final disinfection. The feed to the activated sludge process is a combination
of the treated pond effluent and the high carbon, raw sewage feed;
• sludge treatment – anaerobic or aerobic digestion and dewatering.
Raw
Wastewater
A
Bar
Screen
Preliminary Treatment
Manual
Screening
Activated Sludge Reactor
Activated Sludge Reactor
Nitrification
Towers
Grit
Removal
RAS Pumps
Parallel Integrated Ponds / BNR Activated Sludge Process
BENEFITS
Bioclarifier
Bioclarifier
Anaerobic Ponds
Anaerobic Ponds
Aerobic
Digestion
Sludge
Sludge
Chlorine Disinfection
Sludge
Drying Beds
Figure 2. Parallel configuration of integrated ponds and BNR processes
The benefits of integrating ponds and BNR activated sludge processes are:
AP
Recycle
Pumps
Polishing
Wetlands
9101-A9
Plant
Effluent
A

• smaller sized activated sludge reactors due to the lower COD load feed to the BNR process.
The bulk of reactor tankage is typically required for carbon removal. This can be reduced
substantially with pre-treating the wastewater in anaerobic ponds;
• improved COD : N : P ratios optimise the overall performance of the treatment process;
• multiple barriers are put in place to provide at least partial treatment of the wastewater before
discharge, should any individual treatment component experience an upset condition;
• existing infrastructure is integrated with modern technology to improve treatment and to
successfully achieve strict Nitrogen and Phosphorus discharge standards;
• some of the redundant oxidation ponds on existing treatment plants can be retrofitted as
wetlands to provide final polishing of the treated effluent. This also provides an additional
barrier to protect against breakthroughs in the event of process or plant upsets;
• stormflow attenuation can be done by utilising some of the ponds for this purpose. This will
protect the downstream processes against excessive hydraulic loads that may wash the
biomass out.
CHALLENGES
Ponds exhibit variable performance due to seasonal variations in temperature and solar radiation,
with the consequent drop in performance. Treatment plants should therefore be designed with
sufficient flexibility to enable adjustments to be made to improve the effluent quality. Variability in
feed to either the BNR process or anaerobic ponds can be done by controlling the percentage feed
to either of the two processes. The entry point of the treated anaerobic pond effluent into the BNR
process can also be adjusted to any of the zones.
PRACTICAL APPLICATIONS AND TARGETS
The concept of integrated pond and BNR activated sludge treatment processes have been
implemented at a number of wastewater treatment plants now under construction and some in
operation.
Wastewater treatment plants under planning and construction include the following:
• Mothutlung - Madibeng Municipality (2.4 Ml/d)
• Mahwereleng - Mogalakwena Municipality (5.8 Ml/d)
Wastewater treatment plants in operation include those at:
• Emthonjeni – Highveld District Council (1.5 Ml/d) Parallel configuration
• Rietgat – Tshwane Metropolitan Council (27.5 Ml/d capacity) Series configuration
Raw Wastewater Quality
Parameter
Table 1. Raw wastewater quality data.
50 percentile 95 percentile
Alkalinity (mg CaCO3/l) 229 350
TSS (mg/l) 325 565
TKN (mg N/l) 46 167
Ammonia (mg N/l) 22.8 88
COD (mg COD/l) 821 1508
TP (mg P/l) 10.5 23.6
PO4 (mg P/l) 8.7 21.3

Typical domestic wastewater quality treatment in integrated Pond-BNR activated sludge processes
are summarised in Table 1.
The typical final treated effluent quality from integrated Pond-BNR activated sludge processes is
summarised in Table 2.
Table 2: Final effluent quality data.
Treated wastewater Quality Parameter Based on the 50 percentile conditions
TSS (mg/l) < 20
Nitrate (mg N/l) < 8
Ammonia (mg N/l) < 3
COD (mg COD/l) < 60
PO4 (mg P/l) < 1
CONCLUSIONS
The conclusions of the full scale integration of ponds and BNR activated sludge processes are as
follows:
• it is not necessary to abandon ponds when upgrading a plant to achieve strict N and P
standards, in fact to abandon existing ponds in favour of “modern” technology may be a
wasteful use of available infrastructure
• treatment of wastewater in ponds will modify the desirable COD : N : P ratio for biological
nutrient removal. This can be rectified by a number of process refinements including:
- side stream flow of high carbon (COD) containing streams directly to the BNR activated
sludge treatment process
- optimised Nitrogen removal in the pond system by incorporation of nitrification trickling
filters on a recycle stream
• seasonal adjustment to the integrated pond and BNR activated sludge system may be required
to account for the variable performance of pond treatment systems.