United States	Office of Water
Environmental Protection	EPA 820-F-20-005
ASencV	March 2021
OPTIMIZING BIOLOGICAL PHOSPHORUS
REMOVAL IN MINNESOTA
Optimizing advanced treatment systems reduces nutrients and saves money
Optimization efforts are often focused on improving
nutrient removal in conventional systems, but
operators of advanced systems can optimize their
plants' performance, too. This fact sheet describes
the collaborative approach employed by Metropolitan
Council Environmental Services (MCES) and its
operators to improve nutrient removal and reduce
chemical costs through low-cost operational changes
at two of MCES's nine publicly owned treatment works
(POTWs).
The first MCES plants to experiment, Eagles Point and
Empire, located near Minneapolis, are both designed
for enhanced biological phosphorus removal (EBPR)
and both have a 12-month moving average effluent
total phosphorus (TP) permit limit of 1 mg/L. Staff
successfully reduced TP discharges, stabilized effluent
concentrations, and eliminated expensive chemical
addition.
Eagles Point POTW
The Eagles Point POTW has a design capacity flow of
10 million gallons per day (MGD) and an average daily
flow of 5 MGD. The plant has two primary clarifiers,
four activated sludge aeration basins, two secondary
clarifiers, UV disinfection, and one gravity thickener. In
the aeration basins, flow first enters a pre-anoxic zone,
where return activated sludge (RAS) is fed; followed
by an anaerobic zone; and then three aerobic zones in
series. Process control consists of automatic air control
using dissolved oxygen (DO) probes in the aeration
basins. Although the Eagles Point POTW was designed
for EBPR, the effluent TP concentration could not be
maintained below 1.0 mg/l without adding aluminum
sulfate to the primary influent.
Eagles Point POTW
MCES suspected that high nitrate loads in the RAS
were hindering performance by impeding growth of
phosphate accumulating organisms (PAOs) in the
anaerobic zone. They devised a three-step experiment
to improve EPBR performance and reduce aluminum
sulfate addition.
1.	Gradually decrease the RAS rate from 50% to -30%
to reduce nitrate loads from the RAS to the pre-
anoxic zone and carry over into the anaerobic zone.
2.	Turn off the aluminum sulfate addition system.
3.	Monitor phosphorus release at the end of the
anaerobic zone and effluent TP concentration while
maintaining clarifier performance.
To maintain clarifier performance, staff set targets
for a sludge blanket depth less than 2 ft and a sludge
volume index (SVI) of 100 mL/'g, while monitoring total
suspended solids (TSS) concentrations in the RAS.
Operators began experimenting on August 31,
2017 and were able to stop chemical addition on
September 9, 2017.
National Study of Nutrient Removal and Secondary Technologies
Nutrient removal through optimizing plant operations

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Eagles Point POTW Effluent Total Phosphorus
—^ Monthly Average	12-Month Rolling Average
Eliminating chemical addition not only reduced
operating and maintenance labor, but also saved
the plant about $100,000 per year in chemical
costs.
Since making the changes, the 12-month rolling
average effluent TP concentration reached an
historical low of 0.3 mg/L.
Empire POTW
The Empire POTW has a design capacity flow of
24 MGD and an average daily flow of 11.5 MGD. The
plant has six primary clarifiers, five activated sludge
aeration basins (three normally in use), four secondary
clarifiers, UV disinfection, and two gravity thickeners.
The activated sludge system begins with a pre-anoxic
zone with RAS feed; followed by four anaerobic zones
in series, the second of which receives primary clarifier
effluent; and then six aerobic zones in series. Process
control consists of automatic air control using DO
probes in the aeration basins. Despite being designed
for EBPR, the plant produced high and variable
effluent TP concentrations during the few warm
months of the year.
MCES again suspected that high RAS nitrate loads were
hindering EPBR performance. Starting in April 2016,
Empire staff began decreasing the average sludge
return rate from 44% to 41%, in 1 - 2% increments, while
closely maintaining secondary clarifier performance,
using the same targets as Eagles Point, and monitoring
TP release in the anaerobic zone.
After reducing the RAS rate, Empire operators
quickly saw lower and more consistent effluent TP
concentrations. In just six months, Empire POTW cut
its average effluent TP concentration in half, from 0.4
mg to 0.2 mg/L.
Empire POTW

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Empire 2016 Effluent Total Phosphorus versus Return Activated Sludge (RAS) Rate
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Optimization Started
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Advice for Operators: EBPR Troubleshooting Checklist
Through experience at Eagles Point and Empire POTWs,
MCES developed a Process Troubleshooting Checklist for
Enhanced Biological Phosphorus Removal to guide other
MCES plants. Operators interested in troubleshooting
biological phosphorus removal at their plant should
start by ensuring that high effluent TP concentrations
are in fact related to poor EBPR performance and not
some other cause, such as a new influent contribution,
high solids in the effluent (since phosphorus can be in
particulate form), or new chemicals entering sewers that
can inhibit the EBPR process.
MCES advises operators to gather operating data
when the plant is running well to establish a baseline.
Knowing your plant's "normal" makes field test results
easier to interpret, as all plants are different.
Suggestions from the checklist for troubleshooting
EBPR performance include:
» Review influent conditions (e.g., BOD and COD
concentrations; BOD:TP and BOD:TN ratios) to
confirm flow has sufficient carbon to support PAO
growth and denitrification. Don't assume; sample.
» Review aeration basin and secondary
clarifier DO and TP profiles to identify '^wSS
potential secondary phosphorus releases. If
needed, reduce the secondary clarifier sludge
blanket depth to maintain clarifier performance.
» If high nitrate loads are impeding PAO growth,
determine if you can lower the RAS ratio.
» Examine anoxic/anaerobic zone conditions for
potential to reduce or cycle mixing. This allows
some settling and promotes volatile fatty acid
(VFA) generation from the RAS.
» Set target DO concentration in the initial aerated
zone of at least 1.5 mg/L to support ammonia
oxidation.
» If air mixing is used in the anoxic or anaerobic
zones, adjust the airflow to ensure the DO levels
are as close to zero as possible to encourage VFA
uptake by PAOs.
» Increase gravity thickener sludge blanket depths to
allow VFA production and increase VFAs in gravity
thickener overflow.
Acknowledgements
All nutrient monitoring data, photos, and chemical cost savings were provided by MCES.

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