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Featured Projects | Case Study

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Educational Testing Service - Princeton, NJ

MBR retrofit promotes denitrification

H2O Innovation picture


The Educational Testing Service (ETS) collaborates with government, businesses and academic institutions to conduct research to improve educational assessments including management of

the nation’s SATs. The ETS campus near Princeton, NJ includes offices, boardrooms, a

conference center and hotel. This infrastructure was serviced by an ageing WWTP in need of upgrades. In addition, the effluent from the WWTP is sent to a trout stream and a more stringent discharge requirement was given to improve the effluent quality of the plant. The existing

WWTP included a conventional activated sludge system with an aerobic bioreactor and clarifier.

To address this, a membrane bioreactor (MBR) retrofit of the WWTP was proposed.


The following improvements were included in this retrofit design:

  • Increase in treatment capacity to accommodate the growth of the ETS campus

  • Improvement of treatment plant design to provide denitrification

  • Retrofit of plant without interrupting operation during construction

  • Provide upgrade within the footprint of the existing treatment plant

Before Retrofit

Flow Chart 1

After Retrofit

Flow Chart 2

MBR technology was chosen to meet these design requirements. As shown in the above

process flow diagrams, the existing rectangular clarifiers were converted to anoxic bioreactors.  

Flow of mixed liquor was reversed and is now sent to the pre-existing aerobic zones. The mixed

liquor is pumped to above-grade stainless steel membrane tanks and a return activated sludge

(RAS) stream overflows back to the converted anoxic zones.

To expedite onsite installation and simplify the execution of this project, H2O Innovation provided

a pre-engineered, pre-fabricated equipment building. The building arrived to site on a truck with

all pumps, blowers, valves, instruments and control panels pre-installed. The equipment was

fully plumbed and all electrical wiring was completed so that the onsite installation effort was

limited to interconnecting plumbing between the building and the process tanks.

The project was commissioned in two phases to allow the plant to continuously treat incoming wastewater. One train was retrofitted into an MBR while the other continued to operate as a conventional activated sludge system. Since the existing plant had been operating as a

conventional activated sludge system, seeding of the MBR was expedited. After operation of the

initial MBR train for two weeks, the second conventional train was taken offline and retrofitted.

Since most of the commissioning had already occurred, the second MBR train was

commissioned, seeded and started up very quickly, treating wastewater to effluent specifications within 24 hours.


Despite the significant challenges associated with operating the wastewater treatment plant

while construction activities continued onsite, the start-up of the ETS MBR treatment plant was

very successful. The use of a pre-fabricated building with the equipment installed ahead of time

saved months of onsite construction effort.


The plant was commissioned through May and June of 2018 and has been producing excellent effluent quality as shown in the table below from August 2018:


MBR technology provides excellent wastewater treatment and is particularly well-suited for

retrofits of existing treatment facilities. MBR retrofits can provide increased capacity within the

same footprint, removal of phosphorus and nitrogen and improve removal of BOD and TSS.

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