Sturbridge, Massachusetts POTW: Greater Capacity for High Flow Events, With Enhanced Clarification for new Regulations


Sturbridge, Massachusetts historically suffered from periodic blooms of filamentous bacteria that caused bulking in the secondary clarifiers of their three activated sludge package plants. The elevated clarifier solids loading rates (SLR) during high flow events often caused excessive sand filter backwash cycle times and occasional diversion of excess flow to a neighboring publicly owned treatment works (POTW).  Sturbridge’s need for additional treatment capacity in a highly constricted footprint, coupled with tighter permit limits for BOD, TSS, total nitrogen (TN) and phosphorus (TP), created the need to upgrade the wastewater treatment plant.


Initially, Sturbridge considered installing a membrane bioreactor (MBR) system to achieve the required limits on contaminant removal and deliver the additional capacity needed without expanding the footprint of the plant.

After examining the capital and operating costs of the MBR solution, Sturbridge investigated and ultimately chose to run a pilot of the innovative BioMag™ System from Evoqua that uses magnetite ballasted technology to enhance settling rates and increase performance. The trial, on one of its three activated sludge trains, was designed to measure results based on three main objectives. First, was to demonstrate that the BioMag System was a cost effective alternative to MBR.  Second, was to identify whether converting the activated sludge system to the BioMag System could increase overall plant capacity from 0.75 MGD to 1.6 MGD without adding additional bioreactor or clarifier tanks. Finally, the pilot would determine whether the increased effi ciency of the BioMag System would enable the plant to meet all permit limits, achieving < 10 mg/L TN and < 0.2 mg/L TP. The BioMag System exceeded all expectations.


Concerned by the expected tightening of phosphorus limits coupled with a desire to have a reliable tertiary treatment process to follow the BioMag System, Sturbridge initially focused on expanding its conventional media filtration system.  But a demonstration by Evoqua engineers proved that the CoMag™ System can produce effluent quality far superior to conventional alternatives and at lower life-cycle costs. Further, after comparing costs to an expanded sand filter, the advantages of the CoMag System with its smaller footprint, no loss of productivity to clogging, plugging or backwashing, and a process guarantee of < 0.05 mg/L of effluent phosphorus made the long-term choice for Sturbridge self-evident.

The full trial period for BioMag and CoMag Systems in Sturbridge ran from November 2007 through May 2008. The high performance process was since commissioned and has been fully operational since Fall 2011.

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Sturbridge, MA POTW
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