The Westerly Wastewater Treatment Plant is an advanced wastewater treatment facility designed to handle a daily average flow of 2.89 million gallons per day. The facility treats sewage from the westerly portion of Marlborough (west of Route 495) and the Town of Northborough under an inter-municipal agreement.
The facility operates under a National Pollution Discharge Elimination System (NPDES) permit issued by the EPA and Massachusetts DEP. The new NPDES permit for this facility has been issued.
The primary change in this new permit is the requirement to treat phosphorus to a level of 0.1 mg/l. This is an extremely low level and will require a significant upgrade to the Westerly Wastewater Treatment Plant. The cost for this upgrade is expected to be about $40 million.
Marlborough’s flow to the Westerly Plant is currently at its available capacity and will require an increase in the design flow of the facility in order to accommodate expected growth in the area contributory to this facility.
The new permit does not contain language allowing additional flow discharge from the facility. Regulations exist, however, that allow the performance of an anti-degradation study to prove increasing flows to the receiving water, the Assaber River, does not negatively impact the river. This study is ongoing. When completed, the city is optimistic that the permit may be modified to allow increased flow to the river.