CSOs in Somerville

The majority of Somerville is served by a combined sewer system, which is common in older cities. Most of the time, Somerville’s combined sewer system transports all the wastewater and stormwater to the Massachusetts Water Resources Authority’s (MWRA’s) Deer Island Treatment Plant, where it is treated and then discharged to the Atlantic Ocean. However, during a rainstorm, stormwater runoff rushes quickly into sewers and can cause a dramatic increase of water flowing into and through the combined sewer pipes. When this happens, the total amount of this combined wastewater and stormwater can exceed the capacity of the sewer pipes.

For this reason, combined sewer systems such as Somerville’s are designed with special overflow structures that can release the excess directly to a nearby water body and prevent it from backing up into buildings or out of manholes. These combined sewer overflows (CSOs) contain not only stormwater but also untreated wastewater and debris and are a public health and water pollution concern for Somerville, as well as the approximately 770 other U.S. cities with combined sewer systems. Somerville owns two CSO outfalls that discharge to the Alewife Brook and the Mystic River. The City of Cambridge and MWRA own additional CSO outfalls that discharge into these two rivers.

Separated systems are designed and constructed to convey only stormwater to the rivers and only sanitary waste to a treatment plant. In the recent years, Somerville has increased its sewer separation and stormwater management efforts because of stricter environmental compliance regulations and a desire to provide a better quality to residents' daily lives. To see what Somerville is doing to address pollutants from stormwater runoff please visit our stormwater management website and learn how you can also help reduce stormwater pollution.