Combined Sewer Overflows

Public Works

Today, the City's sewer and drain collection systems consists of both separated and combined stormwater and sewer systems. During significant rain storms combined sewer systems may fill up beyond their capacity with a mixture of sanitary waste and rainwater. Combined Sewer Overflows (CSO) act like a relief valve allowing sewerage to discharge into waterways preventing sewerage backups into homes, businesses and streets. Cambridge’s combined sewer system ties into the Massachusetts Water Resources Authority's (MWRA) wastewater collection system which is treated and discharges at Deer Island. During periods of heavy rain both MWRA and Cambridge’s wastewater systems can become overwhelmed by rainwater releasing CSOs into both the Charles River and the Alewife Brook.

Combined Sewer Overflow Diagram
To view a map of all the outfalls and contributing catchment areas in Cambridge, click here

What is being done about CSO's?

As part of the Federal District Court Order in the Boston Harbor Case (D. Mass. C.A. No. 85-0489-RGS), the MWRA is required to undertake certain corrective actions to reduce or eliminate CSO discharges along Boston Harbor, the Mystic, Charles and Neponset rivers and Alewife Brook. MWRA developed a CSO Control Plan in 1994 which was implemented based on design and construction schedules mandated by the Federal Court as part of the Boston Harbor Court Case.

MWRA and its communities with permitted CSO outfalls, including Boston, Cambridge, Chelsea and Somerville, have been reducing CSO discharges continuously since the 1980s. MWRA, Boston Water and Sewer Commission and the City of Cambridge have completed a number of projects to reduce CSOs to the Charles River. These projects have reduced average annual CSO discharge volume to the Charles River basin by 98% since 1988.

Seven of the MWRA Long-Term CSO Control Program projects have contributed to the control of CSO discharges to the Alewife Brook. These completed projects have closed six CSO outfalls (half of the outfalls that were active in the late 1980s) and are predicted to reduce average annual CSO volume to Alewife Brook by 85% and reduce the frequency of CSO discharges among the six remaining outfalls from sixty-three activations a year to seven activations a year. Remaining CSO discharges are predicted to comply with Class B “fishable/swimmable” water quality standards 98% of the time.

Major improvements to the regional wastewater collection and treatment system by MWRA and local system improvements by the communities, including the separation of combined sewers with construction of new storm drain systems, have contributed to the closing of many CSO outfalls and an 88% reduction in the total volume of CSO discharge overall. The CSO work is in part also guided by the CSO Variances issued by the Massachusetts Department of Environmental Protection for the Lower Charles River/Charles Basin and Alewife Brook/Upper Mystic River.

For more information about these and the other projects in the Long-Term CSO Control Plan, their CSO control benefits, and water quality conditions and improvement, see MWRA’s final CSO Annual Progress Report, filed with the Federal Court in March 2016 and MWRA CSO Control Plan Goals and Costs by Receiving Water (PDF, 2016). 

Six CSO outfalls along Alewife Brook remain following the completion of the MWRA’s Alewife Brook Long-Term CSO Control Plan (1 owned by MWRA, 4 owned by the City of Cambridge and 1 owned by the City of Somerville). 

CSOs in the Alewife Brook  

CSOs in the Charles River


There are 15 CSOs in the lower Charles River. Seven of these CSOs are within Cambridge, including 2 MWRA CSO treatment facilities and 5 City of Cambridge CSO outfalls. Currently 2 of the 5 Cambridge CSO outfalls are temporarily closed pending further study.

Sewer Systems:

In Cambridge, the original sewer lines were engineered over 150 years ago as a combined system: sewage and rain water ran through the same pipes and discharged directly into the Charles River through tide gates. 

Separation of the combined sewer system began in the 1930’s. Separated systems are designed and constructed to convey only stormwater to the rivers and only sanitary waste to a treatment plant. During the past twenty years the City 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. The city‘s collection system currently include approximately 113 miles of sanitary sewer, 99 miles of stormwater drains, and 40 miles of combined sewer. Approximately 55% of the collection system owned and maintained by Cambridge has been separated.

The City has developed a Ten Year Sewer and Drain Infrastructure Plan to prioritize construction and rehabilitation of the sewer and stormwater systems. 

The goals of sewer separation projects are:

  • Improving the quality of waterways in Cambridge
  • Reducing or eliminating combined sewer overflows
  • Reducing or eliminating sanitary sewer backups
  • Reducing flooding

To date, the City’s efforts have resulted in measurably cleaner rivers and significantly reduced street flooding. Today, dedicated sewer lines and the remaining combined sewer lines are conveyed to the Massachusetts Water Resources Authority’s Deer Island Treatment Plant through 15 miles of high-capacity state-owned combined sewer interceptors. During significant rainfall events combined sewer overflows can still occur and discharge untreated sewerage into the Charles River or the Alewife Brook.

Important Health Information

During large storms CSO's are released to the Charles River and Alewife Brook. Because of increased health risks associated with CSO discharges, public health officials recommend avoiding contact with bodies of water during rainstorms and for at least 48 hours following a CSO release, and wearing protective clothing such as gloves and boots if contact is unavoidable. Contact with floodwaters should also be avoided as they may contain similar contaminants. 

Reporting and Data about CSOs

CSO Activations

CSOs that are activated during heavy rainstorms are tracked annually and compiled in yearly reports that are submitted to the United States Environmental Protection Agency (EPA), the Massachusetts Department of Environmental Protection (DEP) and the Massachusetts Water Resources Authority (MWRA). Online records will be updated as data becomes available and is processed. To view the annual reports, please click here

Annual Reports

Annual Reports have been prepared in accordance with Part I, Section D of permit No. MA0101974 (the Permit), issued to the City of Cambridge Department of Public Works on September 30, 2009 (effective February 1, 2010) under the National Pollutant Discharge Elimination System (NPDES) program. The Permit authorizes the City of Cambridge to discharge twelve Combined Sewer Overflows (CSOs) to receiving water bodies named in the Permit. The twelve CSOs are associated with eleven regulator structures. To view these annual reports, please click here

Page was posted on 7/24/2019 1:32 PM
Page was last modified on 10/31/2019 7:42 PM
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