Combined Sewer Overflows

The City of Cambridge, the City of Somerville, and the Massachusetts Water Resources Authority are proud to present the Draft Updated Combined Sewer Overflow Control Plan (Draft Plan) – an achievement that reflects years of collaboration, innovation, and shared commitment to a cleaner and healthier future for the receiving waters and the communities that rely on them. Grounded in rigorous data analysis and shaped by extensive public and stakeholder engagement, this forward-thinking plan balances environmental responsibility with community priorities, construction impacts, and affordability. With an estimated investment of $1.3 billion, the Draft Plan will dramatically reduce CSOs across all three waterbodies and, even in the face of increased precipitation driven by climate change, is projected to eliminate CSOs in a 2050 Typical Year. As the first CSO plan in the nation to explicitly incorporate climate change, it sets a new standard for integrated, resilient water management. Built upon four years of dedicated technical work and partnership, this plan is designed to deliver meaningful water quality improvements and positions the region as a national leader in addressing the complex challenges of CSO control, climate resilience, and long-term environmental stewardship.

To learn about the plan, please visit the Draft Updated CSO Control Plan website.

What are Combined Sewer Overflows?

Like many older European and U.S. cities with 19th century infrastructure, Cambridge also has a combined sewer system where both sanitary sewage and stormwater travel through a single pipe network. In combined sewer systems, a CSO outfall functions as a hydraulic relief point that helps prevent sewage backups into streets, homes, and businesses. During dry weather (no rain or snow) and small or less intense storms, these single-pipe systems convey all flow to regional treatment plants. However, during significant rain events, stormwater volumes can exceed the capacity in the system. To prevent backups into homes, businesses, and streets, excess flows during these significant events are diverted through CSO outfalls to nearby waterways—resulting in a CSO event (see image)

Cambridge’s combined sewer system connects to Massachusetts Water Resources Authority (MWRA) wastewater collection system. In dry weather or during smaller storms, the system works fine, and all wastewater and stormwater flow end at the MWRA Deer Island Treatment Plant, where it is treated and then discharged into the Atlantic Ocean. During big storms, both Cambridge and MWRA systems can overflow, sending CSOs into the Charles River and Alewife Brook.

The Alewife Brook and the Charles River all flow through highly developed, mostly urbanized lands. Pollution sources include stormwater, dry weather inputs (illicit connections), upstream inputs, and CSOs. All these inputs contribute bacteria and other pollutants loads that can make the three waterbodies unsafe for recreation, like swimming and boating.
While CSOs can help prevent backups, flooding, and property damage, they can affect the water quality and ecosystems in our rivers and streams. All Cambridge’s CSO discharges are untreated. That’s why reducing CSOs is a big priority for Cambridge.

The Cambridge Sewer System

Cambridge’s CSO discharges are regulated at the federal and state through the NPDES CSO Permit No. MA0101974 issued by EPA and MassDEP and the 2024 Variance.

Cambridge’s NPDES CSO Permit authorizes discharges from eleven (11) CSO outfalls, only 7 of which are open:

• CAM001, CAM002, CAM401A and CAM401B in the Alewife Brook.
• CAM005, CAM007 and CAM017 in the Charles River. Two CSO outfalls in the Charles River are temporarily closed: CAM009 and CAM011.

What’s has been done about CSOs?

Regionally, since the 1980s, MWRA and its partner communities, Cambridge, Boston, Chelsea, and Somerville, have worked to reduce combined sewer overflow (CSO) discharges. Projects by Cambridge, MWRA, and the Boston Water and Sewer Commission, have cut CSO flows to the Charles River by 98% since 1988.

For Alewife Brook, seven MWRA long-term projects have helped close six CSO outfalls. These efforts are expected to reduce CSO volume by 85% and cut discharge frequency from 63 times a year to just 7. Remaining discharges are predicted to meet Class B “fishable/swimmable” water quality standards 98% of the time.

Major upgrades to the regional wastewater system and local improvements, like separating combined sewers and building new storm drains, have closed many CSO outfalls and reduced overall CSO volume by 88%.

Separation of the combined sewer system began in the 1930’s. Over the past 50 years, the City has been making significant infrastructure investments to separate the stormwater system from the sewer system, but there are still parts of the City that have combined sewers.

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.

Continued CSO reductions at the remaining outfalls require complex solutions. CSOs were purposely built into the system as a hydraulic relief tool for sewage conveyance and to protect the public from sewage discharging directly into people’s homes or city streets; reducing and/or eliminating them is a difficult balance to ensure that the risk and consequence is not just moved downstream.