Combined Sewer Overflows (CSOs)

2021-2024: Continuing Work 

The Final CSO Post Construction Monitoring Program and Performance Assessment Report published in 2021 lists the compliance status of the outfalls and plans for making further progress. Development of an Updated CSO Control Plan for Variance waters. MWRA and CSO Communities continue efforts to reduce CSOs. 

The Federal Court overseeing the Boston Harbor Project including the Long Term CSO Control Plan, has required MWRA to provide ongoing reports and briefings on the further progress toward meeting the long-term plan goals.

Not all sewer systems are created equal. While modern systems transport rainwater and sewage from homes and businesses through separate pipes, some older systems like Boston's have "combined" sewers that carry both flows together. During normal conditions flows are delivered to treatment plants. During very heavy rains, when flows sometimes double and even triple, these systems become overloaded. Built-in overflows (called combined sewer overflows or "CSOs") must then act as relief points by releasing excess flows into the nearest body of water. This prevents sewage backups into homes and onto area streets, but the discharges can impact water quality. Most of the active CSOs in MWRA's service area receive treatment such as screening, disinfection and dechlorination.

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Why CSOs Must Be Controlled

CSOs occasionally discharge a combination of rainwater and sewage into Boston Harbor, the Charles and Mystic Rivers, and Alewife Brook.

Public Health and Environmental Impacts of CSOs

Public health officials recommend avoiding contact with active CSO receiving waters during rainstorms and for 48 hours afterwards as there may be increased risks due to bacteria and pollutants associated with urban stormwater runoff and CSOs. See map for the locations of active CSOs in MWRA's service area.

Where CSOs are Located

Boston, Cambridge, Chelsea and Somerville have combined sewer systems that connect to MWRA's system. In 1987, a total of 84 active, uncontrolled CSOs discharged combined flows into Boston Harbor and into the Charles, Mystic Neponset Rivers.

As of December, 2015 CSO discharges have been eliminated from 34 of the 84 outfalls and virtually eliminated from 5.

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How CSO Facilities Work

MWRA operates facilities that screen and chlorinate combined wastewater (sewage and storm water) prior to discharge or further treatment.

MWRA's CSO facilities at Union Park (Boston), Cottage Farm and Prison Point (Cambridge) store and pump CSO flows. Screened and chlorinated wastewater can be stored and pumped back into the system for processing at the Deer Island treatment plant. Any treated wastewater that exceeds a facility's storage capacity is discharged through CSO outfalls.

At the Somerville Marginal facility, combined wastewater flows enter and leave the facility by gravity, not pumping. The wastewater is screened, chlorinated and dechlorinated. The disinfected wastewater overflows into its receiving water as quickly as it arrives at the facility.

CSOs and the Charles River

The Massachusetts Water Resources Authority, 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 already reduced average annual CSO discharge volume to the Charles River basin by 98% since 1988.

Evidence of a cleaner Charles River is emerging:

• The river is now generally safe for recreational boating.
• Healthy populations of river herring in the Charles provide the Division of Marine Fisheries with fish to stock other Massachusetts Rivers.

Other wastewater system improvements have also dramatically reduced CSOs to the Charles River by transporting much more flow to MWRA's Deer Island Treatment Plant. Also, MWRA's Cottage Farm CSO facility on the Charles River at the BU Bridge has been upgraded to optimize treatment system performance and minimize potential harm to aquatic life in the river posed by discharges.

The CSO work is in part guided by the Lower Charles River Basin CSO Variance issued by the Massachusetts Department of Environmental Protection. Each year, The US EPA provides a detailed summary of the progress that has been accomplished toward cleaning up the lower Charles River. The "Charles River Report Card" has been released every year since 1995. Since that time, the Charles River's grade has gone from a "D" to a "B" or higher since 2004.

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Besides implementing CSO control projects and conducting its own water quality investigations of the Charles River, MWRA has provided financial support to the Charles River Watershed Association’s studies, and in the U.S. Geological Survey’s Lower Charles River Project.

CSO Control Program Costs

In 1990, MWRA and its engineering consultants recommended a $1.3 billion dollar CSO plan that called for an extensive system of pipes that would drop wet-weather flows into deep, bedrock tunnels where they would be stored temporarily until being pumped to Deer Island for treatment. In 1994, a second look at both CSOs and the entire sewer system brought a different solution. The new plan, with a price tag of approximately $906 million, is in keeping with new state and federal CSO policies and takes into account other MWRA facility improvements that also reduce CSO volumes and impacts. This plan recognizes that sewer system improvements have already reduced CSO volumes significantly and provided treatment for the majority of the remaining flows.