Combined Sewer Overflow Green/Gray Infrastructure
Communities throughout the United States have implemented a variety of management approaches to help mitigate the impacts of combined sewer overflows (CSOs).
Learn about integrating green infrastructure into CSO control plans and remedies.
Integrated Planning in Action
City of Atlanta Department of Watershed Management
Atlanta, Georgia
NPDES Permits: GA0037168/GA0038644
In 2001, following several years of public input, the City of Atlanta developed an integrated 25-year plan for reducing combined sewer overflows and improving water quality in the area’s watersheds. As part of this plan the City has invested in green and gray infrastructure projects, including many to meet a 2007 deadline in its federal consent decree for the CSO portion of its sewer system. Green infrastructure projects include the Historic Fourth Ward Park, which is a 17-acre green space that includes a 2-acre stormwater retention basin capable of capturing stormwater from a 100-year storm. Another project is Cook Park, an area prone to flooding where a retention pond and numerous green infrastructure features now capture up to 10 million gallons of stormwater. Gray infrastructure projects have included separating 33 miles of combined sewers and constructing 18 miles of storage, allowing combined sewage to be treated before entering watersheds. Such projects have reduced the number of combined sewer overflows since 2001 by 62 percent and the untreated overflow volume by 97 percent.
Learn more about the City of Atlanta’s holistic water management approach
Clean Rivers Project
District of Columbia Water and Sewer Authority (DC Water)
Washington, D.C.
NPDES Permit: DC0021199
Located at the confluence of the Anacostia and Potomac Rivers, the District of Columbia (DC) has been upgrading its century-old sewer system for years with the help of DC Water’s Clean Rivers Project. The project includes a massive system of tunnels, more than 100 feet deep, designed to reduce CSOs into DC’s waterways by conveying more flow to the city’s wastewater treatment plant. DC Water has also incorporated $100 million of green infrastructure into the project, which has added jobs, improved streetscapes, and provided broad community benefits. DC Water has already reduced CSO volume by 90 percent on the Anacostia River and is on target to achieve a 98 percent reduction on the Anacostia River by 2023 and a 96 percent reduction system wide by March 2030.
Learn more about DC Water’s Clean Rivers Project
Project WIN
Louisville and Jefferson County Metropolitan Sewer District (MSD)
Louisville, Kentucky
NPDES Permit: KY0022411
In 2005, MSD set out to reduce the impacts of CSOs and improve water quality in local streams and the Ohio River. To accomplish this, MSD invested over $1 billion in infrastructure improvements to remedy the community’s aging sewer systems. The investment has allowed MSD to build a new wet weather treatment facility for combined sewer flows, invest in green infrastructure projects, utilize real-time control technology to maximize storage, and construct a tunnel that will prevent 439 million gallons of CSOs from entering local waterways each year.
These projects have reduced CSOs by up to five billion gallons annually, reduced sanitary sewer overflows by 82 percent, and remove approximately 450 million gallons of stormwater from the combined sewer system during a typical year. They also have significantly reduced the levels of bacteria in local streams and the Ohio River. The system-wide improvements have not only improved water quality, but also created green spaces for Louisville residents.
Learn more about Louisville MSD’s Project WIN
Milwaukee Deep Tunnels
Milwaukee Metropolitan Sewer District (MMSD)
Milwaukee, Wisconsin
NPDES Permit: WI0036820
Nearly 30 miles of deep tunnels are helping to store wastewater and stormwater when it rains or snows in Milwaukee, effectively preventing more than 138 billion gallons of polluted wastewater and stormwater from entering Lake Michigan since 1994. As a result of its work, MMSD has treated or captured more than 98 percent of the stormwater and wastewater entering the system. In addition to preventing CSOs, the tunnel system is reducing sewage backups in Milwaukee households. The region is also using green roofs, native plants, river buffers, and other green infrastructure to reduce CSOs, water pollution, and the risk of flooding.
Learn more about Milwaukee’s deep tunnels
Tunnel and Reservoir Plan
Metropolitan Water Reclamation District of Greater Chicago (MWRD)
Chicago, Illinois
NPDES Permit: IL0028053, IL0028061, IL0028070, IL0028088
Since 1975, MWRD has been constructing one of the largest infrastructure projects in the United States – a deep tunnel system that captures and stores stormwater and sewage that would otherwise overflow to local waterbodies during major rain and snow events. The tunnels and reservoirs that make up the system can store about 11 billion gallons during each precipitation event, which will increase to more than 17.5 billion gallons once the final reservoir is complete in 2029. Captured flow is then pumped to one of MWRD’s water reclamation plants for treatment. MWRD is also using green infrastructure to further reduce the amount of stormwater entering the sewer system, lessen flooding, and improve water quality. MWRD is expanding the number of green infrastructure projects in the county by partnering with other communities and public agencies and reimbursing project construction costs.
Learn more about MWRD’s Tunnel and Reservoir Plan
Protecting Seattle’s Waterways
Seattle Public Utilities (SPU)
Seattle, Washington
NPDES Permit: WA0031682
In 2015, SPU developed a long-range plan to reduce CSOs and improve water quality in Puget Sound and other local waterbodies. The plan combines a traditional long-term control plan with an integrated plan to help the city prioritize the most environmentally and financially advantageous projects within a reasonable schedule. The plan is built on a three-pronged approach for reducing CSOs. It includes 1) “fix it first” sewer system improvements, 2) green infrastructure projects to slow, filter, and absorb stormwater before it can enter the sewer system, and 3) tanks and tunnels to store any stormwater the first two options cannot solve. Since the plan was finalized, SPU has constructed four large storage tanks and completed 12 sewer system improvements. Together, these projects have prevented over 100 million gallons of combined sewage from entering Seattle waterbodies.
SPU has also partnered with King County, the regional wastewater treatment provider, on two significant efforts. The first initiative is the 700 Million Gallons program, a mix of utility-led and public private partnerships that are using green infrastructure to prevent stormwater from entering the sewer system. More than 7 miles of green infrastructure have been installed since 2000 and, by 2021, it was managing 465 million gallons of stormwater per year. The second initiative is the Ship Canal Water Quality Project, a large tunnel that will prevent 75 million gallons of combined stormwater and sewage from entering Seattle waterways each year.
Learn more about Seattle Public Utilities’ plan to project their waterways
City of Superior
City of Superior
Superior, Wisconsin
NPDES Permit: WI0025593
Superior, Wisconsin has reduced its CSO outfalls from 28 to 3 by supplementing its wastewater storage and treatment capacity during wet weather. The remaining outfalls are located at three combined sewer treatment plants that are activated during heavy rain or snow events. The plants, which can store and treat about 144 million gallons of wastewater and stormwater, use a series of settling basins, screens, skimmers, and pumps to remove large debris, coarse sediments, and grease. One plant also includes chemical and biological treatment and disinfection. When the storage capacity is not exceeded, wastewater is conveyed to the city’s main treatment facility on Superior Bay for further treatment and disinfection. This system has enabled the city to capture and treat more than 99 percent of combined sewer system flow since 1996.
Learn more about the City of Superior’s combined sewer treatment plants