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Controlling combined sewer overflow outfalls

King County and the City of Seattle are working together to control CSOs.

King County is legally required to control all of its CSO outfalls. A “controlled” CSO outfall can overflow no more than one time each year based on a long-term average, according to the Washington State standard.

King County and Seattle: both working to fix the problem

King County and the City of Seattle are each responsible for specific CSO relief points within the city limits. The history of the sewer system led to this shared responsibility. In the 1950s, voters created Metro to clean up the region’s waterways. Metro built a regional sewer system and took over operation of some of the City of Seattle's system. In 1994, King County assumed authority of Metro. King County is now responsible for treating wastewater for 34 local jurisdictions and agencies, including the City of Seattle. 

Each CSO outfall provides a drainage relief point for specific neighborhoods. King County manages 38 CSO outfalls that serve areas that are greater than 1000 acres. The City of Seattle manages about 82 CSOs that serve smaller areas. View this map to see where County and City CSOs are located.

Decades of successful improvements

Through independent and joint efforts, King County and Seattle have reduced the annual volume of untreated CSOs discharged to local waterbodies.

King County began its CSO Control Program (now called Protecting Our Waters) in 1979. Since then, the County has reduced the annual CSO discharge volume from around 2.3 billion gallons per year to approximately one billion gallons per year on average. More projects are being planned and built to reduce the amount of discharge further. The remaining projects are some of the most complex and expensive.