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CSO treatment facilities

Learn about King County’s combined sewer overflow facilities that treat a combination of stormwater and wastewater so it doesn’t discharge directly into our water bodies.

Our combined sewer overflow (CSO) facilities are designed to treat a combination of stormwater and wastewater that fills pipes to overcapacity in older parts of Seattle during periods of heavy rain. They provide primary treatment (physical settling of solids), disinfection (usually chlorination), and dechlorination of CSOs that would otherwise discharge directly to our water bodies.

Our CSO facilities are built to directly serve the area where they are located. Unlike West Point Treatment Plant, which operates year-round, these facilities operate only during heavy rainfall.

In addition to West Point Treatment Plant, we have 5 local CSO treatment facilities. We’ve also built a roadside rain garden in West Seattle to control CSOs.


 Alki CSO Treatment Facility


 Carkeek Pump Station


 Inlet Structure to the Henderson CSO Storage Tunnel



Elliott West