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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
Alki

Carkeek Pump Station
Carkeek


Elliott West


Georgetown

Inlet Structure to the Henderson CSO Storage Tunnel
Henderson/Norfolk

 

CSO Control Projects in design or construction

Related Topics

Capital projects

Find information on King County’s current and planned wastewater capital projects, including project descriptions, locations, and status, and sign up for project updates. 

Wastewater programs

Learn about our programs to improve water quality, plan and prepare our wastewater system for regional growth, and protect public health and the environment.

Georgetown Wet Weather Treatment Station

The Georgetown Wet Weather Treatment Station, located at the corner of 4th Avenue South and South Michigan Street in Seattle’s Georgetown neighborhood, reduces water pollution and improves aquatic habitat and the overall water quality of the Duwamish River.

West Duwamish Wet Weather Storage Facility

Learn about King County’s plans to build a wet weather storage facility to hold stormwater and wastewater that currently overflows into the Duwamish River during large storms.

Elliott West Wet Weather Treatment Station upgrade

King County is planning to upgrade the Elliott West Wet Weather Treatment Station (EWWTS). The upgrade will ensure compliance with stricter environmental regulations, meet the demands of a growing population, and handle a changing climate.

Barton Roadside Rain Gardens

Learn about the Barton roadside rain gardens, a green infrastructure project to control combined sewer overflows in West Seattle.
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