CSO treatment facilities

Combined sewer overflow (CSO) treatment facilities are designed to provide primary treatment (physical settling of solids), disinfection (usually chlorination), and dechlorination of CSOs.

They are built to directly serve the area where they are located. Unlike the West Point Treatment Plant, these facilities operate only during heavy rainfall.

In addition to the West Point Treatment Plant, King County has four local CSO treatment facilities (and a fifth one under construction). King County has also built a road-side raingarden in West Seattle to control CSOs. 

Alki	Carkeek
Henderson/Norfolk	Mercer/Elliott West

Barton Roadside Rain Gardens

CSO Control Projects in design or construction

• Georgetown Wet Weather Treatment Station
• Ship Canal water quality (joint project with Seattle Public Utilities)
• West Duwamish