Controlling CSOs

This project keeps sewage and stormwater out of Puget Sound

When heavy rains fill our wastewater pipes, Combined Sewer Overflows (CSOs) release sewage and stormwater into Puget Sound and other local water bodies. The chemicals and germs in CSOs can damage water quality, and harm people and animals.

CSOs in King County exist only in older Seattle neighborhoods, where one set of pipes carries both sewage and stormwater. CSOs release 10 percent sewage and 90 percent stormwater. Most of the time, this water goes to a wastewater treatment plant.

Through its Protecting Our Waters program, King County is working to limit its CSOs to one overflow per year on average over the long-term.

The pipe to be replaced was installed in 2015 as part of the South Magnolia CSO Control project to meet this requirement.

Did you know?

Since 1979, King County has reduced its overflows by 90 percent. This has kept more than 2.3 billion gallons of sewage and stormwater out of local waterways. This project will help us continue to reduce overflows in Magnolia. Once the project is complete, overflows will be reduced from approximately 21 overflows per year to one per year, on average.

Graphic of red home showing sewage, roof drain and storm drain draining into CSOs. Graphic also shows where the CSOs travel – either into Puget Sound or into a treatment plant pipe.

Click image for larger graphic
CSOs in Magnolia send sewage and stormwater to Puget Sound during storms.