King County's Combined Sewer Overflow Control Program

Thank you for visiting the Web site for King County's Combined Sewer Overflow (CSO) Control Program Web site. Here, we share information on what a CSO is, why they occur, and where they are found in the King County system. And we explain what we're doing to control them.

January 2012 Update
The end date for providing comments on the Wastewater Treatment Division’s Recommended Combined Sewer Overflow (CSO) Control Plan was December 31, 2011. All comments we received have been forwarded to King County Executive Dow Constantine to help shape the proposal for CSO control that he will forward to the King County Council by the end of March 2012. The Council is expected to review and make any amendments to the Plan through the summer. Adoption of the amended Plan is anticipated by early fall 2012. The public will have the opportunity to comment on the Executive’s recommended plan during the Council review.

Continuing comment by the public is welcome. Comments we receive from January 1, 2012 onward will be included in the review of the Executive’s recommended plan. Learn how to comment and be involved.

2011 CSO Control Program Review Summary of Technical Memorandums (November 2011)

King County Wastewater Treatment Division Recommended Combined Sewer Overflow Control Plan (October 2011)
Our recommended plan calls for controlling all King County CSO locations to an average of no more than one overflow per year at each location. To meet this goal, we propose constructing nine projects in the Lake Washington Ship Canal area and in industrial areas near the Duwamish River.

Real-time CSO notification (Seattle area)

Quick facts

The King County CSO Control Program:

• began in the 1970s
• includes the combined sewer areas within Seattle
• manages 38 CSOs; City of Seattle manages about 90 CSOs
• expects to complete CSO control by 2030.

Background

In the 1950s, more than 20 billion gallons of untreated or poorly treated wastewater flowed from combined sewers into major Seattle lakes, the Duwamish River and Puget Sound. By the 1980s, efforts by King County and Seattle had reduced the CSO baseline to an average of 2.3 billion gallons per year. With construction of CSO control projects since then, King County by 2005 has reduced CSO volume to an average of less than 1 billion gallons per year. Learn more about the relationship between King County's and City of Seattle's management of CSOs.