CSO Reduction Over Time

The following graph shows CSO reductions from the many control projects King County has completed. You can also view a map showing the phasing of future CSO Control projects or view tables displaying CSO Control project status information.

At a Glance: King County/Seattle CSO Control Program

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.

King County and the City of Seattle share management of CSOs based on the size of the drainage basin served by each CSO outfall. The county manages CSOs from basins greater than 1,000 acres. The King County CSO program:

• began in the 1970s
• includes 47.5 square miles of combined sewers (within Seattle); about 20 percent of King County’s service area
• manages 38 CSOs; City of Seattle manages about 90 CSOs
• expects to complete CSO control by 2030 (view project list).

Back to top of page.

Return to:
> CSO Reduction and Cost
> CSO Program Page
> CSO Introduction Page
> CSO Control Program Library Page

For questions about the CSO Program, contact Karen Huber.

Department of Natural Resources
Wastewater Treatment Division
201 South Jackson, Suite 512
Seattle, WA 98104
Phone: 206-684-1246
Fax:206-684-1741

Updated: April 26, 2007