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Combined Sewer Overflow Control

By reducing combined sewer overflows into local water bodies, King County is protecting public health and the environment.
For questions about the King County CSO Control Program, please contact Karen Huber.

Wastewater Treatment Division
King Street Center
201 S. Jackson St., Suite 505
Seattle, WA 98104-3855

Get Directions to our office location in Seattle, Washington.

Real Time CSO Notification

King County's Approved Long-term CSO Control Plan Amendment

A map of approved CSO control projects

King County Council and Executive approved nine projects to control overflows that occur at 14 CSO locations in the regional wastewater system. Click on the project areas of the map below to view a more detailed map of potential site boundary and representative footprint. (This map is also available as a PDF file, pages 4 & 5 of King County Executive's Recommended CSO Control Plan, June 2012.)

Recommended CSO Control projects - click CSO locations to learn more.

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More about the approved projects

The nine approved projects control overflows that occur at 14 CSO locations in the regional wastewater system:

  • Two projects would construct CSO treatment facilities in industrial areas near the Duwamish River and Elliott Bay.
  • Seven projects would control CSOs by building underground tanks or pipes to store the flows until a storm subsides:
    • Four storage projects would be built in the Lake Washington Ship Canal/Montlake Cut area, and three in the Duwamish River/Elliott Bay area.
    • King County and the City of Seattle will continue to work to collaborate on three of the seven projects. If, as they complete their own long-term CSO control plan, the City of Seattle does not recommend joint projects the County has also approved independent projects to take their place.  
    • Four of the seven projects would include further analysis of green stormwater infrastructure as potential components to help reduce the required size of storage projects.

King County Wastewater Treatment Division considered five CSO control methods in developing, comparing, and recommending project alternatives.