Denny Way/Lake Union CSO Project

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Issue No. 12
July 2005

How the System Works:

The Denny Way/Lake Union CSO Control System significantly reduces CSOs to Elliott Bay and Lake Union. Until now, overflows occurred because the decades-old sewer system in Seattle was built to carry both stormwater and sewage, and many of the old pipes were not large enough to handle increased flows during storms.

Artist rendering of new outfalls west of Myrtle Edwards Park.

Overflow points were built into the old system so combined stormwater and sewage could flow into Lake Union and Elliott Bay when the pipes filled up.

The Denny Way/Lake Union CSO Control system now captures increased flows during storms and diverts them to the Mercer Street Tunnel. During dry weather, the tunnel is empty and wastewater flows in the previously existing sewer system. During storms, increased flows are diverted into the tunnel through the south Lake Union pipelines. After a storm, the new Elliott West CSO Control Facility pumps the flows out of the Mercer Street Tunnel to King County's West Point Treatment Plant at Discovery Park.

During large storms, the tunnel could fill completely. When that happens, the CSO facility on Elliott Avenue West automatically begins treating the flows and discharges them through the new pipelines and offshore outfalls west of Myrtle Edwards Park.

This newsletter is also available to view as a Acrobat PDF file (428KB).

For more information, e-mail or call Yvonne Kraus,
community outreach coordinator,
206-269-0229, ext. 12.

Yvonne Kraus
Norton-Arnold & Co.
1932 First Ave., Suite 510
Seattle, WA 98101

Updated: July 5, 2005