Skip to main content
Many King County facilities are closed to the public. Learn how to access services remotely or while practicing social distancing guidelines.  
King County logo

Completed – Spring 2005

The Denny Way/Lake Union Combined Sewer Overflow Control Project is a joint effort of King County and the City of Seattle to control combined sewer overflows (CSOs) into Lake Union and Elliott Bay. CSOs are overflows of sanitary (residential and commercial) sewage and stormwater that are released into water bodies during storms.

Dedication ceremony held Thursday, July 7, 2005

After nearly 12 years of planning and more than four years of construction, the Denny Way/Lake Union Combined Sewer Overflow Control Project is complete. This joint project with the City of Seattle is one of the most significant public investment efforts undertaken by King County and the largest of all county CSO control systems.

We want to express our appreciation to members of the community, organizations and businesses affected by construction—and to agencies, officials, employees, and companies that supported this major effort to protect public health and the environment.

The CSO control facility is an important element of the Denny Way/Lake Union CSO Control Project. The facility has two basic modes of operation:

  1. During rainstorms, the facility will direct combined stormwater and sanitary sewer flows into the new Mercer St wastewater storage tunnel. That is expected to happen about 50 times a year. After each storm subsides, the CSO control facility will pump the stored flows from the Mercer Street tunnel to the Elliott Bay Interceptor, a sewer trunk leading to the West Point Treatment Plant.
  2. During larger storms, about 10 to 20 times a year, the Mercer Street tunnel will fill completely. When that happens, the CSO control facility will automatically begin to treat the stored flows and pump them to the new CSO outfall at Myrtle Edwards Park. Treatment includes screening out floatable materials, disinfection, and dechlorination. Operation of the facility after the storm will be the same as described above.

During very large storms, an average of once a year, flows may exceed the pumping capacity of the CSO control facility. Then, untreated flows will be discharged through the new CSO outfall off Myrtle Edwards Park. New facilities built for this project will thus convey, store and treat combined sewage only during storms. During dry weather, the facilities will be empty, and wastewater will flow through existing pipes to the West Point Treatment Plant.

King County designed this facility with community involvement to ensure its architecture is compatible with the neighborhood. It also incorporates odor control equipment to prevent odors noticeable to nearby businesses and residents. Operation of the facility is automatic, so staff will not be on-site most of the time. Maintenance staff will inspect the facility daily and will be on-site for periodic deliveries of chemicals used in the treatment process. This facility's operation is expected to produce minimal traffic.

Project overview sign 

Ongoing monitoring: Denny Way/Lake Union CSO Control Project Outfall Sediment Monitoring

King County is conducting a long-term sediment monitoring program for the Denny Way/Lake Union Combined Sewer Overflow (CSO) Control Project to fulfill requirements of the Biological Opinion issued by the National Marine Fisheries Service. The chemistry and benthic community data for program monitoring years 1–5 (2006–2010) and Year 10 (2015) are included in the monitoring report: Denny Way/Lake Union CSO Control Project Long-Term Sediment Monitoring Program Data Report Years 2006-2010 and 2015. Although the monitoring program is not complete yet (monitoring years 15 and 20 will occur in 2020 and 2025), this report evaluates the current status of the Denny Way site. 

operations phase

24-hour emergency and odor reporting:

Contact West Point Treatment Plant at  206-263-3801.

King County Wastewater Treatment Division Director Don Theiler addresses the crowd.King County Wastewater Treatment Division Director Don Theiler addresses the crowd

illustration of CSO control facility Elliott West CSO Control Facility