Elliott West CSO Control Facility

CSO Control Facility October 2004

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 Street 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.

Construction of this facility began in fall 2002 and will end in February 2005.

• December 2004 Newsletter
• Elliott West CSO Control Facility Drawing (PDF Format)

Major Facilities | Mercer Street Tunnel | Elliott West CSO Control Facility | Marine Outfalls | Elliott West Pipelines | South Lake Union Pipelines

For more information about the Denny Way/Lake Union CSO project,
contact Yvonne Kraus:

Phone: 206-269-0229
Fax: 206-269-0249
E-mail: e-mail

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

Updated: Dec. 12, 2003