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Wastewater Treatment

King County, Washington

For questions about the Wastewater Treatment Division website, please send an e-mail message or contact us at:

King Street Center
201 S. Jackson St., Suite 500
Seattle, WA 98104-3855
Phone: 206-477-5371
Fax: 206-684-1741
Telecommunication device for the deaf (TTY): 711

Get Directions to our office location in Seattle, Washington.

Staff Contacts

Seattle projects

Fremont Siphon replacement

King County is constructing a new pipeline beneath the Lake Washington Ship canal to carry untreated wastewater from Seattle’s Fremont neighborhood to the West Point Treatment Plant in Magnolia. The current pipeline has been in service for more than 100 years. 

Georgetown Wet Weather Treatment Station

King County will continue design on a new combined sewer overflow (CSO) facility in Seattle’s Georgetown neighborhood that will treat up to 70 million gallons of stormwater and sewage that currently flows directly into the Duwamish River during heavy rains. 

Murray Combined Sewer Overflow Control Project

King County is completing construction on an underground storage tank at its Murray Pump Station in West Seattle. The tank is designed to contain up to 1-million gallons of stormwater and wastewater during heavy rains, which will protect public health and the environment by controlling combined sewer overflows that occur near Lowman Beach Park.

North Beach Combined Sewer Overflow Control Project

The Wet Weather Storage Facility is now operational. While the bulk of construction work was completed in December 2015, the contractor will be completing some work on the pump station site through 2016. This includes work on the new building and final landscaping.

Rainier Valley Wet Weather Storage (Hanford #1 CSO Control) 

King County is beginning construction of a new combined sewer overflow (CSO) underground tank and sewer pipelines in Seattle’s North Beacon Hill and Columbia City neighborhoods, which will control overflows of stormwater and sewage into the Duwamish River during heavy rains.

Sediment Management Program

This program enables King County to address sediment contamination near CSO locations in Puget Sound. The county is continuing its efforts to remove historic CSO contamination and restore habitat at locations in Elliott Bay and the Waterway.

Ship Canal Water Quality Project (external link)

King County and Seattle Public Utilities are working together to build a 2.7 mile underground storage tunnel. This project will keep nearly 60 million gallons of polluted water from Ballard, Fremont, Wallingford, and north Queen Anne out of the Lake Washington Ship Canal, Salmon Bay, and Lake Union each year. Visit the City of Seattle’s project web page (external link) for more information.

South Magnolia Combined Sewer Overflow Control Project

Work on King County's Magnolia Wet Weather Storage Facility at Smith Cove was completed in early 2016! The project's 1.5-million-gallon underground tank can store excess flows of stormwater and sewage from heavy rains to reduce untreated overflows to Puget Sound.

West Duwamish CSO Control Project (West Michigan and Terminal 115 CSO Control)

King County is exploring several alternatives for reducing combined sewer overflows (CSOs) at the Terminal 115 and West Michigan outfalls. There are several options: a 100% “green” solution that could include roadside rain gardens and/or permeable pavement that lets water soak through it, a traditional “gray” infrastructure solution such as an underground storage tank or pipe, or a mix of green and gray.

West Point Treatment Plant improvements 

King County plans a number of improvements at the West Point Treatment Plant. Projects include upgrading and replacing aging equipment to ensure reliable operation and continued compliance with permit conditions