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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

West Duwamish CSO Control Project

Controlling combined sewer overflows at West Michigan and Terminal 115 

Project planning

King County is working closely with Seattle Public Utilities to coordinate our projects and complement our community engagement efforts. To learn more about Seattle Public Utilities drainage projects in South Park, visit:

March 2016 Update
King County has formed a design team to develop a solution to reduce overflows of stormwater and sewage into the Duwamish River. These overflows come from pipes in the Highland Park and South Park neighborhoods of Seattle.

Be RainWise! RainWise, the rebate program that pays property owners to install rain gardens and cisterns on private property, has been available in South Park and Highland Park since 2013. It is available to eligible property owners through 2016. To learn more and check your availability visit

Quick links

Project description

Like many cities around the country, the older parts of King County’s sewer system use a single set of pipes to carry sewage and stormwater to a treatment plant. During storms, the pipes can fill up with stormwater, leading to overflows of polluted runoff and raw sewage into nearby rivers, streams, and Puget Sound. When sewer pipes in the Highland Park and South Park neighborhoods fill with too much stormwater, the polluted water overflows into the Duwamish River. Although these combined sewer overflows (CSOs) prevent sewage backups into homes and streets, they also pose public health concerns.

King County is working with a technical team to explore several alternatives for reducing 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.

King County will select an alternative based on which one does the best job reducing CSOs. Considerations include constructability, operation and maintenance requirements, community impacts and benefits, and cost. During the spring and summer of 2016, the project team will be consulting with community members to learn more about important considerations, such as parking, existing trees, concerns about street or basement flooding, and other issues that will help inform our decision-making.

Project area and vicinity map

Project need

This project is part of a larger Combined Sewer Overflow (CSO) Control Plan that will reduce sewer overflows into local water bodies and protect public health and the environment. The program began in the 1960s. In this plan, there are 14 current or approved projects left to complete to reduce overflows that occur in the regional wastewater system. Duwamish projects, including the South Park and Highland Park GSI project, were prioritized based on what King County heard from the community. 

Project schedule

Project design will begin later in 2016.

Working with the community

This project will protect the investments being made to clean up the Duwamish River and it will continue creating a better, cleaner river for South Park, Highland Park and the region to enjoy.

Rain garden planting event at Highland Park Improvement ClubNew rain garden at the corner of 12th Avenue South and South Southern Street in South Park
(Left) Rain garden planting event at Highland Park Improvement Club
(Right) New rain garden at the corner of 12th Avenue South and South Southern Street in South Park

King County works with communities to provide project information, identify potential impacts, and involve the community in project design where possible. Community members can expect:

  • One on one outreach to directly impacted neighbors
  • Neighborhood meetings
  • Community group briefings 
  • Project newsletters and fliers
  • Project web page updates
  • News releases

Please contact us if you would like to be on our project email and/or mailing lists, and watch this site for updates!

Project updates

  • View project library for project updates, fact sheets and other information 

Related information

Don't feed the Tox-ick monster!
Seven simple actions you can do to protect Puget Sound
Español (PDF)

For more information

Why does sewage overflow on the rainiest days?