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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

Highland Park and South Park Green Stormwater Infrastructure 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 2015 Update 
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 will be available to eligible property owners through the rest of this year. To learn more and check your availability visit

Expect design to start later this year on a capital improvement project that will install roadside rain gardens in planter strips between curbs and sidewalks on streets in both neighborhoods. King County staff will be in your neighborhood with more information once the project gets going.

February 2014 newsletterFebruary 2014 newslettermap of possible GSI Roadside Locations and RainWise eligible boundary
Learn more about our work to reduce sewer overflows and promote green solutions by downloading the February 2014 newsletter (PDF) and map of possible GSI Roadside Locations and RainWise eligible boundary (PDF). 

El Condado de King ayuda a encontrar soluciones al problema que se genera cuando el agua de lluvia corre sobre calles, techos y estacionamientos de South Park Highland Park y ya recogiendo contaminacion.

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 big rain storms, the pipes can fill up with stormwater leading to overflows of stormwater and sewage into nearby rivers, streams, and Puget Sound. Although these overflows prevent sewage backups into homes and streets, they also pose public health concerns. When sewer pipes in the Highland Park and South Park neighborhoods get too much stormwater, the polluted water overflows into the Duwamish River.

This project is exploring the feasibility of reducing overflows using Green Stormwater Infrastructure (GSI) or a combination of GSI and storage for sewer overflows. GSI keeps stormwater out of the sewer system and allows it to slowly soak into the ground using rain gardens, permeable pavement, and/or specially designed tree boxes. Two neighborhoods – South Park and Highland Park – have been identified as possible GSI areas based on relatively flat streets, wide planter strips or roads, and the types of soil that allow stormwater to soak in.

An early action in this area is the implementation of the RainWise incentive program that provides rebates to property owners who install rain gardens and cisterns on their property. The program is available now through 2015.

Any remaining stormwater will be controlled with a storage pipe option starting in 2019.

Project area and vicinity map


Why does King County need to do this project?

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

The project timeline is as follows:

Project definition and research 2013 - 2014
Design 2015 - 2017
2018 - 2019
Storage pipe project definition & research, if needed 2019 - 2025

Combined sewer maps

Rights of way connected to the combined sewer system, Highland Park - West Michigan Basin
Highland Park project area showing streets connected to the combined sewer system
Rights of way connected to the combined sewer system, South Park - 8th Ave Basin
South Park project area showing streets connected to the combined sewer system

What is the impact to my community?

WTD spent several months in 2014 studying the soils and groundwater levels to better design any GSI options. Based on street layouts and results of soils and groundwater testing, King County will discuss options for GSI with the community

How King County works 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

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?