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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

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Staff Contacts

Rainier Valley Wet Weather Storage

Building a new storage tank and new sewer pipeline to safeguard the Duwamish River 

Project design

May 18, 2016

We have come to a major milestone on the project: construction activities are beginning this week on the Bayview portion of the project.

The first activity you will notice is the removal of the small building between the Bartells and Mutual Fish on Rainier Ave S. Additional work that includes some lane closures on Rainier Avenue S and S Bayview Street will begin shortly afterwards. Lane closures of longer than 30 minutes will be done during the weekend hours listed below. Updates about this work and next steps will be provided as construction progresses.

Work hours for this part of the project:

  • Regular work hours fall between 7:00 AM and 3:30 PM Monday through Friday. Work that alters traffic in Rainier Ave. S. for less than 30 minutes (e.g. traffic may be stopped while a truck backs into a driveway) may take place during regular work hours.
  • Weekend hours: work in Rainier Ave. S. and S. Bayview St. will only occur between 5:00 PM on Saturday evening and 2:00 AM on Monday morning. During weekend hours, traffic may be reduced to one lane.

Project information

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Project description

This project will keep sewage and stormwater out of the Duwamish River. A new sewer pipe will be installed near the intersection of Rainier Ave. S. and S. Bayview St. to send sewage and stormwater from a pipe that gets full during storms to another pipe that has more room. 

A storage tank at the intersection of S. Hanford St. and S. 27th Ave. will catch any remaining extra sewage and stormwater to keep sewage and stormwater out of the Duwamish River. 

Project need

King County’s pipes in Rainier Valley and elsewhere to send sewage and stormwater to a treatment plant in Magnolia to be cleaned. Pipes in the northern Rainier Valley get too much stormwater during storms and spill into the Duwamish River. These spills are called “Combined Sewer Overflows” or CSOs. King County will build new pipes and a new 340,000 gallon storage tank to keep more pollution out of the Duwamish.

service area
Area served by Rainier Valley Wet Weather Storage Project
 (PDF), May 2014

Protects public health

Contact with raw sewage and stormwater can make people sick. Unless they are cleaned at a treatment plant, sewage and stormwater can also harm Puget Sound, Lake Washington and the Duwamish River. The pipes and plants we have to clean sewage and stormwater help keep us healthy.

Cleans up the Duwamish River

The new pipes and tank will avoid CSOs from Rainier Valley and keep sewage and stormwater out of the Duwamish River. They will keep the Duwamish River cleaner by keeping millions of gallons of water pollution out of the river each year (on average).

Untreated CSO volume reduction, before and after project


Project location

New and reused pipe at S. Bayview St.
New and reused pipe at S. Bayview St.
 (PDF), May 2014 

new storage tank at S. Hanford St.
Underground storage tank at S. Hanford St.

(PDF, May 2015)

How did King County decide what to build?

The County has different tools it can use to keep Rainier Valley’s sewage and stormwater out of the Duwamish River. Not every tool is right for every neighborhood. The Rainier Valley facility must work for a big, very busy area where a lot of people live and work.

What works for Rainier Valley
What works for Rainer Valley
 (PDF, spring 2014 newsletter)

Project schedule

Project schedule
Project schedule
 (PDF, May 2015)


Caryn Sengupta
24 Hour Project Information Line: 206-205-9178
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Learn more

Why does sewage overflow on the rainiest days?