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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

South Magnolia CSO Control Project

Project Construction

November 10, 2015 Update

Community members are invited to explore some of Magnolia’s history and the field of archeology. On November 19, 2015, King County Wastewater Treatment Division invites all to learn about our award-winning efforts to protect history, and to hear archeologist Alicia Valentino share the story told by remnants of Seattle’s Finntown. Before Alicia’s presentation, you can:

• Learn about how archeologists have recovered and protected cultural resources on King County’s project sites in Magnolia
• See artifacts from Finntown and replicas of artifacts from the West Point Treatment Plant site in Discovery Park
• Visit with the Magnolia Historical Society and learn about their work to chronicle the area’s history.

Thursday, November 19, 2015
Open house and storytelling- 5:30-7:00 p.m.
Presentation- 7:00 p.m., followed by Q &A
Catherine Blaine School, in the cafeteria
2550 34th Ave W, Seattle, WA 98199

In 2014, King County’s contractor unexpectedly unearthed clues to a pre-World War II shantytown during construction of the new Magnolia Wet Weather Storage Facility. Over a month, archeologists recovered over 2,400 artifacts from a site that turned out to be remnants of Finntown, a multicultural community that clung to the tide flats of Smith Cove around 1920-1940. Learn more: view WTD Blog post, June 5, 2015

Qian Long coin from the Ch’ing Dynasty

Do you have a personal story from Seattle’s past you would like to tell? From 5:30 until 7:00 p.m., you can provide video or audio stories to our team! If you are interested or to request special accommodations at this meeting, call or email Monica Van der Vieren, 206-477-5502,

Ongoing November construction activities
King County’s contractor is continuing construction at the Magnolia Wet Weather Storage Facility site at Smith Cove. This 1.5-million-gallon underground tank will store excess flows of stormwater and sewage when heavy rains fill pipes, reducing untreated overflows to Puget Sound.

Crews are re-installing a Seattle City Light street light on 23rd Avenue West south of the Magnolia Bridge in front of the storage site. The light was temporarily removed to accommodate construction. Equipment is being installed in the building that sits atop the storage tank, and landscaping at the site is expected to begin in this month.

King County’s contractor will also test the pipe under 32nd Avenue West. Crews will work mostly near the shoulder on the east side of the street. As needed, flaggers will direct people around the work area. Work hours will be weekdays from 7:00 a.m. to 5:00 p.m.

View construction update (PDF, October 30, 2015).

Construction photos: gravity sewer pipeline and storage tank

Project area for the South Magnolia CSO control project elements (diversion structure, gravity sewer, storage tank)
South Magnolia CSO basin project area
How will the new CSO facility work? View animation.

Project vicinity map
South Magnolia basin and project location.

Young bald eagle
Young bald eagles appear larger and darker than their white-headed parents.

Project description

Storage tank concrete base and sewer pipeline installation complete!

King County is constructing a new combined sewer overflow (CSO) control facility in Magnolia to reduce overflows of untreated stormwater and wastewater into Puget Sound during heavy rains. At the end of 2014, King County’s contractors completed two important milestones. Stellar J Corporation successfully carried out a continuous concrete pour to install a 12-foot thick concrete base slab for the 1.5 million gallon underground storage tank.

The second component of this facility is a new gravity sewer pipeline. Walsh Construction and Mears Group, Inc. completed the final step in the Horizontal Directional Drilling (HDD) process, pulling almost 3,000 feet of pipeline through a borehole extending from 32nd Avenue West to an exit pit on 23rd Avenue West. During wet weather, stormwater and sewage will be diverted through this pipeline to the newly constructed underground storage tank across from Smith Cove Park.

Why does King County need to do this project?

This project is part of a long-term King County plan to help protect Puget Sound and reduce the number of combined sewer overflows (CSOs) in local waterways. In 2013, King County reported a long term average of 22.7 overflows per year from the Magnolia Outfall. The new CSO Control Facility is designed to reduce overflows to Elliott Bay to no more than one per year on a long-term average. The project is included in a federal consent decree that requires completion by December 2015.

What is the schedule for this project?

Planning and design for this project took place from 2007 to 2013. Construction began in 2014 and is scheduled for completion in 2015.

South Magnolia CSO Control Project Timeline, May 2013
View South Magnolia CSO Control Project Timeline (PDF), updated May 2013.

Update on the Magnolia eagles

In recent months, area neighbors have reported sightings of bald eagles that built a nest in Magnolia Boulevard Park above 32nd Avenue West. We are monitoring the eagles until August, when work at the site will be complete. Work activities must adhere to seasonal restrictions to create the least disturbance for the birds. For more information, view the Fall 2014 Newsletter (PDF) or contact

Hotline - South Magnolia CSO Control Project

24/7 Construction Hotline: 206-205-0968

Leave a message, and a project team member will return your call.
Project contact: Monica Van der Vieren, 206-477-5502, or via feedback form.

Construction photos

Gravity sewer pipeline photo album in Flickr
Gravity sewer pipeline

Storage tank photo album in Flickr
Storage tank

Project updates and news releases

  • View project library for current and past project updates, and news releases

In the news

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