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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

July 20, 2015 Update

King County’s contractors are moving along with construction of our new wet-weather facility in Magnolia! If you missed the news, KOMO aired a piece about the archeological discoveries at the underground storage tank site:

Workers unearth centuries-old artifacts under Seattle bridge (external link)
The video footage includes the odor control/electrical facility that will support operations of the storage tank and pumps.

For people who missed the presentation on the archeology story at Museum of History and Industry (MOHAI), we are looking at hosting a presentation this fall in Magnolia to talk about this work and also provide an overview of how King County works to protect our heritage, both historic and prehistoric. If you are new to Magnolia, you may not know that King County’s West Point Treatment Plant in Discovery Park is located on an important cultural site for the region’s tribes. More information about the discoveries at West Point can be found at

CSO Control Project Update – Final paving of 32nd Avenue West this week

Please plan for delays especially if you have important appointments.

The end is very near! Wednesday and Thursday, July 23-24, Walsh Construction will have heavy equipment on the road for two days to complete final paving. Traffic will be reduced to one lane and delayed to allow crews room to maneuver and pave in the road in sections. Newly paved areas will require cooling, which may add to wait times.

Flaggers will direct traffic through the work area, but people should expect extended delays, at times up to 20 minutes or more. Work hours will be 7:00 a.m. to about 5:00 p.m. weekdays. Following final paving, crews will complete plantings and restoration along shoulders of 32nd Avenue West.

Storage facility progress

Brickwork for the odor control/electrical building is also near complete. The roof framing is in, so you can get now see how the building will look. The view will be more complete when the final landscaping is installed. The newly-seeded athletic field on 23rd Avenue West is being watered, and this portion of the field is currently closed but will reopen when grass is established.

View construction update (PDF, July 21, 2015) and Winter 2015 Newsletter (PDF, February 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

Construction photos -- gravity sewer pipeline
Gravity sewer pipeline

Construction photos -- storage tank
Storage tank

Project updates

News releases

In the news

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