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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

South Magnolia CSO Control Project

Project Construction

November 21, 2014 Update -

Continuous concrete pour to occur November 22
Residents and businesses around the Smith Cove area received notice last week about work to build a 12-foot deep slab as the base of the new combined sewer overflow (CSO) tank. Barring any unforeseen issues, the concrete pour will begin at about 12:30 a.m. on Saturday the 22nd, and continue for 16-20 hours. This work is being carried out under an approved City of Seattle noise variance.

Crews will begin mobilizing on site about 9:00 on Friday evening. About 500 truckloads of concrete are needed for this process. Concrete trucks and worker parking will be staged at the Port of Seattle cruise ship parking area. The contractor has coordinated with the Port to provide security for the site. Trucks will be entering and exiting the project site on 23rd Avenue West and the construction entrance by the Magnolia Bridge on-ramp. There will be no traffic detours, but people can expect truck traffic in the area throughout the process.

Latest construction update: Overnight work needed for the new CSO storage tank (PDF, November 7, 2014)

32nd Avenue West
King County’s contractor is continuing underground drilling on 32nd Avenue West to install a new gravity sewer pipeline. The drilling contractor, Mears Group, Inc., will begin the last phase of the drilling process this week. This drilling phase is expected to continue through November and will include many of the same activities that were used during the pilot bore drilling.

The contractor expects to have all pipe fusing activities completed by Wednesday, November 19, reducing the traffic delays neighbors have experienced while pipe sections have been unloaded and strings have been moved. Fused pipeline strings will remain on the east side of 32nd Avenue West until pipeline pullback, expected to occur mid-December.

For more detailed information on Horizontal Directional Drilling, visit
Latest construction update: Construction activities on 32nd Ave W (PDF, November 17, 2014)

Please drive slowly though the temporary detour and construction area.

View the Fall 2014 Newsletter (PDF, September 2014)

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

King County is constructing a new combined sewer overflow (CSO) control facility in Magnolia. A 1.5-million gallon underground tank will temporarily store excess flows of stormwater and sewage when heavy rains fill pipes, reducing untreated overflows to Puget Sound. A second component of this facility is a new 3,000-foot gravity sewer pipeline installed from 32nd Avenue West below Magnolia Bluff that will transfer excess flows to the new CSO control facility on 23rd Avenue West in the Smith Cove area. 

On 32nd Avenue West, construction crews will install the new gravity sewer pipeline that will be drilled under the hill to the Smith Cove Athletic Field to connect to the underground storage tank on 23rd Avenue West. See map on right.

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 control combined sewer overflows (CSOs) in local waterways. When completed, the project will meet current Washington Department of Ecology requirements of no more than one CSO discharge 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.

Being a good neighbor to nearby Magnolia eagles

Magnolia Boulevard Park has a pair of bald eagles that have resided in a cottonwood tree above 32nd Avenue West for several years. During project design and as construction approached on 32nd, Hillary Schafer, King County’s Environmental Planner, worked closely with local and federal environmental agencies to develop plans for work around the eagle nests. Plans in place include seasonal restrictions on certain activities during mating seasons, monitoring and other accommodations 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

Related information