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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

December 18, 2014 Update

King County’s contractor is almost done with the pipeline installation on 32nd Avenue. The sewer line, about 3000 feet long, is one part of a new combined sewer overflow (CSO) control facility in Magnolia that will reduce overflows of stormwater and sewage to Puget Sound during rain storms we are currently experiencing.

This week, the Horizontal Directional Drill (HDD) contractor, Mears Group, Inc., is working mostly on 23rd Avenue West to remove dirt from the bottom of the borehole at 23rd. Once they get this removed, they will be able to pull the tail of the pipe through and finish demobilizing and reopening the street.

Because the final portion of the pullback is taking longer, crews will continue working through the holiday weeks except for Christmas and New Years Day in order to get the drilling equipment on its way and the street cleaned up.

View construction update (PDF, December 18, 2014) and pipeline alignment.

Pipeline tail, located on 32nd Avenue West.
King County’s contractor is almost done with the pipeline installation on 32nd Avenue. View more photos.

In the news: Pipeline to a cleaner Puget Sound (King5 news), December 12, 2014
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

In the news

Related information