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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

April 6, 2014 Update -- Notice of Overnight Work for the Magnolia CSO Control Project

The City of Seattle has approved a night time noise variance beginning April 16, 2014 for King County’s contractor to install a sewer pipeline under the on and off ramps of the Magnolia Bridge. The new pipeline will connect the combined sewer overflow (CSO) control facility site south of the Magnolia Bridge to the County’s existing pipeline on Port of Seattle property north of the bridge. This work will be performed at night during lower traffic periods because it requires sequential closure of the bridge ramps and traffic detours on the bridge to accommodate these closures.

What you can expect

We will provide updates on scheduled dates of work and traffic control as plans are finalized.
• Up to 5 nights of work will be required for each ramp.
• Work will be performed between 7 p.m. and 7 a.m.
• One ramp will be closed at a time, with traffic control directing traffic onto the open ramp and flagging to provide entrance on and off the restricted access to the bridge.

View flier (PDF).

For more information about this project, contact Monica Van der Vieren at, 206-477-5502 or via feedback form.

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.

Project description

King County is building an underground storage tank in the Port of Seattle’s Terminal 91 West Yard south of the Magnolia Bridge. This facility will store peak combined sewer flows so that CSOs are limited to no more than one per year on a long-term average.

Why does King County need to do this project?

In 2008, King County reported that the South Magnolia combined sewer overflow (CSO) facility has 19 overflows per year on average that discharge a total of 31 million gallons of untreated wastewater and stormwater into Puget Sound off of South Magnolia. King County is working to meet current regulations set by the Washington Department of Ecology that require no more than one untreated discharge per year on a long term average.

Project schedule

Upcoming activities

Activities Dates
Community meeting January 26, 2013
Construction procurement process 2013
Pre-construction community meetings Fall/Winter 2013
Construction 2013 - 2015

South Magnolia CSO Control Project Timeline, May 2013
View meeting calendar for upcoming public meetings, and South Magnolia CSO Control Project Timeline (PDF), updated May 2013.

Project updates

News releases

Related information

For more information