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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

Fremont Siphon replacement project

Project construction

July 14, 2016 Update 

Success! SWIZY the tunneling machine finished digging two 465-foot tunnels under the Ship Canal on Tuesday, July 12. Check out the video below to see King County's contractor lifting SWIZY out of the 80-foot deep shaft on the Queen Anne side of the project. View July 11 news release.

King County’s contractor will now install new pipes in each of the tunnels to carry wastewater from north Seattle and beyond to King County's treatment plant in Magnolia. These pipes replace the existing siphon, which is more than 100 years old. The new pipes will be installed by this fall. The project is on track to finish in 2017.

Did you know?

  • The Fremont Siphon Replacement Project was SWIZY’s first project.
  • SWIZY is 7 feet in diameter.
  • SWIZY dug two tunnels 20 feet below the bottom of the Ship Canal, about 100 feet underground.
  • SWIZY removed about 650 cubic yards of dirt for each tunnel - enough to fill 65 dump trucks!

Spring 2016

Queen Anne Project site: W Ewing Alley closed between 3rd Avenue W and 6th Avenue W until fall

In late March, King County’s contractor for the Fremont Siphon Replacement project began building the structure to connect the new siphons to the existing sewer under the W Ewing Alley. W Ewing Alley is closed to through traffic between 3rd Ave W and 6th Ave W until the fall.

Vehicle traffic will be detoured off of W Ewing Alley onto W Ewing St or W Nickerson St. The Ship Canal Trail will remain open. Along with the closure, this work will increase noise and activity around the project site. The contractor will begin by installing shoring for excavation of the new connecting structure.

Ewing Alley
View project update
 (March 16, 2016).

Fremont Project Site: Intersection of 2nd Ave. N.W. and N.W. Canal St. closed until fall 

The corner of 2nd Ave. NW and NW Canal St. will remain closed through the fall while crews install the 9-foot diameter pipe connecting the new siphons to the existing sewer system. Burke-Gilman Trail users should follow the trail detour just south of the normal trail to get around the Fremont Canal Park work area safely.

Intersection of 2nd Ave. N.W. and N.W. Canal St. closed until fall

Have a comment or question? Need more information about the project?
Click here to contact the project team.

Project description

Project areas in Fremont and Queen Anne
Project areas in Fremont and Queen Anne

Fremont Siphon service area
Fremont Siphon service area

Building the Fremont Siphon Tunnel, 1913
Building the Fremont Siphon Tunnel, 1913. Photo provided by Seattle Municipal Archives (external link)

King County will replace Fremont Siphon, a major sewer pipe running under the Ship Canal between Fremont and Queen Anne. The new siphon will be located west of the existing siphon to reduce project risks and impacts to the community. Construction begins in early 2015 and will continue until early 2017. The old siphon will be decommissioned.

Why do we need this project?

The Fremont Siphon has provided safe, reliable sewer service to north Seattle and other cities in north King County for decades. Sewage and stormwater from more than 100 square miles of King and Snohomish Counties pass through the Fremont Siphon every year to be cleaned and safely discharged at the County’s treatment plant in Magnolia. During storms, the pipe carries up to 220 million gallons per day, making it one of the most heavily used pipes in the regional sewer system.

The existing Siphon is nearly 100 years old and has reached the end of its of service life. The new pipes will ensure north Seattle and northern King County continue to enjoy safe, reliable sewer service for decades.

Project schedule

Project schedule

How King County works with the community during construction

King County recognizes that its construction projects can disrupt the daily lives of those near them. During construction, the County’s project team update the community and local jurisdictions on the project’s progress and any unforeseen developments.

The County project team will also work directly with the community to minimize construction impacts to those living or working nearby. Information from the community helps to shape the County’s decisions about ways to address community disruptions, private property impacts attributable to the project and promote public safety around the work site.  

Interested community members will have a number of ways to interact with the project, including:

  • Community meetings with the contractor prior to construction
  • Community briefings on project progress during construction
  • Advance written notice of major construction activities
  • Project status reports available via email and flyer
  • Prompt response to all community inquiries
  • 24/7 construction hotline
  • News releases

What is a siphon and how does it work?

Inverted siphon

The Fremont Siphon is an inverted siphon. Inverted siphons use gravity to push liquids downhill. In this case, sewage and stormwater from north Seattle and elsewhere is pushed through the Fremont Siphon by having sewage and stormwater enter the pipe in Fremont Canal Park at a higher elevation than where it exits the pipe in Queen Anne.  

To help keep the sewage and stormwater moving, inverted siphons in the sewer system are a smaller diameter pipe than those connecting to it on either side. The smaller diameter speeds up the passage of sewage and stormwater pass through the pipe to keep solids moving and avoid clogs.

Project updates / news

  • Project updates and news releases are located on the library page.
  • Meeting calendar and materials are located on the meeting calendar page.

More information

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