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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

September 9, 2015 Update 

Fremont Site: 

King County’s Fremont Siphon Replacement Project contractor finished upgrading a City of Seattle water pipe that runs directly under 2nd Ave. The work had closed access to NW Canal St from 2nd Ave NW for a few weeks. 2nd Ave NW reopened on September 9.

2nd Ave NW will remain open through early October. NW Canal St residents should use 2nd Ave NW to access NW Canal St during the upcoming Fremont Oktoberfest (September 18-20).

The contractor continues to work to replace a City of Seattle storm drain in Fremont Canal Park. Burke-Gilman Trail users should follow the trail detour to get around the work area safely. (See map)

Burke-Gilman Trail users should follow the trail detour.
View project update (pdf, September 9) for more information.

What to expect:
• Work from 7 a.m. to 6 p.m. weekdays
• Increased noise and activity in Fremont Canal Park
• Burke-Gilman Trail shifted south just east of 2nd Ave NW
• Flaggers directing trail traffic around the work areas
• Some short delays on Burke-Gilman Trail while equipment crosses the trail between work areas
• Trail users should access the trail and Fremont Canal Park from 3rd Ave NW or 1st Ave NW
• More trucks on nearby roads

Queen Anne site: W. Ewing Alley and Ship Canal Trail close the week of July 20 as pipe installation begins.

King County’s contractor for the Fremont Siphon will begin installing a 9-foot diameter pipe to connect the new siphon and the existing sewer system on July 20. W. Ewing Alley and the Ship Canal Trail will be closed between 6th Ave NW and 3rd Ave NW in Queen Anne while this work occurs (see map). Vehicles, cyclists, and pedestrians will be detoured to nearby streets through August.

See project update (pdf, July 17) for more information.

Video -- Learn how King County Wastewater Treatment Division creates safe underground work environments using a construction technique called secant piles. Similar to a project in West Seattle, King County will use interlocking concrete cylinders called secant piles to create a watertight ring. The ring keeps groundwater out while the inside of the ring is excavated, which protected nearby homes, streets and utilities from settlement. King County will replace Fremont Siphon, a major sewer pipe running under the Ship Canal between Fremont and Queen Anne.

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 retained by King County for use by the Wastewater Treatment Division as necessary.

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 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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