skip to main content

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

Fremont Siphon replacement project

Project design

July 2014 Update 

Construction begins in late 2014 – watch for your invitation for a pre-construction meeting this 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 is planning to replace the existing Fremont Siphon, a wastewater conveyance system that extends from Fremont to Queen Anne under the Lake Washington Ship Canal. King County will install new pipelines in a separate crossing under the Ship Canal, just west of the existing siphon. After reviewing a range of options to rehabilitate or replace the Fremont Siphon in 2010, King County recommended replacing the existing pipelines in a new location to reduce potential project risks and impacts, to maintain operations during construction and to provide safe, reliable service for years to come. The County will still retain the existing space in the Fremont Tunnel for Wastewater Treatment Division uses.

Why does King County need to do this project?

The Fremont Siphon is a critical conveyance line that carries wastewater from the county’s northeastern service area to the West Point Treatment Plant. The Fremont Siphon conveys wastewater from a service area of about 60 square miles year round, with an additional area of about 54 square miles during the summer. The siphon pipelines, which may convey up to 220 million gallons per day during storms, are located under the Lake Washington Ship Canal, west of the Fremont Bridge. The siphon was installed in the early 1900’s and has provided wastewater conveyance service for almost 100 years.

Project schedule

Fremont Siphon Project Schedule
View upcoming community meetings and more detailed project schedule.

How King County works with the community during project design

During project design, King County will provide the Fremont and Queen Anne communities with information on project details including:

The public can inform the County’s work by learning about the project and helping to identify reasonable solutions for construction impacts such as traffic disruptions.

What is a siphon and how does it work?


Inverted siphon

The Fremont Siphon is an inverted siphon. An inverted siphon is used to pass flow through a valley or under a waterway. King County’s Fremont and Ballard siphons run under the Lake Washington Ship Canal.

Inverted siphons take advantage of the basic rule that water flows downhill. Wastewater is pushed through the siphon by the upstream flow that is at a higher water level than the downstream level.

In a wastewater system, inverted siphon pipes are smaller diameter than the connecting pipes upstream and downstream. This “pinch point” increases the speed of flow to help keep solids moving through and avoid plugging the siphon pipe.


Project updates

News releases

More information

Related project