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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

North Mercer Island and Enatai Sewer Upgrade Project

Learn more about project design  


September 16, 2016 -- Survey work on Mercer Island begins Monday, September 19

Starting Monday, September 19, and continuing into early October, survey crews will be working on Mercer Island along the I-90 Trail and in residential areas near the trail. The trail and nearby city streets will remain open during survey work.

Please see the September 16 project update (PDF) for more detail about this work.

As always, please contact us if you have any questions or concerns about this work. To sign up for email and text alerts, text KING MERCERSEWER to 468-311 for Mercer Island and KING ENATAISEWER to 468-311 for Enatai.

Project need

Installed during the 1970’s, the County’s North Mercer Island and Enatai sewer pipelines are ageing and reaching capacity. Increasing sewer capacity will allow King County to provide reliable wastewater service through the year 2060.

Project description

With over 400 miles of pipes moving wastewater to one of our award-winning treatment plants, King County WTD works hard to maintain our system. We inspect and monitor our regional system and update plans for needed upgrades.

King County WTD needs to replace sewer pipes that serve areas in North Mercer Island, the southwest portion of Bellevue near Enatai Beach, and the Town of Beaux Arts Village (PDF). This pipeline carries wastewater from the North Mercer Island Pump Station to the Sweyolocken Pump Station in Bellevue.

Recommended Alternative, North Mercer Island/Enatai Sewer Upgrade Project
Recommended Alternative

During the planning phase, the project team identifies and evaluates locations for a new pipeline. We always include the existing location as an option because this alignment has proven itself in operations. In this case, where a pipeline is located in the water, we look for additional options to move it upland.

The project team evaluates options using a range of factors: technical, constructability, operations and maintenance, environmental, permitting and property acquisition, community, cost, and risk.

By the end of 2015, King County’s team will recommend a location for the new sewer pipeline. Project design will begin in 2016, followed by construction in 2019. Once installation is complete, expected by 2022, the County’s new pipeline will provide service for the next 50 to 60 years.

What's coming up for your community?

Throughout summer and into September 2016, King County's project team will continue to gather information that will shape our decisions about a final pipeline alignment. You will see crews carrying out field investigations on land and in water to gather information about soils, groundwater, and utilities.

The team will continue evaluating the North Mercer Pump Station. They will determine what changes are needed to keep wastewater moving in the new alignment and to meet current code. The County's team is already discussing upgrades to the City of Mercer Island's Lift Station 11 at Fruitland Landing to support the upland pipeline path.

In late Fall 2016, King County's project team to share what we've learned over the summer and how it has informed the project design:

  • We will host community meetings at Mercer Island and Bellevue.
  • At Bellevue, we'll host an interactive, informational session to help project neighbors understand the method we'll use to install the pipe, and to ask questions.
  • We will host online open houses for people who can't attend a community meeting.
  • We'll provide briefings for interested community groups and organizations.
  • And as always, we will be available to answer your questions and address your concerns.


Project design will begin in 2016. Construction is expected to begin in 2019 and extend until 2022.

Project schedule

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