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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 planning

June 1, 2016 Update

In early June, crews will begin surveying the project, marking the locations of underground utilities, and working from a boat to survey the lake bottom. Later this summer, crews will use drill rigs to measure soil and groundwater conditions. Gathering this information will help us refine a final location for the pipe. See our June 1 project update for more information.

March community meeting documents

A big thank you to the neighbors and community members who took time to meet with the project team during our March community meetings. View the project meeting calendar for locations and meeting materials.

Recommended Alternative, North Mercer Island/Enatai Sewer Upgrade Project
Recommended Alternative
 (PDF). After extensive analysis that considered community needs and priorities,
King County Wastewater Treatment Division (WTD) has identified an alternative for the project that
involves locating an approximately 14,000 foot, below-ground pipeline along an alignment that extends
from the County’s North Mercer Pump Station to the County’s Sweyolocken Pump Station in south Bellevue.
This alternative was selected as the best approach for limiting community and environmental impacts as
well as reducing potential for project costs and risks.

Each of your communities are unique, and King County values what matters to you and your neighbors. We will continue to meet with groups and in neighborhoods along the corridor. If you have not yet heard from our project team and would like to schedule a briefing, call or email Monica Van der Vieren at 206-477-5502, or via feedback form.

As part of its mission to protect public health and the environment, King County Wastewater Treatment Division (WTD) conveys and treats wastewater collected by 17 cities and 17 local sewer utilities in King, Snohomish and Pierce counties, including the Cities of Mercer Island and Bellevue. Your local sewer agency collects your wastewater and sends it to King County’s pipelines. Wastewater from Mercer Island and Bellevue is sent to South Treatment Plant in Renton.

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 (see map below). This pipeline carries wastewater from the North Mercer Island Pump Station to the Sweyolocken Pump Station in Bellevue.

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.

Working with the community

King County’s project team enjoyed a beautiful July weekend with hundreds of visitors to the Mercer Island Summer Celebration, where people could tour our rolling wastewater education center and hear about our upcoming project.

1509_open-house-1 1509_open-house-2
Project team members discuss project options with Open House participants. View more photos.

During Fall 2015, we have been reviewing project options with the Mercer Island and Bellevue communities and area users. The project team hosted in-person open houses in September to introduce the project, present the range of options under consideration, answer questions, and understand community concerns. We are continuing to provide briefings and our online open house continues until November 2, 2015.

The project team will provide WTD management with a recommended proposal by the end of 2015. In early 2016, we will present the recommended option and begin working with our project neighbors and community during the design phase.

Please contact us if you would like to be on our project email and/or mailing lists, and watch this site for updates!

Project area 


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

Project schedule

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News releases

Related information