Massive sewer upgrade project on the way this fall

Most of the construction will take place on north Mercer Island.

King County’s massive sewer upgrade project that will transpire across north Mercer Island and southwest Bellevue has been on the designing table for five years. They’re putting the final touches on the plan, and construction time is inching closer.

The approximately four years of construction on the North Mercer Island/Enatai project are slated to begin this fall. Islanders who live near the construction zones have been apprised of the project through a pair of online open houses, a host of community meetings and more over the last four years, and King County will soon release a mailer to all Island residences to describe the breadth of the project.

“This multi-year project will have sporadic impacts to road and park users at the north end of the Island. But the good news is that King County has shortened major road closures to be as brief as possible and will always allow access to affected homeowners,” according to a city email to the Reporter.

The project, which is coordinating with other planned construction activities on the Island, will feature an upgrade of four miles of sewer pipeline along with an upgrade of the county’s North Mercer Pump Station and the Island’s Lift Station 11 at Fruitland Landing, according to the King County Wastewater Treatment Division. The pipes to be replaced carry wastewater from the local pump station to Bellevue’s Sweyolocken Pump Station.

“The pipes, installed in the 1970s, are aging and reaching capacity,” the division noted in an email to the Reporter. “The project was identified in 2007 as part of King County Wastewater Treatment Division’s conveyance system improvement program, based on the system exceeding its 20-year peak capacity. This project was identified as a high priority project to be completed before 2030.”

With construction expected to be finished by early 2025, some heavily used public spaces and residential areas will be affected while the project moves to fruition.

According to King County, the project shouldn’t affect water usage, but there will be temporary traffic disruptions on residential streets and on North Mercer Way, Interstate 90 Trail detours, equipment storage and staging at the boat launch and Luther Burbank Park’s south parking lot at times, and dust, noise, occasional vibrations and large equipment at the active construction sites.

“Our team will work with residents on streets that will see construction to ensure uninterrupted service delivery and local access to homes and businesses,” the county said.

The county’s team will keep in touch with residents on its construction progress with regular updates on the project website and through its listserv, close coordination with the city and its Let’s Talk Mercer Island page, signs around the Island with project information and more. To receive text alerts, residents can text KING MERCERSEWER to 468-311 or sign up for email alerts on the project website at

The multi-tiered project will also feature new street pavement, new ADA-compliant crosswalks, new trees to replace those that need to be removed and more.

“After construction, residents will also benefit not only from the critical sewerline overhaul, but also from repaved sections of the Mountains-to-Sound (‘I-90’) Trail, which is pretty rough in some places,” according to the city.

Since the project is so immense in the geographic and technical realms, challenges can arise, the county said, adding that its team creates detailed construction plans to ensure smooth project implementation.

“Early and frequent communications with stakeholders and neighbors helps ensure a successful project,” the county said. “As we approach construction, we remain committed to keeping the community informed and being a good neighbor to those who are directly impacted by the project. We provide advance notification of construction activities and maintain a 24-hour construction hotline to quickly solve problems that arise.”

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