King County Wastewater Treatment Division is continuing field investigations to inform design of a sewer upgrade project in Mercer Island and Bellevue. Crews began taking soil sampling, ground surveys and potholing in late January and will continue through March. Photo courtesy of King County

King County Wastewater Treatment Division is continuing field investigations to inform design of a sewer upgrade project in Mercer Island and Bellevue. Crews began taking soil sampling, ground surveys and potholing in late January and will continue through March. Photo courtesy of King County

County planning sewer upgrade for pipe connecting to Bellevue pump station

County prepares to replace an aging pipe running beneath Lake Washington

A sewer line serving Mercer Island is due for a massive overhaul in coming years as the county prepares to replace an aging pipe running beneath Lake Washington.

The Mercer Island – Enatai sewer system begins in north Mercer Island and runs east under Lake Washington through Enatai Beach Park before connecting with the Sweylocken pump station in Bellevue.

The system is owned by King County and was built in the 1970s placing it near the end of its 50-year life-cycle. County officials expect the new 15,000-foot system to last another 50 years.

Kristine Cramer, King County wastewater community relations planner, said the current pipes are experiencing natural aging and corrosion due to their age but no leaks have been detected.

“We don’t want them to break,” she said.

The project will cost $116 million from county wastewater coffers set aside for capital projects. All ratepayers pay into the fund, which is spent on various upgrade and replacement projects across the county, so residents served by the Mercer Island line won’t see a rise in their rates stemming from the project.

Around 8,000 customers will be affected and construction will have significant impacts on local neighborhoods. This will include roads being ripped up to install piping, trenches being dug in residential neighborhoods with heavy equipment and construction noise.

“There’s going to be construction on those residential streets in front of people’s houses,” Cramer said.

This will include not only streets but right-of-way along those streets. Access to roads could be limited to emergency responders and local access during construction, which is slated to begin in 2020 and run through 2023.

The county has been conducting public outreach and Cramer said affected residents will know about construction by the time it starts. An online open house is available at the county Wastewater Department’s webpage.

The old pipe is buried beneath Lake Washington but current regulations stipulate new construction must avoid bodies of water if possible.

“If there’s any way to not put pipes in the water we must follow that alternative,” Cramer said.

Consequently, the majority of piping on Mercer Island will be installed land in an 8-foot trench on land instead off-shore. The segment running from the Island to Enatai Beach Park will be placed in an underwater trench before it is buried.

The Eastside portion of the pipe is buried 110 feet underground and will be replaced with a process known as horizontal directional drilling, which uses computer modeling and a bore to dig out a new hole before the section of pipe is pulled underground in its entirety.

There will be some minor sewer service disruptions but Cramer said the county will let residents know of these in advance. Buildings in the service area connect to side sewers before reaching the county-owned system. The old county pipe network will continue to be used until the new system is in place and service can be switched over.

Increasing customer loads also played into the upgrade as Mercer Island’s population has increased by more than 6,000 since the 1970s. Its current population is around 25,000.

The county’s wastewater website is and Cramer can be reached at 206-477-5415 for more information.

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