City Nears Completion of Critical Sewer Infrastructure Upgrades
The City of Mercer Island is nearing completion of two major infrastructure projects designed to modernize the Island's wastewater system, helping to make the overall system safer and more reliable while also minimizing risks to the environment. The work involved upgrades to three out of the Island’s 17 pump stations that move wastewater off the Island to treatment facilities.
The most significant overhaul occurred at Sewer Pump Station 11, a 60-year-old facility that was no longer equipped to handle the demands of a growing region. In a partnership with King County as part of the broader North Mercer Island/Enatai Sewer Upgrade Project, the City fully modernized the station to accommodate a new pipeline alignment that follows the I-90 Trail. Because of this change, PS11 must now pump wastewater over a much greater distance and elevation than ever before.
The project included the installation of larger, high-capacity pumps, the rehabilitation of existing structures, and the addition of state-of-the-art above-ground control equipment. The station is now fully integrated into the City’s upgraded SCADA system for real-time remote monitoring, with a final flow meter scheduled for installation this month. With the project now 99% complete, the City expects to reopen the Fruitland Landing street end to the public shortly following final minor landscaping.
Further along the shoreline, the City has also successfully commissioned new backup power systems at Sewer Pump Stations 23 and 25. To prevent sewage overflows during the Pacific Northwest’s frequent power outages, the City replaces backup generators on a planned 20-year life cycle.
This specific project involved replacing aging 40 kW diesel generators and required a feat of "ship-in-a-bottle" engineering. Crews had to navigate cramped underground vaults and maneuver nearly five-ton precast concrete vault lids into place, all while ensuring the pump stations remained fully operational. The new generators have already been thoroughly tested under real-world outage conditions and are ready for service. The City extends its sincere thanks to the Mercerwood Shore Club and residents in the 5400 block of East Mercer Way for their patience during these complex logistical challenges.
These vital infrastructure improvements were made possible through the direct contributions of utility rate payers. The replacement of PS11, a regional King County project, was funded by rate payers across the county, including Mercer Island and 16 other neighboring cities from Seattle to Issaquah. In contrast, the generator upgrades at PS 23 and 25 were funded solely by Mercer Island rate payers as part of the City’s local commitment to maintenance. Looking ahead, the City is already planning for the future, with two additional sewer pump station generators scheduled for replacement during the 2027–2028 biennium to ensure the Island’s system remains resilient for decades to come.
