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The City owns, operates and maintains the sewage collection system that serves all of Mercer Island. The Island’s sewage is delivered to a treatment plant at Renton operated by the Metropolitan King County Government. At the Renton plant, the sewage receives primary and secondary treatment.

The City’s system includes a total of 17 pump stations, 2 flushing pump stations, and more than 113 miles of gravity and pressure pipelines, ranging in diameter from 3 to 24 inches which ultimately flow in King County Department of Natural Resources (KCDNR) facilities for treatment and disposal at the South Treatment Plant in Renton. See Figure 1 – Major Sewer Facilities Service Mercer Island.

As of 2014, a total of 7,292 residential and commercial customers were hooked up to the City sewer system.