General Sewer Plan Update

Project Details

The City’s sewer service area encompasses the entirety of Mercer Island and serves a population of approximately 24,000. The system includes approximately 105 miles of gravity mains, 2,400 manholes, 17 pump stations, 1 flush station, and 12 miles of low-pressure Lakeline system.

Background

A General Sewer Plan (GSP) provides a comprehensive, long-range plan for the development, operation, maintenance, and improvement of a municipality’s sanitary sewer system. In Washington State, it is a key planning document required by the Department of Ecology that is typically updated every 6 to 10 years. The City’s current GSP was last approved in January of 2019 and is available here: https://www.mercerisland.gov/publicworks/page/general-sewer-plan

In addition to updating the General Sewer Plan (GSP), the City plans to develop its first comprehensive, systemwide hydraulic model. The current model—developed as part of the 2019 GSP—was limited in scope, focusing primarily on specific infrastructure within the Town Center area. A complete system model will enable the City to more accurately assess sewer capacity, evaluate the impacts of future development and land use changes, and prioritize capital reinvestment and rehabilitation efforts.

Description

The purpose of this project is to update the City’s General Sewer Plan (GSP) in compliance with Washington Administrative Code (WAC) 173-240-050 and Revised Code of Washington (RCW) 90.48.110. This forward-looking plan will guide sewer system investments and improvements over the next 20 years. In conjunction with the update, the City will expand its current hydraulic model to encompass the entire Island, enabling a comprehensive evaluation of system capacity, performance, and future needs.

Project Timeline
Planning/Design: August 2025 – December 2027
 

Estimated Budget

$500,000

Contact:

Christopher Marks, Utilities Engineer, 206-677-1027; chris.marks@mercerisland.gov