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Reducing infiltration and inflow (I/I), which consists of stormwater and groundwater entering a sanitary sewer system, maintains pipe capacity for sewage in the County’s wastewater system. Maintaining capacity for sewage helps prevent overflows and reduces the need for capital projects to add sewer system capacity. King County is engaged in a long-term program, in collaboration with local wastewater agencies, to reduce I/I when cost effective to do so in the separated sewer system.

July 2021 Update

King County, in collaboration with MWPAAC, is currently exploring and developing new program measures to reduce infiltration and inflow (I/I) particularly in private side sewers. Phase 1, Exploring Concepts, has been completed (see reports posed on Library and resources page). Phase 2, Define Programs, is underway.

Phase 2 technical reports:

The King County Wastewater Treatment Division (WTD) serves 34 local wastewater agencies in the regional service area. With the exception of the portions of the City of Seattle that have combined sewers, sewers in the regional wastewater system are designed to convey only wastewater.

However, many of these "separated" sewers also convey groundwater and stormwater that enters through leaky pipes, improper storm drain connections, and other means.

Learn more:

Nicole Smith
Capital Project Manager