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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.

March 2022 Update

Side Sewer Best Management Practices (BMP) Toolkit , December 2021

The Side Sewer BMP Toolkit was developed for MWPAAC agencies and includes a description, instructions to local sewer agencies, example website landing page content, and example content (that can be branded by local sewer agencies) for four BMPs:

  • Unauthorized connection prohibition
  • Courtesy notice to property owner/occupant regarding roots in side sewer connections
  • Side sewer maintenance guidance document
  • Private property I/I source disconnection/redirection public education materials

King County, in collaboration with MWPAAC, recently developed new program measures to reduce infiltration and inflow (I/I) particularly in private side sewers.

Phase 1, Exploring Concepts, was completed in 2019 (see reports posed on Resources page).

Phase 2, Define Programs, was completed in December 2021. This planning process resulted in the definition of two programs options. The regional best management practices were recommended by MWPAAC for implementation by component agencies while the inspector training and certification program was not recommended for implementation at this time. Work on the third program, private side sewer inspection program with financial assistance, was not completed because the program is being considered as part of broader sewer system planning effort.

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