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To explore the feasibility of regional I/I control, the King County Regional Infiltration and Inflow (I/I) Control Program was created in 1999 as part of the Regional Wastewater Services Plan (RWSP).

1999 Program created as part of the Regional Wastewater Services Plan (RWSP).
2000 Comprehensive 6-year, $41 million I/I control study began, led by WTD staff working in a consensus-based approach with local sewer agencies.
2001–2002 Current levels of I/I defined for each local agency tributary to the regional system through extensive flow monitoring and modeling programs.
2003–2004 10 pilot projects in 12 local agency jurisdictions selected and constructed to demonstrate the effectiveness of collection system rehabilitation projects, test technologies, and gain cost information.
2004 Final draft model standards, procedures, policies, and guidelines developed for use by local agencies to reduce I/I in their systems.
2005 Thorough benefit-cost analysis completed to determine the cost-effectiveness of I/I reduction and Executive recommends a regional I/I control program.

Long-term regional I/I control plan developed and then approved by the King County Council.


WTD and local sewer agencies conducted an I/I reduction feasibility analysis and selected three initial I/I reduction project areas.


Initial I/I reduction project in the Skyway Water & Sewer District service area (unincorporated King County) constructed. I/I reduction projects in Bellevue and Issaquah were cancelled in early 2010 due to County budget limitations.

2014 Findings on the initial project published, which will be used to develop recommendations to the King County Council regarding long-term I/I reduction controls.
2015 – 2018  Conducting regional planning work to evaluate programmatic concepts to reduce I/I particularly in private side sewers. This planning effort is in progress, and has resulted in several technical memorandum

What is infiltration/inflow (I/I)?

For more information, please contact:

Steve Tolzman

Contact your local sewer service provider for specific information about the sewer system in your neighborhood.

Steve Tolzman
Capital Project Manager