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Library and Resources - I/I program

Resources and program history

Below are a number of reports, studies, maps, and other resources related to the Regional Infiltration and Inflow Control program.

Resources for local sewer agencies

Side Sewer Maintenance Mailer (2022)

A house on a residential street has a side sewer (a pipe) that carries sewage from the building to the municipal sewage pipe, located under the street. Rain doesn't belong in the sewer, but it can get in through roof drains and foundation drains that are connected to the sewer, uncapped cleanouts on the side sewer, and cracks in the side sewer. Sewage can back up into buildings or yards when pipes are full of water, roots, or things people send down the drain. Rain that gets in the pipes is treated like sewage – which costs everyone money. To learn what you can do, download this flier as a .pdf file at Side Sewer Maintenance Mailer was developed by King County for MWPAAC agencies to distribute to their residents. The educational mailer describes components of a side sewer, private property source disconnection, and side sewer maintenance best practices. The mailer is available in English, as well as: 

Side Sewer Best Management Practices (BMP) Toolkit (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

Note: This toolkit is Appendix A from the Final Regional Best Management Practices Development, December 2021, under Reports / Evaluation of I/I Reduction Concepts to Improve the Regional I/I Program (2017-2021).

Financial Assistance Resources

As part of the Evaluation of I/I Reduction Concepts effort, a list of Existing Financial Assistance Programs for private side sewer inspection or repair was developed that could be accessed by MWPAAC agencies. The programs include both no or low interest loans and grants offered by different sewer agencies and municipalities. Eligibility varies depending on the program.


Phase 1 - These technical memos explore concepts to reduce I/I programmatically and with a focus on private side sewers. Concepts include region-wide side sewer standards and inspection training, interagency coordination to identify and manage I/I, side sewer inspection and repair programs, and side sewer grant/loan programs. This planning phase resulted in three programs being recommended by MWPAAC for further definition and consideration.

Phase 2 – These technical memos further define three program options recommended in Phase 1 including,

  1. Regional sewer and side sewer standards,
  2. A regional inspector training and certification program, and
  3. A private side sewer inspection program with financial assistance.

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.

This report summarizes the history of the project, the work performed, follow-up investigations, and findings about the project’s effectiveness in reducing I/I. It describes lessons learned from the project that can be applied to similar work in the future and outlines additional steps needed in order to use the project results for future decision-making. 

Skyway Infiltration and Inflow Reduction Demonstration Project Evaluation Report  (13.6 MB)

This report presents recommendations for projects to reduce infiltration and inflow (I/I) in portions of King County's regional wastewater conveyance system. Reducing I/I, which consists of stormwater and groundwater entering a sanitary sewer system from various sources, makes more capacity available for sewage in the county's wastewater system. This increased capacity helps prevent overflows and reduce the need for capital projects to add system capacity. King County is engaged in a long-term program to reduce I/I when cost effective to do so, and the projects outlined in this alternatives analysis represent an early test of the effectiveness of I/I reduction measures over a large area.

Initial Infiltration and Inflow Reduction Project Alternatives Analysis Report and Appendices (88 MB)

Initial Infiltration and Inflow Reduction Project Alternatives Analysis Report , no appendices (60 MB)

This report lists the Executive's recommendations for both I/I reduction and long-term I/I control and for program administration and policy. In addition to cost-effectively removing enough I/I from the collection system to delay, reduce, or eliminate some otherwise needed conveyance system improvement (CSI) projects, measures must be in place to maintain I/I reductions long-term and to prevent future increases in I/I throughout the regional system. Long-term I/I control includes policy, administrative, financial, and technical measures that promote an ongoing program of review, maintenance, and repair of the collection and conveyance system.

Transmittal letter from Executive Ron Sims to the King County Council (276KB)

Executive's Recommended Regional Infiltration and Inflow Control Program Report and Appendices (4.6MB)

As part of its Regional Needs Assessment, King County developed a list of conveyance system improvement (CSI) projects. These projects will help accommodate the increasing wastewater flows brought about by growth. To make the most effective use its resources, the County evaluated whether it is cost effective to eliminate or delay projects on the CSI Project List by reducing the amount of I/I in the conveyance system. The benefit/cost analysis compared the estimated costs of constructing conveyance system improvement projects with the estimated costs of I/I reduction projects.

Benefit/Cost Analysis Report (4.8MB)

Appendices (Available on CD upon request)

  • Appendix A1 - Select List Cost-Effectiveness Analysis Package per MWPAAC E&P Planning Assumptions
  • Appendix A2 - Regional Cost Effectiveness Analysis Package per MWPAAC E&P Planning Assumptions
  • Appendix A3 - 30-Percent I/I Removal Cost Effectiveness Package per MWPAAC E&P Planning Assumptions
  • Appendix B1 - Sensitivity Analysis Select List-Cost Effectiveness Analysis Packages per Initial Planning Assumptions

This report presents alternative approaches to controlling regional infiltration and inflow (I/I) and the complex issues that affect I/I control in the region.

Alternatives/Options Report (2.3MB)

The Regional Needs Assessment (RNA) identified conveyance system improvement (CSI) projects and costs in order to provide a baseline for conducting benefit/cost analyses of potential infiltration and inflow reduction projects

Regional Needs Assessment Report and Appendices (2.2MB)

The report gives an overview of the I/I control program, the experiences gained from the pilot projects, and detailed information on project selection, design, construction, costs, and results. The information in the report is technical. Its purpose is not only to record what was done but also to serve as the basis for additional studies on the costs and benefits of I/I control measures.

Pilot Project Report (10.4 MB)

A flow monitoring study was conducted during the winter of 2000/2001 and 2001/2002. This Technical Memorandum addresses the dry weather flow measured during the two flow studies, as well as the wet weather performance of the local agency separated sewer system in 2001/2002.

2001/2002 Wet Weather Flow Monitoring Technical Memorandum (8.9 MB)

Appendices available on CD upon request

A flow monitoring study was conducted during the winter of 2000/2001. This Technical Memorandum addresses the wet weather performance of the local agency separated sewer system.

2000/2001 Wet Weather Flow Monitoring Technical Memorandum (8.9 MB)

Appendices available on CD upon request

The Operational Master Plan, or OMP, explains how King County will implement the Regional Wastewater Services Plan as required by Ordinance 13680, which adopts changes to King County's Comprehensive Water Pollution Abatement Plan.

The ordinance focuses on the policies that drive the RWSP.

RWSP Operational Master Plan (413 KB)

Ordinance 13680 (121 KB)

See also: RWSP Master Plan

Additional resources

Graphic depicting how infiltration and inflow get into the sewer system.

Available as a JPEG or PDF file.

inflow and infiltration in a side sewer example image

Maps of the maximum calculated 20-year peak infiltration and inflow hourly flow rates in the King County system. The data for the following three maps is based on flow monitoring conducted in 2000/2001 and 2001/2002, documented in the Wet Weather Flow Monitoring technical memorandums.

Program history

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

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 memoranda. 

2019-2021 Phase 1, Exploring Concepts of the regional planning effort, formally concluded in 2019. Phase 2, Define Programs, concluded in late 2021 and produced several technical memoranda, and made recommendations on selected programmatic concepts to MWPAAC.

Nicole Smith
Capital Project Manager