Skip to main content
King County logo

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

Files are in Portable Document Format (PDF) .

For more information, please contact:

Steve Tolzman


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 effort is in progress, and additional and final technical memos will be added to this page as they become available.

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.

What is infiltration/inflow (I/I)?

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.

Steve Tolzman
Capital Project Manager