Regional Infiltration and Inflow Control Program

I/I Program Overview

The King County Wastewater Treatment Division (WTD) serves 34 local wastewater agencies in the regional service area. WTD must provide adequate capacity in its system to convey and treat wastewater flows sent by the agencies through their collection systems.

With the exception of portions of the City of Seattle that have combined sewers (designed to convey wastewater and stormwater in the same pipes), sewers in the regional wastewater system are designed to convey only wastewater. However, many of these "separated" sewers also convey clean groundwater and stormwater that enter through leaky pipes, improper storm drain connections, and other means. This clean water, called infiltration and inflow (I/I), takes up capacity that could otherwise be used for wastewater alone and generates the need to build added capacity in pipelines, treatment plants, and other facilities.

This added capacity results in higher capital and operating costs to the regional system that are born uniformly by all agencies and passed on to ratepayers in each jurisdiction.

Recognizing the need to explore the feasibility of I/I control, the King County Council approved I/I control policies as part of the Regional Wastewater Services Plan (RWSP), adopted in 1999 under Ordinance 13680. The policies establish the framework and process for development of a long-term regional I/I control program.

In response to the RWSP I/I Control Program policies, the County, as represented through WTD staff, worked in a consensus-based approach with local agencies to conduct a comprehensive 6-year, $41 million I/I control study. The study began in 2000 and culminates with the Executive's recommendation for a regional I/I control program.

The RWSP defined the following study components:

• Define current levels of I/I for each local agency tributary to the regional system. This was accomplished with a comprehensive flow monitoring program conducted over two consecutive winter seasons in 2000-01 and 2001-02. Approximately 800 meters were placed throughout the regional collection system in local agency service areas to isolate and quantify I/I flows throughout the system. Flow Monitoring reports are available for both years. Originally one year of monitoring was planned but an unusually dry winter in 2000-01 necessitated the second year of flow monitoring.
• Select and construct pilot projects to demonstrate the effectiveness of collection system rehabilitation projects. In 2003, King County and local agencies constructed 10 I/I reduction pilot projects that had been selected in a consensus based process by the local agencies. Results of the pilot projects and lessons learned are documented in the Pilot Project Report.
• Develop model standards, guidelines, procedures, and policies for use by local agencies to reduce I/I in their systems. Draft Standards, Guidelines, Policies and Procedures were developed in a consensus based process between King County and the local agencies. This document will remain draft during implementation of initial I/I reduction projects, after which they will be revised as needed and finalized in a continuation of the consensus based process with the local agencies.
• Identify cost-effective options to remove up to 30 percent of I/I expected to occur in local agency systems during a 20-year peak flow condition. (Peak flow is the highest combination of base flow and I/I expected to enter a wastewater system during wet weather at a given frequency that treatment and conveyance facilities are designed to accommodate.) A thorough benefit-cost analysis was conducted which analysed the cost-effectiveness of I/I reduction using different alternatives and planning assumptions. The analysis and its results are documented in the Benefit/Cost Analysis Report.
• Develop a long-term regional I/I control plan for review and approval by the King County Council. The Executive's Recommended Regional Infiltration/Inflow Control Program was submitted to the King County Council in December 2005 and is awaiting Council review and approval in 2006.

During the I/I control study, the County conducted 10 workshops with local agencies and over 75 work sessions with the MWPAAC Engineering and Planning (E&P) Subcommittee. The County will continue to work collaboratively with local agencies in implementing the regional I/I control program once it has been approved by the King County Council.

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For questions about the I/I Control Program Web site, please contact Maryann Petrocelli at 206-263-7321 or maryann.petrocelli@kingcounty.gov.

Department of Natural Resources and Parks
Wastewater Treatment Division

Updated: Jan. 15, 2008

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