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Infiltration and Inflow Control

King County, Washington

For more information about the Regional Infiltration and Inflow Control Program, please contact Steve Tolzman.

Wastewater Treatment Division
King Street Center
Suite 0505
201 South Jackson Street
Seattle, WA 98104

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Regional Infiltration and Inflow Control Program

What is infiltration/inflow (I/I)?
What is infiltration and inflow (I/I)?

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 system.

With the exception of the 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 groundwater and stormwater that enter through leaky pipes, improper storm drain connections, and other means. This excess 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 wastewater 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.

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).

The purpose of the program is to reduce the amount of peak wet weather flow entering the County's wastewater conveyance system when it is cost-effective to do so. Reduction of I/I in the system has the potential to lower the risk of sanitary sewer overflows and decrease the future costs of conveying and treating wastewater.

Protecting the environment and decreasing wastewater treatment costs are the benefits of a regional I/I control program.