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The King County Wastewater Treatment Division (WTD) serves 34 local wastewater agencies in the regional service area. With the exception of the portions of the City of Seattle that have combined sewers, sewers in the regional wastewater system are designed to convey only wastewater.

However, many of these "separated" sewers also convey groundwater and stormwater that enters 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
  • Generates the need to build added capacity in pipelines, treatment plants, and other wastewater facilities
  • Results in higher capital and operating costs to the regional system that are born uniformly by all agencies and passed on to ratepayers


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), with a goal of, when it is cost-effective, reducing the amount of peak wet weather flow entering the County's wastewater conveyance system.


Reduction of I/I in the system can: 

  • Protect the environment
  • Lower the risk of sanitary sewer overflows
  • Decrease future costs of conveying and treating wastewater
  • Decrease wastewater treatment costs are the benefits of a regional I/I control program.

Nicole Smith
Capital Project Manager