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Infiltration and Inflow Control

King County, Washington

For more information about the Regional Infiltration and Inflow Control Program, please contact Kristine Cramer.

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

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What is infiltration/inflow (I/I)?

Excess water that flows into sewer pipes from groundwater and stormwater is called infiltration and inflow , or I/I. Groundwater (infiltration) seeps into sewer pipes through holes, cracks, joint failures, and faulty connections. Stormwater (inflow) rapidly flows into sewers via roof drain downspouts, foundation drains, storm drain cross-connections, and through holes in manhole covers. Most I/I is caused by aging infrastructure that needs maintenance or replacement.

What is infiltration/inflow (I/I)?
Available as PDF file.

Infiltration

Infiltration is groundwater, or groundwater that is influenced by surface or sea water, that enters sewer pipes (interceptors, collectors, manholes (MH), or side sewers) through holes, breaks, joint failures, connection failures and other openings. Infiltration quantities often exhibit seasonal variation in response to groundwater levels. Storm events can trigger a rise in groundwater levels and increase infiltration flows. The highest infiltration flows are observed following significant storm events or following prolonged periods of precipitation. Since infiltration is related to the total amount of piping and appurtenances in the ground and not to any specified water use component, it is usually expressed in terms of the total land area being served, or in terms of the lengths and diameters of sewer pipe. The unit quantity used in this study is gallons per acre per day (GPAD). See picture above for a graphical view of the sources of infiltration.

Inflow

Inflow is surface water that enters the wastewater system from yard, roof and footing drains, from cross-connections with storm drains, downspouts, and through holes in manhole covers. Inflow occurs as a result of storm events such as rainfall, snowfall, springs or snow melt that contribute to excessive sewer flows. Peak inflow can occur during heavy storm events when storm sewer systems are surcharged, resulting in hydraulic backups and local ponding. See picture above for a graphical view of the sources of inflow.

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