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.
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 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.
Why is I/I a problem?
Extra water in the sewer system is a problem because:
- It takes up capacity in the sewer pipes and ends up at the wastewater treatment plant where it must be treated like sewage, resulting in higher treatment costs. These costs are then transfered to you, the user.
- Requires new and larger wastewater facilities to convey and treat larger volumes of flow, resulting in higher capital expenditures.
- I/I flows contribute to sewer system overflows into local homes and the area waterways, negatively impacting public health and the environment.
Protecting the environment and decreasing wastewater treatment costs are the benefit of the Town's I/I control program.