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How do sewer system inflows and infiltration affect wastewater treatment plants?

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Aerial view of wastewater treatment basins with circular clarifiers; overlaid toilet graphic and arrow with text reading: “Where does it go?” — wastewater treatment All the sewers in your neighborhood end up here.

Inflow and infiltration (I&I) significantly impact wastewater treatment plants by introducing excess water into the sewerage system, which can lead to hydraulic overloading.1

Inflow refers to storm water entering the system through improper connections like roof drains, downspouts, and sump pumps, often during precipitation or snowmelt. Infiltration occurs when groundwater seeps into the system through defective or broken pipes.1

When these events occur, treatment plants may receive volumes of water far higher than their designed capacity. This strain can force utilities to request that residents limit water usage—such as avoiding laundry or dishwashing—to prevent system failure. Because the size and capacity of treatment facilities are determined by estimated sewage volumes plus anticipated I&I, aging infrastructure is particularly susceptible to these issues, which can compromise the efficiency of primary, secondary, or tertiary treatment processes.12