Inflow and Infiltration

What is Inflow and Infiltration?

Water sources that flow into a dedicated sanitary sewer system from groundwater and storm water is called inflow and infiltration (I/I). Sanitary sewers are strictly designed to transport wastewater from sanitary plumbing fixtures, such as toilets, sinks, bathtubs and showers.


Inflow occurs from surface water sources that flow directly into the sanitary sewer via a defined route 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, or snow melt that contribute to excessive sewer flows. Peak inflow can occur during heavy storm events.


Infiltration is groundwater, that inadvertently enters into the sanitary sewer system 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.