What is Inflow and Infiltration?

Inflow and infiltration are terms used to describe the ways that groundwater and stormwater enter into sanitary sewer systems. Inflow and infiltration (I/I) is groundwater or stormwater that flows into the sanitary sewer system due to leaky sewer lines and manholes or from situations where stormwater can flow into the sanitary sewer system through direct connections, such as catch basins, roof drain connections or sump pump connections.

I/I affects the quantity of wastewater that needs to be treated, the capacity of sewer pipes, pump stations, wastewater treatment plant (WWTP) and ultimately, the rate businesses and residents pay to operate and maintain them. I/I takes up valuable capacity in the WWTP and may limit future sewer connections. 

Inflow is stormwater that enters into sanitary sewer systems at points of direct connection to the systems.  Various sources contribute to the inflow, including footing/foundation drains, roof drains or leaders, downspouts, drains from window wells, outdoor basement stairwells, drains from driveways, groundwater/basement sump pumps and even streams. These sources are typically improperly or illegally connected to sanitary sewer systems via direct connections or discharge into sinks or tubs that are directly connected to the sewer system.  An improper connection lets water from sources other than sanitary fixtures and drains to enter the sanitary sewer system.  That water should be entering the stormwater sewer system or allowed to soak into the ground without entering the sanitary sewer system.

Infiltration is groundwater that enters sanitary sewer systems through cracks and/or leaks in the sanitary sewer pipes.  Cracks or leaks in sanitary sewer pipes or manholes may be caused by age-related deterioration, loose joints, poor design, installation or maintenance errors, damage or root infiltration.  Groundwater can enter these cracks or leaks wherever sanitary sewer systems lie beneath water tables or the soil above the sewer systems becomes saturated with water.

Often sewer pipes are installed beneath creeks or streams because they are the lowest point in the area, and it is more expensive to install the pipe systems beneath a roadway.  These sewer pipes are especially susceptible to infiltration when they crack or break and have been known to drain entire streams into sanitary sewer systems.

Sewer pipes are designed to last about 20-50 years, depending on what type of material is used.  Often sanitary sewer system pipes, along with the lateral pipes attached to households and businesses, have gone much longer without inspection or repair and are likely to be cracked or damaged.

Inflow and infiltration reduce the ability of sanitary sewer systems and treatment facilities to transport and treat domestic and industrial wastewater.  As a result of the inflow and infiltration, wastewater treatment processes are disrupted and poorly treated wastewater is discharged to the environment Assessment of the problem is the first step in the reduction of inflow and infiltration into the system.

Assessment of a sewer system is a process that may include defining basins, sub-basins or drainage areas for each section of town, flow monitoring of each designated area, along with rain gauge measurements. Other assessment tools may include interviews with long-term maintenance employees, maintenance records, smoke testing and cleaning with visual inspection of lines through the use of closed circuit television cameras. All of these have their place in the investigation process, and information from the least expensive techniques should be analyzed before going on to televising of the lines. Usually, an adequate diagnosis can be made based on appropriate application of the cheaper techniques.

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