What Is Infiltration/Inflow (I/I)?

Groundwater infiltration and stormwater inflow (I/I) are among the greatest challenges to the proper operation of a sewage and wastewater system. I/I can cause sewage backups into homes and businesses, overflows into streams and yards, and generally increases operational and treatment costs.

Infiltration is water that enters the sewage collection system from the surrounding soil through foundation drains, defective pipes or joints, and faulty connections—primarily groundwater. Inflow is water that enters the system from sources such as roof downspouts, basement and yard drains, or cross connections with drainage lines—primarily surface or rainwater.

If your community has a wastewater collection system, it likely experiences some degree of I/I. For example, a Tennessee engineering study found that in certain systems, I/I accounted for nearly 47 percent of the total flow to the sewer collection system and treatment plants in 2012. While many systems recognize I/I issues, they are often left unaddressed due to the high costs and manpower required to mitigate them.

Since 2018, renewed wastewater system permits in Tennessee have included more extensive reporting requirements for sanitary sewer overflows and backups. The goal of these regulations is to encourage utilities to more effectively reduce I/I and prevent the overflows and backups that it often causes.

Is It Actually I/I?

When an overflow occurs in the system, you may think it is I/I, but it could be:

• debris, roots, or grease in the system, restricting flow.
• sags in lines.
• protruding taps.
• collapsed lines due to corrosion.
• design bottlenecks.
• pump station problems — leaking check valves, improperly sized impeller, improperly sized motor, impeller wear, etc.; or
• forced main issues — corrosion, sediment, air locking, etc.

These are mostly operations and maintenance issues which can be identified and corrected with diligent operations and maintenance.