Skip to main content

Managing Infiltration Inflow in Wastewater Collection Systems

Reference Number: MTAS-587
Reviewed Date: 05/10/2023

What Is Infiltration/Inflow (I/I)?

Groundwater infiltration and stormwater inflow is the greatest challenge to the proper operation and performance of the entire sewage and wastewater system. I/I causes sewage backups into homes and businesses, overflows into streams and yards, and generally causes added costs.

Infiltration is water that enters the sewage collection system from the soil through foundation drains, defective pipes or joints and faulty connections (i.e., ground water).

Inflow is water that enters the sewage collection system from sources such as roof down spouts, basement and yard drains and cross connections with drainage lines (i.e., surface or rainwater).

If you have a wastewater collection system, chances are you have infiltration and inflow (I/I) problems. A third party engineer located in Tennessee says 47 percent of the volume (2012) of water flowing within the sewer collection system and to the treatment plants is I/I. Many systems have recognized I/I problems, but they are not addressed because the problems seem overwhelming — both in terms of money and manpower.

Beginning in 2018 newly renewed wastewater system permits have more extensive reporting requirements when a sanitary sewer overflow or backup occurs. The underlying intent is to force utilities to be more effective in reducing I/I and the overflows and backups which are often caused by I/I.

Is It Really 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.