Inflow and Infiltration Facts

Inflow and infiltration (I/I) are terms that describe clear water that enters wastewater systems, consumes capacity, and increases demand at a wastewater treatment plant. Excessive flows can result in public and environmental health concerns if untreated sewage discharges to basements, rivers, lakes, or other public areas.

Inflow and infiltration each have unique sources, methods of entry, and effects on the wastewater collection system. Both are costly to communities and utility ratepayers due to increased treatment and maintenance costs.

Inflow is typically stormwater that increases peak flow in the wastewater system during and after rainfall events from point sources such as broken manhole covers, sewer cleanouts, sump pumps, foundation drains and rain leaders. 

Infiltration is typically groundwater that gradually enters the wastewater system through cracks and openings in sewer mains, service laterals, joints, and deteriorated manholes.
Infiltration increases base flow in the wastewater system and removes water from the natural hydrologic cycle, which could otherwise recharge the regional aquifers.



Regional Ongoing I/I Mitigation Program

The Metropolitan Council has convened multiple community-based task force committees since 2003 which developed the Ongoing I/I Mitigation Program. The program goals include:

  • Protect public and environmental health by preventing untreated sewage discharges to basements and the environment
  • Minimize regional and local cost to convey and treat excessive I/I
  • Ensure the vitality and sustainability of the regional water resources


Success of mitigationphoto of a pipe with water pouring out

Homeowner, local community, and regional efforts to mitigate I/I from the wastewater systems have resulted in reduced flows across the region.

  • The regional wastewater flow has been reduced by roughly nine billion gallons per year on average. The flow decrease has occurred even as precipitation volumes, rainfall intensities, and population have increased.
  • Two significant rainfall events occurred in October 2005 and June 2014. The 2014 rainfall event included higher precipitation and higher soil moisture, with lower peak flows recorded at the Metro and Blue Lake wastewater treatment plants when compared to the 2005 rainfall event.

What can homeowners do to mitigate I/I?

  • Disconnect unlawful connections to the sewer system. Residents should contact the local Public Works department to help find reasonable alternatives.
  • Be familiar with the plumbing system in the home or business. If roof downspouts are piped underground, a sump pump has no identifiable discharge point, or foundation drains don't discharge to the roadside ditch or other low point on the lot, there may be sources of I/I on the property that are contributing to the problem.
  • Have the private sewer service lateral inspected and, if necessary, repaired.

What are local communities and the Council doing to mitigate I/I?

  • Communities in the region have been working for years to mitigate I/I sources. Between 2007 and 2015, regional communities reported investments of over $157 M in I/I mitigation projects.
  • Communities and property owners offset I/I mitigation expenses using grants. The Minnesota Legislature—
    with advocacy of Metro Cities—has allocated a total of $10.5 M of grant funding to help communities mitigate I/I. Minnesota’s Legacy Clean Water Fund also contributed $1 M to support roughly $3.3 M in investments.
  • MCES invested roughly $88 M on I/I mitigation activities from 2007-2015 and plans to invest about $80 million to $100 million annually in interceptor-related investments through year 2023.
  • Completed comprehensive analyses of the sewer systems to locate I/I sources. Many communities have developed strategies to mitigate I/I as part of comprehensive master plans and capital improvement projects.
  • MCES hosted I/I mitigation workshops and created public outreach videos. Find additional resources here.

photo of long pipes laid out in a pile on lawn

More information

For I/I program questions, please contact:

For financial and grant questions, please contact: