Infiltration and Inflow (I/I)

Many cities and towns around the U.S., including Chanhassen, have sanitary sewer systems that are reaching their life expectancy, and are now in need of major repairs. As with any public infrastructure, such as roads, sanitary sewer systems require regular monitoring and maintenance in order to extend the life of this critical public utility, something the City has been investing in for decades. But even with the investments made in monitoring, maintenance, and upgrades within the public sewer system, infiltration and inflow (I/I) are two ongoing problems the City, alongside the greater metro area, is grappling with.

  1. Erik Henricksen

    Project Engineer
    Phone: (952) 227-1165

What is Infiltration?Picture showing sources of Infiltration and Inflow

The term infiltration is used by wastewater professionals to describe the excess water that sometimes seeps, trickles, or flows into old or damaged collection systems from the surrounding soil. For example, high groundwater or water remaining in the soil after rain or snow often can infiltrate parts of the sanitary sewer system, such as private service laterals that have deteriorated, cracked, sagged, or collapsed.  See image to the right for examples (red circles).

What is Inflow?

Any extra water flowing into wastewater collection systems from above ground sources, either intentionally or unintentionally, is referred to as inflow (purple circles). For example, during storms or snow thaws, large volumes of water may flow into sanitary sewer systems through leaky manhole covers or uncapped sewer cleanouts. In addition, private residences may have roof, cellar, yard, area, or foundation drains inappropriately connected to sanitary sewer laterals. This typically will include a sump pump connection to sanitary sewer lines. In the City of Chanhassen, all of these connections are considered prohibited under the City’s Code of Ordinances Section 19-44.


What is I/I?

When municipalities have older sanitary sewer systems, it often is very difficult to determine exactly how much of the extra wastewater in the system is the result of inflow versus infiltration.  Knowing if it is inflow versus infiltration is important because it helps determine what type of repair is needed.  Because of this uncertainty, wastewater professionals usually refer to the overall problem as “I/I” (pronounced I and I).

What are the Costs of I/I?

The cost of infiltration and inflow, when not corrected, impacts the public directly and communities should be concerned about I/I because:

  •  I/I can cause back-ups of sewers into streets and private properties which requires costly repairs and cleanup.
  • I/I can increase collection system operating costs, e.g. adding to the necessary run times for City pumps and lift stations, which incurs greater costs towards energy, maintenance, and repairs.
  • I/I can increase operation and treatment costs for facilities that receive the additional flows (Chanhassen sends its wastewater to the Blue Lake Wastewater Treatment Plant, pictured below, operated by the Metropolitan Council Environmental Services).   The Metropolitan Council monitors I/I and charges municipalities (and in turn property owners) who exceed allowable daily flows to the treatment facility.

 Photo of Blue Lake Wastewater Treatment Plant

Not only is I/I a pecuniary cost, but it also has environmental impacts.  Because I/I is rainwater and/or groundwater that gets into the sewer system, it is treated as wastewater and discharged into the Minnesota River and sent downstream.  This water would have naturally infiltrated into our groundwater aquifers for use by future generations, but is now lost to Minnesotans.  Another example of the impacts to our natural resources is if a back-up ever occurs due to I/I, such as in the City of Mound in 2014, sewage can enter surface water, such as lakes and rivers, and causes serious degradation to the ecosystems inundated with sewage.

What are We Doing to Remediate I/I in Chanhassen?

The City of Chanhassen has been investing in the abatement and remediation of I/I issues for over 20 years through inspections, maintenance, and repairs of the public sanitary sewer system.  However, more ramped up efforts to repair, upgrade, and reconstruct problematic areas within the public system began in 2006, following the Metropolitan Council Environmental Services (MCES) adoption of a program to reduce I/I throughout the region.  The MCES program aimed to reduce the amount of I/I entering their regional interceptor system through the use of penalties and incentives for the communities it served, which includes the City of Chanhassen.  While many improvements have been installed in the public sanitary system in an effort to avoid these costly penalties, MCES metering and City lift station run time data are showing that certain areas of the sanitary sewer system are still experiencing I/I issues.   

In response to new data from the MCES indicating that up to 80% of I/I can originate from private property connections, the City conducted the Inflow and Infiltration Study Project No. 2019-06, to evaluate local sources of I/I. The study’s focus was to evaluate the impact private property connections have on I/I, as the City has invested a majority of its inspections, maintenance, and upgrades in correcting issues within the public system.  Upon completion of the study and evaluation of the findings by City Council, the city will be implementing a “Mandatory Inspection Program”.  Currently the city has issued a Request for Proposal to Contractors and Consultants for the implementation of this program.

Additional Resources

MCES: Infiltration and Inflow
Inflow and Infiltration Study: Project No. 2019-06