Any extra water flowing into wastewater collection systems from above-ground sources, intentionally or unintentionally, is called inflow. For example, water may flow into sanitary sewer systems during storms or snow thaws 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, these connections are prohibited under the City’s Code of Ordinances Section 19-44.

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.

Not only is I/I a financial 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 on 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 cause serious degradation to the ecosystems inundated with sewage.

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 the 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.