Inflow and infiltration (I/I) are terms for the ways that stormwater runoff and groundwater make their way into sanitary sewer pipes and eventually get treated, unnecessarily, at wastewater treatment plants.
I/I is a problem because it takes up fixed capacity in large regional sewer pipes (interceptors), capacity that will be needed for future households in the region. I/I can also be costly to communities. Once clear water gets mixed in with wastewater, communities are charged for the treatment of all the water.
Building additional interceptor and treatment capacity to handle excessive I/I is not financially prudent; it is much cheaper to eliminate I/I at the source. Metropolitan Council Environmental Services (MCES) and communities in the seven-county region are working hard to reduce I/I in the regional wastewater system in order to preserve capacity as well as protect public health and the environment.
The Council’s I/I program has four elements:
Surcharges and addressing issues with excessive I/I in communities
Fixing MCES pipes to reduce I/I in the regional system
Municipal and private infrastructure grants
Engaging the public in workshops, videos and other education to promote awareness of I/I and action to mitigate it
Inflow increases during major rain events
Inflow is when clear water enters the wastewater system through rain leaders, sump pumps or foundation drains that are connected to sewer lines (illegal in Minnesota since 1968). Infiltration occurs when groundwater seeps into cracked or broken wastewater pipes.
Inflow is the biggest problem because during major rain events it quickly consumes pipe capacity needed for future growth. For example, a sump pump can contribute 7,200 gallons of clear water to the wastewater system in 24 hours, the equivalent of the normal daily flow from 40 homes. In more extreme rain events, inflow can cause sewer backups into homes and businesses.
Infiltration, while it takes up significant pipe capacity, is a steadier, less variable contributor to the problem. Infiltration also causes water that should be filtering down and recharging the region’s aquifers to end up in the Mississippi River and flow out of Minnesota.
Local I/I solutions cost much less than adding regional capacity
In 2006, following a community task force recommendation, the Council instituted a multi-year program to reduce I/I. The cost to fix I/I at the local source was estimated at about $150 million, compared with nearly one billion dollars that would be needed to add collection and treatment capacity to handle excessive I/I.
The Council tracked peak flows and identified 47 communities in the region that released excess I/I into the system. The Council required communities to commence work to eliminate their excess I/I contribution, or face a surcharge on their municipal wastewater bills.
The grant program supports local community efforts in I/I mitigation. In 2012, the Council received $4 million from the state legislature to provide grants to municipalities for improvements to public infrastructure to reduce I/I. In 2013, the legislature allocated $1 million from the state’s Clean Water Legacy Fund for a program focused on mitigation efforts on private infrastructure (for example, foundation drain disconnections). In 2014, the legislature allocated $2 million for grants to cities to make capital improvements in municipal wastewater collection systems to reduce I/I to the regional sanitary sewer system.
Demand Charge Task Force and the ongoing surcharge program
In 2009, a Demand Charge Task Force was formed to develop recommendations to update the program. The task force recommended an ongoing surcharge program in lieu of imposing demand charges that were part of the original program. Now communities can have more time to reduce I/I and the Council can avoid the unnecessary expense of building excess capacity.
The new surcharge program has refocused community attention on excess I/I in the system, offering a chance to develop and implement a work plan on an ongoing basis. And the program is working, as evidenced by a dampening of peak flow rates, as well as avoidance of:
Spending a billion dollars for capacity not needed for sewage collection and treatment
Getting a consent decree from the EPA over spills and backups (which have cost other metro areas billions of dollars)