Inflow & Infiltration (I/I)

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Keeping clear water out of the sewer system

The District’s purpose is to collect and treat wastewater from homes and businesses. But through various ways, excess stormwater and groundwater get into the sanitary sewer system, where it gets cleaned unnecessarily. Reducing clear water coming into the sewer system via inflow and infiltration can save money while protecting human health and the environment.

Sources of inflow & infiltration (I/I) sources at a home, including roof and foundation drains, laterals, manholes, catch basins and sewer pipes.

What is inflow and infiltration?

Inflow and infiltration (I/I) describes excess clear water from groundwater and stormwater sources that flows into the sanitary sewer system.

More specifically, infiltration refers to groundwater that seeps into sewer pipes through holes, cracks, joint failures and faulty connections. Inflow is stormwater that quickly flows into sewers via roof drain downspouts, foundation drains, storm drain cross-connections, and through holes in manhole covers.

Why is I/I problematic?

While some communities have a combined collection system for stormwater and wastewater, the District’s is only a sanitary sewer system. In addition, our collection system and treatment plant both have capacity limitations. I/I, especially during significant storm events, can overload the system and cause costly sewer backups into homes and buildings. I/I can also cause sewer overflows into rivers and lakes.

I/I is also costly. The District charges owner communities for the amount of flow that arrives at the Nine Springs treatment plant; I/I in the system from broken home sewer laterals, leaky sewer mains or improper sump pump connections increases flow and sewer bills. In addition, I/I results in more energy usage to pump the flow and the unnecessary treatment of groundwater and stormwater.

What are the sources of I/I in the District’s collection system?

Aging infrastructure
One cause of I/I in the collection system is the District’s aging sewer infrastructure. Some of our pipes are more than 60 years old and have leaky joints. To remedy this, the District has a program to assess the condition of our gravity mains and look for leaks with special cameras. With this information, we prioritize the rehabilitation of the worst leakers; rehabilitation includes grouting leaky joints and installing sewer liners to seal the insides of our pipes.

The District does not own all sewer pipes in our service area, though; communities are responsible for their own sewer infrastructure that connects to the District’s larger sanitary mains. Community sewer pipes are subject to the same issues of aging and leaking.

Video showing infiltration into a District sewer pipe.

During storm events and snowmelt, water can get into the holes of manhole covers or into leaks and cracks in manhole structures. Flood events can also lift manhole covers out of place.

The District’s Collection System Services team regularly conducts manhole inspections and does repairs as necessary.

Sewer laterals
Sewer laterals are the pipes that connect homes and businesses to your community’s sewer system. Over time, sewer laterals can crack or break due to age. Tree roots can also put pressure on these pipes and cause breakages. When this happens, groundwater can enter the sewer system and contribute to the problems associated with I/I.

In Wisconsin, sewer laterals are the responsibility of the property owner.

Improper connections
Connections between the stormwater and sanitary sewer system, which are prohibited, are significant contributors to I/I. Property owners should not connect sump pumps, gutters, roof drains, foundation drains and other stormwater-handling systems to the sanitary sewer system.

What can I do about I/I?

  • Do not connect sump pumps to the sanitary sewer.
  • Do not connect roof drains and gutters to the sanitary sewer.
  • Do not connect foundation drains to the sanitary sewer.
  • Keep all cleanouts capped, both inside and outside. This will help keep unwanted water out of the sanitary sewer system and prevent sewer gas from entering your home.

What is the District doing about I/I?

I/I is a significant concern for some of our owner communities. In 2019, we invited stakeholders from our communities to join our I/I Technical Advisory Group and we continue working with them to develop a path forward for addressing this issue.

The vision and framework for our I/I Reduction Program were presented in April 2021. The program includes working with owner communities and public outreach. It will be rolled out and implemented over several years, with a ramp-up in funding to correspond with an increase in our shared efforts.

View our Infiltration and Inflow Reduction Program Plan