Sewer Inflow and Infiltration

The Village of Northbrook owns and maintains a 133 mile sanitary sewer system. This system transport wastewater from individual properties to the Metropolitan Water Reclamation District’s (MWRD’s) transmission lines which take it to the Skokie Treatment Plant, where it is processed. As part of its on-going responsibilities and to meet its obligations under MWRD’s Watershed Management Ordinance, the Village must complete inspections of its sewer system as well as private systems to identify and mitigate any defects which may allow stormwater or groundwater from entering the sanitary system. 

What is inflow and infiltration ("I&I")?

Common I-IInflow and infiltration (often called “I&I”) are the two most common ways that storm or groundwater enter the sanitary system. Inflow is entry from an improperly connected drain (such as a sump pump, foundation drain or house gutters). Infiltration is entry from a defect somewhere in a property’s plumbing or from a crack or break in a service line or sewer pipe. The chart to the right illustrates common sources of I&I.

Why is I&I a concern?

I&I reduces capacity in the sanitary sewer system and, during wet weather, it can overwhelm the system and lead to backups into a property’s basement. Backups can happen anywhere where the system might already be slightly obstructed (by roots or a blockage) and there is too much I&I for the system to handle.

What are the most common types of problems on private property?

The most common types of problems in Northbrook are missing or loose clean out caps, illegally connected downspouts or illegal connections (from sump pumps, foundation drains, etc.). If one of these problems is identified on a property, staff will notify the property owner and provide details about the defect and potential remediation methods. 

What is the Village doing about I&I in the public system?

The Village addresses I/I in the public system through two primary methods: (i) annual and special testing to identify defects as well as cleaning of the system and (ii) completing repairs to sewers and manholes to prevent I/I. These repairs often involve the using of lining, which allows for a thin liner to be inserted into the sewer or manhole to prevent more I/I and reinforce the structure of the sewer or manhole. Details on the Village’s year-to-year repair activities can be found in the current five-year Capital Improvement Program on the Village’s website at

Where can I learn more about I&I?

To learn more about I&I, visit MWRD's website