One of the big maintenance headaches for our aging Wilmette sewer systems is Infiltration/Inflow (I/I):


  • Groundwater seeps into cracks and joints in the sewer lines.
  • Water enters via holes caused by tree root penetration.
  • Water enters via manhole covers that have holes or gaps due to a poor fit.
  • Water is forced from the storm to sanitary lines when both are in the same trench, without proper separation.
  • Infiltration increases during storms, especially when the separate storm sewers aren’t clearing runoff.


  • Inflow is when a storm drain is improperly or illegally connected directly into a sewer line.
  • Inflow sources include: sump pumps, downspouts, and drains for roofs, foundations, window wells, basement stairs, and driveways.
Sources of Infiltration/inflow (from Separate Sewer Study, 2010)

Sources of Infiltration/inflow
Source: Village of Wilmette, Separate Sewer Project Compiled Deliverable, by MWH Americas, Inc. (September 2010), p 34.

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