The local sewer system serving early Mount Prospect was comprised entirely of combined sewers.  Today, combined sewers constitute approximately 34% of the Village-owned sewer system. In the 1920s, as was typical of the time, both storm water and wastewater were conveyed using the same pipe. That is, homes and street inlets were connected to the same pipe. Initially, these pipes discharged both stormwater and sanitary wastewater into area creeks. When the MWRDGC interceptor system was constructed, wastewater was transported to the treatment plant in Skokie. During rain events, stormwater and sanitary wastewater flows usually over-taxed the interceptor pipes and were discharged to the creeks.

Typically, homes and buildings were connected to the combined sewer by means of gravity (no sump pumps or ejector pumps).  Some homes also had "catch basins" installed on their service lines (the pipe that connects the home to the main sewer).  These catch basins were designed to collect solid wastes while allowing liquids and stormwater to travel through to the main sewer.  Periodically, the catch basins would have to be cleared to prevent backup of wastewater into the home.  Footing tiles, downspouts, and yard drains were also connected directly to the combined sewer mains.  A prevailing thought at the time, was that numerous sources of fresh water helped to "flush" sanitary wastes downstream and control odor problems.  Typically, buildings serviced by combined sewers do not have overhead sewers and are more susceptible to flooding through floor drains and toilet backups.