Combined Sewer Overflows (CSOs)

Combined sewer systems are designed to collect sewage and stormwater in the same set of pipes.  (This is opposed to separate sanitary sewer systems, which are designed to collect sewage in one set of pipes and stormwater in a separate set of pipes.)  Under normal weather conditions, all wastewater flowing within a combined sewer system is transported to the wastewater treatment plant for treatment and discharge.  However, during heavy rain events, the excess amount of stormwater may overload the capacity of the pipes.  In order to relieve the overloaded pipes, Combined Sewer Overflow (CSO) structures exist within the system.  They allow some of the diluted wastewater to discharge directly into streams prior to reaching the wastewater treatment plant.  Without the CSO structures as relief points, the overloaded pipes may cause wastewater to back up into the basements of homes.

For more information on CSOs, please visit EPA’s website at

MAWC’s service area includes 10 wastewater treatment plants.  Of those, the Jeannette and Mt. Pleasant systems have a portion of their systems classified as a combined sewer system.  The location of the five CSOs in Jeannette and the three CSOs in Mt. Pleasant Borough are shown on the maps below.

Jeannette CSO Locations


Mt. Pleasant Borough CSO Locations