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 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, only the Jeannette WWTP has a portion of its collection system classified as a combined sewer system.  The location of the five CSOs in Jeannette are shown on the map below.

Jeannette CSO Locations


Municipal authorities with combined sewer systems are required to have a Long-Term Control Plan (LTCP) that discusses how CSOs will be managed on a long-term basis in order to comply with EPA’s CSO Control Policy.  MAWC submitted a draft LTCP to the Pennsylvania Department of Environmental Protection (DEP) on 2-1-2018.  DEP tentatively approved it on 2-7-2019.  MAWC is conducting a final public comment period on the draft LTCP.  It is available for review at the MAWC office at 124 Park and Pool Road in New Stanton or by clicking on the link below: