Why do Combined Sewer Overflows (CSOs) exist?
Historically, the combined sewer systems were designed and sized to carry both wastewater and stormwater flow in the same pipe to the treatment plant. As the amount of impervious cover in Philadelphia increased, the amount of stormwater being captured by the collection system also increased. Due to the increase in stormwater runoff from development and poor land use management, combined with population (sanitary) increases, the capacity to carry all the flow in the existing sewer system can be exceeded. To avoid basement flooding, house fixture back-ups, and street flooding, the sewer system was designed to provide for overflow points that will send any flow in excess of treatment or carrying capacity to a nearby waterbody.