What does a Combined Sewer Overflow (CSO) signify?

A Combined Sewer Overflow (CSO) signifies that a combined sewer system, which carries both sewage and stormwater, has reached its capacity limits during periods of heavy rainfall or snowmelt. When the volume of water exceeds the capacity of the sewer system, excess flow is diverted to a discharge point, often into nearby rivers or water bodies, to prevent backups and overflow in streets and homes. This practice is a necessary measure to manage excess flow but can lead to water quality issues in receiving waters due to the introduction of untreated sewage, pollutants, and debris.

While some may think of a CSO in the context of a system being under renovation, a blocked pipe, or the effectiveness of odor control measures, the primary function of the CSO is to indicate that the combined sewer system is unable to handle the water volume under specific conditions. Understanding the implications of CSOs is crucial for addressing environmental and public health concerns associated with combined sewer systems.