What is a combined sewer overflow (CSO)?

A combined sewer overflow (CSO) refers to a discharge that occurs when a combined sewer system, which collects both sewage and stormwater in the same pipe, becomes overwhelmed during periods of heavy rain or snowmelt. When the volume of water exceeds the capacity of the sewer system, untreated or partially treated wastewater is released directly into the environment, typically into nearby bodies of water. This discharge can lead to significant water quality issues, as it may introduce pathogens, nutrients, and other pollutants.

Heavy rain events can lead to the rapid influx of stormwater, which, when combined with already existing sewage in the system, creates an overflow situation. This phenomenon is particularly concerning in urban areas where combined sewer systems are common, and it highlights the need for improved infrastructure and management practices to handle both sewage and stormwater effectively, especially during extreme weather events.

The other options do not accurately describe a CSO. Treated water releases during dry weather, non-compliant effluents, and methods of purifying stormwater do not capture the essence of what happens during a combined sewer overflow event.