What occurs during a combined sewer overflow (CSO)?

A combined sewer overflow (CSO) occurs when the system experiences heavy rainfall or significant stormwater runoff that exceeds its capacity. This can cause untreated wastewater, which typically includes sewage and stormwater, to be released directly into receiving waters like rivers or lakes. The capacity issues arise because combined sewer systems, designed to handle both sewage and stormwater in a single pipe, can overflow during heavy rains when the volume of water exceeds what the treatment facility can manage.

This is a critical issue for environmental health, as the release of untreated wastewater can lead to pollution and pose risks to aquatic ecosystems and public health. Understanding this process helps in recognizing the importance of infrastructure improvements and technologies aimed at preventing CSOs and protecting water quality.