What is a combined sewer overflow (CSO)?

A combined sewer overflow (CSO) refers specifically to the discharge that occurs when a combined sewer system, which handles both stormwater and sewage, is overwhelmed. During heavy rainfall or rapid snowmelt, the volume of water entering the system can exceed its capacity, leading to the release of untreated or partially treated wastewater directly into nearby water bodies. This phenomenon is a significant concern in urban areas where such systems are common, as it can lead to pollution and water quality issues.

The correct answer highlights the potential environmental impact of a CSO event, emphasizing that the overflow consists of a mixture of sewage and stormwater that has not undergone the necessary treatment processes. Understanding this concept is crucial for sewage treatment workers, as it underscores the importance of managing stormwater effectively to prevent such overflows, ultimately protecting public health and the environment.