What is meant by combined sewer overflow (CSO)?

Combined sewer overflow (CSO) refers specifically to a situation where a system designed to collect and treat both sewage and stormwater runoff—known as a combined sewer system—experiences excessive flow during heavy rain or snowmelt. When the capacity of the treatment facility is overwhelmed, the excess volume may be discharged directly into nearby water bodies, such as rivers or lakes. This discharge often contains untreated sewage along with stormwater, which can pose significant environmental and public health risks.

This clarification helps to understand the specific nature of CSOs as they are primarily a concern in urban areas where older sewer systems have not been adequately separated or modernized. The management of CSOs is crucial to prevent pollution and protect water quality in receiving waters.