What is a combined sewer overflow (CSO) situation?

A combined sewer overflow (CSO) situation occurs when there is an excess of stormwater and sewage that exceeds the capacity of the combined sewer system, causing this mixture to overflow into nearby water bodies. This typically happens during heavy rainfall or snowmelt when large volumes of water enter the sewer system quickly. The combined sewer system is designed to carry both sewage and stormwater; however, it can become overwhelmed during significant storm events. As a result, untreated or partially treated wastewater may discharge directly into rivers, lakes, or other water bodies, which can harm aquatic ecosystems and public health.

Understanding combined sewer overflows is crucial in wastewater management, especially in aging urban infrastructure, where many cities still rely on these combined systems. This knowledge highlights the need for systems that can prevent such overflows or effectively manage stormwater separately from sewage to reduce environmental impacts.