What is a combined sewer overflow (CSO)?

• A. A condition when stormwater mixes with untreated sewage
• B. A process for separating solids from liquids
• C. A method for enhancing disinfection efficiency
• D. A technique for removing grit and debris

Answer: (A)

A combined sewer overflow (CSO) refers specifically to a situation that occurs in combined sewer systems, where stormwater runoff combines with untreated sewage during periods of heavy rainfall or snowmelt. In these instances, the capacity of the sewer system can be exceeded, leading to the discharge of excess water that includes both stormwater and untreated sewage directly into local water bodies, such as rivers or lakes. This phenomenon poses significant environmental and public health risks, as it can lead to contamination of water bodies, making them hazardous for recreational activities and harming aquatic life.

Understanding this concept is critical for managing wastewater systems effectively, as CSOs are a significant concern for urban areas where combined sewer systems are prevalent. The correct identification of a CSO helps in developing strategies to mitigate its occurrence, such as improving stormwater management, increasing the capacity of sewer systems, or implementing green infrastructure solutions. The other options relate to processes that focus on the treatment and management of wastewater but do not accurately describe what a CSO is.