What is a combined sewer overflow (CSO) and why is it a concern?

• A. Overflow of a stormwater sewer only during drought.
• B. Overflow of irrigation return flows.
• C. Overflow of wastewater during rainfall, discharging untreated sewage.
• D. Overflow of treated water from a reservoir.

Answer: (C)

A combined sewer overflow happens when a city’s sewer system collects both stormwater and wastewater in one network, and during heavy rainfall the flow becomes too large to be treated. The excess is discharged directly to a nearby water body without treatment. This matters because the release carries untreated sewage along with a mix of pollutants, posing health risks to people who use the water and harming rivers, lakes, and aquatic life. It also stresses downstream water supplies and can trigger sanitary and environmental problems after storms.

The other statements don’t fit CSOs: overflowing only during a drought isn’t correct because CSOs occur with high volumes from rain, not lack of water; irrigation return flows are agricultural and not part of a combined urban sewer system; overflow of treated water would indicate the water has already been treated, not an untreated sewage discharge.