What is backflow in sewer systems, and why is it a hazard after floods?

• A. Backflow is when clean water flows faster than expected.
• B. Backflow is when wastewater flows from the sewer into street drains only.
• C. Sewer water reverses into buildings due to pressure changes; contaminates interiors; mitigate with backflow preventers and system assessment.
• D. Backflow occurs only in new plumbing with no risk.

Answer: (C)

Backflow happens when sewer water reverses its normal flow and is pushed back into a building through the plumbing fixtures, driven by pressure changes in the sewer system, especially during floods when the municipal system is overwhelmed or pipes are damaged. This reversal is a serious health hazard because it brings sewage and its pathogens into living spaces, contaminating surfaces, food areas, and water sources, and it can cause strong odors, mold growth, and property damage. After floods, the risk increases because floodwaters and sewer backup can create pressure differentials that force waste back into homes, making it essential to inspect the entire system and ensure it functions properly. Mitigation focuses on preventing that reversal by installing and maintaining backflow prevention devices like backwater valves or check valves on the building’s sewer line and performing a thorough system assessment after a flood to identify damaged pipes, blockages, or other vulnerabilities. The other descriptions don’t capture the reversal of flow into interiors or the need for preventive devices and system checks, so they don’t accurately convey what backflow is or why it’s hazardous after floods.