Select a possible reason that could cause sanitary sewage to flow through a plumbing fixture into a building.

• A. A blockage in the system
• B. A malfunction of the leaching bed
• C. A blocked vent pipe
• D. A broken water main

Answer: (A)

When the sewer line is blocked, wastewater cannot leave the building as intended and pressure builds behind the blockage. That buildup pushes the sewage back through the connected plumbing fixtures, causing it to appear inside the building. The traps in the fixtures are designed to stop sewer gases, but they can be overwhelmed when a main line backs up, leading to sewage appearing in the fixture or overflowing.

Other issues can cause drainage problems, but they don’t create this direct backflow path. A septic leaching bed problem affects underground disposal and may lead to surface or odor issues rather than backflow into interior fixtures. A blocked vent pipe mainly causes slow drainage and gurgling rather than a full backflow of sewage. A broken water main disrupts water supply, not the sewer line, so it doesn’t directly cause sewage to flow back into the building.