What must be installed to connect a building drain to a public sewer when the upstream manhole is above the drain floor level?

To connect a building drain to a public sewer when the upstream manhole is above the drain floor level, a backwater valve is essential. This device is designed to prevent sewage from flowing back into the building in the event of a blockage or backup in the public sewer system.

When the upstream manhole is at a higher elevation, the risk of sewage backing up becomes significant, especially during heavy rain or when there are obstructions in the sewer line. A backwater valve allows wastewater from the building to flow out to the sewer, but it automatically closes to prevent any return flow. This is particularly important for maintaining sanitary conditions within the building and protecting against potential health hazards associated with sewage leakage.

Other options such as a vent pipe, pump system, or sump basin do not directly address the specific need to prevent backflow under these circumstances. While a pump system could help in moving wastewater against gravitational flow, it does not provide the backflow prevention feature that is crucial for this scenario. Similarly, a sump basin might be used to collect runoff before pumping it away but does not inherently stop sewage from returning into the building. Therefore, the backwater valve is the correct and most effective solution for this plumbing situation.