Backwater, or backflow, is the unintended inflow of water and sewage into sewer mains and laterals which serve homes, businesses and stormwater drainage systems. The system overload usually results from excessive precipitation. Prevention involves a backwater valve, which is similar to a check valve since it only allows water to flow in one direction. The difference between backwater preventer and the check valve is that the backwater preventer uses a rubber flap to prevent the undesirable inflow, whereas a check valve uses a spring-loaded mechanism to ensure water can only pass through it in one direction.
Trenchlesspedia Explains Backwater
A backwater valve isn't the same as a backflow preventer. The backwater valves mission is to prevent raw sewage from backing up into your home or business through the laterals that connected to the sewer main, much like a computer's firewall is the first line of defense against malware infection on a computer. The backflow preventer, on the other hand, works within your home or business. Attached to a potable water source (city water lines or water wellhead), it prevents a reverse flow of greywater into a potable water system.