Backflow is a term in plumbing for an unwanted flow of water in the reverse direction. It can be a serious health risk for the contamination of potable water supplies with foul water. In the most obvious case, a toilet flush cistern and its water supply must be isolated from the toilet bowl. For this reason, building codes mandate a series of measures and backflow prevention devices to prevent backflow.
CHECK "VALVES" PLEASE!
Backwater sanitary valves (also known as "check valves" or "backwater valves") are also often referred to as "backflow preventers" They are intended to prevent backflow of sewage on the sanitary sewer line during a flood or sewer blockage, and have no connection with potable water.
Also sewage lifting stations provide comprehensive protection against sewer backflow. They pump the water above the backpressure level into the sewer. Even when the sewer is completely full