What is Backflow and Why Should It Be Prevented?
Backflow is the flow of any liquid or contaminant back into the potable water supply. This happens by either back-siphonage or back-pressure. Water can be siphoned into the water supply if there is a drop in supply pressure. Back pressure might result from a pressurized system directly connected to the water system.
In other words, water from your water-closet (head) MUST BE PREVENTED FROM being drawn back into the potable water system by siphon, pressure loss, or any other means. The backflow prevention device is used to keep water that passes through it, and into the head, from coming back into the potable fresh-water supply.
For a general description of typical back-flow prevention devices, goto: