Backflow is the undesired reversal of water flow against normal direction. Backflow occurs as a result of Back Pressure or Back Siphonage.
Back Pressure occurs when the pressure in a private water system is greater than the pressure in the City’s water supply system.
Back Siphonage occurs when the pressure in the water supply-system is lower than a private-systems’ pressure. This can result in the reversal of flow between the public and private system.