Backflow is possible in two situations, backsiphonage and backpressure. Backsiphonage occurs when there is a sudden reduction in the water pressure in the distribution system, such as during firefighting or when a water main breaks, water flow can be reversed. This can create a suction effect drawing the non potable substance into the potable water system.
Backpressure is created when pressure in non-potable system, such as in a recirculation system containing soap, acid or antifreeze, exceeds that in the potable system pressure. This can force the potable water to reverse its direction of flow through the cross connection. Non-potable substances can then enter the potable water system.