Backflow prevention devices are used to protect potable water supplies from contamination or pollution due to backflow.
In water supply systems, water is normally maintained at a significant pressure to enable water to flow from the tap, shower etc. When pressure fails or is reduced, as may happen if a water main bursts, pipes freeze or there is unexpectedly high demand on the water system, then such reduced pressure in the pipe may allow contaminated water from the ground, from storage or from other sources to be drawn up into the system. The backflow preventer serves to break the siphon and prevent the water from being drawn back into the water supply.
Watts Regulator