Water normally flows in one direction, from the public water system through the customer’s cold or hot water plumbing system to a faucet or other plumbing fixture. Under certain conditions, water can flow in the reverse direction. This is known as backflow, and it occurs when backsiphonage or backpressure is created in a water line.
Back-siphonage may occur when there is a drop in the supply pressure of the water distribution system. This can be caused by a water line break, water main repair, or during a rapid withdrawal of water from a fire hydrant. This creates a vacuum, which may pull or siphon contaminants or pollutants into the drinking water supply.
Backpressure may be created when a source of pressure, such as a pump, boiler, or other building creates a pressure greater than that supplied from the water distribution system; this may force water to reverse direction.