Backflow is the flow of water or other liquids, mixtures or substances, under positive or reduced pressure into the distribution pipes of a potable water supply from any source other than its intended source. Backflow is caused by either backsiphonage or backpressure. Backsiphonage occurs when a flow of used, contaminated, or polluted water from a plumbing fixture or vessel enters into the public water system, often due to negative pressure in a pipe. It is important to note that a drop in pressure is out of your control and can occur at any time. Backpressure occurs when the pressure is greater than the pressure in the potable water distribution system.