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Backflow is any unwanted flow of used or non-potable water, or other substances from any domestic, industrial, or institutional piping system back into the potable water distribution system. The direction of flow under these conditions is opposite to that of normal flow and is caused by either backsiphonage or backpressure. Backsiphonage is backflow caused by a negative pressure (vacuum or partial vacuum) in the supply piping. Backsiphonage occurs when system pressure is reduced below atmospheric pressure. The effect is similar to sipping water through a straw. Backpressure is backflow caused by pressure in the customer's plumbing being greater than the pressure in the water supply piping. The higher pressure in the customer's plumbing may be from a booster pump, heating boiler, etc. Outside water taps and garden hoses tend to be the most common sources of cross connection contamination at home. The garden hose creates a hazard when submerged in a swimming pool or when attached to a chemical sprayer for weed control. Garden hoses that are left lying on the ground may be contaminated by fertilizers, cesspools, or garden chemicals.
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A cross-connection is a point in a plumbing system where it is possible for a non-potable substance to come into contact with the potable drinking water supply. Common examples of cross-connections include a garden hose submerged in a pesticide mixture, a piped connection providing potable feed water to an industrial process, such as a cooling tower, or a submerged outlet of an irrigation system. Connections to firefighting equipment are other very common cross-connections. Most cross connections occur beyond the customer service connection, within residential, commercial, institutional or industrial plumbing systems. Identifying cross-connections can be challenging because many distribution systems are expanding to serve new customers and changing to accommodate customer needs. Further, temporary and permanent cross connections can be created in existing facilities without the knowledge of the water system managers and operators.