What is Backflow?
Backflow is the undesirable reversal of the flow of water or mixtures of water and other undesirable substances from any source (such as used water, industrial fluids, gasses, or any substance other than the intended potable water) into the distribution pipes of the potable water system. There are two types of backflow conditions: backpressure and back siphonage.
Backpressure: Occurs when the user system is at a higher pressure than the supply water systems, allowing undesirable substances to be “pushed” back into the potable water system. Some causes are: booster pumps, potable water system connections for boilers, interconnection with other piping systems operating at higher pressures, or higher elevations in user systems such as high rise buildings.
Back Siphonage: Occurs when negative or reduced pressure exists in the supply piping allowing undesirable substances to be “drawn” into the potable water supply. Some causes are:
Undersized supply piping, supply line breaks, reduced supply system pressure on the suction side of an on-line booster pump, or sudden upstream high demand. An example of this is a child drinking milk with a straw. The child “sucks” on the straw and the milk flows up the straw and into the child’s mouth. What the child is actually doing is creating a sub-atmospheric pressure in his mouth and the atmospheric pressure (14.7psia at sea level) is pushing down on the surface of the milk and forcing the milk up the straw and into the child’s mouth.
Cross-Connection is defined as any actual or potential connection between a potable water system and any other source or system through which it is possible to introduce into the potable system any used water, industrial fluid, gas, or other substance other than the intended potable water with which the system is supplied. By-pass arrangements, jumper connections, removable sections, swivel or change-over devices and other permanent or temporary devices through which, or because of which, backflow can or may occur are considered to be cross-connections.
There are several different types of mechanical backflow prevention devices. An alternative to a mechanical device is a physical separation, or air gap. The air gap is a physical break in the system. Different types of mechanical devices are used in different situations (if there is backpressure or back-siphonage) and for different degrees of hazard. The degree of hazard is based on the fluid (or other substance) that may backflow into the supply piping system. The fluid may be toxic or nontoxic and could create a “non-health” or “health” hazard. A non-health (non-toxic) hazard cross-connection is any point in a water supply system where a polluting substance may come in contact with potable water aesthetically affecting the taste, odor or appearance of the water, but it is not hazardous to health. A health hazard (toxic) cross-connection is any point in a water supply system where a contaminating substance may come in contact with potable water creating an actual health hazard, causing sickness or death.