Backflow Information

Cross-Connection Control Terms


A back-siphonage condition can occur whenever there is a lowered pressure between the potable and non-potable supply piping. Such conditions typically occur during periods of high demand in the public water main, lowering the supply pressure. For instance during the demands imposed by fire fighting operations, or in the event of a water main break, which suddenly and significantly lowers the city water pressure below that of the non-potable system. This results in a partial vacuum being drawn on the non-potable system, and siphons the pollutants or contaminants into the potable water system through an unprotected cross connection, such as a hose bib or hydronic system make-up connection. Back-siphonage may also occur when a high velocity stream of water passes by a small pipe outlet, such as a residential service tap, due to the "venturi" effect.



A back-pressure condition occurs whenever an elevated pressure exists between the potable and non-potable source. These pressures can be imposed by the installation of pumps which increase pressures above the city water supply pressure, thereby forcing non-potable water in the opposite direction of normal flow into the potable water line. The installation of boilers or other equipment which heat water, causing thermal expansion and resulting in pressures in excess of the incoming water pressure, also can force non-potable water into the potable piping system.


A "pollutant" is any substance which may affect the color, taste or odor of the potable water, but which does not pose a direct threat to human health through exposure or consumption of the water. Pollutants may impose an objectionable odor or appearance to the water, but do not in themselves pose a health threat, and therefore, are considered to be a lesser hazard, when compared to contaminants.


A "contaminant" is any substance which, when introduced into the potable water system, constitutes a direct threat to life or health of a human, if the substance was ingested through consumption, or if the substance came in contact with the skin. A contaminant can therefore be a caustic chemical, a fluid containing bacteria or disease, or any other substance which could threaten human health. Therefore, contaminants compose the highest degree of hazard to the potable water system.