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Backflow is any unwanted flow of used or nonpotable water, or other substances, into the potable water distribution system.  Backflow can be caused by either backsiphonage or backpressure.

Cross Connection
A cross-connection is a point in plumbing system where it is possible for a nonpotable substance to come into contact with the potable drinking water supply. 

Examples of Cross-Connections:

  • A hose submerged in a pesticide mixture

  • A piped connection providing potable feed water to an industrial process, such as a cooling tower

Backsiphonage
Backsiphonage is backflow caused by negative pressure in a portion of the distribution system.  If the system pressure drops due to a water main break or fire hydrant use, it can cause negative flow in portions of the distribution system.

Example of Backsiphonage 
Booster pumps for high-rise buildings can cause backsiphonage if the suction lines of the pumps are being used for service on the lower floors and a temporary or permanent cross-connection on the lower floors exists (e.g., a hose submerged in a bucket of cleaning solution).  If the distribution system pressure drops, the suction pressure can cause the backsiphonage through the lower floor cross-connection when the pump is operating, contaminating the higher floors.

Backpressure
Backpressure is backflow caused when pressure in the supply piping is higher than pressure in the distribution system.  Some possible causes of higher supply piping pressure can be caused by a pump, boiler or elevation difference. 

Example of Backpressure
Compressed air systems such as carbonator can pose backpressure risks.  The pressure of a carbon dioxide tank, for example, can be several thousand pounds per square inch (psi).  This high-pressure carbon dioxide is passed through a regulator and mixed into a water system at anywhere from 60 to 150 psi.  Carbon dioxide from either a tank or a regulator could be introduced to the distribution system pressure if a cross-connection is present and the compressed air system overcomes the distribution system pressure.

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