Backsiphonage – what is it?

Q.
What is backsiphonage?

A.
Backsiphonage occurs when there is a partial vacuum (negative pressure) in a water supply system, which draws the water from a contaminated source into a potable water supply. The water pressure within the distribution system falls below that of the plumbing system it is supplying. The effect is similar to siphoning or drinking water through a straw. For example, during a large fire, a pump is connected to a hydrant. High flows pumped out of the distribution system can result in significantly reduced water pressure around the withdrawal point. A partial vacuum has been created in the system, causing suction of contaminated water into the potable water system. During such conditions, it is possible for water to be withdrawn from nonpotable sources located near the fire -- for example, air-conditioning systems, water tanks, boilers, fertilizer tanks and washing machines -- into buildings located near a fire. The same conditions can be caused by a water main break.

Garden hoses, toilets or similar devices create most household cross-connections. Under certain conditions, the flow in household water lines can reverse and siphon contaminates into the water supply. A toilet installed incorrectly without a "plumbing-code approved" toilet ballcock (air gap) will allow contaminated water to backflow to other water outlets in your house, including the kitchen sink.

The figure below depicts a possible backsiphonage scenario. In this drawing, a motor vehicle collides with a fire hydrant installed on the public water main (B). The crash causes the hydrant to break releasing water from the public main. The ensuing drop in water pressure in the public water main creates suction that draws dirty water from a second floor bathroom in a nearby home into the public main (A).

The second story bathtub presents a cross connection hazard because...

It has elevation (second story). It is a tall column of water that must be constantly supported by the water pressure in the public main. Even a momentary loss of support will cause this column to fall.

The faucets might be installed below the bath tub rim level or overflow drain. In these situations, the faucet (a connection to potable drinking water supply) can become submerged creating a nexus for the introduction of unsanitary water into the drinking water supply.

A temporary hose might be affixed to the faucet and allowed to submerge in the bathwater. This situation would create the same conditions as a submerged faucet described above.