Backflow prevention protects the drinking water system from many types of hazards. There are thousands of reported cases of backflow each year, and some can be fatal. All plumbing codes require the installation of backflow prevention devices where needed, and the testing of these on a yearly basis. The simplest and most effective backflow prevention technique is the air gap. An air gap is simply a space of air between any device that opens into the plumbing, such as a faucet, and any place where water can collect.
One example of an air gap is the space between the rim of a sink and the outlet of the faucet. Even with the sink full of water, there is no way that water could be pulled back up through the faucet if pressure were lost. This loss of pressure is the most common type of backflow, and is called backsiphonage. Backsiphonage often occurs when there is a sudden drop of pressure in the public watermain that supplies water to a house. This can happen if a water main breaks, or if a fire hydrant is turned on.
To further show the value of an air gap in this situation, imagine if it were eliminated by attaching a hose the sink faucet and lowering the hose into the sink. In the event that the water pressure reversed, the water in the sink would be siphoned back through the hose and faucet and into the domestic water supply. If the sink in this example is replaced with a toilet tank or a garden hose whose end is in a puddle of ground water, the importance of backflow prevention is very apparent.