The systems which distribute municipal water supplies are designed with the intention that water flow in one direction only. However, abnormal conditions can occur in water systems that cause water to flow in the opposite direction from that which was intended. This condition is called backflow, and it can result in the contamination of water supplies in unprotected systems. Backflow prevention is essential to keeping a home’s water supply free of contaminants, and there are many ways to implement it.
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 water main 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 to 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 groundwater, the importance of backflow prevention is very apparent.
Air gaps are not the only form of backflow prevention. There are simple devices that can be installed on outdoor faucets, such as hose bibb vacuum breakers, to prevent backflow from occurring at these locations. It is also advisable to install an anti-siphon assembly in a toilet tank to prevent backflow. These tanks are a common backflow hazard, and a serious one, because the water in toilet tanks is often treated with cleansing chemicals that are dangerous if ingested.