An air gap is a vent located between the dishwasher and the main plumbing system to prevent the sewage from siphoning from a plugged or backed-up drain into the machine. In most locations, plumbing codes require installation because the air gap works in all situations.
While installing the air gap prevents contamination of the dishwasher for the home occupant, it also protects the public water supply. In the event of a complete loss of water pressure, sewage that overflows into the dishwasher could move into the water supply.
Plumbers install the air gap on the sink. They use the predrilled hole next to the faucet. If the holes are already in use by a sprayer or soap dispenser, the plumber drills an additional hole through the sink for installation. While the other options free the predrilled holes, they are less reliable, according to plumbing expert Aaron Stickley for About.com.
Check valves are other types of antisiphon devices sometimes used with dishwashers. However, if a small food particle sticks in the valve, it often fails. Most plumbing authorities consider it less reliable than the air gap. The third option, a high loop installation, depends on the force of gravity exceeding the force of the sewer backup. While such installations generally do not cause problems, they are not fail-safe.