What is the only absolutely reliable method of backflow prevention?

The only absolutely reliable method of backflow prevention is an air gap. An air gap involves a physical separation between the end of a water supply pipe and the flood level rim of a receiving vessel. This ensures that there is no chance for backflow to occur, as it creates a barrier that prevents any contamination from entering the potable water supply. The effectiveness of an air gap is not influenced by pressure changes, backpressure, or backflow from other systems, making it a foolproof method.

In contrast, other backflow prevention devices like check valves, reduced pressure zone devices, and vacuum breakers can provide effective protection under many conditions but are not infallible. For instance, check valves can fail due to obstructions or mechanical failure, while reduced pressure zone devices require proper maintenance and function under specific pressure conditions. Vacuum breakers, on the other hand, are effective primarily in preventing backflow under siphonic conditions. Nonetheless, they may not offer complete protection against all forms of backflow. Therefore, an air gap stands out as the most dependable solution for preventing contamination in water distribution systems.