What is the best way to protect the water supply from contamination by cross-connections?

An air gap is the most effective method for protecting water supplies from contamination by cross-connections. It is defined as a physical separation between the source of the water supply and any potential contaminants. This separation ensures that there is no direct connection between the potable water supply and non-potable sources, therefore eliminating the risk of contamination due to backflow, which can occur when water flows in the reverse direction.

An air gap is particularly advantageous because it requires no moving parts and functions solely under the laws of gravity and hydraulics, making it a reliable method that does not necessitate maintenance or regular inspection like other devices might. For instance, unlike backflow preventers, which can fail or require testing and maintenance to ensure they remain functional, an air gap remains effective as long as the gap is maintained according to established standards.

While backflow preventers and check valves can offer degrees of protection, they are mechanical devices that may fail or require regular maintenance. A pressure relief valve is designed to prevent pressure build-up and has a different primary function, focusing more on system safety than on preventing contamination through cross-connections. Thus, the air gap stands out as the best approach for safeguarding water supplies from cross-connections and associated contaminants.