The only completely reliable method for preventing backflow of a contaminated water source into a sanitary water source is to use what?

The most effective method for preventing backflow of contaminated water into a sanitary water source is the use of an air gap. An air gap is a physical separation between the end of a water supply outlet and the flood level of a receiving vessel, such as a sink or tank. This gap prevents any backflow from occurring because it creates a significant barrier that contaminants must cross, thus effectively stopping any reverse siphoning or pressure changes from drawing contaminated water back into the clean water supply.

Air gaps are a simple yet highly reliable means of protection against backflow, as they do not rely on mechanical parts that could malfunction or wear out over time. Instead, they provide a direct and unbroken space that must be maintained to ensure the safety of the water source. The use of an air gap is often mandated by plumbing codes in various jurisdictions for critical applications where water safety is paramount, such as in food service or medical facilities.