The concept of degree of hazard is used to determine which type of backflow prevention devices should be used in a given situation. Which device must be used to protect a cross connection involving sewer-connected waste?

• A. Atmospheric vacuum breaker
• B. Double check valve
• C. Air gap
• D. Reduced pressure principle valve

Answer: (C)

Degree of hazard dictates the level of backflow protection required. When sewer-connected waste is involved, the hazard is high, so the strongest protection is needed: an air gap. An air gap is a physical separation between the potable water outlet and the waste source, such that no contaminated water can reach the supply, even if pressure changes or equipment fail. Other devices rely on valves or seals that can fail under backpressure or back-siphonage, and while they’re used in lower-hazard situations, they don’t provide the absolute barrier needed for sewer-related cross-connections.