Which backflow prevention device is commonly required for protecting drinking water distribution systems from back-siphonage and backpressure?

• A. A check valve
• B. Air gap
• C. Double check valve
• D. A reduced pressure principle backflow assembly (RP)

Answer: (D)

Backflow prevention in drinking-water systems relies on a device that can stop reverse flow under both a pressure drop (back-siphonage) and a downstream overpressure (backpressure). The reduced pressure principle backflow assembly does this by combining two check valves with a relief valve that vent to the atmosphere. If downstream pressure falls or pushes water back toward the supply, the relief valve opens and creates a reduced-pressure zone, preventing any backflow from entering the distribution system. This robust protection is why it’s commonly required for service connections, especially where contamination hazards exist.

Other options don’t meet the same level of protection. A single check valve can fail to seal or be defeated by back-siphonage since it doesn’t vent to atmosphere and can’t maintain isolation if pressure fluctuates. A double check valve improves on a single valve but still lacks the venting mechanism and may not satisfy higher-risk situations. An air gap provides physical separation but is a fixture-specific solution and isn’t a mechanical backflow preventer used on the main service line.