Which backflow prevention device best prevents backflow in high-hazard cross-connection zones and serves as the final barrier?

• A. Gate Valve
• B. Check Valve
• C. Air Gap
• D. Double Check Valve

Answer: (D)

Protecting the drinking-water supply from backflow requires a reliable, testable barrier at the cross-connection that can stop reverse flow even if one internal part starts to fail. A double check valve assembly provides two independent check valves in series, with test ports to verify both valves operate correctly. This redundancy means that if one valve drifts toward leakage, the second valve still blocks backflow, making it a dependable final line of defense in high-hazard cross-connections. It’s designed to be installed on the service line, and its performance can be routinely tested to ensure ongoing protection.

The other options don’t fit as well: a gate valve is only a shutoff with no backflow protection added; a single check valve can fail if downstream pressure pushes it closed or open, lacking redundancy; an air gap is a physical separation that, while effective in some scenarios, isn’t as universally practical or testable as a certified backflow preventer in service-line installations.