what is the most reliable method for preventing blackflow
Backflow prevention device The simplest, most reliable way to provide backflow prevention is to provide an air gap. An air gap is simply an open vertical space between any device that connects to a plumbing system (like a valve or faucet) and any place where contaminated water can collect or pool. A simple air gap has no moving parts, other than flowing water. Many plumbing codes specify a minimum air gap distance required for various circumstances, such as a drain connection for a dishwasher, e.g., BS 6282.