What is commonly used to prevent backflow in water systems?

The answer—check valve—is correct because check valves are specifically designed to allow fluid to flow in one direction while preventing it from flowing backward. This fundamental feature makes them an ideal choice for preventing backflow in water systems, where it is essential to maintain the integrity of the water supply and protect it from contamination that can occur if water flows the wrong way.

Check valves automatically close when fluid starts to reverse direction, effectively blocking any potential backflow. This is critical in various applications, especially in residential and municipal water systems, where backflow could lead to the contamination of drinking water supplies.

While other options like air gaps, pressure relief valves, and flow meters serve important functions in water systems, they do not primarily operate as backflow prevention devices. An air gap can prevent backflow by creating a physical separation between the water supply and potential contaminants, but it requires a specific setup and isn't always practical in every situation. Pressure relief valves are used to manage excessive pressure but do not inherently prevent backflow. Flow meters measure the flow of water but do not play a role in backflow prevention. Therefore, the check valve stands out as the most effective and commonly utilized solution for this purpose.