What device prevents backflow in a plumbing system?

A backflow preventer or air gap is specifically designed to prevent the unintended reverse flow of water in a plumbing system. Backflow can occur when there is a change in pressure within the system, which can lead to contaminated water flowing back into the potable water supply.

A backflow preventer is a mechanical device installed in plumbing systems that ensures water can only flow in one direction. It typically consists of a series of valves that automatically close when backflow is detected, thus stopping potentially hazardous substances from entering the clean water supply. An air gap serves a similar purpose but by creating a physical separation between the water supply and any potential contaminants, such as wastewater, ensuring that there is no pathway for backflow.

While filtration systems and sewer traps serve important roles in water treatment and drainage, they do not specifically address the issue of backflow. A water pressure regulator is used to maintain consistent water pressure but does not prevent backflow. Therefore, the correct answer highlights the most effective devices specifically designed to keep drinking water safe from contamination due to backflow.