The answer to 'how does backflow preventer work' is a backflow preventer is a plumbing device that prevents water from flowing backwards into the public water supply or a home's drinking water system. It does this by allowing water to only flow in one direction, from the main water supply into the home's plumbing system. This prevents contaminated or polluted water from being drawn back into the clean water supply.
Backflow preventers are typically installed at the point where the main water line enters a home or building. They contain a series of check valves and air gaps that create a physical separation between the potable water supply and any potential sources of contamination. This ensures that if there is a loss of pressure in the main water line, water cannot be siphoned back into the public water system.