Plumbing codes call for a gap between potable water and a drain. This usually looks like the distance between the faucet and the top of the sink or bathtub (one inch or more). Without the “gap”, waste water could siphon back into the drinking supply. This “does” happen in situations where a city main is shut off causing a drain-down of the system with plumbing that has been done improperly. Sometimes it is a hose left in a mud puddle, or maybe a hose lying in bleach water. People have died this way. The plumbing code helps protect the general public from these dangers. Current improvements include antisiphon faucets for garden hoses, the air gap for the dishwasher, proper installation of water heater T&P relief drains, and backflow preventers for irrigation systems.