What is an anti syphon-valve?

Answer 1

An anti-siphon (or, anti-syphon) valve is a device that prevents liquid from returning to the line from which it came if a siphon action occurs. An anti-siphon device is frequently required on outdoor irrigation systems to prevent possibly contaminated water to be drawn back into the water supply lines if water pressure decreases in the water supply line.

An anti-siphon device is not the same as a check valve. A check valve allows flow in only one direction, and requires pressure to function. An anti-siphon valve opens to the atmosphere when there is reduced or no pressure in the line.