A backflow prevention device is used to protect your water from contamination due to backflow. A backflow preventer device is installed on your home’s water pipes allowing water to flow in one direction but never in the opposite direction. Backflow can occur when pressure changes in the pipes causing water to flow backwards and seep into the main water supply line. When pressure is lost in the pipes, due to various reason, water is no longer being pushed forward into your home and will flow backwards into the city water lines. This can contaminate the public drinking water.
You should have a backflow preventer device on your irrigation system if your water comes from a potable source. In most places, it is illegal to not have an approved type of backflow preventer on your irrigation system if your irrigation water source is potable.