A backflow prevention device is used to protect potable water supplies from contamination or pollution due to backflow.  Backflow can cause very serious health concerns for your home and, potentially, anyone connected to the same water supply.  Backflow from in-ground sprinklers could introduce pesticides, fertilizers, harmful bacteria, fungu, and other contaminants into your drinkling water.  Backflow preventers can also help prevent damage to your plumbing system.  Sudden changes in pressure inside your pipes – which backflow preventers stop – can cause a sudden jolt to your pipes caused by pressure changes, commonly called a water hammer.