This is a backflow preventer. This is use in one of our project. A backflow preventer is a device installed on your home's water pipes that allows water to flow in one direction only, but never in the opposite direction. Its sole job is to prevent drinking water from being contaminated due to backflow. Backflow happens when contaminated water or other liquids flow backward into the clean water supply. This can happen in a number of situations, such as when a garden hose is submerged in a pool, or when a washing machine or dishwasher malfunctions.