Backflow devices protect your water supply by preventing wastewater from flowing backwards in your pipes and contaminating your potable water. A typical and very reliable form of backflow valve is an Atmospheric Vacuum Breaker. They are elbow-shaped and use an atmospheric vent to prevent water from reentering the potable water supply. They are typically constructed of brass and employ pressure to seal the drain line opening when the water is not draining. Double-check valves are an excellent choice for further backflow prevention. They employ a ball valve, which opens and shuts when water flows through it.