Backflow prevention valves ensure fluids move in one direction through a system. Backflow is the reverse flow of media and occurs as back-siphonage or -pressure. Back-siphonage happens during a sudden reduction in pressure or stoppages. Back-pressure occurs when inlet pressure is lower than outlet pressure or flow exceeds line capacity. Check valves rely on mechanical force to actuate the discs and stop back-siphonage. Vacuum breaker valves have vents that create air gaps to prevent back-siphonage. Backflow preventers can contain many components, such as pressure relief, shut-off, and check valves, in one housing to stop back-siphonage and -pressure. Triple duty valves have spring-loaded silent check valves preventing backflow in low-flow, shut-off valves for installation and maintenance, and balancing valves to distribute media over multiple system branches. Backwater valves prevent wastewater from contaminating a water supply in drainage systems.