Backflow is just what it sounds like: water flowing in the opposite direction from its normal flow. With the direction of flow reversed (due to changes in pressure), backflow can allow contaminants to enter the potable water system through cross-connections. Without proper backflow devices, something as useful as your garden hose has the potential to contaminate your home’s water supply and the public water system.
A potentially hazardous cross-connection occurs every time someone uses a garden hose sprayer to apply fertilizer or herbicides to their lawn. Without a backflow prevention device between your hose and the spigot, the contents of the hose and anything it is connected to can backflow into the home’s water system and contaminate your drinking water.