Potable clean water flows under hydraulic pressure through municipal water lines to homes and businesses. Backflow is the reverse flow of contaminated water through the clean water lines. Water can be contaminated with feces, pesticides, chemicals, fertilizer and other hazard materials, and can cause a serious health hazard to the public water system in the event of a reverse flow. Backflow can occur at cross-connections between potable and non-potable water in any residential, commercial or industrial water system. This creates a backflow condition and the potential for contaminated water to enter the drinking water distribution system.  

A backflow prevention assembly most commonly consists of a pair of mechanical check valves that prevent dirty water from flowing back into the clean public water supply in the event of an unexpected or drastic change in water pressure. If properly installed and maintained, a backflow preventer will keep our drinking water safe and free of contaminants.  

The California Code of Regulation Title 17 requires public water suppliers to protect their water systems from cross-connections and prevent backflow situations. Public water suppliers must conduct cross-connection control surveys of their water customers’ property to evaluate cross-connection hazards. If a homeowner or property owner is found to have a potential or actual cross-connection hazard, the customer will be required to eliminate the hazard and install an appropriate backflow prevention assembly at the water service point of connection. All Backflow prevention assemblies are required to be tested annually to ensure they are working properly.

Backflow can occur in two ways: by back siphonage and backpressure. 

Back siphonage occurs when there is a sudden reduction of pressure in the public drinking water system, such as during a fire fighting or when a water main breaks. These instances can create a suction effect, drawing potential contaminates into the public water system. 

Backpressure occurs when any water system connected to the public drinking water supply becomes greater than the supply pressure. Backpressure typically occurs in areas that contain boilers, booster pumps, and re-circulating systems such as pools, spas, water fountains and irrigation systems.

When conditions create a greater pressure than the potable water supply pressure, water flow will be reversed and potential contaminants from these uses can enter the public drinking water system.