Backflow Protection of Water Supply

Backflow and Cross-Connections Discussion
An important part of the design and use of the drinking (potable) water supply system is preventing contamination of potable water. Contamination can occur when the potable water supply is intentionally or unintentionally connected to a contaminant source. A cross-connection is a connection between the potable water supply and a potential contaminant source. Backflow is when material (usually liquid) travels in the reverse of the intended direction within a cross-connection. Contamination occurs when contaminated material backfl ows into the potable water system through a cross-connection. Some cross-connections are intended. Examples of intended cross-connections include toilet tank fill valves, automatic fill systems for swimming pools, and lawn irrigation systems.

Some cross-connections are unintended. Examples of unintended cross-connections include spray hoses connected
to a laundry tub faucet and garden hoses attached to chemical sprayers. In these examples, a sudden loss of water pressure in the potable water supply system or an increase in pressure at the contaminant source could allow a contaminant to be drawn back into the potable water system. If the liquid were weed killer in a spray bott le at the end of a garden hose, the weed killer could be drawn into the potable water system.

Backflow into the potable water system can occur by back pressure or by backsiphonage. Back pressure
occurs when the pressure in the cross-connection source exceeds the pressure in the potable water supply system. Contaminated material is forced under pressure into the potable water system. Sources of backpressure include pumps, liquid storage tanks at a higher elevation than the cross-connection point, and thermal expansion from a heat source such as a water heater. Backsiphonage occurs when the pressure in the potable water supply system falls below atmospheric pressure. Air pressure can force contaminated material into the potable water supply system or negative pressure in the potable water supply system can draw contaminated material into the potable water supply system. The IRC requires protections of all potable water supply outlets and all intentional cross-connections by an air gap or by an approved backflow prevention device.