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Understanding Backflow

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Causes of backflow

The American Backflow Prevention Association (ABPA) defines backflow as “the undesirable reversal of flow of non-potable water or other substances through a cross-connection and into the piping of a public water system or consumer’s potable water system.”

Cross-connections occur when a temporary or permanent link is created between public drinking water and a source of non-potable or contaminated water, usually from sewage or irrigation systems. On both commercial and residential properties, cross-connections to auxiliary water systems, irrigation systems and cooling systems are the three leading backflow culprits. In these systems, there are two physical conditions that commonly cause non-potable water to drain back into the water supply: backpressure and backsiphonage.

Backpressure occurs when an output system creates a higher pressure than exists at the point of public supply. The superior pressure in a fertilizer injection system, for example, will force pressurized water back through an unprotected cross-connection, carrying with it fertilizers, bacteria and animal traces.

Backsiphonage is the opposite of backpressure. It occurs when there is an unexpected pressure reduction in the public water supply. Undersized piping, elevated water withdrawal rates, broken lines and booster pumps all create the conditions that can cause backsiphonage.

Both of these backflow threats can be best described as conditions of unbalanced pressure. Factors like the type of irrigation equipment used and site elevation are also going to affect that balance of pressure. So do sites that sit at a higher elevation than the water supply.

For these reasons, understanding the pressures created by an irrigation system, and local factors like the elevation of the site, are central to gauging the threat from backflow.

Irrigation systems equipped with pumps, pressurized tanks and injectors represent high-hazard cross-connections. Take the time to understand the nature of the hazard at any irrigation site that you plan to work on.

Devices designed to protect against backpressure may not be effective against backsiphonage and vice versa. Since there are no ordinances dictating which backflow prevention devices need to be installed on which residential properties, you have to understand the needs of the jobsite you’re working on, and how best to combat the threat posed by pressure differences.

Any unsecured cross-connection runs the risk of transporting substances and contaminants from the point of use into the potable water supply. If these substances pose no health risk, they are considered pollutants. If they in fact do pose a health risk, then they are considered contaminants.

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