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March 26, 2007
feature article

Backflow prevention devices keep contaminants out of drinking water

Level of protection depends on the risk to water sources

Backflow prevention device

There's no faster way to contaminate farm water supply than a backflow of polluted or contaminated water. And one of the quickest ways to eliminate that risk is with a backflow prevention device.

Backflow is the unwanted reversal of flow in a water distribution system. When water moves in the opposite direction of the normal flow, water sources can become contaminated. Water systems on the farm can easily become tainted with bacteria, chemicals or animal wastes.

Backflow prevention devices can run anywhere from $15 to more than $2,000, depending on needs. The most basic back flow prevention device is free and is called an air gap. Wherever possible water systems are designed to accommodate an air gap, such as a kitchen tap, above the sink water level. With water safety in the news and increased awareness of the dangers of backflow, more farmers are installing backflow prevention devices.

Breaking down the causes of backflow

While there are numerous causes for backflow, there are just two types. The first is a loss of pressure in the supply main called back syphonage. The second can be attributed to backpressure, which occurs when one water source is at a higher pressure than a second source.

Back syphonage generally occurs following a loss of water pressure which can cause a vacuum in the water supply pipe. For example, this can happen in a farm water system that has a stock trough with a submerged float valve or inlet hose.

"If the check valve in the pump leaks, and the power goes off then the water in the pressure system will drain into the well, says Bob Buchanan, agricultural water specialist with Alberta Agriculture and Food. "Since the water level in the well is lower than the water in the stock trough, a siphon will be created and the contents of the stock trough will siphon down the well."

Backpressure occurs whenever the pressure downstream exceeds the normal supply pressure. This could happen if a stock tank with a submerged valve or inlet hose is at a significantly higher elevation than the rest of the distribution system. For example, a tank 50 feet higher will create a backpressure of approximately 20 pounds per square inch (psi). If someone is using a lot of water elsewhere in the system, and the pressure drops below 20 psi, then the polluted water in the stock tank will have enough backpressure to flow backwards down the supply line.

Cross-connections in a water system

A cross-connection occurs when contaminated or polluted water is connected to potable water. There are a variety of common cross-connections on the farm. A yard with a well and a dugout, both with pressure systems and piped together, create the opportunity for backflow. Other examples include a good well and a well with poor quality water connected together, a stock tank with a submerged valve or a garden hose submerged in a puddle or tank.

"We see many examples of farmers using the same water system for more than one source, be it for livestock, the house or the garden," says Buchanan. "Ordinary garden hoses are likely the most common type of cross-connection. You can't take cross-connections for granted on a farm. If there's a chance that backflow can happen, over time odds are it will."

Match risk with protection

Where a non-potable substance might come in contact with drinking water, the risk at the cross-connection point determines the type of prevention assembly required to protect against the type of potential hazard.

The most inexpensive backflow protection option, a "hose bibb atmospheric vacuum breaker," is installed on faucets and hydrants with hose connectors in order to prevent backflow from a hose.

The mid-priced choice is a "dual check valve," which is designed to be plumbed into the water line. Dual check valve assemblies are also available with test ports to test the integrity of the valves. "These valves have safeguards in two different places and provide a higher level of protection," says Buchanan. "They're best for most non-hazardous situations and are the most common backflow prevention device used on farms."

Where there is a high risk of contamination, such as when a farm is connected to a municipal water supply, a "reduced pressure zone assembly backflow preventer" may be required. These have safety checks in place to protect the integrity of the municipal water supply.

The appropriate backflow prevention device should be used when potable and non-potable water supplies are plumbed together. Proper installation is critical especially if the backflow device is more complex as the risks and effects of contamination are extremely high.

Contamination check

There are a number of ways one water source can contaminate another. For example contamination found in one well can easily spread to those of neighbouring farms, especially if the wells share an aquifer. Or, when a livestock trough is hooked directly to a hydrant, a submerged hose can back syphon into the well.

Improper use of a hydrant and hose assembly located at the top of a well for filling a sprayer tank can also contaminate water. "We don't recommend that farmers fill their sprayer tank directly from the water source," says Buchanan. "As a safety check, we suggest they first fill a nurse tank and haul it to the field to mix with their chemicals." This eliminates any risk of chemical being back syphoned into the well.

All water systems on a farm, including backflow prevention devices, should be checked annually to make sure they are functioning adequately. Backflow preventers have internal seals, springs, and moving parts that can break down. They must also be protected from freezing.


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