Understanding Backflow

IN MAY OF 2000, residents living around the subdivision in Pineville, North Carolina, known as Walden Pointe discovered that their drinking water had become contaminated with raw sewage. The contamination reached around 60 homes and more than 100 Walden Pointe residents fell ill. The affected citizens sued their municipality and received a $1.2 million settlement to cover their damages and medical expenses.

In June, 2004, thousands of residents living in the Detroit, Michigan, suburb of Novi were issued a ‘boil water alert,’ even though municipal water systems were supposedly EPAcompliant. The alert forced Novi schools to shut off sinks and drinking fountains and purchase hundreds of cases of bottled water. There are no reliable estimates of how many people may have been hurt by the contamination.

In 2006, a Georgia man stepped into his shower to get ready for work. Due to a caustic contaminant in the water supply from a nearby plant, the spray from the showerhead seared nearly 80 percent of the skin off his body.

Three different locations. Three separate public health incidents. One common cause: contaminated backflow.

Unfortunately, these incidents are not uncommon. Commercial and residential irrigation sites are a frequent source of water contamination—so frequent, in fact, that a number of companies have dedicated years of engineering research and development to creating backflow prevention devices that shield property owners and contractors from liability while protecting the public health. The good news for irrigation contractors is that contamination can be contained with adequate backflow devices and a better understanding of backflow prevention.

For commercial sites, the laws and regulations have already caught up with the threat. Municipal codes dictate strict guidelines for the installation, operation and maintenance of backflow prevention devices. For residential sites, however, the regulations are not as robust. The lack of legal obligations, however, doesn’t preclude an ethical and professional duty placed squarely on the shoulders of the contractor.

“Because residential properties are not inspected by public operators or municipal supervisors, the contractor actually has an elevated level of responsibility for controlling backflow,” says Rick Fields, national sales manager for Wilkins. “Contractors are becoming better educated about the right way and wrong way to install a [backflow prevention] system.”

If you’re a contractor doing commercial landscape and irrigation work, chances are you’ve had some experience with installing and maintaining backflow prevention systems. If you specialize in residential properties, there may be a bit of a learning curve. Getting up to speed on backflow prevention can give you a step up on your competition.

{::PAGEBREAK::}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.

{::PAGEBREAK::}Backflow devices

Pete Chapman, general manager at Conbraco, says that the four most common backflow prevention systems are atmospheric vacuum breakers (AVBs), pressure vacuum breakers (PVBs), reduced pressure principle assemblies (RPs) and double check valve assemblies (DCs). Each system has its own strengths and weaknesses.

AVBs are designed to prevent backsiphonage and not backpressure. They rely on standard air-gap technology to break up the vacuum that pulls polluted water back through a cross-connection into the potable water supply.

The standard AVB set-up includes a disc-float assembly and an atmospheric vent area (the air gap) that automatically seals off a cross-connection when pressure conditions likely to cause backsiphonage are present. Gravity causes the disc float to fall, plugging the atmospheric vent area and injecting air into the line. ABVs must be installed at a six inch elevation relative to downstream piping and outlets attached to the suspect irrigation system, otherwise the disc float won’t act as intended.

PVBs, just like AVBs, are only effective at preventing backsiphonage backflow. These are mechanical systems that consist of a spring-loaded check valve and a spring-loaded air inlet valve. The air inlet valve opens when the internal pressure rises above atmospheric pressure. This action prevents polluted and contaminated water from an application site being siphoned back into a public well. The springs in a PVB replace the gravitational force used by AVBs.

RPs, according to the ABPA, “consist of two independently acting, spring-loaded check valves with a hydraulically operating, mechanically independent, spring-loaded pressure differential relief valve between the check valves and below the first check valve.”

When a cross-connection protected by an RP is acting normally, the pressure between the two check valves is lower than the supply pressure. When the pressure in this RP protection zone begins to approach the supply pressure, the relief valve kicks in to prevent both backflow and backsiphonage. Because RPs are uniformly effective at preventing all forms of backflow, they are the recommended devices for irrigation sites that expose the potable water supply to fertilizer and chemical contamination.

Last, but not least, the DC is a backflow prevention device that also relies on spring-loaded check valves. Elevation is not as important for DC installation, but many contractors report that DCs installed belowgrade have proven difficult to maintain and drain for freeze protection.

Just like the RP, the DC is effective against backpressure and backsiphonage, but it is not recommended for use on cross-connections between potable water and irrigation systems. This is because the DC only protects against pollutants and not contaminants.

For almost every irrigation site that you are going to be asked to maintain, an RP is the ideal choice for backflow prevention. But selecting a device is only the tip of the iceberg; your biggest challenge after installation is going to be responding to changes in the physical condition of the site, and maintaining a regular inspection schedule. Unless this function has been delegated to a plumber, it’s the responsibility of the irrigation contractor to inspect all backflow prevention devices after installation, and run annual maintenance to make sure the system remains in proper working order.

Backflow preventers are complicated machines, with an abundance of moving parts that operate in a dynamic, pressurized environment. A unit’s internal seals and springs are at a high risk for wear and fatigue. Annual inspection is the bare minimum necessary to ensure that a specific unit is functioning as intended.

For simple air gap systems, a cursory visual inspection is sufficient to confirm that the size and elevation of the gap are substantial enough to meet best practice standards. You should invest in properly calibrated gauge equipment for inspecting the more complex, mechanical systems.

Before installation, it is recommended that you identify suitable sites for backflow devices that are easily accessible for inspection. There is a high probability that you will be the one performing regular inspection and maintenance, so installing the system in an accessible location will only make your job easier down the road.

{::PAGEBREAK::}Emerging concerns

It is worth noting that backflow devices have been targeted in a rash of thefts spanning the entire country. Maybe it’s just the sour economy, but the last five years have seen a marked increase in copper fittings being stolen off of backflow preventers, on both commercial and residential land.

When essential parts are stolen off a backflow preventer, it not only diminishes the device’s efficacy as a backflow countermeasure, but it can actually create the conditions for backflow to occur where they were not already present. If one of your customers is a victim of these thefts, he or she will be the one left on the hook for replacing the looted device—and may also be exposed to contaminated drinking water.

These incidents have caught the attention of backflow prevention device manufacturers. Some companies are responding by engineering cages to go around the devices. Other companies, Wilkins being an example, are attempting to make their backflow devices less attractive targets by manufacturing the units with less brass and copper.

Almost all irrigation systems pose an extremely high risk for backflow contamination. If you’re working on a commercial site, the local codes will inform you about your legal obligations. Your residential sites are not likely to be inspected by an external authority, but you can mine the same local codes and ordinances for guidance when deciding what system to install.

Don’t become another headline about water pollution and EPA fines. Identify the backflow risks unique to the site. Select and install a mechanical prevention device that meets that site’s needs. Properly maintain that device after installation, and your client—not to mention the public at large—can continue to enjoy uncontaminated drinking water.