Backflow Prevention

Know the risks.

Backflow Prevention Device

It is natural for us to assume that our tap water flows in only one direction as it makes its journey from the water treatment plant to our home, institution, or business. The water is certainly not always flowing by gravity, but pumps that are positioned at critical locations maintain water pressure, and keep the water moving in the same direction.

This is the normal situation, but one must remember that water can flow in two directions inside a pipe or hose. What can be said with certainty is that water always moves in response to differences, or changes, in pressure. The situation in which water flows in a direction that is opposite from the intended flow is called backflow, and can potentially put a drinking water supply in danger. More specifically, the term backflow is generally used to indicate the undesirable reversal of flow such that non-potable water (i.e., water that is unsafe to drink), or water that at least contains unwanted substances, moves into the potable water system. The location at which this backflow occurs, where a customer’s water service line and the main supply line are joined, is called a point of cross-connection.

Backflow can be caused by two situations. Backsiphonage is backflow caused by a negative or reduced pressure in the distribution lines of the potable water system. The effect here is similar to sipping a drink through a straw. Sipping reduces the pressure at the end of the straw in your mouth, and it results in liquid flowing in a direction opposite to what would be caused by gravity. Backsiphonage is generally caused by a major water main break or a massive draw on the water system, such as would occur from the use of a hydrant(s) during a fire. In this situation, if a downstream homeowner with no backflow protection has a garden hose immersed in a bucket of weed-killer, or even in a tub of soapy water, the contaminated water could possibly be drawn through the garden hose into the main water supply line at the street or road.

Backpressure backflow is caused by pressure within a customer’s system that is greater than the potable water supply pressure. Such pressure increases can be created by pumps, high temperatures in boilers, and elevated tanks. These situations are more commonly seen in institutional and industrial facilities, but may also occur in high-rise residential buildings where pumps increase the pressure to deliver water to the upper floors. Backflow may also result from a combination of backpressure and backsiphonage.

So how can water quality be maintained when backsiphonage and backpressure are such real issues? The answer is by a variety of types of backflow prevention assemblies. These are devices that can be installed on the water supply lines of residences, institutions, and businesses to prevent backflow into the potable water supply. (The most basic and effective means of preventing backflow is a physical separation called an air gap, which either eliminates a cross-connection or provides a barrier to backflow. However, air gaps are not always practical). There are several types of backflow prevention assemblies, and the proper device to use depends upon the specific situation and degree of hazard. For instance, some devices are effective only in situations of backsiphonage, while others are able to prevent backflow due to both backpressure and backsiphonage. Some devices are used where both health and non-health hazards exist, while others are used to isolate only non-health hazards.

Due to the potential serious nature of backflow and unprotected cross-connections, the Virginia Department of Health (VDH) lists strict guidelines in its Waterworks Regulations (12 VAC 5-590). This document serves as the foundation for the Cross-Connection and Backflow Prevention Program that the Albemarle County Service Authority (ACSA) has established. The VDH lists a variety of facilities at which an approved backflow prevention device must be installed. Among many other facilities, the list includes:

  • hospitals and veterinary clinics
  • laboratories
  • carwashes and laundries
  • greenhouses and nurseries
  • sewage treatment plants
  • extermination companies
  • health clubs
  • high-rise buildings
  • facilities and homes with fire service systems
  • facilities and homes with irrigation systems

The Waterworks Regulations also state that a device shall be installed at each service connection where, in the judgment of the water provider, “…a health, pollution, or system hazard to the waterworks exists.” The ACSA believes this need extends to most commercial facilities but, to date, has not required a device to be installed at each residential connection.

The large majority of backflow incidents in residential neighborhoods involve the use of a garden hose in which the hose is used to spray fertilizer or weed-killer from an attached container, is used to bathe a dog or wash a car, and therefore may be immersed in a tub or bucket of water and detergent, is immersed in a swimming pool during filling, or is used in various other manners. For this reason, a state-wide building code has required all houses built since 1988 to have outdoor faucets within which a hose bib vacuum breaker is designed. This device prevents backsiphonage of the water within the hose to the potable water supply by means of a spring-loaded check valve. A significant cross-connection incident in Roanoke in 1979, in which the toxic insecticide chlordane was backsiphoned from a barrel through a garden hose into the city’s water supply, could have been prevented by this inexpensive device attached to the faucet.

If you do not have this type of outdoor faucet, the ACSA strongly recommends that you purchase a hose bib vacuum breaker for each of your outdoor faucets. These are available at any plumbing supply store and can easily be attached to your faucets. Even with hose bib vacuum breakers in place, care should always be taken when using the garden hose for anything other than watering and rinsing. In general, never leave a hose immersed in anything you would not want to drink.

If you feel you have had a backflow incident, or have any further questions, please contact Tim Brown at 977-4511, ext. 119 or at