Backflow prevention devices are designed to prevent backflow, which is the reversal of the normal and intended direction of water flow in a water system. Backflow is a potential problem in a water system because it can spread contaminated water back through a distribution system. For example, backflow at uncontrolled cross connections (cross-connections are any actual or potential connection between the public water supply and a source of contamination or pollution) can allow pollutants or contaminants to enter the potable water system. More specifically, backflow from private plumbing systems, industrial areas, hospitals, and other hazardous contaminant-containing systems, into public water mains and wells poses serious public health risks and security problems. Cross-contamination from private plumbing systems can contain biological hazards (such as bacteria or viruses) or toxic substances that can contaminate and sicken an entire population in the event of backflow. The majority of historical incidences of backflow have been accidental, but growing concern that contaminants could be intentionally backfed into a system is prompting increased awareness for private homes, businesses, industries, and areas most vulnerable to intentional strikes. Therefore, backflow prevention is a major tool for the protection of water systems.

Backflow may occur under two types of conditions: backpressure, and backsiphonage.

Backpressure is the reverse from normal flow direction within a piping system that is the result of the downstream pressure being higher than the supply pressure. These reductions in supply pressure occur whenever the amount of water being used exceeds the amount of water being supplied, such as during water line flushing, fire fighting, or breaks in water mains.

Backsiphonage is the reverse from normal flow direction within a piping system that is caused by negative pressure in the supply piping (i.e., the reversal of normal flow in a system caused by a vacuum or partial vacuum within the water supply piping). Backsiphonage can occur when there is a high velocity in a pipe line; when there is a line repair or break that is lower than a service point; or when there is lowered main pressure due to high water withdrawal rate, such as during fire fighting or water main flushing.

Aging water systems, leaking sewer connections, contaminated groundwater, cross-over connections, and growing numbers of users all contribute to the potential for backflow in a system because they can lead to unintended connections between different parts of the system or leaks that can contribute contaminants to the system. Therefore, backflow preventers are typically installed at critical points in a distribution system to prevent contamination. Currently, backflow preventers are mandated in many jurisdictions at the point where the backflow may occur, such as at a hose bib or at a feed point to a fire sprinkler system. However, security concerns dictate that wider use of backflow preventers be considered.

It should be noted that water systems are typically designed with numerous interconnections so that water can routinely flow in either direction in many areas of the water system. This improves the system hydraulics while minimizing the required sizes of water mains. It is desirable that each point in the system be fed from at least two points so that a maintenance problem can be isolated within the smallest possible area. Therefore, the use of backflow preventers within the water service network may have limited applicability. However, they may be applicable in other places in the network, such as at user connections.

The appropriate type of backflow preventer for any given application will depend on the category of hazard which may flow into the potable water supply if backflow occurs. Municipalities define their own hazard classifications, which usually include two or three general classifications, depending on the municipality. These categories include:

As noted above, the appropriate type of backflow preventer for any given application will depend on the category of hazard which may flow into the potable water supply if backflow occurs. The primary types of backflow preventers that are appropriate for use at municipalities and utilities are:

Each of these types of backflow preventers is manufactured to achieve certain standards. For example, the American Water Works Association (AWWA), the American Society of Sanitary Engineers (ASSE), the American Society of Mechanical Engineers (ASME), and the International Association of Plumbing and Mechanical Officials (IAPMO) have standards for the construction materials, design, workmanship, testing, and delivery of several types of backflow prevention devices. Interested parties can consult these standards and verify with vendors that their products meet these requirements. Each backflow preventer type is described in detail below.

An air gap is a non-mechanical backflow prevention method that is effective against backsiphonage or backpressure conditions. An air gap system is implemented by physically separating the supply pipe from the receiving vessel (see accompanying figure). This breaks the pressure between the inlet and the outlet, and thereby prevents backflow. According to standard engineering design practice, the distance between the supply pipe and the receiving vessel should be at least twice the diameter of the water supply outlet and never less than one inch. An air gap is acceptable for use in applications to protect against contaminant or pollutant hazards. In addition, an air gap may be the best means of protecting against accidental contamination from lethal hazards.

An air gap system may be constructed using commercially available plumbing components, or it may be purchased as separate components, which are then integrated into existing plumbing and piping configurations. Because an air gap breaks the pressure between the inlet and the outlet, a booster pump is usually needed downstream to ensure downstream pressure, unless the flow of the water by gravity is sufficient for the downstream water use. The air gap drain is a very effective way to prevent accidental contamination of the water system; however, it is important to note that an air gap is not always practical and can easily be bypassed. If the distance between the supply pipe and receiving vessel is compromised either purposely or inadvertently to prevent excessive splash, the air gap is defeated. Also, with an air gap, water is exposed to the surrounding air; therefore, the aspiration effect could potentially drag down airborne pollutants or contaminants into the receiving vessel.

AMSE standards A112.1.1 and A112.1.2 and IAPMO PS 65 provide standards for air gap drains. Some of these standards are for specific applications (for instance, IAPMO PS 65 is for water conditioning equipment).

When it is not possible to design an air gap into a system, designers may opt to install mechanical backflow prevention devices, which provide physical barriers to backflow. Physical backflow prevention devices are described below.

A double check valve is a mechanical device that consists of two single check valves coupled within one body, and two tightly closing gate valves, one located at each end of the unit. Each check valve consists of a physical plate connected to the top of the pipe by a hinge. The hinge is oriented such that flow in the intended flow direction keeps pressure on the plate and keeps it open, permitting the passage of fluid in the intended flow direction. Thus, under normal conditions, the check valves remain open. In the absence of water flow, the plate is not being held open by flow in the correct direction, and the valves close until the normal water flow resumes. In the event of backflow, the flow is against the direction of the hinge, so the plate remains closed. A double check valve may be used under continuous pressure. It can be effective against either backpressure or backsiphonage, and may be used to protect against pollutant hazards. It should be noted that double check valves are susceptible to interference from materials within the piping system. For example, grit or fibers can catch under the valves, causing them to remain open and potentially allowing leakage back into the system.

AWWA standard C510-97 and ASSE standard 1015 cover double check valve backflow prevention assemblies.

The principle behind a reduced pressure principle backflow prevention device is to reduce a negative pressure differential between the upstream and downstream ends of a line, thereby preventing backflow. A reduced pressure principle assembly is a mechanical backflow preventer that is essentially two check valves with an automatically operating pressure relief valve placed between the two checks. This system is designed such that this "zone" between the two checks is always kept at a lower pressure than the supply pressure. Under normal flow conditions, the check valves remain open and the relief valve is closed. In the event of backsiphonage, the relief valve will open to allow the induction of air to break the vacuum. In the event of backpressure, the opened relief valve routes the contaminated water out of the system (drainage can be provided for such spillage). The reduced pressure principle assembly also contains two shut-off valves upstream and downstream of the check valves and a series of test cocks for periodic testing of the valves.

A reduced pressure principle assembly is effective against either backpressure or backsiphonage, and may be used to protect against pollutant or contaminant hazards. Reduced pressure principle assemblies may be used under constant pressure, and are commonly installed on high hazard installations.

AWWA standard C511-97 and ASSE standard 1013 cover reduced pressure principle backflow prevention assemblies.

The principle behind a pressure vacuum breaker (PVB) backflow prevention device is to break the vacuum created during a backsiphonage event, thereby preventing backflow. A PVB consists of a spring-loaded check valve which closes tightly when the pressure in the assembly drops or when zero flow occurs, plus an air relief valve (located on the discharge side of the check valve) that opens to break a siphon when the pressure in the assembly drops. The assembly also includes two shut off valves and two test cocks for periodic testing of the assembly. The air relief valve ensures that no non-potable liquid is siphoned back into the potable water system.

PVBs prevent the backflow of contaminated water into a potable drinking main line, but they are not designed for backpressure conditions. PVBs may be used under continuous pressure, but the air inlet valve may become stuck in the closed position after long periods of continuous pressure. A PVB may only be used against backsiphonage and may be used to protect against pollutant or contaminant hazards.

ASSE standard 1020 covers PVB backflow prevention assemblies.

A summary of the typical applications for the backflow prevention devices discussed above is provide in the following table.

The implementation of backflow prevention devices within a water or wastewater system can be complex. Because backflow and cross connections can occur at so many different places within a typical system, and because many systems have large numbers of connections, it is not practical for a municipality to implement backflow preventers completely on their own to protect their system. Therefore, most municipalities have adopted ordinances requiring end users to install and maintain appropriate backflow preventers.

Determining where the implementation of backflow prevention devices is appropriate or feasible is an important consideration in any backflow prevention program. For example, as discussed above, the reversal of the direction of flow is a normal condition within an average municipal water system. As a result, backflow preventers are not practical for use in many areas of a water system. However, backflow preventers can be used at a point where water is fed to individual users to prevent flows back into the water system.

In order to be effective in reducing the potential for tampering, backflow preventers can be installed within secured locations, such as within locked underground vaults or within secured rooms within a building. However, the need to secure the backflow prevention devices must be balanced with the need to access the devices for testing.

As mentioned above, many municipalities, and many end-users, have implemented backflow prevention programs. These backflow prevention programs typically require periodic testing of each backflow prevention device (typically on an annual basis) to ensure that it is functioning properly. These programs typically require that testing be conducted by a trained and certified technician.

The primary factor affecting cost of a given type of backflow prevention device is the size of the pipe for which it is designed. The following will also contribute to the total cost for installing a backflow preventer: system design (including consultation as to which products are appropriate); on-site delivery; installation and retrofit; maintenance; and inspection, testing, and surveying. Costs for individual backflow preventers or backflow preventer systems will vary depending on the product brand and vendor. However, some general prices are provided below. These prices are capital costs for the backflow preventer and do not include installation or service costs.

As discussed above, backflow prevention devices must be tested on a periodic basis. Testing must be conducted by a trained and certified technician. Testing time for an individual backflow prevention device will vary with the size of the device and its accessibility. Typically, testing time can range from half an hour for a small, easily accessible device to several hours for larger units located in areas that are not easily accessible. When these requirements are extrapolated to include testing for each backflow prevention device within a system, costs for a backflow prevention testing program can be considerable.