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  • James Howey

Back(flow) to the future!

Marty would have seen much better backflow prevention in 2015 than 1955!

Backflow prevention is crucial to maintain hygiene in drinking water supplies. The first efforts to address backflow risks date back to 1930, with pressure vacuum breakers developed and installed for some connections in Los Angeles. The first (recorded) widespread incident was in Chicago in 1933, where defective plumbing and cross connections resulted in 98 deaths.

While perhaps not high on his list of priorities, Marty McFly may have noticed a significant improvement in backflow prevention from his adventures in 1885 to 2015!

Importance of Backflow Prevention

Treatment plants, and even a high chlorine residual are not sufficient to protect the supply system from contamination where the source is within the network.

Supply systems are at greatest risk when pressure within the supply system drops. When the supply pressure drops, back siphonage is likely, particularly where a property has cross-connections, is subject to a flood event or backflow prevention devices have failed. Even when a supply system is at normal operating pressure, back pressure caused by equipment failure may cause back flow. Under these conditions, backflow may allow the ingress of pathogens or chemical contamination that may result in a widespread public health risk.

New connections and other major building work are should be verified to ensure appropriate backflow protection is included in the hydraulic design. Increasing attention is also being given to backflow prevention for existing connections in water supply systems, including a recent push by Northern Territory Power and Water.

While consumers are responsible for providing appropriate backflow prevention to meet the National Construction Code and AS3500:2015, it is the responsibility of water service providers to ensure compliance within their supply system to maintain the safety of the water supply for all consumers.

Principles of Backflow Prevention

The management of backflow prevention requires both the identification of risk associated with a customer’s premises as well as monitoring of backflow prevention devices.

Backflow is possible from any connection, although some types of facility represent a higher hazard than others. AS3500:2018 Plumbing and drainage – Water services provide guidance for hazard levels of various facilities and installations, and the type of protection to be provided. A hospital presents a significantly higher risk than a residential connection, and this is reflected within the standard.

Depending on the hazards within a connection, backflow prevention may be provided by a number of layers:

  • Individual Protection: Used to protect a water service from a specific hazard from a fixture, appliance or other device

  • Zone Protection: Used to protect the water supply within a residential or commercial service from backflow from one or more hazards within the facility

  • Containment Protection: Used to protect the drinking water system from backflow hazards from connected service.

It is important to protect the water distribution system against all hazards. Individual or zonal protection should be used in combination with containment protection where significant hazards are present within a connection.

Which device is appropriate?

Backflow prevention can be provided by a number of devices, including:

  • Reduced Pressure Zone Devices (RPZDs)

  • Dual Check Valve Assemblies (DCVs)

  • Single Check Valves with Test Ports (SCVTs)

  • Pressure Type Vacuum Breaker with test ports (PVBs)

  • Registered Air Gaps

  • Registered Break Tanks

These devices can be used as containment protection, zone protection or individual protection as appropriate to the hazard. Each device provides protection against different failure modes, varying testing requirements and suitability for different types of connections. Devices should be carefully selected to ensure they are appropriate for the application.

It is important to note that a 'standard' check valve without test ports is not considered a backflow prevention device!


Management of backflow prevention requires active participation from consumers as well as water service providers.

To determine the required backflow prevention within a service, consumers should:

  • identify hazards that may affect water safety within their service network, property and adjacent properties

  • assess the level of risk associated with each hazard

  • select and locate backflow prevention devices to isolate hazards both within their service and to isolate all hazards from the supply system

  • install and test appropriate backflow prevention devices.

Once installed and tested, consumers should regularly inspect and test the device. An installed device that is not functional will not protect against backflow!

Water service providers play a key role in this process. Regulatory requirements vary by jurisdiction, however water service providers have a duty of care to ensure their network is safe for all users. A well managed backflow prevention program will typically include:

  • approval process for new connections

  • ensuring an understanding of backflow risks within a supply system

  • monitoring of the supply system for changes in use

  • ensuring installed devices are tested at appropriate intervals

  • maintaining a register of connections that pose a risk to the supply system

  • maintaining a register of backflow prevention devices and testing records

  • enforcement actions to ensure the safety of the supply system

It is important for providers to maintain a clear policy to guide consumers through the process, ensure that all consumers understand their responsibilities and communicate how the policy will be enforced by the provider.

For assistance in managing backflow risks within your supply system, don't hesitate to contact Viridis.

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