Backflow (Cross-Connection) Prevention

In water supply systems, water is normally maintained at a pressure to enable water to flow from the tap, shower etc. When water system pressure drops or declines, contamination may be drawn into the system.

Key Cross-Connection Terms and Definitions
Any actual or potential connection between the public water supply and a source of contamination.
The flow of water or other liquids, mixtures, or substances into a potable supply of water. Backsiphonage is one type of backflow.
Backflow that occurs when the pressure in non-water system sources exceeds the pressure in the water supply.
Results from negative pressures in the potable water supply.

 Where Can Cross-Connections Occur?

Cross-connections can occur at many points throughout a distribution system and a community's plumbing infrastructure. Cross-connections can be identified by looking for physical interconnections between a customer's plumbing and the water system. Some specific examples are:

  • Lawn chemicals backflowing (backsiphoning) through a garden hose into the distribution system.
  • Backsiphonage of "blue water" from a toilet into a building's water supply.
  • Carbonated water from a restaurant's soda dispenser entering a water system due to backpressure.
  • Backsiphonage of chemicals from industrial buildings into distribution system mains. Backflow of boiler corrosion control chemicals into an office building's water supply.
  • Backflow or cross-connection with fire suppression systems containing anti-freeze. 
Cross-Connections in Household Plumbing Page 1
Cross-Connections in Household Plumbing Page 2

What Technologies are Available to Control Cross-Connections & Prevent Backflow?

The type of backflow that is most likely to occur in your system (either from backpressure or backsiphonage) and the related health effects will determine which backflow prevention technology is best for your water system. The available technologies are described briefly below. Backflow prevention devices can protect your health and are mandatory to safeguard public health.

Atmospheric Vacuum Breaker
  • Consists of float check, check seat, air inlet port, and possibly a shutoff valve immediately upstream.
  • Allows air to enter the downstream water connection to prevent backsiphonage.
  • Used for backsiphonage conditions only.
Pressure Vacuum
Breaker Devices
  • Consist of vacuum breakers with a loaded check valve and a loaded air inlet valve.
  • Used for backsiphonage conditions only.
Double Check Valve Devices
  • Consist of two independently acting, tightly closing, resilient seated check valves in series with test ports.
  • Have tightly closing, resilient seated shutoff valves attached at each end of the assembly.
  • Prevent backflow under backsiphonage and backpressure conditions.
  • Typically approved for only low to medium hazards.
Air Gaps
  • Physical separation between a potable water system and a receiving vessel or source of contamination.
  • Air gap between the outlet of the potable system and the flood level rim of the receiving vessel or any source of contamination must be at least twice as large as the diameter of the potable water outlet and never smaller than 1 inch.
  • May require additional pumping downstream of air gap.
  • Safest and simplest means under backsiphonage and backpressure conditions.
  • Useful for all hazard levels.
Reduced Pressure Zone Backflow Devices
  • Similar to the double check valve devices, but also contain an independently acting pressure relief valve between the two check valves (which sits lower than the first check valve).
  • Protect against high water pollution hazards.
  • Protect against backsiphonage and backpressure.

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