What is backflow?

An example of backflow would be if a hose is left lying in a garden pond when there is a burst water pipe, the contents of the pond could be sucked into the water pipes.

There are two main causes of backflow:

Backsyphonage can occur when the pressure of the supplied water falls to very low levels. This can be caused by a burst water pipe, by the fire brigade drawing large volumes of water to fight a fire, or when we turn off the supply for short periods during repair or replacement works.
This causes a vacuum which can suck substances back into the pipework, in a similar way that petrol can be siphoned from a car’s fuel tank. If any substance affects the quality of the water, then it is called contamination and, of course, most substances will do this.

Backpressure occurs when the pressure at the outlet from the pipework rises above that of the water in the pipework. This is a less common problem than backsyphonage and will usually occur due to an incorrectly installed pump running in reverse or in processes where compressed air or other substances at high pressure are injected into the water. Examples would include drinks machines or dental equipment.
In this case, the higher pressure pushes substances back into the pipework. If these substances affect the quality of the water, then it is called contamination.

There are many devices designed to prevent backflow. Different arrangements are used depending on the risks posed by the substances likely to affect the high quality of the public water supply.

These substances are divided into Fluid Categories which take into account their likely affect on the well being of anyone who came into contact with them if backflow occurred.