Backflow and Cross Connection
Backflow is the reversal of water flow through pipes due to siphonage or back pressure.
The water supply system is designed to ensure that water flows to your property under pressure. If this pressure is not maintained, there is a chance that water could be drawn back into the water mains.
Water pressure can be affected when:
A cross connection is a point in the piping system where the potable water is connected directly or the possibility exists of being connected to a source of contamination, pollution, or non-potable substance.
If there is a cross connection between your water and a contaminated source and there is a pressure drop in the main that creates a siphoning effect, water from that contaminated source can be drawn back into the drinking water supply. This is known as a backflow incident. (Examples of contamination sources that one could expect to find on our island would include hot tubs, automatic sprinkler systems, garden sprayers, and booster pumps.)
A backflow incident is a risk to public health. To protect from this risk, backflow prevention devices ensure that when there is a cross connection, the drinking water supply cannot be contaminated. The devices prevent water from flowing back into the water mains. There are different types of devices which can be installed depending on the potential hazard - from a simple device that can be installed on your outside faucets (<$5) to the more complex device which is installed on your water connection ($300+).
The following illustration shows a commercial example of controlling cross connection. When there is no backflow preventer device installed, as shown in the "Wrong:" schematic, non-potable water from the boiler could flow back into the water system due to backflow, thus contaminating the water supply. The system is protected when there is backflow preventer installed as shown in the "Right:" schematic.