Ideally, the water supply in your house should only flow in one direction; that is, the water should always flow from the supply side to the demand (outlet/tap) side before exiting the house as wastewater. However, there are situations in which this doesn't happen as simply as that, and contaminants find their way into your fresh water supply.
Two Major Causes of Backflow
There are two main reasons why backflow can occur and contaminate your fresh water supply.
Back siphonage occurs when a partial vacuum develops inside a water supply system, creating a negative pressure that allows water from a contaminated source to be drawn into the fresh water supply pipe. A good example is when a fire hydrant reduces the supply of water in a neighborhood, thereby reducing the water supply pressure. In such a case, a garden hose can pick up contaminants in the yard that eventually travel to and contaminate the fresh water in your house. As you can imagine, that would be a terrible thing for your household's health because you may end up drinking or using contaminated water while thinking it is clean.
Back pressure occurs when the water pressure in a plumbing system exceeds that of the water supply lines. A good example is when water in an improperly connected water heater exceeds that of the supply lines. In that case, water from the boiler may flow back into the supply lines. If that happens, it can easily send water from the boiler into your fresh water pipes.
The good news is that a properly installed plumbing system will have integrated backflow preventers. This means you have no cause for alarm if your plumbing system was designed and installed by a professional plumber. You only need to worry if your plumbing system has been damaged—for example, if some parts have frozen and burst or if you have tinkered with your plumbing system in the name of DIY repairs.
If that is the case, then you should consult a plumbing contractor to diagnose the problem and determine whether the backflow prevention system has been interfered with. The plumber will use any of the multiple backflow prevention methods to keep your fresh water from becoming contaminated.
One way is to install a mechanical backflow preventer that acts as a barrier to allow water to flow only in one direction. Such a contraption includes a spring assembly that closes the affected pipe so that no backflow occurs; at the same time, a valve opens up to discharge the backflow water.
Another way is to install a breaker that monitors the water pressure; this type is mostly used in outdoor water fixtures such as garden hoses. The breaker includes a system to turn off the water flow and avoid backflow if a drop in pressure is detected.