Simply put, backflow contamination is where the normal flow of water is reversed. This means that contaminated water can enter the drinking water supply. Backflow contamination can be very detrimental to health, and should always be avoided. If backflow contamination occurs then you need to call a specialist plumber such as FAB Gas to correct the problem as quickly as possible.
There are several sorts of backflow contamination found in the plumbing world. Back-siphonage is usually created when the water supply is stopped – for repairs of the water main, for instance. Perhaps the easiest way to imagine it is that the action is similar to that of drinking through a straw, which induces a flow in the other direction. Now that the most popular boiler in the UK is the combi boiler, back-siphonage can be an increased problem for plumbers. Combi boilers have a flow rate of around two gallons per minute, meaning that, for instance, there is not enough water to satisfy the flow of two taps. If, for example, you ran your bathroom sink, and then turned on your kitchen tap it is probable that the sink in the bathroom will stop running, although the tap will still be open. Should your bathroom sink have a temporary shower hose connected to it, with a sink full of dirty water, this could easily cause natural siphonage, and the contents of the sink could back-siphon into the mains water supply. For this reason, garden hoses can be equally dangerous if left submerged in fish ponds, or in an elevated location above the outside tap. Backflow prevention systems, fitted by plumbers, should prevent these problems from occurring, but it still pays to be careful with taps and hoses.
Backpressure backflow is when the normal water flow in a system is reversed due to an increase in the downstream pressure in the water system of a property, which is above that of the mains water supply pressure. This is usually seen in heating systems, pressure-producing systems and elevated tanks. Most heating systems are pressurised, meaning that water is fed into the system directly from the water mains via a connection, which is then sealed. If the pressure is affected in combination with a reduction of the mains water supply the backpressure backflow can occur, and cause the contaminated boiler water to flow into the mains water supply. Backpressure backflow can be extremely dangerous and, if you suspect it has happened, you must call a plumber immediately to fix the problem.
The final plumbing term that is often used is cross connection. This is simply a direct arrangement of a piping line that allows mains water to be connected to a line, which contains a contaminant. The garden hose is, somewhat surprisingly, the most common offender. This is because it can easily be connected to the water supply, and then used for potentially dangerous uses. For instance, hoses are often left in buckets containing garden chemicals, or in ponds that can have dangerous algae in them. It can also occur if pipes have been incorrectly connected in the first place. Your plumber will be able to fix these problems if they have occurred.