Water Treatment, Legionella Control and Backflow Protection
This domain tests water quality and the two risks that dominate it: legionella and backflow.
Water treatment
Hard water scales heat exchangers; base-exchange softeners remove hardness but add sodium, so a hard drinking-water tap should be left unsoftened. Heating systems need correct chemical treatment, inhibitor at the specified concentration, and a magnetic filter on older systems.
Legionella
Bacteria multiply between about 20 °C and 45 °C and are killed above 60 °C. Control means storing hot water at 60 °C, distributing at 50 °C at outlets within one minute, keeping cold below 20 °C, removing dead legs, flushing little-used outlets and controlling aerosols from showers and cooling systems. Risk assessment, sentinel monitoring, sampling and records follow national guidance.
Backflow
Water regulations classify risk into fluid categories 1–5. Category 5 needs an air gap; lower risks may use a verifiable single or double check valve. Hose union taps, filling loops, bidets and submerged inlets are classic failures.
Worked example
A caretaker connects a hosepipe to a tap and leaves the end in a drain. That is a category 5 risk, because drain water could be drawn back into the mains if pressure drops. The correct protection is a permanent air gap or a hose union tap with appropriate protection - not reliance on the caretaker's behaviour.
Traps that cost marks
Thinking a non-return valve suits every risk; assuming soft water is corrosive or hard water is safe; neglecting records; treating TMV adjustment as a legionella control.