55 Watt UV Steriliser, Stainless Steel

Ultraviolet light at 254 nanometres passes through the quartz sleeve into the water and damages the DNA of anything living in it: E. coli, coliforms, Giardia and Cryptosporidium cysts, viruses. They cannot reproduce, so they cannot make you sick. No chlorine, no taste, no chemicals to top up, no change to the water except that it is safe.

It does not remove anything. Sediment, iron, manganese, hardness and chemicals pass straight through, and worse, they shade the light. UV goes last in the line, after filtration, and the water reaching it should be clear: turbidity under 1 NTU, iron under 0.3 mg/L, hardness under 120 mg/L, or the sleeve fouls and the dose drops.

Rated 2 720 litres an hour, which is 45 litres a minute: a full household on a 1 inch line with a booster pump. If the water is marginal, halve the flow with a regulator and the dose doubles. For a single kitchen tap or a small cottage, the same unit at lower flow is simply safer, not wasted.

Horizontal or vertical, indoors or in a weatherproof box, after the last filter and before the house. The ports are 3/4 inch male BSP; the combo with the AquaGuard includes the 1 inch adaptors. Leave a metre clear at the lamp end so the sleeve and lamp can be withdrawn. Mount the ballast dry, plug it through a surge protector, and fit a bypass valve so a lamp change does not mean a dry house.

The lamp is the only part that wears: 9 000 hours, which is a year. It still glows blue at twelve months, but the germicidal output has dropped by a third, so change it on the calendar. Wipe the quartz sleeve with a soft cloth and vinegar or alcohol every six months. Lamps, sleeves and ballasts are all in stock, and a spare lamp on the shelf is a good habit.