UV Disinfection

Ultraviolet disinfection is a lamp sealed inside a housing that water flows through on its way to the tap. The light is a wavelength too short for the eye to see, and it damages the genetic material inside bacteria and viruses so they can no longer reproduce or cause illness. Nothing is strained out and nothing is added to the water. One unit normally treats a single line rather than the whole supply, and treatment stops at the housing: there is no lasting residual, so anything entering the pipes beyond the lamp is untreated.

Dose and clarity govern whether it works. Dose is the light an organism receives, counted in millijoules per square centimetre - about 20 deals with common bacteria while many viruses need closer to 45, and units certified to the usual household standard deliver at least 40. Clarity decides whether that dose arrives, because ultraviolet light reaches only about 5 to 8 cm (2 to 3 in) into water and a speck of silt casts a shadow an organism rides through untouched. A sediment filter ahead of the lamp is required rather than optional. Iron and manganese colour the water and absorb the light, and hard water scales the clear quartz sleeve around the lamp, so the sleeve needs regular cleaning or a softener upstream.

It pairs with a fine filter such as a ceramic element, which strains out the hardy parasite cysts the lamp cannot reliably disable while the lamp inactivates viruses small enough to pass the element. It needs power continuously and fails without any sign: lamps fade rather than burn out, losing about a fifth of their output in the first hundred hours, so a lit lamp is not proof of a working one. Upkeep runs on a clock rather than on symptoms - clean the sleeve, change the lamp after about 9000 hours of use, and test the treated water for bacteria on a fixed cadence.