UV Purifiers

A lamp emitting ultraviolet light at 254 nanometers sits inside a quartz sleeve in a stainless steel chamber, and water flows around it in the dark on its way to the house. Nothing is added and nothing is removed. Bacteria, viruses and cysts passing through absorb enough energy to damage their DNA. This leaves them unable to reproduce and so unable to make anyone sick.

Dose is the measurement that matters, stated in millijoules per square centimeter and equal to lamp intensity multiplied by the time water spends in the chamber. Flow rate is a safety setting rather than a convenience one, since water rushing through gets a smaller dose. NSF/ANSI 55 splits this in two: Class A delivers 40 millijoules per square centimeter and is the only class rated for water of unknown microbiological quality. In contrast, Class B delivers 16 and is meant only to polish water already known to be safe.

Clear water is a hard requirement, because ultraviolet works by line of sight. Turbidity above roughly 1 NTU lets particles shade organisms from the lamp, and iron, manganese and hardness coat the quartz sleeve until very little light reaches the water. A 5 micron sediment cartridge belongs ahead of every unit, and hard water areas need the sleeve cleaned on a schedule rather than when someone remembers.

Countertop gravity filters are the alternative for the same problem and behave oppositely. They need no power and treat about a gallon an hour, while an ultraviolet unit treats 8 to 12 gallons a minute for the whole house and stops the instant the power fails. This is often exactly when a well gets contaminated. Carbon does not disinfect at all, and reverse osmosis is not a certified barrier on its own. Well owners with a coliform positive result, cabins drawing surface water and homes under repeated boil notices are the buyers.

Choose Class A if the water is not already known to be safe, match the flow rating to household peak demand, and prefer a unit with an alarm or a solenoid valve that stops flow when the lamp fails. Replace the lamp every 12 months even though it still glows, since germicidal output drops around 20 percent a year while visible light does not. Wipe the sleeve at each change, and remember ultraviolet leaves no residual, so contaminated plumbing downstream can recontaminate water the unit just treated.