What Do UV and UF Water Purification Actually Do?

A UV purifier passes water through a quartz sleeve alongside a low-pressure mercury lamp emitting at 254 nm. That wavelength is absorbed strongly by nucleic acids, and the damage it causes prevents bacteria, viruses and cysts from replicating. It is fast, chemical-free and leaves no taste.

What it does not do is remove anything. The inactivated organisms are still in the glass you drink, along with every dissolved salt and every particle that entered. For treated municipal supply that is entirely adequate — an inactivated bacterium cannot colonise you — but it is not what most people picture when told water has been purified.

A UV lamp's germicidal output decays steadily and is typically down to an inadequate level after roughly 8,000-9,000 hours — about a year of continuous running — while still emitting visible light. The purifier looks as though it is working. Nothing about its behaviour tells you otherwise.

Better units fit a UV sensor or a lamp-hours alarm that cuts the outlet when the lamp is due. Cheaper ones do not. Either way, the single most valuable habit with a UV purifier is replacing the lamp annually on a calendar. A UV purifier with a two-year-old lamp is a storage tank with a light in it.

Ultrafiltration uses a bundle of hollow fibres with pores around 0.01-0.1 microns. Water passes through the fibre walls; anything larger stays on the outside. That covers bacteria at 0.2-2 microns, Giardia and Cryptosporidium cysts at 4-15 microns, silt, rust and colloidal matter comfortably.

The two properties that make UF genuinely valuable in India are that it needs no electricity — ordinary tap or gravity pressure is enough — and that it wastes no water. That makes it the correct technology for gravity-based purifiers, for homes with frequent power cuts, and for anyone who objects to discarding two litres to drink one.

The two technologies cover each other's weaknesses almost exactly. UF removes the sediment that would otherwise shadow the UV lamp, and removes bacteria and cysts outright. UV handles the viruses small enough to slip through UF's pores. Neither wastes water and neither strips minerals.

For a household on treated municipal supply measuring under about 200 ppm, a well-built UV+UF purifier at ₹7,000-12,000 is the technically correct purchase. The ₹18,000 RO unit a salesperson would rather sell is solving a problem that supply does not have, at roughly ₹17,000 more over five years.

• Reduce TDS. Not by one ppm. If your meter reads 800, it will still read 800 after both stages.
• Remove hardness. Scale on your utensils and geyser is calcium and magnesium, and both technologies leave them untouched.
• Remove arsenic, fluoride, nitrate or lead. These are the contaminants that are invisible, tasteless and genuinely dangerous, and they need RO.
• Improve the taste of salty or bitter water. That taste is dissolved content, so nothing here will change it.
• Handle borewell, tanker or coastal supply adequately. Those need RO, with UV behind it.