What UV does
As water passes the lamp, 254 nm light penetrates microbial cells and damages their DNA so they cannot reproduce — inactivation. A correctly dosed UV system is a formidable barrier: bacteria (E. coli, Salmonella, Legionella), most viruses, and — its special distinction — the protozoa Cryptosporidium (chlorine-resistant) and Giardia (chlorine-tolerant) are inactivated at the standard 30 – 40 mJ/cm² doses. The one gap worth naming is adenovirus, which shrugs UV off at those doses: the EPA's four-log virus credit is about 186 mJ/cm², so where virus inactivation has to be proved, a standard-dose UV unit does not prove it. It does this in seconds of contact, with no chemicals, no taste change, no by-products, and no possibility of overdose. As a final barrier at point of entry or point of use, it is hard to beat for simplicity per log of protection.
What UV does not do
It removes nothing. The microorganisms — inactivated but physically present — flow on, along with every particle, every milligram of dissolved solids, iron, hardness, chlorine, odour and colour. A UV unit changes no number on a water analysis except the microbiological ones. Customers regularly expect a "steriliser" to fix taste or staining; that is a filtration and chemistry conversation, with UV as the final step after it.
It leaves no residual. Protection exists only in the instant of passage. Downstream of the lamp, the water is as vulnerable to recontamination as any other clean water — a dirty tank, a dead leg, a garden-hose cross-connection all reinfect UV-treated water freely. Where water is stored after treatment or reticulated over distance, UV alone is an incomplete answer: either treat at the last point before use, or combine UV with a low-level chlorine residual for the network (the subject of the UV-versus-chlorination article).
It cannot see through dirt. Particles physically shadow microbes from the light, and Cryptosporidium-sized particles are exactly the size that shelters them. This is why sediment prefiltration ahead of UV is mandatory, not advisory — covered in its own article.
It struggles in low-UVT water. Colour, organics and iron absorb the light before it reaches the far side of the chamber. UV on tea-coloured or iron-rich water without pretreatment is a lamp warming water.
The two classic misapplications
- UV as a substitute for treatment. A borehole with iron, turbidity and bacteria gets a UV unit only; the water still stains, the UVT is poor, the dose is compromised, and the one job UV had is done badly. UV belongs at the end of a treatment train, polishing water the rest of the train has already made clear.
- UV ahead of storage. Unit at the borehole, then a tank, then the house: everything growing in the tank arrives at the tap untreated. The lamp goes after the tank — or the tank gets its own management (sealed, dark, cleaned) plus a residual.
Rule of thumb: UV inactivates everything and removes nothing, at the instant of passage only. Place it last in the train and as close to the point of use as practical — and if there is storage or long pipework after it, the design is not finished yet.