Two ways to make germicidal light
Low-pressure mercury lamps — the technology in almost every installed UV system — pass current through mercury vapour, which emits very efficiently at 254 nm, conveniently close to DNA's peak sensitivity. The design is mature: predictable output, well-understood 9 000-hour lamp life, validated dose models, and decades of field history.
UV-C LEDs are semiconductors emitting in the 260 – 280 nm band. They bring the LED virtues — instant on/off, no warm-up, no mercury, no glass lamp, compact and rugged packaging, output that can be driven up and down on demand — and one large present-day weakness: electrical efficiency. A low-pressure mercury lamp converts roughly a third of its power into germicidal UV; commercial UV-C LEDs still convert only a few percent. Per watt and per rand of germicidal output, mercury remains far ahead.
What that difference means in practice
| Mercury lamp | UV-C LED | |
|---|---|---|
| Germicidal efficiency | High (~30%+ at 254 nm) | Low today (single digits), improving yearly |
| Warm-up | Minutes to full, stabilised output | Instant |
| Switching tolerance | Cycling shortens lamp life | Unaffected — designed to cycle |
| Physical format | Long lamp + quartz sleeve | Compact module, flexible geometry |
| Mercury content | Yes — handling and disposal care | None |
| Lamp/module life | ~9 000 h continuous | Varies widely with drive and cooling; check the datasheet, not the brochure |
| Validated municipal-scale history | Deep | Emerging |
| Sensible duty today | Continuous flows, larger systems | Small, intermittent, point-of-use |
The switching row is the strategic one. A mercury system serving an intermittent tap must either run continuously (wearing the lamp while treating nothing) or cycle (ageing the lamp faster than the hour meter suggests). An LED unit simply lights when water flows — for a point-of-use tap drawing a few litres at a time, that converts the LED's efficiency handicap into a net win, because it only spends power during the seconds that matter.
Where each belongs in 2026
Choose mercury for anything with sustained flow or serious duty: whole-building point-of-entry, borehole and process systems, and anywhere a validated 30/40 mJ/cm² claim at a real flow rate must be met economically. It is the incumbent for good reasons, and the consumables (lamps, sleeves, ballasts) are stocked commodity items.
Consider LED for small point-of-use duty with intermittent draw — under-counter drinking taps, appliances, dispensers — and for applications valuing its packaging: portability, battery/solar operation during load-shedding, tight spaces, mercury-free policies. Scrutinise LED claims with the same three questions as any UV unit — dose, at what flow, at what UVT, at end of life — plus one more: the thermal design, since LED lifetime is set by how well the module sheds heat.
The trajectory is one-directional — LED efficiency improves year on year and mercury faces regulatory pressure (the Minamata Convention exempts germicidal lamps for now) — so expect the crossover point to keep moving up the flow range. It has not arrived for mainline duty yet.
Rule of thumb: mercury for continuous and large, LED for intermittent and small. If a supplier claims LED parity for a whole-building system, ask for the dose-at-flow-at-end-of-life figure and the price of the replacement module — the two numbers that still decide the argument.