How does UV disinfect water

UV light is a naturally occurring component of solar radiation. It falls in the region between visible light and X-Rays in the electromagnetic spectrum. Generally, UV light is considered as falling between the wavelengths 100nm – 400nm, however UV can be categorized even further into separate regions.

What is UV-C light?

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Although scientists hold varying opinions as to the exact boundaries of these regions, they are generally considered to be: Far UV (or vacuum) 100nm – 220nm, UV-C 220nm – 290nm, UV-B 290nm – 320nm and UV-A 320nm – 400nm. Of these, UV-C is recognized as having significant ‘germicidal’ properties. UV-C light is almost entirely filtered out by the Earth’s atmosphere, so to utilize its germicidal properties we have to artificially generate it here on earth using commercially produced UV lamps.

How does a UV-C light disinfect?

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Strong sunlight is known to kill bacteria, viruses, moulds and spores. Almost a century ago scientists identified the part of the spectrum responsible for this well-known effect: wavelengths of 240-280nm, primarily in the UV-C spectrum.

UV light damages the DNA in bacteria, viruses, moulds and some protozoa, leaving them unable to perform cellular functions and multiply. UV is particularly highly effective against Cryptosporidium and Giardia – organisms resistant to chlorine that are a major risk to human health. Another advantage of UV is the absence of taste and odour.

UV lamps contain a small amount of mercury, either in a free state within the lamp tube or embedded within the tube’s surface. When electricity is applied to the lamp electrode, electrons flow between them, vaporizing the mercury which, when bombarded with electrons, emits UV light.

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Figure 1 – Electromagnetic Spectrum

The exact wavelengths emitted depend on the vacuum pressure within the lamp tube itself. Low pressure (LP) UV lamps are evacuated to relatively ‘low’ pressures (between 1-10 Pa) and emit UV-C light at a single wavelength of approximately 254nm. Medium pressure (MP) lamps are evacuated at a ‘medium’ pressure and emit a broader spectrum of UV light, with higher intensities between around 254 – 265nm (figure 1).

Low pressure and so called ‘Amalgam’ lamps are twice as efficient at converting electrical energy into UV-C light compared to medium pressure lamps. However, medium pressure lamps emit far more UV-C energy per unit length over a wider wavelength than low pressure or amalgam lamps. Various considerations therefore need to be taken into account when choosing which of these lamps is suitable for a given application.

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