Ultraviolet (UV) is that part of electromagnetic light bounded by the lower wavelength extreme of the visible spectrum and the X-ray radiation band.The spectral range of UV light is, by definition between 100 and 400 nm and is invisible to human eyes. Using the CIE classification the UV spectrum is subdivided into three bands:
- UVA (long-wave) from 315 to 400 nm
- UVB (medium-wave) from 280 to 315 nm
- UVC (short-wave) from 100 to 280 nm
UVC purification has a long and honourable history in cleaning room air. However, growth in other applications such as high-tech volume liquid treatment and domestic ponds has expanded, whilst surface treatment of food has been used to extend shelf life in supermarkets, resulting in less waste food and lower stockholdings. Whilst UVC can be used as the exclusive solution in some applications, it is often used in tandem with other techniques. It follows that a single technology solution approach is unlikely to be ideal. It also follows that since UVC is so simple and energy effective, it is perhaps wise to consider this option first. Micro-organisms such as bacteria, moulds, yeast’s and protozoa can be destroyed or removed by physical, biological and chemical methods. UVC works using a photolytic effect whereby the radiation destroys or inactivates the micro-organism so that it can no longer multiply.
For DNA it does this by causing adjacent thymine bases to form a chemical bond thus creating a dimmer and if sufficient of these are created, DNA cannot replicate. Some micro-organisms can repair themselves by absorbing UVA. In other cases UVC (and indeed UVA or UVB) can cause bond splitting in a molecule resulting in the creation of free radicals, which are often highly labile and which can react together to produce an inert end product. For purifying these effects are produced by wavelengths below 320 nm, with the optimum effect occurring at around 260 nm. The phenomenon whereby micro-organisms can be disfigured or destroyed is independent of host state (fluid or solid). Indeed with pH or temperature, the important feature of the action is that radiation can reach the organism; this means that a bacterium shadowed by another or by a particle will escape attack. Unlike other techniques, UVC photolysis rarely produces potentially dangerous by-products.