General

UV disinfection

Worldwide growing and more dense populated urbanisations result in a growing demand on the natural resources and cause a stronger load on the natural environment. The growing awareness to maintain the ecological balance requires a careful reconsideration of the industrial processes applied. As a result more and more national and local authorities are developing stringent environmental concerned standards to assure a sustainable growth. One of such technologies is water disinfection by ultraviolet (UV) radiation; arguably it is the most reliable and environmentally friendly method of water disinfection without formation of by-products and does not require the use of chemicals. These days popularity for UV disinfection technology is growing rapidly throughout the world. For decades LIT Technology has been introducing and successfully applying this method.

Germicidal effect

UV-c radiation at the wavelength of 254 nanometer, destroys the genetic structure of micro-organisms and inhibits their ability to replicate and as such prevent colony formation and thus causing their extinction. In nature germicidal ultraviolet is a part of the sun's radiation; however, most germicidal radiation (UV-C) is filtered by the atmosphere and does not reach earth.

Modern technologies can be used to convert electrical energy into germicidal ultraviolet power in an efficient manner. One of these technologies is a mercury gas-discharge lamp that generates UV-C radiation, which can be used to inactivate micro-organisms by destroying DNA.

UV radiation, in particular UV-C, effectively kills airborne, surface and water living bacteria, viruses and cysts. Especially important is the fact that UV radiation destroys some types of viruses that are resistant to conventional chlorination. One of the characteristics of UV lamps is the ability to selectively radiate at 254 nm close to the optimum wavelength of 260 nm (see below) and not to emit radiation that generates ozone or other carcinogenic substances.

Germicidal ultraviolet does not change the chemical composition and flavour of water in contrast to reagent methods of disinfection (chlorination, ozonation). Resistivity of micro-organism to UV radiation strongly depends on its type. Most sensitive to UV radiation are viruses and bacteria in vegetative forms (bacillus, coccus), for example, well know micro-organisms such as Salmonella typhosa, Vibrio cholerae, Shigella dysenteriae, Hepatitis virus, Mycobacterium tuberculosis.

A modest UV dose is required to inactivate Lamblia cysts and Cryptosporidium oocysts, while a large UV dose is necessary to destroy spores. It is always possible to choose a UV dose that provides proper disinfection in any particular case, since there is no negative overdosing effect.

Best performance

Inactivation efficiency of UV radiation is demonstrated by the so-called germicidal curve that shows relative quantity of destroyed micro-organisms versus wavelength of UV radiation. The germicidal effect of ultraviolet radiation is considerable in the 205-315 nanometre range of the spectrum, with the maximum disinfection efficiency achieved at 260 nm. The spectral distribution of radiation from mercury low-pressure germicidal lamps used in LIT UV systems is peaks at 254 nm, which is very close to the wavelength (260nm) where micro-organisms have the maximum energy absorption and thus the best disinfection performance. LIT UV units provide reliable disinfection over a wide range of water quality parameters due to the adjustable UV dose in a LIT UV system. Where typically ozone and chlorination processes require long retention time (several minutes) in large contact tanks, disinfection by ultraviolet radiation only takes a few seconds to inactivate most bacteria and viruses in treated water. This allows a fast process, compact UV project design and does not require any large civil constructions for contact tanks.