Anyone with access to printed, online, or broadcast news is likely to well aware that, among our other assaults on the planet, we have managed to create holes in the ozone layer. This rare form of oxygen, with three atoms rather than the two of the regular molecule, forms a layer within the stratosphere, surrounding the earth and extending from between 10 and 17 km to 50 km above its surface. Its main role is to protect all life below from the harmful effects of ultra-violet (UV) rays of the sun. With typical human ingenuity, we have learnt to generate UV rays and utilise their properties for water treatment and for several other equally important applications.
The effect of this short-wavelength radiation on living cells can be quite extreme. Depending upon the wavelengths selected, they can result in anything from a rich golden tan or painful sunburn to the breakdown of nucleic acids and disruption to DNA that could lead to mutations in plants and malignancy in animals. However, the latter and more extreme effects have also been applied as the basis of a process known as UV germicidal irradiation, or UVGI. The process leverages the mutagenic properties of shortwave radiation to kill or inactivate microorganisms. In this manner, UV rays provide a highly effective disinfection option for use in water treatment.
While the operators of water-purification plants first embraced this technology in the mid-’50s, its popularity has grown steadily within the industry. By the turn of the century, more than 6000 plants were believed to have adopted UV germicidal irradiation as part of their routine. Nevertheless, its use is still more widespread within the medical field. In fact, the Danish physician Niels Ryberg Finsen received the Nobel Prize for Medicine in 1903 for his successful use of UV rays, not to disinfect water, but for the treatment of a tuberculous skin condition known as lupus vulgaris.
The main benefit of this technology is that it offers an alternative to methods of disinfection or sterilisation that depend upon the use of dry heat, steam, boiling, or chemicals, any or all of which may be either unsuitable or uneconomical for certain applications. It is perhaps ironic that, while halogens such as iodine, bromine, fluorine, chlorine, and their carbon compounds, once used as propellants in aerosol cans, are cited as the main cause of ozone depletion, prior to the introduction of UV radiation for water treatment, chlorination was the method of choice and is still widely used.
The latter method, however, offers several advantages. Firstly, the residual chlorine remaining in the disinfected liquid serves to protect it from re-infection within the distribution network. In addition, it can act on any organisms that may be embedded in particulate matter which could effectively protect them from the action of UV radiation. Despite this, many consumers still prefer to eliminate the taste of chlorine with a secondary home filtration system. UV water treatment is probably most effective when applied to the disinfection of the highly purified product obtained by the use of reverse osmosis. To overcome the risk of embedded organisms, disinfection is usually preceded by a pre-filtration stage, which also serves to clarify the product and improve the transmission of ultraviolet light. Care must be taken to regulate the flow rate as, if it is too high, some microorganisms could remain unaffected while a rate that is too slow could cause the lamp to overheat and, eventually, malfunction.
The use of UV radiation provided a form of water treatment that is a purely physical process with no potential environmental consequences. While the use of chemical disinfectants can sometimes be ineffective, for example in the elimination of common parasites like Giardia and Cryptosporidium, these species are highly susceptible to the disruptive action of concentrated ultraviolet rays. In sewage treatment too, this technology is frequently being used to replace chlorination. This is to avoid the risk that chlorine, in combination with any organic compounds present, could form long-lived toxic compounds that might pose an environmental threat.
In-line systems are available for the UV treatment of water circulated in aquarium tanks and ornamental ponds while there are many further uses for this type of disinfection in laboratories and the food and beverage industry. For most applications, WaterIcon offers the guarantee of quality products and professional support.