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| Ultra Violet
Light
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Ultraviolet Radiation (ultra violet radiation)It has been established scientifically that ultraviolet light has germicidal properties. Low-pressure mercury arc lamps easily produce ultraviolet light. The radiation of ultraviolet light kills or deactivates pathogens. While bacteria are killed with relatively low amounts of radiation, viruses are more resistant to this radiation. Cysts and worms are unaffected by ultraviolet light radiation. It must be kept in mind that the efficiency of these low-pressure mercury arc lamps decreases with age and therefore, these lamps must be replaced annually. Another factor that reduces the efficiency of these lamps is the color, turbidity and organic impurities in the water. These also reduce the purity of water to unsafe levels. Additionally, there are no residual products of radiation and thus there is no disinfectant effect beyond the treatment period. This is a major limitation of ultra violet treatment that must be borne in mind. UV (ultra violet) part of the light from sun has higher frequency than visible light but lower than the x-rays. Ultra Violet is also used as a water treatment technique as it is effective disinfectant because of its strong germicidal (inactivating) ability. Hence, ultra violet disinfects water containing bacteria and viruses but it does not have any effect on Giardia lamblia or Cryptosporidium cysts that is also present in water. Ultra Violet was used as a drinking water disinfectant in the early 1900s but was given up due to high operating costs, not very reliable equipment, and the ever increasing popularity of disinfection of water by chlorination process. Recently, the safety of chlorination has been questioned by many people and so ultra violet has again started becoming a popular method of water disinfection. The process of ultra violet disinfectant has recently experienced increased demand from both municipal and household systems. The number of large scale ultra violet water treatment plants in Europe is more than 2000. Though, in the United States there are few such plants but they slowly but surely are increasing in number owing to the recent popularity. To ensure that the amount of chlorine used for disinfection is less, most municipal systems are nowadays using Ultra Violet in conjunction with chlorine. This is in an attempt to reduce the disinfection byproducts (DBPs) that are the chemicals associated with chlorination. Some studies have established that few disinfection byproducts such as trihalomethanes are linked with increases in certain cancers. While the main advantage of ultra violet treatment
is that it does not require any chemical input, its major disadvantage
is that ultra violet treatment does not have any residual disinfection
in the water delivery system. This kind of residual disinfection
is available with a chemical treatment system such as chlorination.
Therefore, a secondary disinfection method, such as chlorine or
ozone may be a requirement for a ultra violet system. This has made
a combination of Ultra Violet and Chlorine a popular
choice in recent times.
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