Unlike chemical approaches to water disinfection, UV light provides rapid, effective inactivation of microorganisms through a physical process. When bacteria, viruses and protozoa are exposed to the germicidal wavelengths of UV light, they are rendered incapable of reproducing and infecting.
 
Microorganisms are inactivated by UV light as a result of damage to nucleic acids. The high energy associated with short wavelength UV energy, primarily at 254 nm, is absorbed by cellular RNA and DNA. This absorption of UV energy forms new bonds between adjacent nucleotides, creating double bonds or dimers. Dimerization of adjacent molecules, particularly thymine, is the most common photochemical damage. Formation of numerous thymine dimers in the DNA of bacteria and viruses prevents replication and inability to infect.

For many decades, chlorine was successfully used in drinking water treatment. However, there are chlorine-resistant organisms, such as Cryptosporidium and Giardia, that are harmful to humans. For example, in 1993, over 400 people died and over 400,000 became ill as a result of a Cryptosporidium outbreak in the drinking water supply of Milwaukee, Wisconsin, USA. 
 
UV is highly effective at treating these chlorine-resistant organisms and produces no disinfection by-products. Many large cities (including New York City) have installed UV as part of a multi-barrier strategy to ensure their drinking water is safe.

Ultraviolet (UV) light is a form of light that is invisible to the human eye. It occupies the portion of the electromagnetic spectrum between X-rays and visible light. The sun emits ultraviolet light; however, much of it is absorbed by the earth’s ozone layer.
 
A unique characteristic of UV light is that a specific range of its wavelengths, those between 200 and 300 nanometers (billionths of a meter), are categorized as germicidal – meaning they are capable of inactivating microorganisms, such as bacteria, viruses and protozoa. This capability has allowed widespread adoption of UV light as an environmentally friendly, chemical-free, and highly effective way to disinfect and safeguard water against harmful microorganisms.

Cryptosporidium is a harmful protozoa that can cause severe illness and even death in some individuals. Fortunately, this chlorine-resistant parasite can be easily inactivated with a low UV dose.

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By employing UV as the primary disinfectant, the required chemical contact time can be reduced or eliminated (dependant upon local regulations).

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UV is a perfect fit as an additional barrier in a multi-barrier strategy to provide additional public safety.

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Giardia can be found worldwide and is one of the most common waterborne parasites. Low UV doses can inactivate Giardia without forming disinfection by-products.

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