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.

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.

UV offers a key advantage over chlorine-based disinfection, due to its ability to inactivate protozoa that threaten public health – most notably Cryptosporidium and Giardia. UV does not add any chemicals to the water and does not produce disinfection by-products. 
 
Chlorine gas is a highly toxic chemical that must be transported and handled with extreme caution. It is a strong oxidizing agent that can be extremely dangerous to humans. The increasing concerns about the hazards of chemical disinfection to the public, the operators, and the environment have dramatically increased the cost of chemicals and the administrative efforts required to meet safety regulations. As a result, UV disinfection has increasingly become the choice for wastewater disinfection due to some significant advantages over chemical-based disinfection.

The vast majority of wastewater treatment plants around the world provide secondary treatment through conventional activated sludge processes. Some plants have tertiary treatment processes that use an additional filtration process to further improve water quality. The resulting effluent typically requires a disinfection step before discharge back to the environment.

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UV disinfection has been proven effective in challenging applications such as combined sewer overflows (CSO), storm and sanitary sewer overflows (SSO), blended effluents as well as disinfection of primary treatment effluent.

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Chlorine gas has traditionally been used for wastewater disinfection for many years. Sodium Hypochlorite (liquid bleach) is an alternative to chlorine gas; however, it is more costly and has the same environmental concerns. UV disinfection can easily be retrofitted into an existing chlorine contact tank and has significant benefits compared to chlorine disinfection.

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