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.
How UV Disinfection Works
Unlike chemical approaches to water disinfection, UV 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. UV light has demonstrated efficacy against pathogenic organisms, including those responsible for cholera, polio, typhoid, hepatitis and other bacterial, viral and parasitic diseases. In addition, UV light (either alone or in conjunction with hydrogen peroxide) can destroy chemical contaminants such as pesticides, industrial solvents, and pharmaceuticals through a process called UV-oxidation.
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.
Effectiveness of UV
A significant body of scientific research has proven UV light’s ability to inactivate an extensive list of pathogenic bacteria, viruses and protozoa. 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. The release of these harmful microorganisms into receiving lakes and rivers by wastewater facilities utilizing chlorine disinfection increases the potential of contamination in communities that rely on these same bodies of water for their drinking water source and recreational use. Drinking water treatment plants can benefit by using UV since it can easily inactivate chlorine-resistant pathogens (protozoa), while reducing chlorine usage and by-product formation.
Photochemical damage caused by UV may be repaired by some organisms if the UV dose is too low via photoreactivation or dark repair. However, studies have shown that there is little to no potential for photoreactivation at doses higher than 12 mJ/cm2. In fact, it has been shown that some organisms, like Cryptosporidium, do not exhibit any evidence of repair under light and dark conditions following low-pressure or medium-pressure lamp irradiation at UV doses as low as 3 mJ/cm2. UV systems should be designed with enough UV dose to ensure cellular damage cannot be repaired. Sizing of a system should be based on bioassay validation (field testing) to ensure proper disinfection.
Safety Advantages of Disinfection
UV is a chemical-free process that adds nothing to the water except UV light
UV requires no transportation, storage or handling of toxic or corrosive chemicals – a safety benefit for plant operators and the surrounding community
UV treatment creates no carcinogenic disinfection by-products that could adversely affect quality of the water
UV is highly effective at inactivating a broad range of microorganisms – including chlorine-resistant pathogens like Cryptosporidium and Giardia
UV can be used (alone or in conjunction with hydrogen peroxide) to break down toxic chemical contaminants while simultaneously disinfecting
Cost Advantages of UV Disinfection
Annual lamp replacement and electrical consumption comprise the operating costs of UV disinfection
UV eliminates or reduces the immediate safety threat of chlorine gas without creating new long term costs associated with chemicals, transportation and delivery
Costs for leak response, administration, risk management and emergency planning and operator training are minimized and/or eliminated with UV
Municipalities do not pay a premium for the significant safety advantages of UV