Ultraviolet light is part of the light spectrum, which is classified into three wavelength ranges:
UV-C, from 100 nanometers (nm) to 280nm
UV-B, from 280nm to 315nm
UV-A, from 315nm to 400nm
UV-C light is germicidal - i.e., it deactivates the DNA of bacteria, viruses and other pathogens and thus destroys their ability to multiply and cause disease. Specifically, UV-C light causes damage to the nucleic acid of microorganisms by forming covalent bonds between certain adjacent bases in the DNA. The formation of such bonds prevent the DNA from being unzipped for replication, and the organism is unable to reproduce. In fact, when the organism tries to replicate, it dies.
Ultraviolet technology is a non-chemical approach to disinfection. In this method of disinfection, nothing is added to the water, which makes this process simple, inexpensive and requires very little maintenance. Ultraviolet purifiers utilize germicidal lamps that are designed and calculated to produce a certain dosage of ultraviolet (usually at least 16,000 micro-watt seconds per square centimeter but many units actually have a much higher dosage.) The principle of design is based on a product of time and intensity - you must have a certain amount of both for a successful design. In the real world, Ultraviolet treatment technology is very useful and effective at controlling bacteria in bulk flows of water being processed in order to help prevent active biological passage into downstream processes or storage. Ultraviolet is, however, not designed to eliminate all possibilities of downstream biological growth or contamination and other additional methods of biological control, such as chlorination, are necessary for potable water storage and distribution type water systems.