Ultraviolet irradiation is the sterilization of either water, air, or objects through the use of a particular wavelength of ultraviolet light. This is a common way of disinfecting wastewater and drinking water for houses and institutions. This method of disinfection does not result in the toxic by-products that can be produced by the chlorination of water.
Ultraviolet radiation is not visible to the naked eye and emits shorter wavelengths than visible light. These wavelengths are measured in nanometers (nm). There are several different types of ultraviolet light (UV), and each has differing wavelengths. Sunlight emits all of these, but the short-wave ultraviolet light (UVC) used in ultraviolet disinfection is screened out by Earth's atmosphere. Thus, microorganisms do not have resistance against it.
A typical UV disinfection system is a special mercury vapor lamp made of quartz glass that emits a wavelength of 254 nm, known as a germicidal lamp. The maximum wavelength at which DNA absorbs is 260 nm. Thus, at 254 nm, the DNA of the microorganisms is damaged, and they cannot reproduce. This ultraviolet irradiation does not kill them, but they are irreparably damaged. Such a UV sterilizer is effective against bacteria, viruses, parasites, fungi, and spores.
In order for ultraviolet irradiation to succeed, the microorganism must be in direct contact with the UV light rays. Also, for a UV water purifier to function, the water cannot be turbid. If it is cloudy, it must be filtered first, or treated with activated charcoal. It is important that the light stay strong, so lamp maintenance is an important part of an ultraviolet system. Home units should have an alarm to sound a warning if the lamp is not working properly.
Given public concern about the toxic by-products of chlorine treatment, ultraviolet irradiation is being used more frequently for water purification of both drinking water and wastewater. One problem is that there is no residue left in the water to continue the disinfection after the light has penetrated it. Therefore, if the water becomes contaminated after the initial treatment, it will stay contaminated. For this reason, the ultraviolet water purifier should be located close to the location of final use, to minimize any chances of subsequent contamination. In home units, ultraviolet irradiation is frequently the final step in a process that can also include some other form of treatment, like water-softening or reverse osmosis.
Ultraviolet irradiation is particularly useful in treating the water of homes that draw their supply from wells, lakes, or streams. People that are concerned about the effects of chlorine by-products, typically de-chlorinate their water and use UV irradiation to disinfect it after this treatment. Commercial establishments frequently find it easier to use a UV disinfection system than to chlorinate water. Bottled water is also often disinfected in this manner.