UV is very effective
UV is a superior method of disinfection
How UV is generated
How UV disinfects
Ultra-violet radiation (or UV) is a proven process for disinfecting water, air or solid surfaces that are microbiologically contaminated.
The effects of UV as a disinfection agent have been documented since the early days of research in biology and the physics of light waves.
The U.S. Environmental Protection Agency recognizes the use of ultra-violet as a proven, viable technology;
"Ultraviolet (UV) radiation has been found to be an effective disinfectant…. Simplicity of installation, ease of operation and maintenance, and low costs relative to chemical disinfection, make UV a useful small systems disinfection technology option."
Small System Compliance Technology List for
Surface Water Treatment Rule, U.S. EPA, August 1997
Ultraviolet Light Disinfection Technology in Drinking Water
Applications - An Overview, U.S. EPA, 1996
Ultraviolet is recognized as superior compared to other methods of disinfection;
UV is very safe. There are no dangerous chemicals to handle or monitor. Disinfection results are immediate.
UV has a low initial system cost and a very low cost of operation.
UV is environmentally friendly. There are no byproducts from the UV process, and nothing is discharged into the environment.
UV causes no change to the taste or odor of the water disinfected. It is impossible to over-treat water with UV.
UV does not remove any of the minerals, which are a health benefit and provide water with its good taste
UV systems are very easy to install and maintain.
UV disinfection is compatible with all other forms of water treatment.
Ultraviolet light is electromagnetic radiation in the spectrum with a wave length between 100 and 400 nanometers (nm). The ultraviolet spectrum can be divided into three bands (several different divisions of this band exist);
UV-A 320 to 400 nm
UV-B 280 to 320 nm
UV-C 100 to 280 nm
The UV-C band contains the wavelengths (250-270nm) which have been found to be very effective in destroying many microorganisms (optimum wavelength is 265nm).
Low-pressure mercury discharge lamps (similar in construction and operation to fluorescent lamps), emit a wavelength of 253.7nm, which has been found to be a good source for UV radiation to perform the disinfection process. An electric arc, struck the length of the lamp, travels through an inert gas containing mercury. The arc heat vaporizes some of the mercury, which becomes ionized in the electric arc and gives off UV radiation. The UV lamp is constructed using a special quartz glass, which easily passes the UV radiation through it. The UV lamp is slipped into a quartz glass sleeve which is submerged in the stream of water. As the water flows past the lamp the microorganisms in the water are exposed to UV radiation. The quartz sleeve prevents the water from contacting the lamp, which would change the temperature of the lamp glass (and affect the pressure of mercury in the lamp and in turn the level of UV output).
Ultraviolet light penetrates the cell wall of a microorganism and causes a reaction in the microorganisms DNA (deoxyribonucleic acid), which breaks the C=C carbon bond in the molecules of the microorganism. This causes cellular death, rendering the microorganism incapable of growing and multiplying.
Through research, biologists have determined the amount of UV required to destroy different kinds of microorganisms. The amount of UV required is termed the dosage, and is a function of a certain intensity of UV (expressed in power or microwatts), delivered for a given period of time (seconds), over a given area (square centimeters).
Power X Time X Area or microwatt-sec/cm2 (µW-s/cm2)
A short exposure time at high intensity can be as effective as a long exposure time at lower intensity, as long as the product of power multiplied by time is the same.
The UV reactor design is critical to obtaining proper UV dosage of water. The water entering the UV chamber must be sufficiently free of suspended solids such that the microorganisms (some of which, like viruses, are extremely small) cannot hide behind or "in the shadows of" particles floating in the water. Then the flow-rate of the water must match the power of the lamp so that the microorganism has time to absorb the UV radiation and be destroyed. Good design of the UV chamber itself promotes uniform water speed in the hydraulic flow through the chamber, insuring maximum average UV exposure.