Ultraviolet energy is found in the electromagnetic spectrum between visible light and x-rays and is best described as invisible radiation. Ultraviolet radiation has been widely used in laboratory water systems as a germicidal treatment for water. In order to kill microorganisms, the UV rays must actually strike the cell. UV energy penetrates the outer cell membrane, passes through the cell body and disrupts its DNA preventing reproduction. UV disinfection treatment does not alter water chemically, nothing is added except energy. The sterilized microorganisms are not removed from the water. UV disinfection does not remove dissolved organics, inorganics or particles in the water. The degree of inactivation by ultraviolet radiation is directly related to the UV dose applied to the water. The dosage, a product of UV light intensity and exposure time, is measured in microwatt second per square centimeter (µws/cm2). Most UV units are designed to provide a dosage greater than 30,000 µws/cm2 after one year of continuous operation. Certain contaminants in water can reduce the transmission of UV light through the water, which reduces the UV dose that reaches the bacteria. These UV absorbing contaminants include turbidity, iron, and humic and fulvic acid, common to surface water supplies. Suspended particles are a problem because microorganisms buried within particles are shielded from the UV light and pass through the unit unaffected.
UV disinfection is most effective for treating high-clarity purified water through processes such as reverse osmosis distillation, ultra-purification and others.
Types of UV Disinfection
Special low-pressure mercury vapor lamps produce ultraviolet radiation at 254 nm, the optimal wavelength for disinfection and ozone destruction. The UV lamp never contacts the water; it is either housed in a quartz glass sleeve inside the water chamber or mounted external to the water which flows through UV transparent Teflon tubes. Some ultrapure water systems use 185 nm UV units for reducing TOC (total organic carbon).