Ultraviolet Radiation


Background

Ultraviolet (UV) light has long been known to have germicidal properties, but equipment and methods using it effectively in water supply systems are quite new. UV radiation was one of the earliest recognized methods of disinfection , however its use was discontinued around the the turn of the century because of the advent of chlorination and ozonation. Recently, there has been a renewed interest in UV radiation as an alternative to chemical treatments largely as a result of concern over toxic chemical byproducts.

Ultraviolet light occupies the spectral range of 4,000 to 1,200 angstroms, which is from just below visible light to soft X-rays. Ultraviolet radiation at about the center of the range has been found to kill or deactivate many pathogens.

Chemistry

UV treatment is unique in its mode of action in that it does not necessarily kill the target organism, instead the radiation alters cell DNA so that the organism is sterilized. This serves to inactivate the pathogen so that it cannot proliferate and cause disease.

The minimum recommended dose is 16,000 MWs/cm2 (60 billionths of a kilowatt hour/cm2) but pathogens differ widely in the amount of radiation required to kill or deactivate. Bacteria are the easiest group to treat and differ the least in amount of radiation required. Viruses are most resistant and variable. Recent research indicates that Polio and Rotarians virus required 45,000 MWs/cm2 or nearly three times the minimum recommended dose. Cysts and worms are unaffected by ultraviolet light so if present they must be treated by another procedure.

Devices and Applications

Main categories of UV application systems are:

  • Unsubmerged systems where the process flow stream is routed through transparent tubes placed adjacent to externally mounted low-pressure mercury arc lamp.
  • In channel submerged systems consisting of lamps mounted on racks that can be lowered into the flow channel. Each lamp is inserted into a transparent tube and sealed to protect it from exposure to the water. For this type of system, water level variations should be kept to a minimum to avoid short circuiting.
  • Enclosed submerged lamp systems much like the other submerged system except that the lamps are fixed in place. This arrangement allows for the possible use of new high intensity lamps for enhanced treatment capabilities.

    For information on other devices used for smaller and private water systems. see the KK Technology International Homepage.

    Advantages and Disadvantages In Portuguese

    Ultraviolet treatment has the advantage of adding nothing to the water while not requiring the addition of treatment materials as long as the system used is maintained in good operating condition. Extensive contact time is not required in this process making it a time efficient treatment option.

    The major disadvantage is that there is no residual for treatment beyond the device. If contaminants enter after treatment, another disinfection method such as chlorination must be used to sanitize the system and treat the water. Some pathogens deactivated by UV light may be reactivated when exposed to oxygen. Ultraviolet light is easily absorbed by solids, including particulate matter in the water or deposits on the lamp surface. As a consequence, ultraviolet light treatment should only be attempted on clear water.

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    Page created by Steve Boglarski and Shyam Telikicherla, Fall 1995