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Ultraviolet

Ultraviolet Radiation (UV) units are usually installed as Point-of-Entry (POE) systems. They are generally placed at the entrance of the water system into the house to service all incoming water.

Ultraviolet (UV) systems expose the water to light from a special lamp.  The light is at a specific wavelength between 100 and 400 nanometres (nm) with light under 300nm being most effective at killing common bacteria. UV adds nothing  to the water, produces no tastes or odors, and usually  requires only a few seconds of exposure to be effective. The minimum dosage  for an ultraviolet light water treatment device is 16,000 Ws/cm at a wavelength of 253.7 (nm) at maximum flow through the UV device.

UV is effective on pathogens but not on protozoan cysts such as those of Giardia Lamblia, responsible for giardiasis.  Ultraviolet light physically alters the DNA material in cells so that bacteria, viruses, molds, algae and other microorganisms can no longer reproduce. When a microbe can no longer multiply it is considered dead.

UV systems, however, cannot process water which is turbid or has suspended solid particles of inorganic or organic matter because these can shield organisms against the light. Because of the possible presence of protozoan cysts, pre-filtration to clarify the water or micro filtration after UV processing must be added to ensure that the system is completely disinfected.  

Ultraviolet effective for:

  • Bacteria :  Coliform Bacteria, 
  • Bacteriophage (E. coli), 
  • Salmonella Typhosa (Typhoid Fever)

  Considerations:

Because the process of UV sterilization is generated by a light source the light chamber through which the water passes must be kept clear of algae, scale build-up and sediment which can foul the light chamber.  UV lamps are separated from the water by a tube, it is these tubes upon which algae grows, recently Teflon tubes have been replacing quartz tubes decreasing the algae problem. Consider maintenance. 

UV lamps have a functional life expectancy of between 12 and 18 months after which their ability to emit sufficient UV emissions is reduced below the level at which bacteria will be killed or sterilized.  Consider maintenance. 

Special consideration must be given to "Dwell-time," if the Gallon Per Minute (GPM), or flow rate is only slightly too high for the power of the UV lamp, there is little net effect on the microorganisms.

UV sterilization is most effective when: 

  • Total iron is less than 0.3ppm

  • Manganese is less than 0.05ppm

  • hydrogen sulphide is less than 0.05ppm

  • suspended solids are less than 10ppm

  • Water hardness below 7 gpg
    Have your water tested!

Holding UV-treated water within and without the water system may cause increase in the risk of contamination from reintroduction of bacteria.  For example, larger water systems and the storage water in the refrigerator for a cold drink or reconstituted drinks.

 

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Last modified: November 13, 2018