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About UV Water Sterilization Systems

Ultraviolet Radiation (UV) units are usually installed as Point-of-Entry (POE) systems. They are 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 µWs/cm² at a wavelength of 253.7 (nm) at maximum flow through the UV device.

UV systems damage the DNA of pathogens (listed below) that pass by the UV light.  Their DNA is damaged to the point where the pathogen cannot reproduce.   When a microbe can no longer multiply it is considered dead.

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Many UV systems are effective on pathogens but not on protozoan cysts such as those of Giardia Lamblia, responsible for giardiasis be very careful to verify that the equipment you purchase handles all of the pathogens.  Ultraviolet light physically alters the DNA material in cells so that bacteria, viruses, molds, algae and other microorganisms can no longer reproduce. 

UV systems, however, cannot process water which is turbid or has suspended solid particles of inorganic or organic matter because these things 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.

Standard Ultraviolet effective for:

  • Bacteria :  Coliform Bacteria, 
  • Bacteriophage & Escherichia Coli (E. coli), 
  • Hepatitis Virus
  • Influenza Virus
  • Legionella Pneumophila
  • Salmonella Typhosa (Typhoid Fever)
  • Salmonella Paratyphi

Advanced UV is additionally effective for:

  • Giardia Lamblia
  • Cryptosporidium


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 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!

UV Transmittance:

Water isn't always "gin clear" even when to the eye it appears to be colourless and without suspended impurities.  Testing will provide you with the UV Transmittance factor to help you select the UV system for your needs. 


NSF Certified products to NSF Standard 55 Class A -are tested to stringent standards by NSF International, an independent testing organization and the leading authority on water treatment systems.  This standard requires a minimum of 40 mega joules of UV light emission per cubic meter ( 40 mJ/cm2).

Third-Party Validation

The U.S. EPA UV Disinfection Guidance Manual has defined third-party validation as “the process by which a UV reactor’s disinfection performance is determined relative to operating parameters that can be monitored. The reactors are validated to indicate that they achieve a certain delivered UV dose for a range of flow, UV intensity and water quality conditions.”

There are many different ways to obtain a third-party validation for a UV system.

For residential (point-of-entry) applications (under 30 gpm), there is the NSF 55 validation, which only tests for one specific UV dose level. NSF defines a point-of-entry system as “a system used to treat all or part of the water for the facility at the point where the drinking water comes into the facility. For Class A systems, a single-family dwelling shall be considered a facility.” The only condition that the NSF 55 protocol tests for is the delivered dose of the system when it is in alarm, not over a range of water quality levels or conditions. A UV system can be in alarm for various reasons such as low UV intensity (which can be due to low water quality or stained quartz) or a lamp failure.

Regardless of the protocol followed, the focus of a UV validation is the biodosimetry/bioassay analysis. Biodosimetry can be defined as the procedure used to determine the performance or dose delivery of a UV system. Through a measurement of the inactivation of a challenge microorganism (MS-2 bacteriophage) following UV exposure, the reduction equivalent dose of a UV reactor can be determined following a dose-response curve comparison. A dose-response curve is generated through a controlled laboratory test, called a collimated beam, which measures both the exposure time and UV intensity at which the MS-2 is exposed and inactivated.

MS-2 is typically used as a challenge microorganism due to its high resistance to UV, the ease to culture and its highly reproducible inactivation results. MS-2 is also a non-pathogenic microorganism; therefore, there is no concern of possible infection when working with MS-2.


Holding UV-treated water within and outside 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