Drinking Water Treatment with UV Irradiation

Ultraviolet (UV) rays are part of the light that comes from the sun. The UV spectrum is higher in frequency than visible light and lower in frequency compared to x-rays. This also means that the UV spectrum has a larger wavelength than x-rays and a smaller wavelength than visible light and the order of energy, from low to high, is visible light, UV, than x-rays. As a water treatment technique, UV is known to be an effective disinfectant due to its strong germicidal (inactivating) ability. UV disinfects water containing bacteria and viruses and can be effective against protozoans like, Giardia lamblia cysts or Cryptosporidium oocysts. UV has been used commercially for many years in the pharmaceutical, cosmetic, beverage, and electronics industries, especially in Europe. In the US, it was used for drinking water disinfection and treatment in the early 1900s but was abandoned due to high operating costs, unreliable equipment, and the expanding popularity of disinfection by chlorination.

Because of safety issues associated with the reliance of chlorination and improvement in the UV technology, UV has experienced increased acceptance in both municipal and household drinking water treatment systems. There are few large-scale UV water treatment plants in the United States although there are more than 2,000 such plants in Europe. There are two classes of disinfection systems certified and classified by the NSF under Standard 55 – Class A and Class B Units.

Class A — These ultraviolet water treatment systems must have an ‘intensity & saturation’ rating of at least 40,000 uwsec/cm2 and possess designs that will allow them to disinfect and/or remove microorganisms from contaminated water. Affected contaminants should include bacteria and viruses
“Class A point-of-entry and point-of-use systems covered by this Standard are designed to inactivate and/or remove microorganisms, including bacteria, viruses, Cryptosporidium oocyst and Giardia cysts, from contaminated water. Systems covered by this standard are not intended for the treatment of water that has obvious contamination or intentional source such as raw sewage, nor are systems intended to convert wastewater to drinking water. The systems are intended to be installed on visually clear water.”

Class B — These ultraviolet water treatment systems must have an ‘intensity & saturation’ rating of at least 16,000 uw-sec/cm2 and possess designs that will allow them to provide supplemental bactericidal treatment of water already deemed ‘safe’. i.e., no elevated levels of E. coli. or a standard plate count of less than 500 colonies per 1 ml. NSF Standard 55 “Class B” UV systems are designed to operate at a minimum dosage and are intended to “reduce normally occurring non-pathogenic or nuisance microorganisms only.” The “Class B” or similar non-rated UV systems are not intended for the disinfection of “microbiologically unsafe water.”

Therefore, the type of unit depends on your situation, source of water, and your drinking water quality. Transmitted UV light dosage is affected by water clarity. Drinking water treatment devices are dependent on the quality of the raw water. When turbidity is 5 NTU or greater and/or total suspended solids are greater than 10 ppm, pre-filtration of the water is highly recommended. Normally, it is advisable to install a 5 to 20 micron filter prior to a UV disinfection system.

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