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A bit of theory

Bactericidal water treatment with ultraviolet irradiation is a crucial final stage in every water treatment process. Thanks to simplicity and disinfection efficiency, ultraviolet technology is largely used in daily lives and manufacturing – everywhere pure water is needed.

Water disinfection with ultraviolet (UV) rays is part of physical (reagents free) methods of water treatment. Major benefit of such method is the ability to preserve composition and properties of treated water. The disinfection effect of the UV irradiation primarily is a result of photochemical reactions in molecular structure of DNA and RNA occurring under UV influence. These molecular structures are the informational basis of the reproductive apparatus of any living organism, in particular, bacteria and viruses. Photochemical reaction results in irreversible damage of DNA and RNA, as well as in damage of membrane structure and cell walls of microorganisms, leading to their demise.

One of the positive factors is that during water purification, peroxide compounds do not accumulate and, under the influence of UV irradiation, some pyrogenic substances, caught in water, are deactivated.

Numerous studies proved that the bactericidal effect of UV irradiation is stronger than chlorination on both most common pathogenic vegetative bacteria and spore bacteria. In addition, UV irradiation has an inactivating effect on viruses, causing their death at an activation energy of 16–40 mJ/cm2, surpassing the ozonation method in this index. As a comparison, to achieve high virucidal effect, an ozone dose of 0.5–0.8 g/l and contact duration of up to 12 minutes are required, while during UV irradiation, viruses inactivation occurs in seconds.

It is known that some types of bacteria are able to form strains that are insensitive to the action of chlorine. To deactivate such microorganisms, UV irradiation methods are irreplaceable: bacteria do not develop resistance to UV bactericidal action. It is important to note that there is no limitation of the upper dose threshold for UV irradiation, therefore, by increasing the dose of UV radiation, the desired level of disinfection can always be achieved.

Irradiation does (mj/см2), required for inactivation of 99.9% if viruses and bacteria

Adenovirus type III4.5Agrobacterium lumefaciens8.5Clostridium botulinum11.2Legionella dumoffill5.5Leptospira interrogans6.0Rhodospiril-lum rubrum6.2Serratia marcescens6.2Staphyloco-ccus albus5.7Streptococcus pyogenes4.2
Bacteriophage6.6Bacillus anthracis (anthrax)8.7Corynebac-terium diphtheriae6.5Legionella pneumophila (Legionnaires disease)12.3Mycobacterium tuberculosis10.0Pseudomonas fluorescens6.6Shigella dysenteriae - Dysentery4.2Staphylococcus aureus6.6Streptococcus salivarius4.2
Coxsackie6.3Bacillus megatherium Sp. (veg)2.5Dysentery bacilli4.2Leptospira canicola – Infectious Jaundice6.0Neisseria catarrhalis8.5Salmonella typhi7.0Shigella flexneri - Dysentery3.4Staphylococcus epidermidis5.8Streptococcus viridans3.8
Infectious hepatitis8.0Bacillus megatherium Sp.5.2Eberthella typhosa4.1Legionella longbeachae2.9Phytomonas tumefaciens8.5Salmonella types15.2Shigella paradysenteriae3.4Streptococcus faecaila10.0Vibrio comma (Cholera)6.5
Flu6.6Bacillus paratyphosus6.1Escherichia coli6.6Legionella gormanil4.9Proteus vulgaris6.6Salmonella enteritidis7.6Shigella sonnei7.0Streptococs hemolyticus5.5Vibrio cholerae6.5
Rotavirus24.0Bacillus subtilis11.0Legionella bozemanii3.5Legionella micdadei3.1Pseudomonas aeruginosa10.5Salmonella10.5Spirillum rubrum6.2Streptococs lactis8.8Micrococcus candidus12.3


William V. Collentro, “Treatment of Water with Ultraviolet Light – Part I”, Ultrapure Water.

James E. Cruver, Ph.D., “Spotlight on Ultraviolet Disinfection”, Water Technology.

Dr. Robert W. Legan, “Alternative Disinfection Methods-A Comparison of UV and Ozone”.

Rudolph Nagy, Research Report BL-R-6-1059-3023-1, Westinghouse Electric Corporation.

Myron Lupal, “UV Offers Reliable Disinfection”, Water Conditioning & Purification.

John Treij, “Ultraviolet Technology”, Water Conditioning & Purification.

Bak Srikanth, “The Basic Benefits of Ultraviolet Technology”, Water Conditioning & Purification.

“The Use of Ultraviolet Light for Microbial Control”, Ultrapure Water.

Note: inactivation doses rates may vary slightly between different sources.