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Using UV light to purify water may not work

In theory ultraviolet (UV) light kills the majority of bacteria in drinking water. In terms of public safety, UV light achieves this in a way that does not introduce harmful chemicals into the water. However, new research suggests that this sanitization method is not completely effective. Not only is the method seemingly ineffective, instead of killing all bacteria, the water-purification treatment may simply drive some into dormancy. This paves the way for harmful microbes to be revived at a later time.

UV treatment works by damaging bacterial DNA (this stops the cells from growing and dividing.) The action of the light on the DNA directly blocks bacterial replication. However, UV light does not disrupt the cell membrane. Destruction of the cell membrane happens with other water purification techniques, such as chlorination. The good news for consumers is that many water treatment facilities first disinfect with chlorination and then use UV light as a secondary measure. However, other, smaller scale water plants use only the UV treatment. It is with these facilities that there could be a degree of risk.

To examine the risk of treating water only with UV, environmental engineer Xin Yu of the Chinese Academy of Sciences’ Institute of Urban Environment exposed two types of pathogenic bacteria (Escherichia coli and Pseudomonas aeruginosa to varying doses of UV light. Post-treatment, the scientists assessed the surviving microbes and measured the expression of a bacterial gene within the survivors. While the treatments reduced colony growth by as much as 0.0001 percent, the scientists found no changes in the expression of the genes, suggesting that the bacteria were still able to synthesize proteins. This means that they potentially continue to be a threat.

In fact the bacteria were able to generate new growth within 24 hours E. coli showed regrowth regardless of the level of UV treatment, while P. aeruginosa were only able to restart after exposure to low levels of UV.

In summing up the implications, the authors’ write: “The occurrence of a viable but nonculturable state in bacteria may dramatically underestimate the health risks associated with drinking water. These results systematically revealed the potential health risks of UV disinfection and strongly suggest a combined disinfection strategy.”

The new findings have been published in the journal Environmental Science & Technology. The article is titled “UV Disinfection Induces a Vbnc State in Escherichia coli and Pseudomonas aeruginosa.”

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Written By

Dr. Tim Sandle is Digital Journal's Editor-at-Large for science news. Tim specializes in science, technology, environmental, business, and health journalism. He is additionally a practising microbiologist; and an author. He is also interested in history, politics and current affairs.

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