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Study finds benefits in combined chlorine-UV disinfection

A combined treatment with chlorine and UV radiation can be highly effective in drinking water disinfection, a University of Eastern Finland study has found.

The study isolated different coliphages from treated municipal wastewater. Coliphages are viruses that infect E.coli bacteria and can be used as models for human enteric viruses in disinfection studies.

Nearly half of the isolated coliphages were highly resistant to chlorine or UV radiation, which meant neither chlorine nor UV radiation alone were effective against all coliphages.

“This highlights the importance of a combined treatment,” said University of Eastern Finland Doctoral Student Alyaa Zyara, MSc, who presented the results in her doctoral dissertation.

When coliphages were first exposed to a low chlorine concentration (0.1 or 0.5 mg Cl/L) for 10 minutes followed by low UV radiation (only 22 mWs/cm2), more than 99.9% of all of the coliphages studied became inactivated.

However, when the order of the treatment was reversed (UV first, chlorine second), disinfection was much less effective.

“It is more effective to first use a low dosage of chlorine followed by a low dosage of UV radiation than to use high chlorine or UV dosages alone," she said. "The order of treatment is also important: using UV radiation first and chlorine second was less effective. In other words, the combination treatment using chlorine first and UV second can be recommended as a disinfection method for viruses.”

The study also tested novel UV-LED technology to determine its effectiveness in disinfecting drinking water.

The study used UV-LEDs operating at a wavelength of 270 nm and with a 120 mW irradiation capacity, which have not been used in disinfection studies before.

As little as two minutes of this UV-LED treatment was enough to cause a 90-99.9% reduction in the coliphages tested in a 5.2-litre reactor.

Irradiation time of 10 minutes in the same reactor increased the reductions to 99.99 – 99.999%. A traditional mercury UV lamp at a 254 nm wavelength caused similar or slightly higher reductions in two or 10 minutes, but the water volume was only 10 millilitres. 

“UV-LEDs are a promising method for disinfecting water, since they consume less energy than traditional mercury UV lamps," Zyara said. "Furthermore, as UV-LEDs do not contain any mercury, they are safer for the environment.”

Author: Robin Hackett, Deputy Editor, WWT and WET News
Topic: Treatment
Tags: drinking water , treatment , chlorine , Finland

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