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Chlorine plus UV light can detoxify water affected by harmful algae blooms

Treatment plants use a combination of tools to keep toxins and contaminants out of drinking water. Researchers with the University of Cincinnati have examined two such tools in addressing a toxin produced by harmful algae blooms, which are becoming increasingly common in waters around the world.

Blue-green algae can reproduce en masse in waters laden with nitrogen, phosphorus or other excess nutrients. These algae "blooms" also can form when water levels drop during droughts or when bottom sediments heavy with nutrients get churned up in a storm, said Minghao Kong, a doctoral graduate of UC's College of Engineering and Applied Science.

When the blue-green algae bloom and die, they release toxins into the water that can be harmful or even lethal to people and pets, Kong said.

"Boiling the water or even filtering it won't help because that doesn't remove the toxins, which target your liver," he said.

Kong now studies environmental health at Emory University. He is lead author of a paper in the journal Environmental Science & Technology examining the benefits of one particular water treatment method.

"Cyanotoxins are not a new phenomenon on Earth. In fact, there is speculation that dinosaurs, whose fossils were discovered near water sources in Madagascar, may have been killed by toxins produced by prehistoric harmful algal blooms," Kong said.

Often, these algae blooms produce toxins at extremely dangerous levels, as was observed in Clear Lake, California, in 2014 and in Lake Okeechobee, Florida, in 2021, prompting advisories warning residents not to drink their water and that boiling tap water would provide no health protection.

In the lab of the late UC Professor Dionysios Dionysiou, Kong and his co-authors examined the combination of ultraviolet light and chlorine to detoxify water laden with toxins from cyanobacteria.

Their experiments demonstrated that the combination of ultraviolet light and chlorine significantly enhanced the degradation of toxins compared to chlorine alone.

"The integration of UV radiation and chlorination offers an efficient strategy for the treatment of toxins in drinking water with both low chemical demand and energy consumption," researchers concluded.

One concern, Kong said, is that chlorine and ultraviolet light in combination can create disinfection byproducts, including carcinogens toxic to human health. But UC's lab-scale experiments showed that UV/chlorine treatment effectively reduced the level of toxins below World Health Organization guidelines without generating harmful byproducts.

"Our tests confirmed that resulting byproduct concentrations remained within safe limits," Kong said.

"We found that chloride ions present in the water enhanced the process by forming reactive molecular chlorine," Kong explained. "Our work provides a comprehensive assessment of using UV/chlorine as a final barrier against harmful algal bloom events, especially in mitigating toxins while minimizing unintended toxic consequences."