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Researchers at Duke's Pratt School of Engineering have developed a new way to measure microbes' exposure to ultraviolet light. The tool could bolster efforts to use UV light to improve the quality and safety of tap water in the United States.
The novel "microsphere dosimeter" technique is the first direct test of how much UV light microorganisms in fluids have been exposed to, said the researchers -- a critical step in validating the use of UV light treatment for preventing the spread of infection through drinking water. The technique was reported in the Nov. 15, 2005, issue of Environmental Science and Technology by Karl Linden and colleagues.
Linden's technique uses fluorescent microspheres, which become bleached with exposure to UV light, to mimic pathogenic microbes in water flowing through a UV reactor. By measuring the bleaching of the microspheres, the researchers can obtain precise measures of the full distribution of UV doses that a pathogen may experience -- information critical for gauging the treatment's capacity to kill disease-causing bacteria or parasites before they reach the public.
The new method comes at a key time as the Environmental Protection Agency (EPA) is set to introduce regulations in December requiring water treatment plants at risk of infection to add UV reactors as an additional line of defense against pathogenic contaminants in the water supply, said Linden, associate professor in Pratt’s Department of Civil and Environmental Engineering.
"The use of UV will certainly lower the public's risk of microbial pollution because it offers a second barrier of defense," Linden said. UV treatment will also reduce the need for chemical disinfection, he added.
In the U.S., chemical treatment with chlorine remains the primary method for disinfecting drinking water, he said. However, chlorine can produce chemical byproducts that have been linked to cancer. Such byproducts are also coming under stricter regulations in the new EPA rules for drinking water, according to Linden.
Chlorine also fails to kill some infectious microbes, such as the protozoan parasite Cryptosporidium. Known as "Crypto," the parasite is a common cause of waterborne disease in the U.S. In 1998, water quality researchers, including Linden, discovered UV is very effective in killing Crypto, a finding that subsequently became a fundamental basis for the new EPA regulations.
"While chlorine attacks the cell membrane, UV attacks organisms by breaking down their genetic material," Linden said. "Combining the two disinfection methods will better protect against known threats like Crypto and against the next 'bug of the month.'
"The microsphere dosimeter tool we've developed is an important step in advancing our understanding of how UV treatment works to disinfect drinking water," he said.
In addition to improvements in the fight against waterborne illness, UV also offers a more environmentally-friendly water treatment method, Linden added. Its implementation will allow a reduction in chlorine use, which will lower the concentration of chemical byproducts found in the water supply and help to meet the new byproduct standards.
Similar to the more familiar Giardia parasite, which often infects hikers who drink untreated water, Crypto can be transmitted through ingestion of drinking water, person-to-person contact or other exposure routes, according to Linden. Symptoms of the condition include acute diarrhea, abdominal pain, vomiting and fever that can last up to two weeks in healthy adults, but may be chronic or fatal in those with a compromised immune system. In 1993, a massive outbreak of Crypto transmitted through the public water supply infected an estimated 400,000 people in Milwaukee and led to more than 100 deaths, he added.
In the last two decades, Crypto has become recognized as one of the most common causes of waterborne disease among people in the United States, according to the Centers for Disease Control and Prevention. The parasite may be found in drinking water and recreational water worldwide.
The use of fluorescent microspheres to measure the UV dose distribution will augment earlier devised methods for characterizing UV systems, thereby raising confidence in the reactors' ability to combat health threats in the water supply, Linden said. Traditional methods simply test disinfection by spiking a water sample with bacteria or other surrogate microbes and measuring the number present after treatment to infer the delivered UV dose. That method provides only the average exposure when, in reality, microorganisms are exposed to a range of UV energy, depending upon the path they take in traveling through the UV system, he said.
"Many microbes display nonlinear responses to UV levels, which can make the average exposure a poor predictor for the actual efficacy of the reactor against different pathogens," Linden said. Microbes also differ in the level of UV required to successfully kill them, he added.
The new findings offer many fundamental and practical advances for UV reactor evaluation and testing, according to Linden. The tool also will help validate and improve the accuracy of mathematical models for estimating the efficiency of water treatment with UV.