U Ideas in Science -- March 2001University of Illinois at Urbana-Champaign
Contact: James E. Kloeppel, Physical Sciences Editor (217) 244-1073; [email protected]
WATER QUALITYSequential disinfection process provides safer drinking water
CHAMPAIGN, Ill. -- Fresh from the faucet, a killer may be lurking in your glass.
Cryptosporidium parvum is a parasitic protozoan that can infiltrate a city's water supply -- as happened in Milwaukee in March 1993, when more than 400,000 people were infected. With symptoms similar to food poisoning, outbreaks of cryptosporidiosis can prove deadly for individuals with immune system deficiency problems. Researchers at the University of Illinois are developing a cost-effective treatment strategy for providing drinking water free of this harmful contaminant.
"Most surface-water disinfection systems in the U.S. were originally designed, or subsequently modified, to control contamination by another dangerous microbe, Giardia lamblia," said Benito Marinas, a UI professor of civil and environmental engineering. "Unfortunately, the disinfectant concentration and contact time in these systems are generally inadequate for killing C. parvum."
Destroying the parasite is also complicated by the fact that, outside its host, C. parvum enters a spore-like dormant stage, Marinas said. "Encased in a dense wall of proteins and lipids, this 'oocyst' is extremely resistant to chlorine -- the disinfectant most commonly used in water treatment plants."
Marinas and graduate students Amy Driedger and Jason Rennecker have found that sequentially applying two disinfectants -- such as ozone and chlorine -- is much more effective in killing C. parvum than either treatment alone. The primary disinfection step can result in secondary disinfection rate increases of up to 2,200 percent, compared to the rates for a single disinfectant.
While some water treatment plants already use ozone to kill G. lamblia, they are not designed to kill C. parvum, which requires 25 to 40 times greater ozone exposure. But, by first using ozone -- at levels to kill G. lamblia -- and then following with chlorine, the researchers can easily destroy C. parvum.
"Ozone not only attacks the oocyst wall -- thereby opening the door for the next disinfectant -- it also oxygenates the wall and changes the very nature of the material, making it more susceptible to chlorine," Marinas said. He and his students are currently characterizing the synergistic effects that take place, and optimizing the sequential disinfection process.
Using chlorine as the secondary disinfectant also carries an additional benefit, Marinas said. "Unlike ozone, which decomposes rapidly, chlorine will remain in the distribution system for a long time, offering protection against any subsequent contamination."
Because the sequential disinfection process works most effectively at low temperatures, it offers a potential solution to killing C. parvum oocysts during the wintertime in regions where the water temperature approaches the freezing point, Marinas said.
The researchers published their latest findings in the January issue of Water Research, a journal of the International Water Association. The U.S. Environmental Protection Agency and the Illinois Water Resources Center provided funding for the work.
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