|Ask The Expert: Canada: UV & Log Reduction
|Question: We just acquired WC&P's March 2000 issue in which you review various ultraviolet (UV) systems and their effectiveness against Cryptosporidium oocysts (T.M. Hargy, "UV Equipment Proven Against Cryptosporidium," pp.36). Your article has great relevance for the people of Erickson, British Columbia. Here in Canada, we are, as a community, resisting chlorination that's insisted upon by our Medical Health Officer (public health). We're instead trying to have approved a UV system and articles like yours further strengthen our resolve and give us increased data with which to argue our position.
Because I'm not a scientist I don't know what "log" means when quoting success rates for the various technologies. Does it mean that a certain number of cysts are inactivated but not all, so that infection doesn't occur in normal healthy individuals? I ask this because in the same magazine issue Dr. Kelly A. Reynolds in her article on high-risk segments of the population writes that UV isn't sufficient as a treatment method for immunocompromised individuals. Please clarify.
Sam Moore -- Erickson, Canada
Answer: Thank you for your questions. The term "log," as in log removal or inactivation (reduction) refers to an order of magnitude of change. For example, if a given volume of water containing 1,000,000 (one million) organisms is treated so that it now only contains 1,000 organisms, this is a 3-log reduction. If the one million were reduced to 10 (ten), this would be a 5-log reduction. And if it's treated so that only one remains, this is 6-log reduction. If the original water only contains 100 organisms, and is treated to the point where it contains only one, this is 2-log reduction. Another way to state this is: 90% = 1 log 99% = 2 log 99.9 % = 3 log...
The studies discussed in the "UV Equipment Proven" article, as well as more recent ones by other researchers, indicate greater than a 4 log (99.99%) inactivation of Cryptosporidium can be achieved by as little as 10 milliJoules per square centimeter (mJ/cm2)—a unit of measure of UV light exposure. UV systems for drinking water typically apply 16-to-40 mJ/cm2, which would be quite effective.
Dr. Reynolds article included advice from the Centers for Disease Control, regarding dependability provided by various bottled water treatments with respect to Cryptosporidium. This was published in 1997, or just about the time research by Clancy Environmental Consulting was finding UV to be very effective at inactivating the parasite. Hargy's article notes that earlier (pre-1996) research into UV and Cryptosporidium used analytical methods that severely understated UV's effect. Unfortunately, this outdated misinformation is still widely cited. For instance, the recently released Sierra Legal Defense Fund's document, "Canada's Drinking Water Report Card," published in January 2001, states UV isn't effective against Cryptosporidium.
Regulators, however, are moving to accept UV in drinking water systems based on new awareness of its effect on Cryptosporidium and Giardia. See the article —- also by Hargy -— "New California Guidelines Drafted for UV System Installations" (WC&P, October 2000. pp. 82) for a discussion of California's initiative to incorporate UV for drinking water. The USEPA is following suit. In Canada, the Ontario Ministry of Environment also is looking favorably on a plan to install UV as the primary disinfectant at Waterloo's treatment plant, where a Cryptosporidium outbreak occurred in 1992.
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