Refuting UV drawbacks

Dear Editor:
Your FAQ section is excellent! I have a question that I can’t seem to find an answer to elsewhere. UV lights are becoming popular in this area but many people are unaware of their drawbacks. I’m aware UV is ineffective against protozoa (Cryptosporidium and Giardia) without prior filtration. My first question is about the type of filter, i.e., down to what size particles must be removed—4 microns? UV also has difficulty with turbidity and iron/manganese. What level of turbidity? Surely a filter capable of removing protozoa would suffice. What level of iron/manganese? Is hardness a problem?

Jim Hayward
Nova Scotia, Canada

Editor’s note: Your perception of UV’s ineffectiveness against Cryptosporidium and Giardia is understandable. It’s been the prevalent thinking of water professionals and microbiologists until only recently. In the last four to five years, however, research using better test methods demonstrates UV is in fact very effective against these protozoa, even more so than against viruses and most bacteria. In the latest research, application of 10 mJ/cm2 (10,000 µWsec/cm2) will achieve at least 3 log (99.9%) inactivation of Cryptosporidium (Clancy, et al., Journal AWWA, September 2000, p. 97-104), and even greater inactivation of Giardia (Shin, et al., Proceedings of the AWWA Water Quality Technical Conference, November 2000). Another discussion of this can be found in an article by Tom Hargy in WC&P (March 2000, p. 36-38). The Clancy and Hargy articles include evidence of effective UV treatment of Cryptosporidium in waters with high particle content (recycle backwash water and water park water). Factors you mention that affect UV are of two types. Turbidity acts as a direct shield to target organisms in the water. The other parameters over time cause fouling or scaling of the lamp’s protective sleeves, reducing the amount of irradiation applied to the water. Turbidity in drinking water has been shown to have little effect on virus inactivation by UV at levels below 3 NTU (see Jim Malley’s slide presentations at the International UV Association’s website, www.IUVA.org). Fouling/scaling parameters need to be evaluated on a site-specific basis, using a pilot unit to determine the extent of problems. UV systems with manual or automatic wiper systems are available to deal with fouling/scaling of lamp sleeves.

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