UV Experience for Inactivating Cryptosporidium in Surface Water Plants

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The disinfection of pathogenic microbes in drinking water has been largely successful over the last century due to the use of chlorination. However, research conducted in the 1970's revealed that by-products formed during the chlorination process are potentially carcinogenic and that there is a direct correlation between the concentration of chlorination by-products and the probability of certain cancers and other health problems. Following these discoveries, drinking water regulators have struggled to find a balance between the benefits of chlorination and the harmful side effects caused by chlorination, within the confines of technological and economic limitations.

In the U.S.A., the Surface Water Treatment Rule (SWTR) of 1989 mandates inactivation levels for giardia cysts and enteric viruses, and also sets treatment standards for Trihalomethanes (THMs). The SWTR provides guidance to drinking water facilities through 'CT' tables that prescribe the inactivation efficacy of various processes under varying water quality conditions. By following this guidance, most water treatment plants were able to provide an adequate degree of disinfection while not compromising their Disinfection By-Product (DBP) limits and without requiring major changes to their plants. However, continuing DBP health effect research indicated that even the DBP standards required in the 1989 SWTR produced an unacceptable level of risk and the SWTR was amended in 1996 to further lower DBP standards. In addition, a major outbreak of cryptosporidiosis in Milwaukee in 1993, and other minor cryptosporidiosis outbreaks caused regulators to create a removal requirement for cryptosporidium oocysts in the 1998 Interim Enhanced Surface Water Treatment Rule (IESWTR) and most likely a disinfection requirement in the final ESWTR (LT2ESWTR). The new DBP standards have caused many plants to fall out of compliance, requiring either extensive plant modifications or new disinfection strategies. The LT2ESWTR will include a cryptosporidium disinfection requirement and many surface water plants will fall out of compliance due to the very poor efficacy of chlorination for cryptosporidium. Therefore, due to these apparently conflicting conditions, a void was created for a water treatment technology that is effective for protozoa and viruses, does not create DBPs, and is economically feasible.

UV technologies have long been known to be effective for viruses and bacteria in drinking water and guidelines for the disinfection of viruses exist in the Alternative Disinfectants and Oxidants Guidance Manual. However, UV was widely considered to be ineffective for encysted protozoa as it was thought that the UV light would not penetrate the cyst membrane, and since giardia is the controlling microbe for chlorine dose determinations, no reductions in chlorine usage could be gained by using UV. Therefore, UV Disinfection was not used for surface waters in North America.

New breakthrough research conducted by Calgon Carbon Corporation in 1998 however proved that UV disinfection is, in fact, very effective for inactivating cryptosporidium and giardia at low UV doses. Subsequent to Calgon Carbon's research, the USEPA created a UV subworkgroup to report to the Federal Advisory Committee (FACA) on issues and costs related to UV disinfection. In advance of new guidance manuals for UV disinfection, many utilities have begun to consider UV disinfection in their plants either as an additional barrier for protozoa disinfection or to get 'CT' credits for UV for giardia so that chlorine doses can be lowered to meet the 1998 DBP standards.

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