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Log-reduction in Water Treatment

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The log-reduction terminology was developed by engineers as a way to express levels of decreased biological contamination in water by factors of 10 that could be easily converted to percent reduction. The most commonly used logarithmic base is 10 because it is compatible with our base-10 decimal system. The log of 10 in the base 10 logarithmic system is 1 and the log of 100 is 2, with the log of 1000 being 3, etc.

Log reduction relates to the percentage of microorganisms physically removed or inactivated by a given process.

1-log reduction = 90%
2-log reduction = 99%
3-log reduction = 99.9%
4-log reduction = 99.99%


How is this rule applied for public health? Short of being an engineer, there are some very practical reasons for such a complicated contaminant removal system. The EPA has determined that the presence of microbiological pathogens in public water supplies is a health concern. If finished water supplies contain microbiological contaminants, illnesses and disease outbreaks may result. Twelve waterborne cryptosporidiosis outbreaks caused by contamination in public water systems were reported to the Center for Disease Control and Prevention between 1984 and 1998. In 1993, Cryptosporidium caused more than 400,000 people in Milwaukee, WI, to experience intestinal illness. More than 4,000 were hospitalized and at least 50 deaths were attributed to this cryptosporidiosis outbreak. Other recent cryptosporidiosis outbreaks attributable to public water system contamination occurred in Nevada, Oregon, and Georgia. The Interim Enhanced Surface Water Treatment Rule (IESWTR) set enforceable drinking water treatment technique requirements to reduce the risk of Cryptosporidium from surface water for systems serving at least 10,000 persons.

The LT1ESWTR extends further this necessary protection from Cryptosporidium to communities of fewer than 10,000 persons. Today's rule for the first time establishes Cryptosporidium control requirements for systems serving less than 10,000 persons by requiring a minimum 2-log removal for Cryptosporidium. The rule also strengthens filter performance requirements to ensure 2-log Cryptosporidium removal, establishes individual filter monitoring to minimize poor performance in individual units, includes Cryptosporidium in the definition of GWUDI, and explicitly considers unfiltered system watershed control provisions. The rule also reflects a commitment to the importance of maintaining existing levels of microbial protection in public water systems as plants take steps to comply with newly applicable DBP standards.

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