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Research Findings on Heat Treatment of Pathogens and Indicator Organisms
Citation:
MECKES, M. C. Research Findings on Heat Treatment of Pathogens and Indicator Organisms. Presented at WEFTEC 2011 Workshop 208: WEF/WERF Better Safe Than Smelly: An Action Plan to Remedy Bacterial Regrowth, Odors, and Sudden Increase (ROSI) in Biosolids , Los Angeles, CA, October 16, 2011.
Impact/Purpose:
To inform the public.
Description:
Heat or thermal treatment has been used for many years for reducing the densities of pathogens in food and beverages. Its application for disinfection of municipal sludges has occurred only recently. One method for achieving Class A biosolids is to thermally treat sludges either before or during the digestion process. The duration of the thermal treatment has been specified by the U.S. Environmental Protection Agency (EPA), for different sludge types/thicknesses, and is dependent on the temperature that is used. This time-temperature requirement would ideally produce biosolids that are free of pathogens and would meet the fecal coliform (FC) requirement of less than 1000 MPN/g Dry Solids (DS). Over the last several years, utilities and researchers have reported that significant increases in indicator bacteria could occur immediately after dewatering of digested biosolids. This “sudden increase” or SI (sometimes greater than 4 orders of magnitude) was followed by additional increases in density when the cake was stored. The increase during storage has been termed “regrowth”. The effects of heat treatment on pure cultures of fecal coliform organisms and specific pathogens were evaluated in our laboratory. Interestingly, we found that following limited heat treatment at 55°C, these organisms could recover. These results were compared to work conducted by others who identified heat shock proteins in cells which survived similar heat treatments. Here we discuss these findings and discuss the possibility of how heat shock proteins may be responsible for the observed sudden increase following heat treatment of sludge.