Development of a Qualitative Pathogen Risk Assessment Methodology for Municipal Sludge Landfilling
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The report addresses potential risks from microbiological pathogens present in municipal sludge disposed of in landfills. Municipal sludges contain a wide variety of bacteria, viruses, protozoa, helminths and fungi. Survival characteristics of pathogens are critical factors in assessing the risks associated with potential transport of microorganisms from the sludge-soil matrix to the groundwater environment of landfills. Various models are discussed for predicting microbial die-off. The order of persistence in the environment from longest to shortest survival time appears to be helminth eggs > viruses > bacteria > protozoan cysts. Whether or not a pathogen reaches groundwater and is transported to drinking-water wells depends on a number of factors, including initial concentration of the pathogen, survival of the pathogen, number of pathogens that reach the sludge-soil interface, degree of removal through the unsaturated and saturated soil zones, and the hydraulic gradient. The degree to which each of these factors will influence the probability of pathogens entering groundwater cannot be determined precisely. Information on the fate of pathogens at existing landfills is sorely lacking. Additional laboratory and field studies are needed to determine the degree of pathogen leaching, survival and transport in groundwater in order to estimate potential risks from pathogens at sludge landfills with reasonable validity.