Encouraged by a finding that the pathogen analyses of numerous samples of the final product of the Metropolitan Water Reclamation District of Greater Chicago, Illinois (District), met the U.S. Environmental Protection Agency (U.S. EPA) Class A criteria, the District optimized and codified the operation of its sludge processing trains (SPTs), submitted a petition to the U.S. EPA Pathogen Equivalency Committee for obtaining certification of its SPTs as equivalent to a process to further reduce pathogens, and conducted a 3year full-scale study. The objective of the study was to determine whether or not the District's SPTs consistently produced a Class A biosolids final product. The primary conclusion drawn from this optimized and codified operation study was that all batches of the final air-dried product complied with the Class A criteria as specified by U.S. EPA in the Part 503 regulations. Water Environ. Res., 72, 413 (2000).

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The literature on microbial emissions from wastewater treatment plants is reviewed and a major study of microbial aerosol emissions from such a plant is described and discussed. The literature was found to be repleted with studies which showed that microbial aerosols are emitted from wastewater treatment plants but rapid die-away was shown once the aerosols reach the ambient air. Epidemiological studies of wastewater treatment plant workers and of communities nearby such plants gave overwhelming evidence to show that wastewater treatment plants do not affect the health of the local populace nor the plant workers themselves. The paper describes a major study of the microbial aerosol emissions of a waste-water treatment plant operated by the Metropolitan Sanitary District of Greater Chicago. In this study, it was found that aerosols containing microorganisms were emitted from the aeration tank surface at rates of 208 to 386 standard plate counts per cubic meter of air at a distance of 0.3 m above the surface. However, it was found that there was rapid die-away of microbial emission with distance from the aeration tanks both horizontally and vertically. It was concluded, based upon the literature review and the major study described, that microbial aerosol emissions from wastewater treatment plants were not a significant factor in the health of the surrounding populace.

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