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Title: Estimated occupational risk from bioaerosols generated during land application of Class B biosolids

item Brooks, John

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2008
Publication Date: 11/1/2008
Citation: Tanner, B.D., Brooks, J.P., Gerba, C.P., Haas, C.N., Josephson, K.L., Pepper, I.L. 2008. Estimated occupational risk from bioaerosols generated during land application of Class B biosolids. Journal of Environmental Quality. 37:2311-2321.

Interpretive Summary: Biosolids land application has been under national scrutiny due to the potential off-site transport of disease-causing microorganisms via soil, water, or air. Biosolids are the solid by-product of sewage wastewater treatment, and those who work closely with it are at a greater potential risk of infection than those not associated with the operation. Despite regular hygienic practices, such as hand-washing, these workers could still be at risk of exposure to these microorganisms via aerosol generation. Aerosols are generated throughout the land application process, and can be comprised of biosolids, soil, liquid, or plant material. The purpose of this research was to quantify the risk of infection associated with aerosols during land application of biosolids. Air samples were collected downwind of biosolids operations throughout the nation and were analyzed for various sewage-associated microorganisms. It was determined that the majority of these microorganisms were aerosolized during the loading stage of the operation, often involving a front-end loader. During this operation, biosolids are loaded into a manure-spreader wagon and it was estimated that the worker was at his/her most risk of infection from aerosolized biosolids-borne bacteria. The risk assessment calculations were made using a wide-range of assumptions, allowing for conservative risk assessment. Overall the amount of exposure to the occupationally exposed was less than that of wastewater treatment workers, however quantitative microbial risk analyses demonstrated that risk of infection would realistically range from less than 1% to 2% per year.

Technical Abstract: It has been speculated that bioaerosols generated during land application of biosolids pose a serious occupational risk, but few scientific studies have been performed to assess levels of aerosolization of microorganisms from biosolids and to estimate the occupational risks of infection. This study investigated levels of microorganisms in air downwind of land application of biosolids and used the data to estimate the occupational risk of infection. HPC bacteria were ubiquitous in air near land application sites whether or not biosolids were being applied. Coliform bacteria were detected in 22% of air samples (68 of 309) at concentrations ranging from 1.5 to 6,000 colony-forming units per cubic meter of air (cfu/m3) and were only detected when biosolids were being applied to land or loaded into land applicators. Coliphages were detected in 2% of air samples (7 of 309), at concentrations ranging from 26 to 270 plaque-forming units per cubic meter of air (pfu/m3) and were only detected when biosolids were being loaded into land applicators. Occupational risks of infection and illness from aerosolized Salmonella and enteroviruses were estimated for a variety of land application scenarios. Realistic exposure scenarios carried estimated occupational risks of Salmonella infection ranging from 0.0001% to 0.013% per year and estimated occupational risks of enterovirus infection ranging from 0.78% to 2.1% per year, depending on whether the worker was applying biosolids to land or loading them into the applicator. The occupational risk of infection from land application of biosolids is probably about 0.78% to 2.1% per year (with the most conservative exposure scenario resulting in an annual risk of infection of 34%).