Biological aerosols generated from the land application of biosolids: Microbial risk assessment

Abstract

In the United States greater than 6 million dry tons of biosolids are produced nationwide, with greater than 60% being land applied. Although most counties utilizing land application are practicing this beyond nearby homes, the increase in population has begun to blur the line between rural and urban communities. This study was conducted to investigate the occurrence of biological aerosols (bioaerosols) containing microorganisms and endotoxins, and assess the human health risk involved in these practices. Aerosol samples were collected for 2 years from land application sites located at various locations throughout the U.S.A., which represented different climatic conditions and different application practices. Land application practices involved the use of liquid biosolids spray and "cake" biosolids applicators depending on location. Bioaerosols were collected via the use of six SKC Biosamplers, impinging air at a rate of 12.5 L/min for a total of 20 minutes. Samples were collected from both downwind of land application and background sites from distances ranging between 2 m and 70 m downwind. Microbial concentrations were measured within these aerosols, measurements included: heterotrophic plate count bacteria (HPC), coliphage, Clostridium perfringens, total coliforms, Escherichia coli, endotoxin (lipopolysaccharide), enteroviruses, norovirus, and Hepatitis A virus (HAV). In addition a model was developed to predict viral transport. Overall the levels of aerosolized indicator bacteria and phage were at or below detection limits. Three samples were positive for the presence of norovirus viral RNA via reverse transcriptase polymerase chain reaction, although their viability was unable to be determined based on current available techniques. Calculated microbial risks of infection were determined to be at or below the acceptable risk of annual infection from drinking water proposed by the Environmental Protection Agency, 1:10,000. Biosolids loading scenarios presented the greatest risk of infection, partly due to the point source of exposure. All other portions of biosolids land application operations yielded risks of infection well below the annual 1:10,000 risk of infection. Overall the microbial aerosol exposures brought about by land applied biosolids are minimal and hence minimal overall risks of infection.