News Release

MADISON, WI, OCTOBER 27, 2008–Class B biosolids are sewage sludges that have been treatedto contain fewer than 2.0 x 10⁶ fecal coliforms/dry gram. TheUSEPA estimates that 6.3 million tonnes of Class B biosolidsare generated in the United States each year, and that by 2010,the amount generated per year will increase to 7.4 million tonnes.Biosolids produced during municipal sewage treatment are mostcommonly applied to land as a fertilizer at agricultural sitesthroughout the United States. Class B biosolids,which are the principal type of biosolids applied to land, containa variety of enteric pathogens.

Land application of biosolids has received national attention due to the potential for off-site transport of disease-causing microorganisms through soil, water, and air. Workers face greater exposure to bioaerosols from biosolids than those not associated with the operation. A new study published in the November–December issue of Journal of Environmental Quality investigated levels of microorganismsin air immediately downwind of land application operations andestimated occupational risks from aerosolized microorganisms.

The authors report that risks of aerosol-borne infection for biosolids workers are generally low, at less than 1 or 2% per year. Overall, occupational exposure to bioaerosols from biosolids appears to be less risky than similar exposures among wastewater treatment workers.

In all, more than 300 air samples were collected downwind ofbiosolids application sites at various locations within theUnited States. Coliform bacteria, coliphages, and heterotrophicplate count (HPC) bacteria were enumerated from air and biosolidsat each site. Concentrations of coliforms relative to Salmonellaand concentrations of coliphage relative to enteroviruses inbiosolids were used, in conjunction with levels of coliformsand coliphages measured in air during this study, to estimateexposure to Salmonella and enteroviruses in air. The HPC bacteriawere ubiquitous in air near land application sites whether ornot biosolids were being applied, and concentrations were positivelycorrelated to windspeed. Coliform bacteria were detected onlywhen biosolids were being applied to land or loaded into landapplicators. Risksfrom aerosolized microorganisms at biosolids land applicationsites appear to be lower than those at wastewater treatmentplants, based on previously reported literature.