Survival of pathogenic and indicator bacteria in biosolids

Abstract. Concentrations of surviving Escherichia coli, Clostridium perfringens, and Salmonella spp. were determined temporally in mechanically dewatered biosolids derived from anaerobic–mesophilic digestion and applied to agricultural land. Following applications in different seasons, repeated assessments of bacterial concentrations in biosolid clumps, using most-probable-number (MPN) techniques, found sustained high levels of these bacteria. Bacterial concentrations were often well above soil background levels at 6 months, and in some cases 11–12 months, after land application. Survival in surface-applied biosolids was similar to that for biosolids incorporated into the soil, and between application rates of 10 or 30 dry t/ha. Salmonella concentrations in applied biosolids were not predicted from, and could exceed those of, the indicator organism E. coli. Multiple plot analyses indicated regrowth of E. coli and Salmonella can occur within biosolids, up to several months after application. However, Salmonella serovars likely to pose a significant risk to animal health were not detected among isolates from the dewatered biosolids. Reduced accessibility for grazing livestock by soil incorporation, together with the time taken for normal pasture establishment practices, and the limited pathogenicity of the vast majority of salmonellae present in biosolids may significantly reduce the risk of spread of these organisms to the human food chain