Risk evaluation for pathogenic bacteria and viruses in sewage sludge compost

Composting can be regarded as the most available option for recycling of sewage sludge. However, the existence of pathogenic bacteria and viruses in the compost has been scarcely investigated until now. So there is little information on the infectious risk through agricultural activities or gardening in using the compost. In this study, several kinds of composts were investigated for detection of pathogenic bacteria (Salmonella spp. and Escherichia coli O 157) and enteric viruses. It was concluded from the result that these bacteria and viruses could not be detected in 1.0 g-wet of any kinds of composts. Infectious risks through agricultural activities or gardening were evaluated by Monte Carlo simulation in the case that the compost was polluted by Salmonella spp., E. coli O 157:117 and Poliovirus 1. Criteria satisfying the acceptable risk (less than 10⁻⁴ per year) for these pathogenic bacteria and virus in the compost were determined from the result of simulations. 1.0 [CFU or PFU/g-wet] was available as the criteria for E. coli O157 and Poliovirus 1 in the compost. On the other hand, the criterion for Salmonella spp. in the compost should be established on a lower concentration than 0.001 CFU/g-wet.