Earthworm activity reduces bacterial pathogen loads in sewage sludge

Wastewater processing plants (WWTPs) produce large amounts of sewage sludge that are mainly disposed of on the land. This represents a health hazard due to the high loads of human bacterial pathogens (HBPs) the sludge generally contains. Vermicomposting, a mesophilic process, can reduce HBPs in organic wastes. This raises the question as to how earthworms eliminate HBPs. We aimed to determine whether earthworms reduce the levels or eliminate HBPs from sewage sludge and to establish whether the reduction/elimination occur during active (earthworm casts) or maturation (vermicompost) stages of vermicomposting. To this end, we analyzed HBPs (Escherichia coli O157, Listeria monocytogenes, Salmonella spp., and total E. coli) in sewage sludge samples from three WWTPs and the fresh earthworm casts and vermicomposts by using qPCR, to assess the impact of earthworms on reducing and/or eliminating HBPs. We did not detect either Salmonella spp. nor E. coli O157 in any of the samples. Earthworms removed or significantly reduced the amounts of E. coli and L. monocytogenes (mean relative abundances of 2.4 × 10^-7 and 1.5 × 10^-8 respectively) in the sewage sludge. Thus, the abundance of these HBPs was lower (or not present) in casts than in sewage sludge. This was also observed in vermicomposts (95% and 43% reduction in E. coli abundance in casts and vermicompost, respectively). Finally, our findings indicate that vermicompost derived from sewage sludge meets the requirements of organic fertilizers at least regarding the microbial pathogen content. Thus, vermicomposting seems to be a promising technique for managing municipal waste.