Influence of composting and thermal processing on the survival of microbial pathogens and nutritional status of Nigeria sewage sludge

Purpose Sewage sludge samples from a water treatmentplant in Nigeria were subjected to an in-vessel composting(using sawdust as a bulking agent) and thermal sludge processingto improve its quality for agricultural applications.Methods Treated samples were analyzed for physicochemicaland microbiological properties using standardanalytical and aerobic culture protocols.Results Microbiological analysis of the initial fresh mixture(sewage sludge/sawdust) showed that the total heterotrophicbacteria was 1.17 9 106 CFU/g of fresh compost, coliforms4.7 9 104 CFU/g, Salmonella sp., and Shigella sp.7.3 9 104 CFU/g, yeasts and moulds 9.0 9 104 CFU/g.These values were significantly (p = 0.05) reduced after40 days of in-vessel composting to 4.3 9 104 CFU/g for totalheterotrophic bacteria, 7.4 9 102 CFU/g for coliforms, whileyeasts and moulds, Salmonella and Shigella sp. were notdetected in the final compost. The results of thephysicochemistry revealed variation in pH, temperature, andnutrients status of treated sludge.Conclusion Salmonella sp., Staphylococcus aureus, andShigella sp. were eliminated, while a 2-log reduction incoliform counts occurred after 40 days of composting.Composting had a better processing impact by increasingthe ash as well as reducing the carbon/nitrogen ratio oftreated sludge, while thermal processing improved thesulfate and phosphate components of treated sludge. Thetreated sludge (biosolids) met the permissible limits ofmicrobiological and nutritional standards recommended byUS EPA for land application of sludge and could, therefore,be used as a biofertilizer, soil conditioner and also forland reclamation.