Microbial Risk Assessment of Mature Compost from Human Excreta, Cattle Manure, Organic Waste, and Biochar

Abstract

Lack of sanitation is the underlying cause of many diarrheal infections and associated deaths. Improving sanitation through the set-up of ecological sanitation dry toilets, followed by the thermophilic composting of human excreta, could offer a solution. In addition, treating the excreta via thermophilic composting allows us to recycle the nutrients to be used as fertilizer for agriculture. However, for this purpose, the compost should be free of pathogens. We conducted a thermophilic composting trial over 204 to 256 days with human excreta, along with vegetable scraps and teff straw, with and without biochar. A sawdust–cattle manure mixture with the same supplements served as a control treatment. To evaluate the hygienic quality of the mature compost, the bacterial indicators Escherichia coli and Salmonella were assessed using the cultivation-based most probable number method. In addition, Ascaris lumbricoides eggs were quantified through light microscopy. The amount of detected E. coli was below the thresholds of German and European regulations for organic fertilizer. Salmonella and Ascaris eggs were not detected. No significant differences between the treatments were observed. Thus, the composting process was efficient in decreasing the number of potential human pathogens. The mature compost fulfilled the legal regulations on organic fertilizer regarding potential human pathogens.