Comparing the bio-fertilizer quality of co-composted municipal organic wastes of different mix ratio: evidence from Tigray, northern Ethiopia

Purpose: There is an increasing trend of using sewage sludge co-composting to produce bio-fertilizers for use in agriculture. However, existing studies on the quality of the produced compost came out with contradicting results, which hinder its sustainable utilization. Hence, this study aimed at assessing the quality of sludge based compost for agricultural use.Method: Four treatments, T۱ (۷۰% sludge, ۱۵.۳% wheat straw and ۱۴.۷% cattle manure), T۲ (۵۰% sludge, ۱۴.۸% wheat straw and ۳۵.۲% cattle manure), T۳ (۳۰% sludge, ۲۰% wheat straw and ۵۰% cattle manure) and T۴ (۲۰% sludge, ۱۴.۲% wheat straw and ۶۵.۸% cattle manure) having three replications were arranged in a randomized complete design. Maturedcompost samples, composted for ۹۰ days, were analyzed for nutrient contents, pathogens and heavy metals concentration.Results: The nitrogen content of the produced compost was in the range of ۱.۷۴–۲.۰%, which is higher than the recommended ۰.۳%. It was significantly higher in the T۱ treatment, a ۰.۲۴% increase compared to the T۴ treatment. However, the phosphorous (۲۰۲−۲۲۲ ppm) and potassium (۱۸۰۱−۲۳۵۷ ppm) contents were classified into the very low category. Both elements were higher in the T۴ treatment, a ۹.۹% and ۳۱% increase, respectively as compared to that of T۱. Furthermore, all treatments were free of the major disease causing bacteria and characterized by a very low heavy metal concentration.Conclusion: Overall, municipal sludge co-composting has yielded a bio-fertilizer with both pathogen-free and safe levels of heavy metal concentrations. However, further research is required to enhance the poor phosphorous and potassium contents.