Evaluation of Market Waste as a co-substrate for enhanced biogas production: Case of Kasubi C/U Primary School

Abstract

Urban sanitation in sub-Saharan Africa remains a critical public health challenge, with large volumes of faecal sludge generated daily in densely populated cities presenting both a management burden and an untapped resource for renewable energy recovery. Bio-latrine systems, which treat faecal sludge anaerobically to produce biogas, have emerged as a promising technology for simultaneously addressing sanitation and energy needs in institutional settings. However, mono-digestion of faecal sludge is often constrained by its low carbon-to-nitrogen ratio, resulting in poor biogas yields that undermine the economic viability of these systems. Co-digestion with carbon-rich organic substrates offers a practical solution to this limitation, and the large quantities of biodegradable market waste generated in urban areas present a locally available and largely untapped co-substrate opportunity. This study therefore evaluated the potential of jackfruit peel waste from Kasubi Market as a co-substrate for enhancing biogas production from faecal sludge, using the institutional bio-latrine system at Kasubi Church of Uganda Primary School in Kampala, Uganda as the case study. A market waste composition study conducted at Kasubi Market established that jackfruit peel waste constituted the largest fraction of the waste stream at 36.65% of total waste generated, with an estimated daily availability of 1,900–2,160 kg, approximately five to six times the quantity required to sustain the digester. Physicochemical characterisation revealed complementary substrate properties: faecal sludge had a C/N ratio of 12.1:1, well below the recommended range of 20–30:1, while jackfruit peel waste recorded a high C/N ratio of 48.8:1, with volatile solids contents of 78.65% and 95.75% respectively, confirming their suitability as anaerobic digestion feedstocks. Biochemical methane potential (BMP) experiments were conducted at five jackfruit peel waste to faecal sludge mixing ratios under mesophilic conditions (37°C). Biogas yield data were fitted to a quadratic mixture model using Design-Expert software, which was statistically significant (F = 7.61, p = 0.0304, R² = 0.7526). The 50:50 jackfruit peel waste to faecal sludge ratio achieved the highest experimental biogas potential of 517.51 mL/g VS, with a maximum production rate of 58.53 mL/g VS/day among the co-digestion ratios. Numerical optimisation using the desirability function approach identified the optimal mixing ratio as 38.4% jackfruit peel waste and 61.6% faecal sludge, with a predicted biogas yield of 586.39 mL/g VS at a desirability value of 0.971. Process stability monitoring confirmed that all co-digestion ratios maintained VFA/alkalinity ratios well below the 0.4 threshold throughout the experiment, while mono-digestion of jackfruit peel waste alone resulted in severe acidification and process failure. These findings were derived from batch experiments and would require validation under continuous feeding and pilot-scale conditions before full implementation. The study confirms that co-digestion of faecal sludge with jackfruit peel waste from Kasubi Market is a technically viable, cost-effective, and locally appropriate strategy for improving biogas recovery in institutional bio-latrine systems, addressing both the school's energy needs and the market's waste management challenge simultaneously.