10th U6+ Consortium of African Universities International Conference

Anaerobic co-digestion offers an integrated route for renewable energy generation and organic-waste management, but digestion of protein-rich fish offal is frequently constrained by rapid acidification, volatile fatty acid (VFA) accumulation and pH instability. These challenges are particularly relevant in Kenya, where bioenergy remains a major component of final energy demand even as renewable electricity dominates grid generation. Water hyacinth (Eichhornia crassipes), an invasive aquatic weed in Lake Victoria, offers a carbon-rich co-substrate that can improve nutrient balance and divert problematic biomass from aquatic environments. However, co-digestion alone does not necessarily prevent acid accumulation under varying substrate loading and solids conditions. This study proposes to optimize calcium carbonate (CaCO₃) buffering during anaerobic co-digestion of water hyacinth and fish offal using Response Surface Methodology based on a three-factor, three-level Box–Behnken Design. The independent factors will be water hyacinth proportion (20–70%), CaCO₃ dosage (0–10 g/L), and total solids concentration (5–15%). The principal responses will be total VFA concentration, pH stability and methane yield, with biogas yield, methane composition, alkalinity and hydrogen sulfide considered as supporting responses. Feedstocks will first be characterized for moisture, total solids, volatile solids, ash, carbon, nitrogen, C/N ratio, protein, lipid and structural carbohydrates. Batch digestion will be conducted in 500-mL serum-bottle reactors under mesophilic conditions of 37 ± 2 °C for 35 days. The 15-run Box–Behnken matrix will be used to estimate linear, interaction and quadratic effects. Second-order polynomial models will be assessed using ANOVA, coefficient of determination, adjusted and predicted R², lack-of-fit testing and residual diagnostics. Numerical desirability optimization will identify conditions that minimize VFA accumulation, maintain pH within a methanogenic range and maximize methane yield. Confirmation experiments will compare predicted and observed responses. The study is expected to provide a validated process model and practical operating window for stabilizing fish-offal/water-hyacinth co-digestion, thereby contributing to renewable-energy generation, invasive-weed management and circular bioeconomy development in Kenya.