Optimization of Biogas Production from Anaerobic Co-Digestion of Poultry Manure and Cocoa Pod Husks: Effect of Carbon-to-Nitrogen Ratio and Temperature Regime

Abstract

The growing challenges of organic waste accumulation, energy insecurity, and environmental degradation in West Africa underscore the need for sustainable waste-to-energy solutions. Poultry manure (PM) and cocoa pod husks (CPH), two abundantly available agricultural residues in the region, exhibit complementary characteristics for biogas production: high nitrogen and buffering capacity in PM, and high carbon and energy potential in CPH. This study investigated the optimization of biogas production from the co-digestion of poultry manure (PM) and cocoa pod husks (CPH), with a particular focus on the effect of carbon-to-nitrogen (C/N) ratio and digestion temperature on the biogas yield. Substrate characterization was performed to determine physicochemical and nutritional properties. Batch anaerobic digestion assays were conducted under mesophilic (37 °C) and thermophilic (55 °C) conditions with C/N ratios of 20, 25, 30, and 35 using the ANKOM gas production system over 30 days. Cellulose was included as a positive control to confirm inoculum microbial activity and system reliability. Biogas yields were monitored daily, and the cumulative production was statistically analysed using two-way ANOVA followed by Tukey’s HSD test at a 95% confidence level. As a result, CPH displayed higher volatile solids (84.06%) and energy content (HHV: 19,987 kJ/kg) but had a high C/N ratio (51.16) and lignin content (23.71%), limiting its biodegradability. PM, though rich in nutrients (e.g., N, P, Ca, Mg) and alkalinity (pH 8.70), exhibited high ash content (25.84%) and low C/N ratio (16.07), raising the risk of ammonia inhibition. The mono-digestion trials yielded 244.65 ± 8.48 mL/g VS for CPH and 210.73 ± 27.12 mL/g VS for PM under mesophilic conditions (37°C). Co-digestion significantly improved biogas yields, with the highest cumulative yield (348.65 ± 10.44 mL/g VS) recorded at C/N 25 under mesophilic conditions and contributing to 65.45% improvement over PM and 42.51% improvement over CPH. Statistical analysis confirmed a significant effect of C/N ratio (p = 0.001), as well as its interaction with temperature (p = 0.031) on biogas yield, while temperature alone had no significant impact (p = 0.259). Thermophilic digestion improved performance for carbon-rich conditions (C/N 30 and 35), while mesophilic conditions provided greater stability at lower C/N ratios (C/N 20 and 25). The co-digestion of PM and CPH demonstrates strong synergistic effects, yielding higher and more stable biogas production than the mono-digestion. Optimal performance was then achieved at a C/N ratio of 25 under mesophilic conditions, demonstrating that proper adjustment of substrate mixing ratios and digestion temperature can substantially improve the efficiency of biogas systems. This study contributes to knowledge on the anaerobic co-digestion of nitrogen-rich and lignocellulosic residues in West Africa, especially in Togo, providing experimental evidence to support waste-to-energy strategies, climate change mitigation, and sustainable agricultural development.