Anaerobic digestion of Gliricidia sepium inoculated with pig dung using a portable bio-digester for process optimization

• Physicochemical and spectroscopic analysis shows that alkaline modification improves substrate pore structure and surface properties, enhancing hydrothermal pretreatment effectiveness. • ANOVA analysis shows mixing ratio and hydraulic retention duration impact biogas production. The 1:1.55 ratio achieved peak biogas yield (0.0764 m 3 /kg) for Gliricidia sepium and pig dung co-digestion. • The following model was derived from the Response Surface Methodology; CBY G. sepium (M 3 ) = + 0.101517 + 0.011245A - 0.0076625B + 0.00302576C - 0.00548597A 2 - 0.0384239B 2 • The high coefficient of determination (R² = 0.9928) and additional statistical parameters demonstrate that the model provides a robust and reliable representation of the anaerobic digestion process for biogas production using pig dung and Gliricidia sepium substrate. Energy is essential for development, and sustainable energy systems are required for long-term development. Anaerobic digestion (AD) provides a sustainable approach to managing organic waste and supplying energy. This study focus on the optimization of anaerobic digestion of Gliricidia sepium with pig dung using response surface methodology (RSM). Central Composite Design was used for experimental design to generate the best process level and predict the optimal process. The three variable used were: mixing ratios of substrate/inoculum (S/I) ratios (1:1, 1:1.55, and 1:3.5), hydraulic retention time (HRT) (20 to 30 days), and temperature) using 25 Litre-biogas plastic digesters. Substrate characterization was performed using standard methods to assess physico-chemical and microbial properties. The model's viability was statistically tested using ANOVA to identify significant differences. The hemicellulose concentration reduced from 9 to 3 after the thermo-alkaline pretreatment. T.alkalinity, T.nitrogen, T.phosphate, T.carbon, Iron, zinc, aluminum, copper, BOD, COD, as well as potassium, sulphate, calcium, magnesium, total solids, volatile solids and manganese all showed an increase after pre-treatment. The difference between Predicted R² (0.8966) and Adjusted R² (0.9848), the model shows strong correlation and agreement with experimental data. The model's significance is confirmed by a p-value of 0.0001. This indicates a high degree of correlation between experimental data and the model. Co-digesting Gliricidia sepium and pig manure yields 0.0764 m 3 /kg with 58.26% methane content, suitable for small-scale applications.