Anaerobic Co-Digestion of Cassava Peel and Pig Manure: Effects of Substrate Ratio and Organic Loading Rate on Methane Production and Digestate Characteristics

Anaerobic co-digestion offers an approach for improving energy recovery from organic wastes while producing a nutrient-containing digestate. This study investigated the effects of cassava peel-to-pig manure ratio and organic loading rate on methane production, substrate degradation, process stability and digestate quality. Cassava peel and pig manure were co-digested at ratios of 100:0, 80:20, 60:40, 50:50, 40:60, 20:80 and 0:100 for 35 days under anaerobic conditions. Biogas production, methane concentration, pH, volatile fatty acids, alkalinity and volatile solids reduction were determined. The best-performing substrate ratio was subsequently evaluated at organic loading rates of 1.0–3.5 g VS/L/day. Methane production varied with substrate composition, with the 60:40 cassava peel:pig manure ratio producing the highest methane yield of 306.8 ± 8.9 mL CH₄/g VS added and methane concentration of 58.7 ± 1.4%. The same treatment achieved the highest volatile solids reduction (62.29%) and maintained a favourable final pH of 6.96 with a VFA/alkalinity ratio of 0.21. Methane yield increased with organic loading up to 2.5 g VS/L/day, where the highest yield was obtained, but declined at higher loading rates. Digestate from the best-performing treatment showed increased pH, nitrogen, phosphorus and potassium, with reductions in organic carbon, total solids and volatile solids. The findings demonstrate that appropriate substrate balancing and organic loading can improve methane recovery from cassava peel and pig manure while producing a potentially nutrient-rich digestate.