Evaluation of chemical, thermal and thermo-chemical pretreatment effects on lignocellulose in cattle manure for enhanced biogas production

The increasing demand for sustainable energy has highlighted the importance of utilizing renewable resources like cattle manure for biogas production. However, the recalcitrance of lignocellulosic biomass primarily composed of lignin, cellulose, and hemicellulose, limits its biodegradability and methane yield during anaerobic digestion (AD). This study explored chemical, thermal, and thermochemical pretreatment methods to enhance substrate digestibility and methane production. Chemical pretreatment involved using NaOH at pH levels of 8, 10, and 12 for contact times of 6, 15, and 24 hours. In batch systems with a substrate-to-inoculum ratio of 2:1 at 35°C, optimal results were observed at pH 12 for 15 hours, yielding 149 mL-CH4/gCODadded and achieving a biodegradability of 43%. In fed-batch systems with an HRT of 20 days and OLR of 3.75 gCOD/L-d, maximum biodegradability reached 35%, 40%, and 37% for pH 8, 10, and 12, respectively. At pH 10 and 12, lignin, cellulose, and hemicellulose removal ranged from 12%-13%, 55%-58%, and 55%-58%, respectively, and were notably higher than at pH 8. Thermal pretreatment was conducted at temperatures of 50°C, 70°C, and 90°C for contact times of 6, 15, and 24 hours. In batch systems, methane yields of 161 mL-CH4/gCODadded and biodegradability of 46% were achieved at 70°C and 90°C for 24 hours. In fed-batch systems under the same conditions, biodegradability reached 34%, 36%, and 37%, respectively, with the highest lignocellulosic removal observed at 90°C: 21% lignin, 63% cellulose, and 56% hemicellulose. Thermochemical pretreatment combined NaOH (pH 8, 10, and 12) with temperatures of 50°C, 70°C, and 90°C for contact times of 6, 15, and 24 hours. The optimal conditions were pH 12, 90°C, and 24 hours, which resulted in a maximum biodegradability of 51% and a methane yield of 180 mL-CH4/gCODadded. In conclusion, this study demonstrated that chemical, thermal, and thermochemical pretreatments improve the anaerobic digestion of cattle manure by enhancing lignocellulosic degradation. Among the methods tested, thermochemical pretreatment proved to be the most effective in maximizing methane production.