Sustainable Valorization of Cattle Manure: Efficacy and Trade-Offs in Post-Digestion Strategies

This study evaluated thermal and thermo-alkaline post-treatment of digested cattle manure (DCM) as a strategy to increase methane recovery and improve the flexibility of biogas systems within hybrid renewable energy alternatives. A 10 L mesophilic CSTR was operated for 311 days, producing lignin-rich digestate that was subjected to a statistically designed range of post-treatment conditions varying temperature (50–90 °C), pH (8–12), and contact time (6–24 h). Biomethane potential assays and lignocellulosic fractionation were used to determine changes in solubilization, biodegradability, and methane production kinetics. Thermal treatment provided modest improvements, reaching 84 mg SCOD g−1 PCOD solubilization and a 26 mL CH4 g−1 COD increase in methane yield. Thermo-alkaline treatment produced substantially higher enhancements, with the most severe condition (90 °C-pH 12–24 h) achieving 493 mg SCOD g−1 PCOD solubilization, 66% removal of structural carbohydrates, and a 60.2 mL CH4 g−1 COD increase in methane yield, corresponding to a 16% rise in biodegradability and a twofold increase in methane production rate. Gompertz modeling indicated accelerated kinetics and minimal lag time. A strong linear correlation (R2 = 0.90) between severity index and solubilization supported predictable scalability. These results demonstrate that thermo-alkaline hydrolysis can significantly enhance post-digestion methane recovery and strengthen the role of agricultural biogas in integrated renewable energy systems. The techno-economic analysis revealed that, despite higher operating costs for thermo-alkaline post-treatment than for the control, the main drivers are chemical costs and the price of renewable energy, and thus the application of post-treatment as a sustainable solution for animal manure treatment will likely improve as renewable energy prices increase in the future.