Comparison between thermophilic-mesophilic two-phase and thermophilic single-phase process in anaerobic digestion of rice straw: Biomethane production, energy balance and microbial structure

To fill the research gap in two-phase anaerobic digestion (TPAD) of rice straw, this study assessed the performance of a thermophilic–mesophilic TPAD system over 180 days in terms of methane production, energy balance, and microbial community structure and compared it with a thermophilic single-phase anaerobic digestion (SPAD) system. Results showed that SPAD achieved a methane yield of 170 ± 14 mL CH4/g-VS, with VS and cellulose removal efficiencies of 39.8% and 59.5%, respectively. In contrast, TPAD produced 101 ± 8 mL CH4/g-VS and achieved 36.9% and 49.1% removal of VS and cellulose. The lower CH4 conversion in TPAD might be caused by the mismatch between the recalcitrant structure of rice straw and conventional temperature configurations in two-phase process. The economic assessment suggested that although TPAD reduced the net energy deficit by 56% compared with SPAD, both configurations operated with negative net energy balance. Microbial analysis further revealed that community structure and functional characteristics were strongly shaped by process configuration. This study will provide some guidance for future optimization of lignocellulosic biomass valorization.