Integration of hydrothermal pretreatment and anaerobic co-digestion strategies to improve the biomethane yield from rice straw and waste algal biomass

The lignocellulosic complexity and high carbon-to-nitrogen (C/N) ratio of rice straw (RS) hinder its anaerobic digestion (AD). This study explored integrating hydrothermal pretreatment and anaerobic co-digestion (AcoD) to address these challenges and enhance biogas and biomethane production. Hydrothermal pretreatment (200 °C, 20 % total solids, 10 min) was applied to RS to disrupt its structure. AcoD of untreated and pretreated RS (RS200) with algal biomass (AB) at C/N ratios of 20–35 was then conducted to correct nutrient imbalance and regulate microbial activity. AcoD of untreated RS with AB improved biogas and biomethane yields by 23.82–89.01 % and 32.29–74.79 %, respectively. RS200 co-digested with AB at a C/N ratio of 30 achieved the highest biogas and biomethane yields of 686 ± 32 mL/g VSinput and 315 ± 16 mL/g VSinput, showing increases of 151.28 % and 164.71 %. These gains were supported by improved biodegradability, synergistic effects, and volatile solids reduction. Metagenomic analysis revealed a higher abundance of hydrolytic and acidogenic bacteria (e.g., Bacteroidota, Chloroflexi) and acetoclastic methanogens (notably Methanothrix) in the AcoD reactor. Thus, integrating hydrothermal pretreatment with co-digestion significantly enhances RS and AB valorization, supporting sustainable waste management and renewable energy production.