Combined pretreatment of biogas slurry immersion followed by freeze-thaw cycle for methane production enhancement of rice straw.

The challenge associated with agricultural straw pretreatment before anaerobic digestion (AD) lies in overcoming biomass recalcitrance at a low cost while minimizing the wastewater that requires additional purification. Biogas slurry from AD itself, rich in ammonia nitrogen and microorganisms, can pretreat straw instead of ammonia reagents or microbial agents. This study explored a synergistic pretreatment approach, particularly relevant in cold regions that offer natural cold resources, involving the immersion of straw in biogas slurry, followed by freeze-thaw cycles. Research findings indicated that freeze-thaw treatment significantly enhanced the lignin removal of biogas slurry immersion. Although sealing offered advantages for biogas slurry pretreatment, open was also feasible in terms of lignin removal and volatile fatty acids production, which helps further reduce operating costs. The hydrolytic acidification mediated by microorganisms, such as Clostridium sensu stricto 1 and Comamonas, dominated the biogas slurry immersion process, rather than the ammonolysis driven by free ammonia. The delignification process of rice straw underwent significantly interactive effects by several factors, reaching maximal lignin removal rates of 63.27% and 39.16% under sealed and open immersions, which corresponded to the optimal pretreatment conditions: immersion temperature of 30.70 and 43.20 °C, immersion durations of 4 days and 20 h, biogas slurry-to-straw ratios of 16.33:1 and 14.62:1, both followed by four freeze-thaw cycles. The specific methane yields under the two optimal conditions reached 281.37 and 262.18 L/kg VS. The combined pretreatment presents a promising low-cost operational strategy, especially in cold regions, while also facilitating on-site utilization of biogas slurry.