Enhancement of maize stover degradation and methane conversion by synergistic pretreatment with oxygen nanobubble water and composite microbes

Enhancing the hydrolysis efficiency of maize stover is crucial for its efficient anaerobic fermentation (AF) to produce methane. This study investigated the effects of combined pretreatment with oxygen nanobubble water (O₂-NBW) and composite microbes on the degradation characteristics of maize stover and its subsequent AF for methane production. Firstly, the effects of a combined pretreatment using composite microbes and O₂-NBW on the degradation characteristics of maize stover were investigated at different inoculation ratios (ranging from 1:1 to 4:1). This pretreatment method is both environmentally friendly and efficient. It overcomes the long cycle time typical of conventional biological pretreatments and avoids the high cost commonly associated with single physicochemical approaches. Following the determination of the optimal inoculation ratio, the effects of four pretreatment methods—namely, deionized water (DW), a single microbial consortium, O₂-NBW, and the combined composite microbes with O₂-NBW—on the methane production performance of maize stover during anaerobic fermentation were compared. An exergy analysis of the anaerobic fermentation process was subsequently conducted. Results demonstrated that the optimal inoculation ratio for the composite microbes was consistently 4:1 across all tested conditions. The O₂-NBW pretreatment group achieved significantly higher peak levels of reducing sugars and chemical oxygen demand (COD) compared to the control. At this optimal ratio, the synergistic pretreatment combining O₂-NBW and composite microbes outperformed the deionized water (DW) pretreatment, improving the exergy efficiency of anaerobic fermentation by 70.99%. Specifically, it increased the peak daily methane production by 24.40% to (14.02 ± 0.3) mL/(g·d), and raised the cumulative methane yield by 37.40% to (139.57 ± 4.5) mL/g. Kinetic analysis using the modified Gompertz model revealed that the combined pretreatment group had the shortest lag phase (λ=1.63 days), which was 23.8% shorter than the control group (2.14 days), indicating faster microbial acclimatization and metabolic initiation. The maximum methane potential (M₀) of 139.57 mL/g confirmed more efficient substrate conversion in this group. Notably, the maximum methane production rate reached 13.83 mL/(g·d), representing a 35.3% improvement over the control group 10.22 mL/(g·d), demonstrating significant enhancement of reaction kinetics through pretreatment. Process monitoring during the methanogenic phase showed that the combined pretreatment group exhibited rapid hydrolysis-acidification (sharp pH drop and reducing sugar surge) followed by highly efficient methane production (peak methane concentration of 66.33% ± 1.2%). The pH value in this group decreased rapidly from the initial 7.2 to 6.65 ± 0.04, while the peak reducing sugar concentration reached (221.80 ± 3.36) mg/L, a 31.5% increase compared to the control group (168.00 ± 4.61) mg/L, confirming the effective depolymerization of lignocellulose by the pretreatment. Exergy analysis highlighted the energy efficiency of the combined pretreatment, which achieved an exergy efficiency of 15.80%, surpassing composite microbes alone (14.92%), O₂-NBW alone (10.07%), and the DW control (9.24%). This improvement was attributed to O₂-NBW's micro-oxygen environment, which enhanced the activity of composite microbes -derived ligninolytic enzymes and promoted lignocellulose deconstruction, thereby reducing energy losses and optimizing metabolic efficiency. In conclusion, the combined pretreatment of O₂-NBW and composite microbes significantly enhanced the degradation of maize stover, shortened the microbial lag phase, and thereby improved methane conversion efficiency. From an energy conversion perspective, this approach effectively reduced irreversible system losses while increasing methane yield. These findings demonstrate its potential as a feasible strategy for developing low-energy, short-cycle, and high-efficiency pretreatment processes for the anaerobic fermentation of lignocellulosic biomass.