Augmented methanogenic performance in straw-manure co-digestion via micro/nano bubble-enhanced syntrophic metabolism

This study aimed to investigate the enhancement of methane production by comparing air-nanobubble water (air-NBW) and conventional micro-bubble aeration during the anaerobic co-digestion (AcoD) of rice straw and pig manure. Remarkably, the addition of air-NBW (T3-NBW), as the most effective gas supplementation strategy, resulted in the highest cumulative methane yield of 489.49 mL/g·VS with a minimal lag phase of 0.57 days, representing a 59.80 % increase over the control (306.49 mL/g·VS, p < 0.001). This finding aligns with the kinetic fitting data (477.85 ± 5.84 mL/g·VS), wherein the maximum methane production rate for the T3-NBW was 41.89 mL/g∙VS·d^-1. Mechanistic insights revealed that air-NBW dramatically elevated the activity of β-glucosidase (hydrolysis) and coenzyme F₄₂₀ (methanogenesis) by 82.72% and 133.8 %, respectively, compared to the control. These results suggested that the efficient conversion of intermediate products played a critical role in enhancing overall methane yield. Microbial community analysis identified acetotrophic methanogenesis as the dominant pathway, with Methanosaeta abundance reaching 91.66 % in NBW-amended treatments. Additionally, the co-occurrence network analysis showed that the air-NBW system promoted microbial interaction and stabilized the ecological network. Crucially, the NBW treatment exhibited the lowest expression of energy-dependent antioxidant enzyme genes (e.g., superoxide reductase, SOR), suggesting reduced oxidative stress and enhanced energy allocation toward microbial growth and methanogenic activity. By enhancing oxygen dispersion and microbial intracellular redox balance, NBW technology provides a promising approach for improving methane recovery in sustainable waste-to-energy systems.