Strengthening the syntrophic pathway for acetate oxidation-hydrogenotrophic methanogenesis by biogas stirring for effectively mitigating acidification in anaerobic digestion

Anaerobic digestion (AD) frequently encounters challenges such as acidification under high organic loadings, leading to system instability or failure. This study demonstrates that biogas stirring (RB) effectively mitigates acidification during the AD of FW (FW), outperforming traditional mechanical stirring (RM). The RB system sustained stable methane production, reaching 249.1 mL CH₄/g VS_added/d, whereas the RM system encountered significant acidification, with a pH dropping to 5.0 ± 0.2, impeding methanogenesis. For sludge characteristics, biogas stirring in the RB system significantly decreased particle size to 78 μm, which were anticipated to facilitate mass transfer and substrate conversion. Focus on the microbial communities evolution, employing biogas stirring facilitated a rational microbial collaboration with the enhanced capability for volatile fatty acids (VFAs) conversion and methane production. In addition, Methanobacterium, a typical methanogen for hydrogenotrophic methanogenesis (HM), was predominant in RB, occupying in an important position (Centrality = 0.9232) in the microbial network. Furthermore, the increased H₂ partial pressure enriched the concentration of F₄₂₀ by 63.1 %, facilitating its uptake and supporting the growth syntrophic acetate-oxidizing (SAO) bacteria. The consequent activation of the SAO-HM pathway is key to rapidly restoring methane production post-acidification. The findings of this study revealed the underlying role of biogas stirring for SAO-HM pathway and provided a potential strategy to facilitate acidification alleviation in AD.