Acidic-alkaline shocks in vinasse fermentation shape methanogenesis and sulfate reduction dynamics

Efficient two-phase anaerobic digestion (2nd-AD) of sugarcane vinasse hinges on effectively suppressing methanogenesis within the initial sulfate-reducing stage (acidogenesis) to maximize downstream methane production and mitigate safety risks associated with H₂S/CH₄ co-production. This study investigates the strategic in-process application of sequential acidic and alkaline pH shocks to achieve this critical control. An anaerobic structured-bed reactor (AnSTBR), reactivated from prolonged storage (5 months) to mimic off-season conditions and test long-term system resilience, was fed with vinasse for 90 days at 30 °C across six operational stages. The results demonstrate that these pH shocks hindered the hydrogenotrophic and, mainly, acetoclastic methanogenesis, reducing methane content to 3% while restoring sulfidogenesis to 82% (Stage VI) even after the system returned to original conditions. This robust sulfate removal in high-rate fermentative systems yielded an effluent rich in acetate (>3.0 g-HAc L^-1) with enhanced buffering capacity, ideal for subsequent acetoclastic methanogenesis. Microbial community analysis identified Desulfovibrio (28.69-49.38%) as the dominant and most active dissimilatory sulfate reducer, while Bacteroides (6.49-3.44%) and Aminobacterium (1.73-8.82%) were key acetate producers driving fermentative metabolism. This work establishes a novel operational strategy to efficiently modulate microbial pathways in vinasse biorefineries, advancing biogas production, environmental protection, and sustainable waste management.