Compositional variability shapes sulfate reduction during thermophilic vinasse fermentation: Implications for operational stability and microbial function

High sulfate concentrations and seasonal compositional variability hinder sulfur management during the anaerobic digestion of sugarcane vinasse, particularly in first-stage fermentative-sulfidogenic reactors. This study evaluated operational strategies for maintaining stable thermophilic sulfidogenesis under variable sugarcane vinasse composition. Two identical anaerobic structured-bed reactors were operated at 55 °C using vinasse batches collected during different periods of the sugarcane season. The evaluated strategies included increased bicarbonate alkalization, recirculation of fermented or methanized effluents, and supplementation with glycerol, lactate, or both as external electron donors. Carbohydrate-rich vinasse induced rapid acidification and inhibited sulfate reduction, while recirculation of methanized effluent restored circumneutral pH but resulted in only moderate and composition-dependent sulfate removal (29–54%) and promoted methanogenic activity, with methane reaching up to 29% of the biogas. Electron donor supplementation was more effective for stabilizing sulfidogenesis. Co-supplementation with lactate and glycerol increased sulfate removal efficiencies to 75±7%, while glycerol supplementation alone was associated with a short-term sulfate-removal response of 62±8%. Microbial analyses indicated that the operational phases were accompanied by changes in taxa with sulfate-reducing potential, while taxonomy-based functional prediction suggested a low sulfate respiration potential associated with Desulfovibrionaceae; these predictions do not constitute direct evidence of in situ sulfate-reducing activity. These outcomes reveal that vinasse composition plays a key role in sulfidogenesis stability and highlight that industrial-scale applications will require adaptive operational approaches tailored to the compositional quality of each vinasse batch, rather than uniform process control.