Temperature-dependent mechanisms of volatile sulfur compound transformation and emission during mesophilic and thermophilic anaerobic digestion of sludge.

Volatile sulfur compounds (VSCs) are key odorants in sludge anaerobic digestion (AD), but their temperature-dependent transformation mechanisms remain unclear. This study compared mesophilic (35 °C) and thermophilic (55 °C) AD by integrating VSC profiles, microbial communities, sulfur-related enzymes, methionine degradation, and EPS characteristics. Mesophilic digestion produced 24.25 μg·g-1 TS VSCs, with H2S accounting for 96.91%, whereas MM, DMS, and DMDS were marginal. Thermophilic digestion increased total VSCs to 180.27 μg·g-1 TS and shifted dominance to MM (61.04%), followed by H2S (29.87%), DMDS (7.79%), and DMS (1.30%). Methionine degradation increased from 37.55% to 67.24%. Putative sulfur-oxidizing bacteria reached 15.1% and 20.9% under mesophilic and thermophilic conditions, respectively, with Arcobacter and Acinetobacter as dominant genera, while Caldisericum emerged after day 6 at 55 °C. Thermophilic digestion increased indicators associated with APS kinase, sulfate reductase, thioredoxin reductase, thiol peroxidase, and sulfide quinone oxidoreductase by 80.49%, 166.55%, 137.47%, 151.84%, and 85.81%, respectively. It also altered LB-EPS and TB-EPS fluorescence and increased protein-like, fulvic-acid-like, and microbial byproduct-like components. Overall, temperature acted as a metabolic switch, redirecting sulfur transformation from H2S-dominated emissions toward MM/DMDS-dominated organic sulfur volatilization through coordinated microbial, enzymatic, and EPS regulation.