Dynamics of Bacterial Communities and Predicted Functional Profiles During Hyperthermophilic Composting of Anaerobically Digested Sludge

Hyperthermophilic composting (HTC) is a promising approach for rapidly stabilizing organic waste and valorizing municipal biological residues; however, the microbial mechanisms underlying biomass conversion under hyperthermophilic conditions remain insufficiently understood. In this study, a pilot-scale HTC process was applied to anaerobically digested sludge, and temporal changes in bacterial communities and predicted functional genes were investigated. The process rapidly exceeded 90 °C within 2 days without external heating and maintained an average of approximately 77 ± 11 °C, demonstrating stable hyperthermophilic conditions. Moisture content concurrently decreased from 46.7% to 31.8% during the 40-day process. Bacillota and Actinomycetota dominated at the phylum level, while thermophilic genera including Thermobifida, Planifilum, Compostibacillus, and Oceanobacillus prevailed at the genus level. Functional prediction indicated increasing abundance of genes associated with carbohydrate metabolism and lignocellulose degradation throughout the process. Predicted gene profiles related to fengycin biosynthesis and oxidative stress responses suggested enhanced microbial adaptation to extreme conditions and potential suppression of pathogenic microorganisms. These findings provide integrated insights into microbial dynamics under hyperthermophilic conditions and highlight the potential of HTC as an efficient sludge stabilization strategy.