Navigating the trade-off: Thermal hydrolysis as a winning pretreatment for simultaneously enhancing methanogenesis...

While conventional sludge pretreatments focus on enhancing bioenergy recovery from anaerobic digestion (AD), their disparate impacts on heavy metal stabilization raise critical concerns for long-term environmental safety. This study systematically evaluated four pretreatment methods including ultrasonic (ULP), thermal hydrolysis (THP), acid (ACP) and Fenton (FTP) pretreatments using a unified framework that simultaneously prioritizes methane production and cadmium (Cd) bioavailability. Results demonstrated THP uniquely improved sludge solubilization and hydrolysis, leading to a 48.8% (p < 0.001) increase in cumulative methane production compared with Cd-laden untreated sludge, while reducing Cd bioavailability by 12.3% (p < 0.01). Metatranscriptomic analysis revealed THP enhanced the abundance of methanogens and up-regulated key functional genes related to methanogenesis, sulfate reduction, and Cd resistance, which collectively contributed to enhanced Cd immobilization and methanogenesis. In contrast, ACP and FTP caused severe microbial inhibition and process failure despite effective Cd immobilization. ULP showed limited improvement in both methane yield and metal sequestration. This work establishes a viable strategy for managing metal-laden sludge, demonstrating a practicable path toward sustainable sludge treatment within the circular economy framework.