Enhancing methane production from waste activated sludge via iron-mediated low-temperature thermal hydrolysis

Thermal hydrolysis is widely applied as effective sludge pretreatment to enhance organic matter solubilization and methane production during anaerobic digestion, yet its reliance on high operating temperatures results in substantial energy consumption and limits overall sustainability. This study proposes an iron-mediated thermal hydrolysis strategy to achieve temperature compensation and improved digestion performance. The roles of Fe2+, Fe3+, and Fe6+ in organic matter release, dissolved organic matter transformation, and methane generation were systematically evaluated. The results demonstrate that iron addition significantly enhances sludge disintegration and anaerobic digestion efficiency, with Fe6+ showing the strongest effect. At moderate temperatures of 130–140°C, Fe6+-assisted thermal hydrolysis achieved a soluble chemical oxygen demand (SCOD) release of 20,200 ± 450 mg/L and a cumulative methane yield of 212.4 ± 5.2 mL/g ΔCOD at a low dosage of 20 mg/L, which are fully comparable to the values obtained via conventional, energy-intensive 170°C treatment (20,500 mg/L and 198.5 mL/g ΔCOD, respectively). Mechanistic analysis indicates that superior performance of Fe6+ is attributed to its strong oxidative capacity and valence-state cycling, which promote cell and EPS disruption while reducing the humic-like Maillard fluorescence intensity by 43.9% and suppressing formation of refractory melanoidin-like compounds. Economic evaluation further reveals that Fe6+-mediated thermal hydrolysis at a 100 t/day scale can reduce thermal energy demand by 32.5% and enhance biogas recovery, netting approximately USD 72000 in additional annual revenue. Overall, this study provides an effective pathway for low-temperature, energy-efficient sludge valorization and supports the development of sustainable and energy-positive wastewater treatment systems.