Iron-bound organic carbon formation governs the dual role of iron in anaerobic digestion of waste activated sludge: A stage-specific dosing strategy for enhanced methane recovery.

Iron salts are widely used in sludge treatment, but their dual role in stimulating methanogenesis and immobilizing bioavailable organic carbon during anaerobic digestion remains insufficiently understood. This study investigated the effects of FeCl3 dosage and dosing timing on methane production, Fe-bound organic carbon (Fe-OC) formation, and microbial metabolic activity during anaerobic digestion of waste activated sludge (WAS). The methane yield increased from 92.50 mL g-1 TS in the control to 108.88 mL g-1 TS at 20 mg g-1 TS, but decreased to 65.33 and 54.04 mL g-1 TS at 80 and 100 mg g-1 TS, respectively. High FeCl3 dosages promoted the substantial immobilization of organic carbon into the solid phase, where Fe-OC accounted for up to 67.62% of the TOC (at 100 mg g-1 TS). This profound carbon sequestration significantly restricted substrate bioavailability for methanogenic conversion. Furthermore, dosing iron during active digestion (day 5) yielded the highest methane production (147.8 mL g-1 TS), achieving a 73.1% increase over the control and outperforming day-0 and day-10 dosing by 37.4% and 58.8%, respectively. The dosing-time experiments showed that FeCl3 addition during the active digestion stage was more favorable than addition at the initial or late stage. The superior methane recovery achieved by day-5 Fe addition is attributed to a synergistic combination of reduced Fe-OC formation, optimized microbial community richness, sustained methanogen populations, and maximized enzymatic and electron transfer efficiencies. This study provides a practical strategy for improving methane recovery from WAS.