Operational performance and mechanistic analysis of the anaerobic digestion of iron-rich waste activated sludge

The effects of high iron content on the anaerobic digestion of waste activated sludge were investigated in this study. Biochemical methane potential tests indicated that waste activated sludge with high iron content (54.7 mg/g total solids) significantly reduced cumulative methane production and methane content compared to waste activated sludge with low iron content (25.98 mg/g total solids). Furthermore, the addition of magnetite led to negligible enhancement of methane production. Physicochemical analyses indicated that pH, oxidation–reduction potential and alkalinity were maintained within normal ranges during the anaerobic digestion of iron-rich waste activated sludge, whereas the solubilisation rate of particulate organic matter was notably reduced due to the physical binding of iron ions, iron oxides and organic particles. Microbial community analysis indicated that dissimilatory iron-reducing bacteria ( Longilinea ) were enriched under high-iron conditions, competing with methanogens for electrons and thereby inhibiting methane production. Based on microbial community structure and metabolic function analyses, acetotrophic methanogenesis and hydrogenotrophic methanogenesis were identified as the dominant metabolic pathways for methane production. Overall, anaerobic digestion alone may not be effective for the treatment of iron-rich waste activated sludge due to low energy recovery and high operating costs. This study provides new insights into the mechanisms underlying the poor anaerobic digestion performance of iron-rich waste activated sludge and offers practical guidance to improve methane recovery and sludge disposal. • High iron content in sludge inhibited methane production during anaerobic digestion. • Magnetite had a negligible effect on the anaerobic digestion of iron-rich sludge. • Solubilisation and hydrolysis were limited by flocculation and iron mineral binding. • Dissimilatory iron-reducing bacteria were enriched under high-iron conditions. • Integrating iron removal with anaerobic digestion may enhance methane recovery.