Humic metal complexes as dominant anaerobic digestion inhibitor Multidimensional diagnostic framework from 16,200 tonne full scale sludge treatment plant

This study investigates a 16,200 ton full-scale anaerobic digestion (AD) plant facing poor organics reduction and limited biogas recovery. Biochemical methane potential tests using active exogenous sludge confirmed that the inefficiency stemmed from the intrinsic properties of the waste activated sludge, not insufficient microbial activity. Alkaline thermal hydrolysis increased methane yield by 69%, yet the maximum output remained low (76 mL CH4/g VS), suggesting deeper inhibition. Further diagnostic based on the detection of physical properties, biological structures, material composition, and microbial communities are applied to localize root causes. Results revealed that humic-metal complexes were the main limiting factor, along with stable biomass microstructures and residual facultative anaerobes. Two improvement strategies were proposed (1) upstream source control of flocculants and lignin-rich wastewater, and (2) downstream valorization of humic rich digestate. This study offers systematic diagnostic framework and resolution strategy for full-scale AD inefficiencies, providing practical guidance for similar large scale sludge treatment operations.