Response of anaerobic digestion of waste activated sludge to residual ferric ions

[Display omitted] •The solubilization of WAS was increased with the increase of residual FI.•Performance of acidogenesis and methanogenesis depended on the contents of FI.•The fates and distributions of FI in liquid, EPSs and solid phases were analyzed.•The responses of microbial community to different contents of FI were revealed.•The findings provide some solutions for resource recovery from WAS contained FI. This study was conducted to investigate the effects of residual ferric ions (FI), released from iron or its oxides for wastewater or waste activated sludge (WAS) treatment, on anaerobic digestion of WAS. Herein it was found that the anaerobic digestion process was greatly affected by FI dosages as well as FI distributions. The responses of performance and microorganism suggested that a low FI (e.g., 0.125 mmol/g volatile suspended solid (VSS)) enhanced methane production by 29.3%, and a medium FI (e.g., 0.3 mmol/g VSS) promoted short chain fatty acids accumulation to reach the maximum of 247 mg chemical oxygen demand /g VSS, conversely, a high FI (e.g., 0.9 mmol/g VSS) led to severe inhibition on acidogenesis and methanogenesis. The findings may provide some new insights for mechanism understanding on anaerobic digestion process influenced by iron or its oxides, as well as the disposal of WAS contained FI.