New insight into effects of waste scrap iron on sludge anaerobic digestion: Performances, microbial community, and potential metabolic functions

The low sludge degradation efficiency and hydrogen sulfide (H 2 S) generation impair the development of sludge anaerobic digestion. This study explored the effects of waste scrap iron (WSI) addition on methane (CH 4 ) production and H 2 S control in sludge anaerobic digestion. The maximum cumulative CH 4 yield of 205.02 mL/g VSS was obtained at the WSI dosage of 0.7 g/g VSS, which increased by 21.34 % compared to that of control without WSI addition. The increase in CH 4 yield was attributed to the enhancement of sludge degradation and soluble organic conversion. Meanwhile, the cumulative H 2 S yield reduced by 62.00 % compared to that of control. The pH value and content of insoluble sulfide precipitate increased with the increase in WSI dosage, which were two main reasons for H 2 S control. Microbial community results indicated that relative abundance of unclassified_f_Clostridiaceae_1, Bacteroides, vadinBC27_wastewater-sludge_group, Clostridium_sensu-stricto-1, Macellibacteroides, and hydrogenotrophic methanogens increased with WSI addition, meaning that WSI addition promoted syntrophic interaction between key bacteria and hydrogenotrophic methanogens. Gene prediction analysis demonstrated that WSI addition promoted microbial proliferation and respiration, carbohydrate and amino acid metabolism, and activity of butyrate kinase, acetate kinase, pyruvate ferredoxin oxidoreductase, and coenzyme F420 hydrogenase. In view of the enhancement of ABC transporters, lipopolysaccharide biosynthesis, quorum sensing, two-component systems, and ribosome metabolic pathway, it could be considered that WSI addition promoted information recognition and substance exchange among microorganisms, increasing microbial metabolic activity of anaerobic system.