Enhanced anaerobic digestion for energy recovery from brewery wastewater employing nano zero-valent iron loaded biochar prepared by residual sludge

• A novel strategy for EPS-doped biochar loaded with NZVI is proposed. • NZVI@EPSBC has been proven to reinforce methane accumulation with a 40% increase in efficiency. • NZVI@EPSBC significantly increased electron transfer efficiency in microbial respiration. • Both hydrolytic acidifying bacteria and methanogens were enriched in the presence of NZVI@EPSBC. Anaerobic digestion is an effective wastewater energization technology, but suffers from low energy conversion efficiency. In this study, a novel biochar was successfully prepared by incorporating extracellular polymeric substances (EPS) to residual sludge, and the anaerobic digestion performance could be significantly improved by employing this biochar loaded with nano zero-valent iron (NZVI@EPSBC). In comparison to the control, the cumulative methane production with 100 mg/L NZVI@EPSBC was raised by approximately 1.4-fold. The NZVI@EPSBC did not significantly change the morphology of the inoculated granular sludge, but induced the secretion of proteins and humic substances in the extracellular EPS, which are beneficial for carbon conversion and extracellular electron transport at the microscopic scale. Meanwhile, the quantitative analysis of the electron transport system also confirmed that NZVI@EPSBC could significantly improve the electron transfer efficiency in microbial respiratory chain throughout the experimental cycle, in which the highest respiratory efficiency increase of 26.51 % at 48 h. Taxonomic analysis of microbial communities showed that NZVI@EPSBC exhibited complete retention of bacteria and archaea compared to the raw sludge, while realizing an efficient cooperative pattern of syntrophic bacteria by regulating the relative abundance of specific functional bacteria, like Anaerolineaceae and Methanosaeta . Overall, the study offers a valuable strategy for achieving resourcefulness of residual sludge and EPS along with facilitating efficient energy recovery from brewery wastewater.