Effect of metal ions on moving bed biofilm reactor (MBBR) for nitrogen removal in wastewater: Performance characteristics, flocculation efficiency and bacterial community

Advanced operational moving bed biofilm reactor (MBBR) has demonstrated to achieve simultaneous sludge yield minimization and pollutants removal. However, effect of different metal ions on MBBR performance for nutrients removal in wastewater under low carbon to nitrogen ratio is still unclear. Fate of NH4+-N and NO3 --N by MBBR were explored under the influence of Mn(II), Cu(II), and Fe(II) at carbon to nitrogen ratio of 5. The result showed that Mn(II) and Fe(II) significantly improved the NH4+-N and TN removal with a removal ratio of 71.0% and 80.5%, and 77.8% and 87.5% compared to the absence of added metal ions, and Cu(II) reduced the nitrogen removal. Biofilm characteristics including extracellular polymeric substances (EPS), electron transport system activity, and reactive oxygen species were investigated. The result indicated that both Mn(II) and Fe(II) enhanced the extracellular polymeric substances activity, electron transfer, and flocculation efficiency and hydrophilicity of biofilm. Furthermore, nitrification and denitrification processes of MBBR were further evaluated by X-ray photoelectron spectroscopy, fourier transform infrared reflection, cyclic voltammetry, differential pulse voltammetry, and Tafel curves. The result revealed that the main functional groups (e.g., C=O, C-O, and O-C=O bonds), and manganese and iron cycling might be involved in the nitrogen and metal ions removal. Finally, microbial community and co-occurrence network of biofilm were studied, showing that Proteobacteria and Bacteroidetes were identified as the dominant phyla for nitrogen and metal ions removal. This study aimed to provide the technical support for the startup and engineering applications of MBBR in wastewater treatment.