Effect and mechanism of biological nitrogen removal driven by moving bed biofilm reactor: A new perspective based on sustainable verification of manganese redox cycling

• Manganese redox cycling was disrupted and difficult to recover. • Nitrogen removal decreased by Mn(II)-deficiency manganese cycling. • Humic acid could be converted into denitrification carbon source. • Mn(II) formed crystals to promote the manganese redox cycling. • Proteobacteria was positively correlated with the nitrogen removal. A limited understanding on microbial interaction and mechanism of nitrogen removal driven by manganese redox cycling in moving bed biofilm reactor (MBBR), has hampered the establishment between micro-level interaction and macro-level nitrogen removal. In this study, MBBR with distinct manganese cycling was developed to investigate the nutrient removal, metabolic characteristic, and differential mechanism for treating nitrogen-containing wastewater with a low carbon-to-nitrogen ratio. Results indicated that NH 4 + -N and NO 3 – -N removal by Mn(II)-deficiency manganese redox cycling decreased by 2.8 and 1.8 times compared to Mn(II)-dependent manganese redox cycling. Kinetic analysis revealed that Grau’s second-order kinetics effectively described the removal of NH 4 + -N, NO 3 – -N, total nitrogen, and Mn(II). Compositions and spectral characteristics of extracellular polymeric substances (EPS) were analyzed by parallel factor analysis, showing that the more EPS was secreted under Mn(II) stimulation for contributing to biofilm formation and stability, and humic acid secreted by biofilm could be converted to carbon source to promote the denitrification process. Structure and community of biofilm were investigated. Results showed that overproduction of reactive oxygen species (ROS) induced by changes in Mn(II) might lead to the intracellular metabolic dysfunction including Mn(II)-deficiency manganese redox cycling, both Mn(II)-dependent manganese redox cycling and Mn(II)-deficiency manganese redox cycling had a significant impact on the cell activity. 16S rRNA and correlation indicated that phyla Proteobacteria and Bacteroidetes were mainly involved in the nitrogen removal. Furthermore, the mechanism of sustainable manganese redox cycling drive nitrogen removal was explored, indicating that biofilm catalyzed the oxidation of Mn(II) adsorbed on the cell surface to BioMnOx through enzymatic action for nitrogen removal. X-ray diffraction and X-ray photoelectron spectroscopy showed that Mn(II) formed the manganese crystals in the simultaneous nitrification and denitrification processes, which could serve as an electron donor for nitrogen removal. Mn(II) was removed through redox process and valence changes. This study can provide a preferable reference for practical applications for manganese cycling driven MBBR.