Moving bed biofilm reactor for nitrate removal driven by iron and manganese binary redox cycling: Denitrification...

Traditional biological denitrification process is limited by the lack of organic carbon source as electron donor in a nutrient poor environment. So, Fe(II) and Mn(II) are introduced as alternative inorganic electron donors to solve the problem of low denitrification efficiency caused by insufficient carbon sources. In this study, the moving bed biofilm reactor (MBBR) was developed to investigate the effect of denitrification under different Fe/Mn ratios. The result showed that when the Fe/Mn ratios were 5:5, 7:3, 10:0, and 0:10, the NO₃^--N removal efficiency was above 94% at influent NO₃^--N concentration of 12 and 24 mg L^-1. The variation characteristics of Mn(III) and biofilm were investigated by confocal laser scanning microscope. The result suggested that Fe(II) significantly inhibited the oxidation of Mn(II), and Mn(II) participated in the energy metabolism and cells redox at Fe/Mn ratios of 5:5 and 0:10, which directly affected the synthesis and secretion of protein. The fluorescence characteristic and concentration of extracellular polymeric substances (EPS) in biofilm were explored including protein secondary structure. Results showed that Fe-Mn binary redox cycling promoted the production of soluble microbial products and enhanced the nitrogen removal, and the protein concentration in EPS gradually migrated from the outer layer to the inner layer as the MBBR operated. The electron binding energy and oxidation states of the EPS was detected, and the microbial responses on iron-manganese binary redox cycling driven nitrate removal was proposed by high-throughput sequencing. The result showed that the relative contents of C=O were 54.4% and 28.1% when the Fe/Mn ratios were 0:0 and 10:0, the addition of iron and manganese significantly promoted enrichment of Gammaproteobacteria at the class level. In conclusion, effect of Fe(II) on microbial communities was greater than that of Mn(II), and removal of NO₃^--N had a strong correlation with the Gammaproteobacteria and Planctomyetes. This study provides new insights into efficient biological nitrogen removal by MBBR based on iron-manganese binary redox cycling.