Study On The Effect And Mechanism Of Fe³⁺ On Nitrogen Transformations During Simultaneous Nitrification And Denitrification Process

Simultaneous nitrification and denitrification (SND) has drawn the attention of many experts, due to its high nutrient removal efficiency and low energy consumption. Optimizing nitrogen immigration and transformation during SND processes, to achieve higher contaminant removal efficiency and less nitrous oxide emission, has becoming a research hotspot. Iron is a necessary element for both microbial activities and biological nitrogen removal process. It is a new kind of biological treatment method, to add ferric salt into sewage disposal system to improve sludge characteristic.In this study, the SND process was achieved by using anaerobic-aerobic SBR through strictly control of dissolved oxygen; different concentration of Fe3+ was maintained in each reactor, and nitrogen immigration and transformation were monitored, in order to investigate the effect of Fe3+ on nitrogen immigration and transformation during SND process; Microbial community structure in SND reactors under different concentration of Fe3+ were analyzed by using molecular biology techniques, to elucidate the microbial mechanism of Fe3+ affect nitrogen immigration and transformation during SND process. The main conclusions are as follows:(1) Appropriate concentration of Fe3+ during SND process could improve removal efficiency of nitrogen compounds, but adverse effect was observed at high Fe3+ concentration. When the concentration of Fe3+ was 20 mg/L, the effluent concentration of nitrate decreased significantly, and the removal rate of TN improved by 9.8%, compared with the blank reactor (R3). Whereas when it turned to 60 mg/L, much more nitrite accumulated, resulting in lower TN removal rate, which was 23.7% less than the blank reactor.(2) Effect of different concentration of Fe3+ on N2O emission during SND process was diverse. When the concentration of Fe3+ was 20 mg/L, the conversion rate of N2O was 5.22% lower than the blank reactor. Whereas when it turned to 60 mg/L, much more N2O was released, and the N2O conversion rate increased 70% compare with the blank reactor.(3) Fe3+ has significant effect on the activity of dehydrogenase. When the concentration of Fe3+ was 20 mg/L in the reactor, the relative abundance of Hydrogenophilaceae family, which is related to dehydrogenation, increased apparently, leading to enhancement of dehydrogenase activity by 34.9%. While it was opposite turned to 60 mg/L, and the activity of dehydrogenase decreased 29.8%, compared with the blank reactor.(4) Fe3+ could increase abundance and diversity of microbial community in SND reactors. Comparing with the blank reactor, when the concentration of Fe3+ was 20 mg/L and 60 mg/L, the OTUs of microbial community in the SBRs increased 15% and 13%, respectively. When the concentration of Fe3+ was 20 mg/L, the relative abundance of Proteobacteria phylum improved 4% more than the blank reactor. Whereas when it turned to 60 mg/L, the relative abundance of δ-proteobacteria class was improved.(5) The effect of different concentration of Fe3+ on nitrifiers and denitrifiers was diverse. When the concentration of Fe3+ was 20 mg/L, growth and development of denitrifiers were promoted apparently, resulting in its lower accumulation of nitrate and higher TN removal efficiency. Whereas when it turned to 60 mg/L, much more nitrite accumulated and lower TN removal efficiency was achieved. This is mainly because that high concentration of Fe3+ decreased abundance of denitrifiers contain nirS gene.