Performance Of Nitrogen Conversion And Phosphorus And Manganese Removal In Biofilms Driven By Manganese Redox

Low-concentration nitrogen and phosphorus wastewater（e.g.,industrial tailwater）was characterized by high discharge and low concentrations,which could lead to eutrophication and black odor in water bodies,posing a potential threat to the ecological environment and human health.Therefore,the advanced treatment of wastewater containing low concentration of nitrogen and phosphorus has become the focus.In this study,a micro-aeration sequential batch moving-bed biofilm reactor（MBBR）was constructed for advanced treatment of low-concentration nitrogen and phosphorus wastewater,compared the performance of MBBR with different C/N ratios for simultaneous nitrification denitrification and denitrification phosphorus removal.The performance of MBBR driven by redox of manganese for simultaneous nitrification denitrification and denitrifying phosphorus removal,and MBBR driven by in situ generated biogenic manganese oxides（BioMnOx）for nitrification denitrification and denitrifying phosphorus removal were investigated.The main results were obtained as follows:（1）The change of influent C/N ratio had a small effect on NH₄⁺-N removal（from98.05%to 99.60%）;NO₃^--N and TN removal efficiencies decreased with the reduction of C/N ratio,while the removal of COD and TP showed a trend of first increased and then decreased.When the influent C/N ratios were 15,10,5 and 2,the average effluent concentrations of COD met the primary A standard in the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant（DSP-MWT,GB 18918-2002,China）.More than 89%of the nitrogen was converted to N₂when the influent C/N ratio was 20,15,10and 5.With the decrease of C/N ratio,the protein（PN）to polysaccharose（PS）ratio（PN/PS）in extracellular polymeric substances（EPS）and electron transport system activity（ETSA）reduced by 2 and 9-fold,respectively,while nitrate reductase（NAR）activity was significantly inhibited.Metagenomic sequencing showed that genes encoding nitrite reductase（nir K）,oxynitrite reductase（nos Z）and nitrate/nitrite transport proteins（nrt P and nas A）were key genes in the nitrification denitrification,and microorganisms associated with biological denitrification and phosphorus removal include Acidobacteria,Nitrospirae and Chloroflexi.（2）When the influent carbon source was sodium succinate,the removal efficiencies of TN,NO₃^--N,NH₄⁺-N,TP,COD,Mn²⁺and the simultaneous nitrification and denitrification efficiency(E_SND)were 65.13%,79.63%,92.79%,51.57%,91.42%,68.10%and 84.33%,respectively.The kinetic analysis of TN removal by the modified Stover-Kincannon model showed that the maximum substrate removal(U_max)were 11.03 and 10.05 mg TN·L^-1·h^-1when sodium acetate and sodium succinate were used as carbon sources,respectively.Fourier transform infrared spectroscopy（FTIR）analysis showed that functional groups such as C-O,Mn-O and C-N bonds were involved in the removal and conversion of nitrogen,phosphorus and Mn²⁺.X-ray photoelectron spectroscopy（XPS）characterization of the biofilm showed that there were Mn²⁺,Mn³⁺and Mn⁴⁺in the biofilm.Mn³⁺in the effluent was’capture’by pyrophosphates,Mn²⁺could be first oxidized to Mn³⁺by microorganisms,and then further converted to Mn⁴⁺.High-throughput sequencing showed that Proteobacteria,Bacteroidetes,Planctomycetes,Acidobacteria and Leptothrix were involved in the removal of nitrogen,phosphorus,COD and Mn²⁺redox.（3）The average removal efficiencies of NO₃^--N,NH₄⁺-N and TN in the experimental MBBR(Mn_inf²⁺=10 mg·L^-1)reached 89.00%,70.64%and 76.06%,respectively,which were 1.3,1.0 and 1.2 times higher than those in the control group(Mn_inf²⁺=0 mg·L^-1).Mn²⁺promoted the secretion of total EPS in the biofilm,increased the activity of ATPase and improved the electron conduction ability of biofilm.Laccase and catalase（CAT）activities in the experimental group were 3.5 and 1.9 times higher,respectively,than in the control group.In addition,under anoxic conditions,microorganisms were able to reduce BioMnOx and the removal of NH₄⁺-N during the reduction via the ammonia oxidation pathway,and the Mn²⁺generated in this process could provide the electron donor for denitrification.Betaproteobacteria,Cytophagia,Alphaproteobacteria and Sphingobacteriia were involved in the removal of nitrogen and phosphorus,and Ohtaekwangia.sp played an important role in biofilm formation and EPS secretion.The micro-aeration enabled the MBBR to form a local aerobic/anoxic environment,which inhibited the activity of nitrite oxidizing bacteria and thus achieved a partial short-cut nitrification in the MBBR.The C/N ratio and the type of carbon source had the greatest influence on the denitrification.At a C/N ratio of 5,MBBR maximized the simultaneous removal of NH₄⁺-N,NO₃^--N,TN,TP and COD,and sodium succinate was more readily available to microorganisms and possessed a higher denitrification rate when used as a carbon source.Under anoxic conditions,there was a microbial-mediated redox cycle between Mn²⁺and BioMnOx,which could provide electron donors and electron acceptors for the removal of NO₃^--N and NH₄⁺-N and contribute to the enhanced denitrification of MBBR.The results of this study revealed that manganese redox enhanced the biological nitrogen removal ability of MBBR,and the simultaneous removal of nitrogen,phosphorus,COD and Mn²⁺was realized,which provided technical support for the advanced treatment of low concentration nitrogen and phosphorus wastewater.