Effect Of Iron-carbon Composites On The Treatment Of Low C/N Municipal Wastewater By The A/A/O-MBR Process

In recent years,the discharge of municipal wastewater has been increasing and shows a trend towards low carbon nitrogen ratio（C/N）.In order to reduce the addition of organic carbon sources in low C/N municipal wastewater nitrogen removal treatment,an algal biochar supported nano-zero-valent iron composites（ABC-n ZVI）were prepared to enhance the biological nitrogen removal process.Firstly,the effects of influent C/N,hydraulic retention time（HRT）and other factors on nitrogen removal in A/A/O-MBR reactor were investigated.Secondly,the KOH activated algal biochar（ABC）and iron-carbon composites were prepared and characterized to investigate their physical and chemical properties.Thirdly,batch experiments were conducted to verify the nitrogen removal performance of different ABC-n ZVI doses combined with microorganisms.Finally,iron-carbon composites were used in A/A/O-MBR reactor to study the nitrogen removal process of low C/N municipal wastewater.The main results of this study are as follows:（1）The A/A/O-MBR reactor was operated in three phases under different C/N and HRT factors to explore the effects of nitrogen and carbon removal.The experimental results showed that the chemical oxygen demand（COD）removal efficiency of the three phases was about 86%,and the ammonia nitrogen（NH₄⁺-N）removal efficiency was about 90%.The average concentration of total nitrogen（TN）in effluent was 8.6 mg/L,6.3 mg/L and 7.9 mg/L,respectively.The experiments showed that influent C/N and HRT factors had an impact on the TN removal efficiency,among which influent C/N had the greatest influence on the TN removal efficiency.In addition,the denitrification performance became worse under the condition of C/N≤4,resulting in a higher concentration of nitrate nitrogen（NO₃^--N）in the anoxic tank and effluent.（2）Iron-carbon composites were prepared by liquid phase reduction method,scanning electron microscopy（SEM）,specific surface area and X-ray photoelectron spectroscopy（XPS）were used for characterization.The specific surface areas of ABC and ABC-n ZVI were 1731.7m²g^-1 and 862.4 m²g^-1,respectively,and the Fe 2p electron spectrum of ABC-n ZVI showed a Fe（0）binding energy peak at 706.33 e V.The results of characterization test proved that nano zero-valent iron（n ZVI）had been successfully loaded onto the biochar.The hysteresis loop test of ABC-n ZVI showed that the remanent magnetization was 30.64 emu/g and the coercivity was about 50 Oe,which indicated that the iron-carbon composites were magnetic.（3）In the blue-capped bottle system containing 100 m L anoxic sludge,the experimental results of nitrogen removal performance verification with different doses of ABC-n ZVI showed that the optimal dose of ABC-n ZVI was 300 mg/L,and the removal efficiency of total inorganic nitrogen（TIN）reached 84.94%,and the effective time of nitrogen removal was 24 h.The nitrogen removal efficiency was not obvious and the nitrogen removal performance was poor as the added ABC-n ZVI dose was 100 mg/L.Abiotic reaction occurred and NH₄⁺-N concentration increased as the added ABC-n ZVI dose was 500 mg/L in the system.In the abiotic group with only 300 mg/L ABC-n ZVI,the removal efficiency of TIN was 3.17%.The results indicated that microorganisms played an important role in nitrogen removal process.（4）ABC-n ZVI was added to the anoxic tank of A/A/O-MBR reactor to enhance biological nitrogen removal of low C/N wastewater.The optimal dose of ABC-n ZVI in A/A/O-MBR system was 250 mg/L,the duration of nitrogen removal in anoxic tank could reach 9 h,the concentration of TIN in effluent reached the lowest after 2 h of reaction,and the concentration of NH₄⁺-N in effluent meet the national level 1 A emission standard.The dose of ABC-n ZVI was increased to 300 and 350 mg/L,during the reaction period of 2～6 h,the NH₄⁺-N concentration of the effluent was more than 5 mg/L,which violated the level 1 A emission standard.What’s more,the duration of nitrogen removal was reduced to 7 h.（5）Illumina high-throughput sequencing analysis showed that adding ABC-n ZVI increased the biodiversity of anoxic and aerobic tank.At the genus level,the addition of ABC-n ZVI had a significant effect on the abundance of norank＿f＿Saprospiraceae,norank＿f＿Caldilineaceae,OLB12 and Azospira,and the relative abundance of Azospira was increased.Through PICRUSt2 prediction analysis,it was found that adding ABC-n ZVI could enrich the abundance of key enzymes and functional genes in nitrogen removal process.With ABC-n ZVI added at 250 mg/L,the relative abundances of nitric oxide reductase（Nor）and nitrous oxide reductase（Nos）increased to 50%and 52%,respectively,and the relative abundances of nor BC and nos Z,which encode these two enzymes,increased to 44.13%and52.20%,respectively.The results indicated that the addition of ABC-n ZVI could promote the denitrification process.In conclusion,in the experiments for the start-up and operation of the A/A/O-MBR reactor,it was concluded that the influences of influent C/N on TN removal were the greatest.In batch experiments,it was concluded that the optimum dose of iron-carbon composites was 300 mg/L,and the effective time at the optimum dose was 24 h.In the continuous experiment,the optimal dosage of iron-carbon composites was 250 mg/L in the anoxic tank of A/A/O-MBR reactor,and the nitrogen removal duration was the longest,and the effluent TIN concentration was low.In addition,the addition of iron-carbon composites increased the diversity of microorganisms and enriched the bacteria related to iron autotrophic denitrification.