Preparation And Nitrogen Removal Mechanism Of Composite Self-releasing Carbon Biofilm Electrode

In recent years,the total nitrogen in the tail water of urban sewage plants has become a concern.The lack of carbon source in denitrification process leads to the restriction of denitrification process,which has become a technical difficulty in denitrification.Therefore,in order to realize the efficient supply of carbon sources in the denitrification process of sewage technology,this study studied the preparation of different composite self-releasing carbon biofilm electrodes,studied their carbon release performance,static denitrification characteristics and microbial population changes,and selected the optimal composite self-releasing carbon biofilm electrode as the third pole of the three-dimensional biofilm electrode reactor（3D-BER）to study the denitrification characteristics and denitrification mechanism.The following research conclusions are obtained:（1）The optimum component ratio and carbon release performance of composite self-releasing carbon biofilm electrode were explored,and the component ratio of corn cob powder,sponge iron and polyvinyl alcohol prepared by composite self-releasing carbon biofilm electrode was determined to be 3:0.45:1.The corn cob was pretreated by mechanical grinding（JX）,mechanical grinding+alkali treatment（JJ）and mechanical grinding+biological treatment（JS）,which was used as the core substrate of electrode.Polyvinyl alcohol was used as the binder to bond it with sponge iron,and JX,JJ and JS composite self-releasing carbon biofilm electrodes were prepared.The cumulative amount of static carbon release was JJ electrode(116.139 mg·g^-1)>JX electrode(93.200 mg·g^-1)>JS electrode(78.079 mg·g^-1),TN and TP release was close to 0 mg·（g·d）^-1on the whole,JJ electrode had good carbon release capacity and stable release rate.The release process is close to the diffusion model.The soluble fluorescent organic compounds released by the three electrodes were all tyrosine/tryptophan and tyrosine/tryptophan protein substances,which were easy to biodegrade.The structure of corn cob after mechanical grinding and alkali treatment makes the release of carbon source more stable,and is suitable for microbial attachment and growth.（2）The nitrogen removal performance of the composite self-releasing carbon biofilm electrode and its effect on the microbial population structure were studied.The TN removal rate of the three electrode groups in the stable period was all over 90%,and that of the JJ electrode group was up to 97.3%.The average COD of effluent from JX,JJ and JS electrode groups are 15.412,29.690 and 19.877 mg·L^-1,respectively,during the 10-45 d denitrification process.The theoretical utilization rate of COD of JJ electrode group is stable at 90.34%.The average removal rates of TP by JX,JJ and JS electrodes were 76.50%,80.20%and 73.50%.The microbial diversity analysis showed that the microbial community richness and diversity of JX,JJ and JS electrode groups were significantly increased compared with that of inoculated activated sludge.At the genus level,there was a great difference in microbial community structure,which was mainly manifested in dominant bacteria such as Thauera and Dechloromonas.Among 36 dominant bacteria genera in 5 samples,13 were found to have denitrification function,including Thauera,Thiothrix,etc.There were 6 bacteria genera with the function of degrading cellulose and polysaccharides,including Sphingobium and Nannocystis.Among the dominant bacteria genera of JX,JJ,JS electrode groups and DZ group,the abundance ratio of the bacteria with denitrification function in JJ electrode group was the smallest,and the sum of the abundance of the two functional bacteria was the largest.The existence of symbiotic relationship improved the denitrification effect.（3）The nitrogen removal performance of three-dimensional biofilm electrode reactor（3D-BER）based on composite self-releasing carbon biofilm electrode was studied.The nitrogen removal efficiency of 3D-BER was increased by about 20.00%compared with the control group.The removal rate of NO₃^--N was stable between 80.00%and 85.00%,which was about 20.00%higher than that of the control group.The removal rate of NH₄⁺-N in the electrode group and the control group was lower,but the removal rate of NH₄⁺-N in the electrode group was about 10.00%higher than that in the control group during the stable period.The effluent COD concentration of electrode group and control group was stable at12.00 mg·L^-1and 15.50 mg·L^-1respectively.The 3D-BER weak current can stimulate the activity of microorganisms,improve the degradation ability of cellulose-degrading functional bacteria,and the three-dimensional carbon-releasing electrode is more stable.The removal rate of TP in the electrode group was stable at 96.00%,which was higher than that in the control group at 50.00%.In the reaction system of the electrode group,due to the stimulation of micro-current,oxidation occurs and corrosion occurs,resulting in more Fe2+and Fe³⁺.Moreover,plate corrosion also releases a large amount of Fe²⁺and Fe³⁺,so that the removal rate of TP in the electrode group is much higher than that in the control group.（4）Microbial population diversity analysis showed that the microbial samples of anode（JDA2H）cathode（JDB2H）and control group（BJD2H）in 3D-BER reactor except Firmicutes,The relative abundance of other dominant bacteria in JDA2H and JDB2H samples is greater than that in BJD2H samples.Due to the same water quality conditions,the dominant bacteria species in the three samples are similar.However,except for purple omonas,The relative abundance of other dominant bacteria in JDA2H sample or JDB2H sample was higher than that in BJD2H sample,respectively.The reason was that under the action of current in3D-BER reactor,the anode and cathode plate showed different stress effects on bacteria,resulting in a large difference in the relative abundance of bacteria.Under the action of current,the relative abundance of relevant functional bacteria was improved in 3D-BER reactor,which was conducive to the improvement of nitrogen removal effect in 3D-BER reactor.In addition,the bacteria with denitrification functions such as autotrophic denitrifiers Sulfurimonas and Hydrogenophaga were also found in the JDA2H sample and JDB2H sample.The relative abundance of Basidiomycota decreased significantly near the anode and cathode of 3D-BER reactor,while the relative abundance of Ciliophora,Ascomycota and Mucoromycota increased significantly near the anode and cathode of 3D-BER reactor.The relative abundance of Rozellomycota increased significantly only near the anode of 3D-BER reactor.Mortierella omycota and Ochrophyta were detected only near the anode and cathode of 3D-BER reactor.The number of dominant bacteria near the anode and cathode of 3D-BER reactor was much more than that in the control reactor,and the effect of current helped to enrich the fungi.A total of 64 functional genes of carbon,nitrogen,phosphorus,and sulfur were detected,and the largest number of functional genes of carbon,nitrogen,phosphorus,and sulfur were detected in JDA2H samples near the anode of 3D-BER reactor.There were significant differences in the functional genes of carbon,nitrogen,phosphorus and sulfur among the three samples,and different samples showed different amounts of functional genes of carbon,nitrogen,phosphorus and sulfur.The total number of carbon,nitrogen,phosphorus,and sulfur functional genes was JDA2H sample>BJD2H sample>JDB2H sample.Among the three samples,the number of carbon,nitrogen and phosphorus cycle functional genes was dominant,among which the number of carbon,nitrogen and phosphorus cycle functional genes in JDA2H sample was the most,and the sulfur cycle genes in JDB2H sample was the most,and both samples were from 3D-BER reactor.The increase of the abundance of nitrogen and phosphorus cycle functional genes in 3D-BER reactor is helpful to achieve efficient nitrogen removal.