Study On The Influence Of The Biological Nitrogen Removal Process By Bioelectrochemical System In Municipal Wastewater

The current wastewater treatment plants generally has the problem of poor nitrogen removal effect from low C/N municipal wastewater.Bio-electrochemical system（BES）has become a research hot spot in the field of wastewater nitrogen removal in recent years,which could effectively improve nitrogen removal efficiency of low C/N municipal wastewater by enhancing the microbial activity with its internal electrochemical properties and increasing nitrogen removal pathways.The cross-membrane migration of nitrogen ions offers new ideas for the multi-channel synergistic nitrogen removal in BES.However,the effect of electrochemical properties on characteristics of nitrogen migration,transformation and removal remains unclear.Here,by treating wastewater with different nitrogen combinations with the three-chamber bio-electrochemical reactor,the removal of nitrogen in municipal wastewater was studied under different applied voltages to find out the nitrogen removal pathway and the optimal voltage.On such a basis,the effect of COD concentration distribution on the nitrogen removal was further studied in the cathode chamber and the anode chamber.The primary conclusions are as follows:Experiment groups with applied voltages of 0.2 V（R-0.2）,0.6 V（R-0.6）,and 1.0 V（R-1.0）and the control group OC with no applied voltage were built.With simulated wastewater as the system influent,the operating characteristics of the system during the start-up period were investigated.According to the results,the COD removal in the cathode and anode chambers of all 4 groups were above 90%;the higher the applied voltage was,the higher the current intensity would be,which was more conducive to shortening the domestication time of nitrifying bacteria in the cathode chamber（shortened by up to 57%at R-1.0 than OC）.In the meantime,NH₄⁺-N removal in the cathode chamber also increased with the increase of applied voltage.However,applied voltage only had a limited effect on NO₂^--N and NO₃^--N removal in the anode chamber.After the start-up,the nitrogen-only wastewater was used as the intermediate chamber feed,the organic-only wastewater was used as the cathode chamber feed and the anode chamber feed to explore the characteristics of nitrogen migration,transformation and removal in bio-electrochemical reactors.According to the results,at NO₂^--N/NO₃^--N of 1:1（all are 70 mg/L）,the difference in TN removal in the anode chamber was small among the 4 groups;the removal at R-0.2 was the highest,up to 74.37%.When the influent NO₂^--N concentration was 140 mg/L and does not contain NO₃^--N,TN removal in the anode chambers of the 4 groups was enhanced to a certain extent;the removal in the anode chamber at R-0.2 was still the highest.Meanwhile,it was noticed that at least 25%of NO₂^--N can be converted into NO₃^--N through various pathways.When the influent NO₃^--N concentration was 140 mg/L and does not contain NO₂^--N,TN removal in the anode chambers of the reactors decreased;TN removal in the anode chamber at R-1.0 was relatively the highest.The higher the applied voltage was,the better the migration and conversion effect of NH₄⁺-N would be in the cathode chamber.The rate of simultaneous nitrification and denitrification（SND）showed a positive correlation to TN removal in the cathode chamber.The relative size of TN removal capacity in the system of the reactors is in order:R-1.0>R-0.2>R-0.6>OC.In addition,the migration of nitrogen ions followed the first-order reaction kinetics;The migration rate generally increases with the increase of applied voltage,and the migration rates of NH₄⁺and NO₃^-were positively correlated with the average migration and the average biological removal rate,respectively.According to analysis on the microbial community structure and enzyme activity in reactors under different applied voltages,the applied voltage could increase the microbial richness in the cathode chamber,Ferruginibacter,Thermomonas,and Nitrospira.Moreover,with the increase of the applied voltage,the overall activity of the nitrification and denitrification enzyme in the cathode chamber was also enhanced.In the anode chamber,the applied voltage of 0.2 V had a significant positive stimulating effect on nitrite reductase（NIR）and nitrate reductase（NAR）activities;the applied voltage of 1.0 V only enhanced NAR activity.Under the applied voltage of 1.0 V,the system feed water C/N was kept constant（About3.15）,the cathode chamber and the anode chamber feed water COD concentration distribution was changed by setting three sets of reactors,namely R-400（anode chamber:400 mg/L,cathode chamber:200 mg/L）,R-500（anode chamber:500 mg/L,cathode chamber:100 mg/L）and R-600（anode chamber:600 mg/L,cathode chamber:0）,respectively.According to the results,as COD concentration in the cathode chamber declined,the migration and conversion of NH₄⁺-N first improved and then reduced;the R-500 cathode chamber showed the highest NH₄⁺-N removal rate.As COD concentration in the anode chamber increased,the biological removal and migration rate of NO₃^--N also increased.According to high-throughput sequencing and enzymatic activity analysis,R-500 in the cathode chamber mainly promoted Thermomonas growth;as COD concentration declined,the overall activity of the nitrifying enzyme was improved while that of the denitrifying enzyme decreased to a certain extent.In the anode chamber,as COD concentration rised,the relative richness of denitrification-related bacteria Ferruginibacter and Arcobacter increased while Acinetobacter decreased.NAR activity was proportional to COD concentration in the influent water.