| The increase of nitrogen compounds emissions seriously affected the quality of surface water.Traditional nitrogen removal technology required an external carbon source to achieve further denitrification for the wastewater with low carbon to nitrogen（COD/N）ratio（<5）,which was inconsistent with the concept of low carbon.Microbial fuel cells（MFCs）were based on electroactive microbial catalytic electrode reaction,which can realize the reduction of pollutants.Due to its sustainability,strong pollutant removal capacity and low sludge yield,it has become a new low energy consumption water treatment technology that has attracted much attention.However,the nitrogen removal rate of bioelectrochemistry was far lower than that of traditional biotechnology.Therefore,it was necessary to further study how to improve the nitrogen removal rate of MFCs biocathode.At the same time,the pathway and mechanism of MFCs to enhance nitrogen removal were not clear.In this paper,the influences of system structure,cathode influent condition and long-term operation on the power generation and pollutant removal characteristics of the three-anode microbial fuel cells（3A-MFCs）system with an effective volume of 5.6 L adapted to low COD/N ratio wastewater have been investigated,and the structure and operation conditions of the 3A-MFCs have been optimized.From the perspectives of electrochemical characteristics,pollutant removal effect,microbial community,functional gene expression and functional enzyme activities of biocathode,the denitrification pathway have been analyzed,the contribution of the weak electric field and COD to nitrogen removal have been quantified,and the influence mechanism of weak electric field on nitrification and denitrification have been put forward,which have provided theoretical basis and method for bioelectrochemical enhanced nitrogen removal from low COD/N ratio wastewater.The main results and conclusions were as follows:（1）Comprehensive consideration of power generation performance,TN removal efficiency,economy,and long-term performance,the 3A-MFC with GF and PEM was the better structure.The optimum operation conditions were as follows:the initial anode COD concentration was 1150 mg·L-1,the cathode DO concentration was 0.7±0.1 mg·L-1,the initial cathode p H was 7.0,the COD/N ratio was 3.5,the organic matter was Na Ac,and the initial ammonia nitrogen concentration was 65 mg·L-1.At this time,the PMD and OE of 3A-MFCs were 327.1 m W·m-3 and 39.2 m W·h,and the removal efficiencies of ammonia nitrogen and TN were 84.8%±5.6%and 81.6±4.7%,respectively.（2）The cathode organic matter weakened the power generation performance of the system,reduced the activities of AMO and NOR and the abundance of nitrification-related genes（pmo ABC and hao）,reduced the ammonia nitrogen removal efficiency;however,it increased the activities of Nar and Nir and the abundance of denitrification-related genes（nar GH,ncd2,npd,nas A,hcp）,which significantly improved the nitrogen removal performance of the 3A-MFC.The weak electric field increased the abundance of nitrification/denitrification-related bacteria（Candidatus＿Nitrotoga and unidentified＿Nitrospiraceae;Geobacter and Bacteroides）,and functional genes（pmo ABC,nir K,nar GH,ncd2,Npd,nas A,nir K,hcp）,and activities of AMO,NOR,Nar,Nir,which improved the electroautotrophic denitrification in the biocathode,and enhanced the nitrification and denitrification process.At the same time,the contribution of cathode organic matter to the denitrification effect was greater than that of weak electric field.The nitrogen removal pathways of the 3A-MFCs included microbial assimilation,traditional nitrification/denitrification process and electro-enhanced nitrification/denitrification process.（3）The application of-400 m V cathode potential inhibited the nitrification rate,but the application of-200 m V,+200 m V and+400 m V enhanced the nitrification rate.In the nitrification biocathode cultured under+400 m V constant potential,the proportions of Nitrosomonas（AOB）and unidentified＿Nitrospiraceae（NOB）were 5.2%and 12.4%respectively,which were 3.5 times and 1.3 times of that in the open circuit group,the activities of AMO and NOR were 4.8 and 2.2 times of that in the open circuit group,and the ammonia nitrogen removal rate(16.1±0.1 mg·L-1·d-1)was 1.8 times of open circuit group.（4）The application of-400 m V,-200 m V,+200 m V and+400 m V cathode potential increased the denitrification efficiency,and the lower the potential,the stronger the strengthening effect.-400 m V constant potential was favorable for the attachment of microorganisms,and the electroautotrophic denitrification using solid cathode electrons accounted for at least 34.1%of the total denitrification and at most 64.5%,and the related functional bacteria were Thauera（54.6%）.The electron transfer method between denitrifying bacteria biofilm and solid cathode electrode mainly depended on direct electron transfer,and there were at least two electroactive components（related to Nar and Nir）.In addition,the activities of Nar and Nir were 3.4 and 1.8 times that of the open circuit group respectively,the TN removal efficiency and removal rate(83.3±5.7%and 7.8±0.6 mg·L-1·d-1)were 4.5 times that of open circuit group.In summary,the 3A-MFCs for treating low COD/N ratio wastewater achieved the best performance of electricity generation and nitrogen removal under the optimized structure and optimized operation conditions.The nitrogen removal pathway included microbial assimilation,traditional nitrification/denitrification process and electro-enhanced nitrification/denitrification process.Among them,the traditional nitrification/denitrification process had a greater contribution to the TN removal of the 3A-MFCs system than the electro-enhanced process.The electro-enhanced nitrification/denitrification process not only increased the electroautotrophic-denitrification process,but also enhanced the microbial activity（biomass,abundance of functional bacterias and gene,functional enzyme activity）of biocathode under the weak electric field. |
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