| In recent years,the microbial fuel cell(MFC)has attracted wide attentions both at domestic and overseas because of its ability to convert chemical energy into electricity.In this paper,ammonium in wastewater was removerd by multi-anode MFC with simultaneous nitrification and denitrification(SND)in cathode.This study firstly analyzed biological communities on both anode and cathode biofilms in multi-anode MFC reactors and the biological communities on the cathode biofilm under different DO conditions in the cathode of three-anode MFC.Secondly,the performances of denitrification and electricity generation in the three-anode MFC continuous running reactor were studied,and the influences of the low voltage and low current on microorganism in MFC with SND in cathode was analyzed.Finally,Biofilm electrode reactor(BER)was constructed separately to study the influence of low voltage on the SND performances and microorganisms in MFC cathode.The main conclusions are as follows:1)Base on high-throughput technique and PCR-DGGE technique,samples were took from the biological communities on the anode and cathode biofilm in multi-anode MFC reactors and the biological communities on the cathode biofilms under different DO conditions in the cathode of three-anode MFC.The diversity of biological community on biofilm in three-anode MFC was lower than that in single-anode MFC and five-anode MFC.The Shannon index was 4.6 and 4.7 respectively.The functional bacterias were enriched.There were Paracoccus,Bacillus,Pseudomonas on the anode,which belongs to the electricity producing bacteria.The denitrification performance of bacteria strains on cathode were mainly Proteobacteria,Nitrospirae,Planctomycetes.Meanwhile,denitrifying bacteria was existed.Bacillus and Paracoccus were aerobic denitrifying bacteria.Ignavibacterium and Aridibacter were belonging to heterotrophic denitrifying bacteria.2)The continuous-flow MFC performance was studied.With the hydraulic retention time(HRT)increasing,the total nitrogen(TN)removal rate rose firstly and then decreased and the voltage increased.With the inffluent COD increasing slightly,TN removal rate was similar and the voltage increased.With DO increasing in cathode,the total nitrogen(TN)removal rate rose firstly and then decreased and the voltage increased.Series circuit changed to parallel circuit,TN removal rate was reduced and the voltage was increased.But increases the current was increased.Under the optimal condition,the removal rate of close path TN is twice that of the open circuit.The optimum operation condition was selected,which ammonium nitrogen 27 mg/L,COD 550±15 mg/L,HRT 17 h,cathode DO 1.2 mg/L,resistor150?.The total nitrogen(TN)removal rate was 81%.Cathode SND efficiency was up to72.2%.There was no nitrite.Under the same optimal conditions,the load was reduced from31.2 Nmg/(L·d)to 4.325 Nmg/(L·d)when changing the continuous flow to intermittent flow.3)The study found that the optimal conditions of the system which total COD removal rate was 98.4%and COD removal rate in anode chamber was 94.9%.The organic compounds in effluent water from anode chamber is mostly soluble microbial products.A large number of ammonium was degradated by SND in cathode chamber,which ammonia nitrogen decomposed and the nitrate nitrogen no enriched.TN was mainly removed in cathode which removal rate was 50.2±3.2%.It accounted for two-thirds of TN removal of the system.It also showed that the smaller HRT was,the less ammonia consumption in anode batch was.The ratio of anode methane production in the reactor was increased with operation.It meaned that more methanogens were growed,which would compared with exoelectrogens.It made lower coulombic efficiency of MFC.4)To explore the battery performance of continuous-flow MFC in different operation stages,the study showed that maximum power density was increased with reactor operation time,which was as high as 608.0 mV/m~3 at the late period,which was 2.1 times the previous period.The apparent resistance reduced until 20?in the late period.After the operation,the high power density was decreased and internal resistance was increased.In the later period of operation,the cathodic oxidation reducibility was enhanced,which the oxidation potential was 0.6V and the reduction potential was-0.2V.Meanwhile,the cathode capacitance was increased and the electrode performance of cathode was enhanced.The external resistance is inversely proportional to the current and the coulomb efficiency of the anode.The COD concentration of the inffluent is directly proportional to the voltage and current.Hoever,the coulomb efficiency of the anode firstly increased and then decreased.5)The microbial characteristics of multi-anode MFC with continuous flow was analyzed.Most Proteobacteria,Chloroflexi and Firmicutes were increased,which was beneficial to denitrification.Proteobacteria Class was mainly in the alpha,beta and gamma class,BetaProteobacteria was gradually becoming the main class about 1.69-fold increase at the late period.These two main species were accounted on cathode:Paracoccus belongs to aerobic denitrifier and Ignavibacterium belongs to heterotrophic denitrifying bacteria.There were also autotrophic denitrifiers which consists of Thiobacillus,Desulfovibrio and Pseudomonas,nitrogen-fixing bacteria mainly consists of Azohydromonas,a little of anammox bacteria contains Candidatus Kuenenia,ammonia-oxidizing bacteria contains Nitrospirillum and nitrite oxidizing bacteria consists of Nitrospira.The higher the cathode current density was,the larger the proportion of Alphproteobacteria was.When the circuit was broken,the proportion of Alphproteobacteria was smaller.The total proportion of autotrophic denitrifying bacteria using electrode electrons was increased from the broking circuit,the low current density and the high current density were 0.72%,0.96%and 1.12%,respectively.The electricity producing bacteria were Pseudomonas,Rhizobium,Bacillus,Clostridium,Ignavibacterium,Acinetobacter,Planococcaceae_incertae_sedis,and Methanothrix.The activity of ammonia monooxygenase(AMO)was positively correlated with the DO concentration,and the high current density would promote the activity of nitrite reductase(Nir).The activity of nitrate reductase(NOR),Nar enzyme and Nir enzyme at circuit break was significantly lower than that at pathway condition and the activity of NOR enzyme and Nir enzyme in continuous flow was higher than that in intermittent flow.The high current density can promote the expression of nirS gene and the relative abundance of pathway nirS gene was higher than that of open circuit.The existence of weak electric field can improve the permeability of biofilm.The existence of weak electric field is beneficial to the secretion of extracellular polymer and the formation of biofilm.6)In order to investigate the nitrogen removal rate in cathode of multi-anode MFC with continuous flow,two kinds of autotrophic denitrification rate calculation were calculated which were based on charge transfer and comparison.The autotrophic denitrification rate was65.2%and 60.4%respectively.The former calculation formula is appropriate for analyzing the transmission electron for nitrogen utilization rate in MFC.Deficiency in weak electric field was that heterotrophic denitrifying bacteria was also calculated.7)In order to investigate the ability of simultaneous nitrification and denitrification in external weak electric field,the denitrification performance of experimental group which was imposed at 0.7 V voltage on MFC cathodic biofilm was similar with the control group(0 V).enzyme activities of the denitrifying enzyme(NOR,AMO)and denitrifying enzyme(Nir,Nar)were promoted when the cathod was imposed at 0.7 V voltage or 10 mA current.Especially the influence of the current was greater than the voltage for the enzyme activities. |
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