Study Of Microbial Fuel Cells Coupled With Aerobic Granular Sludge For Wastewater Treatment

The growing population has increased environmental problems leading to increasing pollution of air,water and land resources,and high energy consumption and operating costs are the main challenges for current wastewater treatment technologies.At the same time,there is an increasing interest in the treatment of nitrogenous wastewater due to its regulation of eutrophication levels in water bodies and its serious impact on human and animal health.Microbial fuel cells（MFC）can combine electrochemical reactions with microbial metabolism to simultaneously perform biological nitrogen removal and generate electricity.Aerobic granular sludge（AGS）has a rich microbial structure,high biomass retention,excellent biological treatment efficiency and toxicity tolerance,and can achieve effective removal of carbon and nitrogen pollutants.Therefore,this study coupled aerobic granular sludge（AGS）into microbial fuel cell（MFC）and constructed a new electrochemical system MFC-AGS for simultaneous power generation and wastewater treatment,and investigated the effects of influent p H,carbon to nitrogen ratio（C/N）on the removal efficiency of carbon and nitrogen pollutants and power generation efficiency of MFC-AGS system.The microbial community structure and metabolic mechanism in the reaction chamber were also analyzed.In addition,the response surface model was established by Design Expert 8.0.6 software,and the interactive effects of p H,C/N and hydraulic retention time（HRT）on the removal efficiency and power density of MFC-AGS system were investigated.The specific experimental results are as follows:（1）The COD removal efficiency and denitrification performance of the MFC-AGS system showed a trend of increasing and then decreasing under different p H influent conditions,with the best effect at p H=10 and the highest COD removal rate and NH₄⁺-N removal rate of 96.1%and 98.6%,respectively,with the lowest internal resistance of 102.2Ω,the highest output voltage of 459 m V,the highest power density of 116.4 m W/cm² and the highest Coulomb efficiency of 23.3%.The high-throughput analysis showed that the anode microbial community at p H=10 was structurally stable,and the relative abundance of the main bacterial groups consisted of 28.0%of Proteobacteria,43.5%of Firmicutes,and 1.9%of Chloroflexi.In which the massive enrichment of Firmicutes enhances the electron transfer capability of the system.The highest percentage of Clostridia in the p H=10 anode sample was 42.3%at the phylum level,which increased the power production output and power density of the system.78.2%relative abundance of Metabolism at p H=10 indicated that the main function of the bacterial community was related to the removal of the target contaminants,and the genetic information processing function and the environmental information processing function accounted for a higher percentage than the other.The relative abundance of Metabolism in the p H=10 environment was 78.2%,indicating that the main functions of the bacterial community were related to the removal of the target pollutants,and the Genetic information processing function and Environmental information processing function accounted for 7.3%and 6.2%respectively.Carbohydrate metabolism,Amino acid metabolism and Energy metabolism were the main pathways of metabolic clusters in the second level,with relative abundance of 8.1%,8.2%and 4.2%,contributing to substrate degradation and energy conversion.The relative abundance of these functions changed the Metabolic pathways of microbial cells themselves and increased the content of Membrane transport proteins.The increase in the abundance of these functions changed the metabolic pathways of the microbial cells themselves and increased the content of membrane transport proteins.In the third level,Biosynthesis of secondary metabolite,ABC transporters and Quorum sensing play important roles in maintaining the normal metabolism and stable operation of microorganisms in the MFC-AGS system.The inlet water p H changed the biomass density of the anode.The highest biomass density of the biofilm of the MFC anode was 0.24 mg/cm² at p H=10,and the strongest electrochemical activity per unit protein was 0.48 W/g,which effectively reduced the internal resistance of the system.（2）The COD removal efficiency of the anode chamber of the MFC-AGS system did not vary much under the influence of different C/N influent conditions and was75.8%,76.3%,79.3%,and 75.8%,respectively.The best denitrification performance of the system was achieved at C/N=15,with the highest NH₄⁺-N removal efficiency of97.3%and the lowest accumulation of NO₂^--N and NO₃^--N of 4.0 mg/L and 7.6 mg/L,respectively.the system power density varied with increasing C/N of 86.3 m W/m²,92.3m W/m²,116.4 m W/m²,61.9 m W/m²,and the internal resistance was 202.1Ω,184.2Ω,102.2Ω,and 201.6Ω,respectively.The main groups in the cathodic microbial samples were Proteobacteria,Deinococcota,Bacteroidota,Actinobacteriota,Chloroflexi,and Firmicutes.The relative abundance of Gammaproteobacteria and Actinobacteria was highest at C/N=15 with 23.9%and 8.5%,respectively,which enhanced the denitrification performance of the system,with Actinobacteria playing an important role in maintaining the stable form of AGS.At C/N=5,the growth of Anaplasma phylum was inhibited due to the high concentration of ammonia nitrogen in the influent water,and the NO₃^--N in the system effluent was 96.5 mg/L.The Metabolism was the main function for different cathode microbial samples,indicating that the MFC-AGS system was able to have strong adaptability under the impact of different influent loads.The highest level of Carbohydrate metabolism was 8.7%in the second level with C/N=20 influent condition,which promoted the cathode COD removal efficiency up to 17.1%,and the lowest relative abundance of Amino acid metabolism and Membrane transport function were 8.1%and 3.1%,respectively,which affected the synthesis and quantity of transport proteins on the microbial cell membrane and limited the enhancement of electricity production capacity.The relative abundance of Biosynthesis of secondary metabolite in the third level was the highest at 8.3%at C/N=15 water intake,maintaining the granular form of AGS.Microbial metabolism in diverse environments and Two-component system were higher at 5.4%and 1.9%under C/N=5due to the increase in the concentration of ammonia nitrogen influent,according to the cathodic chamber pollutant The MFC-AGS system is maladaptive to the reduction of C/N,based on the analysis of the removal efficiency and the poor performance of electricity production in the cathode chamber.The higher C/N was beneficial to enhance the functional level of nitrogen metabolism,and the change in the abundance of related enzymes in the metabolic pathway might be an important factor to enhance the denitrification capacity.（3）Analysis of variance（ANOVA）of the mathematical models with response values of ammonia nitrogen removal efficiency and power production density showed that the models were all significant.The significance of the effects of influent p H,C/N and HRT on the different response values were not consistent.The effects of C/N on the removal efficiency of ammonia nitrogen were in the order of C/N>HRT>p H,and the effects of p H>C/N>HRT on the power density.The effect of p H is quadratic,and the interaction between p H and HRT has the strongest effect on the power density.The results of validation tests and SEM characterization after data optimization were analyzed,and it was found that the integrity of anode biofilm formation may be the main reason for the pollutant removal efficiency and power production performance of the system.