Study On The Efficiency And Mechanism For Phenolic Wastewater Treatment By Coupled System Of Biochar-electric Field Attached Membrane Bioreactor

The membrane bioreactor（MBR）effluent COD of the coking wastewater treatment process of a steel plant in Liaoning Province is unstable and difficult to meet the standard,which seriously affects the subsequent advanced wastewater treatment effect and increases the enterprise’s environmental protection facility renovation and operation costs,mainly due to the reduction of wastewater treatment efficiency and the increasing membrane fouling caused by refractory organics in phenolic wastewater.In recent years,electric field attached membrane bioreactor（EMBR）has shown potential advantages in the membrane fouling control and improving wastewater treatment efficiency.In addition,biochar has great application prospects in the field of wastewater treatment due to its adsorption and desorption,catalytic oxidation,and electron exchange capacity.In view of the practical engineering problems in phenolic wastewater treatment,this study developed a new biochar-EMBR coupling system,which could not only enhance the phenol wastewater treatment efficiency,but also mitigate membrane fouling.The key operation parameters such as electric field intensity and biochar content were optimized,and then it was applied to treat the high concentration phenol wastewater and real phenolic wastewater.The synergistic effect of biochar adsorption,electrocatalytic and biodegradation in the coupling system was analyzed.Meanwhile,the key enzyme activities,mixture liquid properties,membrane surface characteristics and microbial community structure changes were investigated to clarify the strengthening mechanism of the improvement of phenolic wastetaer treatment performance and membrane fouling mitigation in the coupling system.Firstly,the key operation parameters（electric field intensity and biochar content）of the coupling system were optimized,and the key enzyme activities,the ATP content and membrane fouling were also analyzed.The results showed that when the electric field intensity was 0.8 V/cm and the biochar content was 4 g/L,the phenol degradation efficiency of the coupling system could reach 100%,significantly greater than the sum of the removal efficiency of biochar adsorption and the degradation efficiency of EMBR（4.21%+88.62%=92.83%）,which was mainly attributed to the increase of key enzyme activities and ATP content.This indicated that there was a coupling effect among electro-catalytic degradation,biochar adsorption and biodegradation in the coupling system.The shock load resistance and membrane fouling of the coupling system in the treatment of high concentration phenol wastewater were investigated under the optimal operating parameters.Compared with the traditional MBR,the degradation efficiency of phenol（2000 mg/L）in the coupling system reached 98%in 8 h.In addition,the average growth rate of transmembrane pressure（TMP）was reduced by 53.33%,confirming that the coupling system could significantly reduce membrane fouling.On this basis,considering the toxicity and refractory properties of real phenolic wastewater,the coupling system was applied to treat the real phenolic wastewater.Compared with the traditional MBR,the degradation efficiency of phenol,o-cresol,m-cresol,p-cresol and COD in the coupling system was increased by 22.24%,31.16%,19.62%,20.16%and 32.94%respectively,and the average growth rate of TMP was decreased by 54.67%.High-throughput sequencing was used to analyze the shift of microbial community in the coupling system.The results showed that the coupling system improved the richness,and reduced the diversity and evenness of microbial community.Compared with the traditional MBR,the relative abundance of the dominant phenol-degrading bacteria Pseudomonas,Sphingobium and Stenotrophomonas in the coupling system increased by 14.22%,5.90%and2.75%,respectively,in mixture liquid.The relative abundance of dominant polysaccharide-degrading bacteria Phaselicystis and Brevundimonas increased by2.22%and 0.41%respectively,and the relative abundance of Acinetobacter involving in the formation of biofilm reduced by 7.49%.To clarify the mechanism of mixture liquid properties on membrane fouling mitigation in the coupling system,the characteristics of activated sludge,the content of H₂O₂,SMP and EPS were analyzed.The results showed that compared with the traditional MBR,the MLSS increased,SVI decreased,sludge viscosity decreased and sludge particle size increased in the coupling system,which effectively improved the activated sludge properties.The absolute value of Zeta potential increased significantly,which enhanced the electrostatic repulsion between the membrane pollutants and the cathode membrane.Besides,the average yield of H₂O₂（0.33 mg/L）could mitigate the membrane pollutants in situ（H₂O₂was not produced by traditional MBR）.Meanwhile,compared with the traditional MBR,the content of soluble microbial products（SMP）and extracellular polymeric substances（EPS）decreased significantly.The content of protein（PN）and polysaccharide（PS）and PN/PS of SMP decreased by 60.83%,40.23%and 35.84%,respectively,and the content of PN and PS of EPS decreased by 32.80%,23.74%and 10.34%,respectively.Three-dimensional excitation-emission matrix（3D-EEM）and Fourier-transform infrared spectroscopy（FTIR）were used to analyze the fluorescent components and the change of functional groups in SMP and EPS.The results showed that the fluorescence intensity of tryptophan and humic acid in SMP and EPS decreased significantly,and the stretching vibration peak of proteins,polysaccharides,lipids,nucleic acids,aromatic compounds and other related functional groups were weaker than those in traditional MBR.The analysis of the absolute value of Zeta potential on the membrane surface,the weight of the cake layer and the reduction of EPS showed that compared with the traditional MBR,the electrostatic repulsion between the membrane pollutants and the cathode membrane enhanced,the weight of the cake layer reduced,and the PN and PS content of EPS and PN/PS decreased by 45.35%,23.94%and 28.33%,respectively.In addition,high-throughput sequencing showed the increase of richness,and decrease of diversity and evenness of microbial community on membrane surface in the coupling system,indicating that the microbial community structure of membrane surface in coupling system was more stable compared with traditional MBR.Compared with traditional MBR,the relative abundance of phenol-degrading bacteria Pseudomonas,Comamonas,Simplicispira and Acidovorax on membrane surface of coupling system increased by 9.78%,1.99%,0.88%and 0.33%,respectively.The relative abundance of Luteococcus,Taibaiella and Acinetobacter that produced EPS and participated in the formation of biofilm decreased by 2.31%,2.40%and 8.74%,respectively.