Removal Efficiency And Membrane Fouling Mechanism Of Slow Filtration And Low Pressure Nanofiltration For Antibiotic Resistance Genes In Secondary Effluent

As emerging environmental contaminants,antibiotic resistance genes have been frequently detected in the secondary effluent of wastewater treatment plants,and have become a potential threat to human health and ecological safety.In this study,combined process of slow filtration and low pressure nanofiltration was used for advanced treatment of the secondary effluent,and evaluated the removal effect of the three combined process of direct NF of secondary effluent,slow filtration without biofilm-NF and slow filtration of aerobic heterotrophic biofilm-NF on four ARGs(tet A,tet W,sul I,sul II)and dissolved organic carbon,and analyzed the influence of slow filtration and its surface biofilm on membrane fouling mechanism.The experimental results showed that the optimal filtration rate for slow filtration without biofilm and slow filtration of three kinds of biofilms(aerobic heterotrophic biofilm,nitrification biofilm and denitrification biofilm)to remove ARGs was 20cm/h,of which slow filtration of aerobic heterotrophic biofilm had the highest removal amounts,with removal amounts of 1.92～2.96-log.The slow filtration without biofilm removed more cellular ARGs,and slow filtration of the three kinds of biofilm removed more free ARGs.The combined process of slow filtration of aerobic heterotrophic biofilm-NF had the optimal removal effect on ARGs.The removal amounts of ARGs by direct NF of secondary effluent,slow filtration without biofilm-NF,and slow filtration of aerobic heterotrophic biofilm-NF were3.58～4.79-log,4.22～4.81-log,4.83～5.67-log,respectively.Under the optimal filtration rate(20 cm/h),the removal efficiency of DOC by slow filtration without biofilm and slow filtration of three kinds of biofilm were 27.2%,68.4%,40.4% and65.8%,respectively,the slow filtration of aerobic heterotrophic biofilm had the optimal removal effect of DOC.The slow filtration of aerobic heterotrophic biofilm had the optimal removal effect on tryptophan protein organics,fulvic acid organics,microbial by-product organics,humic acid organics and organics in different molecular weight ranges in the secondary effluent,slow filtration of denitrification biofilm,slow filtration of nitrification biofilm followed,and slow filtration without biofilm was the worst.Compared with direct NF of secondary effluent and slow filtration without biofilm-NF,slow filtration of aerobic heterotrophic biofilm-NF had the optimal removal effect on DOC,fluorescent organics and organics in different molecular weight ranges.There was a significant positive correlation between the concentration of 16 S r DNA,int I 1 and DOC in water and the content of different types of ARGs,the removal effect of the combined process on ARGs could be improved by enhancing the removal of 16 S r DNA,int I 1 and DOC in water.The increase in the proportion of Proteobacteria,Actinobacteria,Firmicutes,Pseudomonas and Acinetobacter in aerobic heterotrophic biofilm made it have the optimal removal effect on ARGs.The microbial population structure of the aerobic heterotrophic filter at different heights indicated that the high filtration rate would bring more nutrients to the deep layer of the filter,which was conducive to the maturation of the deep biofilm,thereby increasing the removal of ARGs.Compared with the direct NF of the secondary effluent and the slow filtration without biofilm-NF,the combined process of slow filtration of aerobic heterotrophic biofilm-NF increased the membrane specific flux,and the membrane fouling resistance(irreversible resistance and reversible resistance)and membrane fouling index were lower.The results of the membrane fouling characteristic curve also showed that the membrane fouling degree of combined process of the slow filtration of aerobic heterotrophic biofilm-NF was lower.The cake layer of slow filtration of aerobic heterotrophic biofilm-NF were thin and loose,which could be easily removed by mechanical washing,and relieved membrane fouling better.In summary,the combined process of slow filtration-NF could effectively remove ARGs and dissolved organic matters in the secondary effluent,and could be used as an effective way for the advanced treatment of the secondary effluent.