Effects And Mechanisms Of UV Disinfection On The Removal Of Antibiotic-resistant Bacteria And Antibiotic Resistance Genes In Secondary Effluent From Wastewater Treatment Plants

For decades,antibiotics have been widely used for the prevention and treatment of diseases in humans and animals,as well as in livestock breeding.Antibiotics in the environment have promoted the generation of antibiotic-resistant bacteria(ARB)and antibiotic resistance genes(ARGs),leading to an increase in antibiotic-resistant pathogens.ARGs in the secondary effluents of a variety of full-scale WWTPs were found at levels well above those in typical aquatic environments.And a significant correlation between riverine ARG levels and upstream capacities of WWTPs had already been observed.Therefore,study on the removal of ARB and ARGs by advanced treatment processes(such as UV disinfection)is of great significance for the control of its dissemination in the environment.To elucidate the effects and mechanisms of UV disinfection on removal of antibiotic resistance in secondary effluent of wastewater treatment plants,the antibiotic resistance profiles and species of cultivable heterotrophic bacteria were investigated,and antibiotic-resistance genes(ARGs)in antibiotic-resistant bacteria(ARB)before and after UV irradiation were also studied in order to investigate the effects of UV irradiation on ARGs located on different positions(genomic and plasmid DNA).In addition,real-time fluorescence quantitative PCR(qPCR)technology was used to detect the abundance of ARGs in secondary effluent before and after UV disinfection,so as to explore the removal effect of ARGs by UV disinfection.The main results are as follows:The multi-antibiotic resistance(MAR)indices of most main genera significantly increased after UV irradiation.For example,MAR index of Aeromonas increased from 0.56 to 0.67 and Enterococcus increased from 0.53 to 0.63.Aeromonas strains resistant to>5 antibiotics accounted for 90%in UV5,but only 76%in RW.The MAR indices of both summer and winter samples were also significantly increased after 5mJ/cm~2 UV irradiation(P<0.05),and the proportions of bacteria resistant to more than 9 antibiotics in both samples were 2 times and 4.3 times of that in RW,respectively.Thus UV irradiation had an obvious selective effect on ARB.With the increasing UV irradiation fluences,the bacterial community structure was significantly changed.The detection frequency of gram-positive bacteria increased gradually while that of gram-negative bacteria decreased.And the alteration of bacterial community led to the changes in detection frequency of bacteria resistant to antibiotics,as the antibiotic resistance profiles among genera were varied.After 10mJ/cm~2 UV irradiation,the MAR indices of water samples in summer and winter were reduced from 0.51 and 0.46 in UV5 to 0.45 and 0.25,respectively,as the genera with relatively low MAR indices replaced that with high MAR indices,and became the dominant genera.For genomic ARGs,the detection frequencies of 5 ARGs(tetA,tetC,tetM,tetW,and sul2)increased after 10 mJ/cm~2 UV irradiation.The average detection frequency of tetracycline resistance genes changed from 56%to 59%.However,the detection frequency of sulfamide resistance genes was significantly reduced from 42%to 28%(P<0.05),which was mainly caused by the alteration of bacterial community.For plasmid DNA,the detection frequencies of 6 ARGs(tetA,tetC,tetM,tet W,tetX,and sul1)decreased after UV irradiation,ranging from 8.9%to 20.4%.The average detection frequencies of tetracycline and sulfonamide resistance genes were reduced by 15%(P<0.05)and 6%,respectively.UV irradiation had no significant effect on genomic ARGs in surviving ARB,but significantly reduced the detection frequencies of plasmid ARGs.The minimum fluence of UV disinfection prescribed by municipal wastewater treatment plants in China is 20 mJ/cm~2,which could achieve great bactericidal effect(>99%),but only remove 42.1%ARGs.The removal rate of most ARGs could reach more than 90%at a fluence of 160 mJ/cm~2,while the removal rate of mex B,a multi-antibiotic resistance gene,was only 50%,and the concentrition of it was still above 10~7 copies/mL in irradiated effluent.After 160 mJ/cm~2 UV irradiation,the relative abundance of 14 out of 23 ARGs significantly decreased,while the total relative abundance of ARGs greatly increased.This was mainly due to the extreme enrichment of mexB(17.31 times),which significantly affected the total relative abundance of ARGs.