| Antibiotic resistance genes(ARGs)are emerging unconventional environmental pollutants that are detected in large quantities in the effluent of sewage treatment systems,posing a potential threat to human health and ecological security.Ultrafiltration(UF)technology is widely used in the actual engineering of reclaimed water treatment.Coagulation-sedimentation pretreatment can effectively control membrane fouling and improve membrane effluent quality.Therefore,based on UF technology,this study used two kinds of coagulants,polyaluminium chloride(PAC)and polyferric sulfate(PFS),to pretreat the secondary effluent.Firstly,the removal efficiency of 6 kinds of ARGs(tetA,tetC,tetG,tetX,sul?,sul?)and organic matter by coagulation(PAC/PFS)sedimentation-UF combination process was explored,and the effect of coagulation-sedimentation pretreatment on the removal of ARGs by combined process was discussed.Finally,the effects of two combined processes on membrane fouling were analyzed.When the dosages of PAC and PFS were 0.85mmol/L(calculated as Al)and 0.50mmol/L(calculated as Fe),the removal rate of ARGs was about 83% by combination processes more than that of direct UF,the removal amount is about 3-log,and the coagulation(PAC)sedimentation-UF removal effect is better.The removal rate of DOC and UV254 is better than that of the direct UF,the removal rate is about 37% and 34% respectively.For fluorescent organic compounds such as fulvic acids,soluble microbial by-products and humic acids,the peak of effluent from the coagulation(PAC/PFS)sedimentation-UF combination process can be reduced by about 10% to 20%.And the concentration of ARGs in different process effluents showed a significant positive correlation with the concentration of class I integron(int I 1),16 S rDNA and organic matter(p<0.05),indicating that the removal of int I 1,16 S rDNA and organic matter in water samples contributed to the reduction of ARGs.The account of cell-free ARGs in the secondary effluent of the wastewater treatment plant is 60%~70%,and the content is higher than the cell-associoted ARGs.For the cell-associoted ARGs,the coagulation-sedimentation pretreatment can remove more than 90%;however,the removal of cell-free ARGs is unstable,the removal amount of tetG and sulI is less than 50%,and the amount of other cell-free ARGs removed is more than 90%.During coagulation-sedimentation pretreatment,the larger floc size and smaller fractal dimension of PAC caused the flocs to be loose and porous and better removal effect on ARGs.Moreover,the different coagulation mechanisms of PAC and PFS cause different removal effects on ARGs.PAC mainly relies on the net electric double layer of positively charged multi-nuclear complexes and the net trapping of amorphous precipitates;PFS mainly relies on the adsorption and net trapping of organic matter and colloidal particles by about 70% of stable iron materials to remove ARGs.After pretreatment,the microbial abundance decreased by about 20%,which promoted the removal of ARGs.There is a certain difference between the pretreatment of microbial community structure and the original water.The changes of the content of ?-Proteobacteria and Sphingomonas are helpful to promote the degradation of organic matter,and Shinella can promote flocculation of microalgae.Compared with direct UF,the membrane resistance and membrane fouling index of coagulation(PAC/PFS)sedimentation-UF are lower,which can alleviate the decrease of membrane specific flux,thus the combined process can effectively alleviate membrane fouling.The membrane fouling index is significantly positively correlated with the organic matter concentration,indicating that the membrane fouling is closely related to the organic matter in the water sample;and the membrane fouling process is complex,and there are many membrane fouling mechanisms in different filtration stages.The main pollution model in the combined process filtration process is standard blockage and filter cake layer pollution.The surface morphology of the contaminated membrane shows that PAC is looser and porous than PFS flocs,and the membrane pollution is lower,easy to be backwashed and improved the removal effect of ARGs.The coagulation sedimentation-UF combination process is used to treat secondary effluent in this study.The ARGs and organic pollutants can be effectively removed,and membrane fouling can be alleviated,which is beneficial to control the spread of ARGs in the sewage plant to the environment and provide protection for human health and ecological security. |
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