Removal Efficiency And Membrane Fouling Mechanism Of Opportunistic Pathogens In Secondary Effluent By The Process Of Granular Activated Carbon-Nanofiltration

The secondary effluent from municipal wastewater plants contains a large number of opportunistic pathogens,and its regeneration and reuse can pose a serious threat to human health and public health safety.In this study,two filtration units of granular activated carbon（GAC）and bioactive carbon（BAC）were used in combination with nanofiltration（NF）for the deep treatment of secondary effluent from wastewater plants.And evaluate the effectiveness of three treatment processes,direct NF,GAC-NF and BAC-NF,on the removal of three opportunistic pathogens（Legionella,Pseudomonas aeruginosa and Mycobacterium avium）,Escherichia coli and organic matter from secondary effluent.To analyze the membrane contamination mechanisms of different combined processes and to explore the influence of the interfacial interaction energy between the contaminants in water and the membrane on the membrane contamination mechanisms.The test results showed that all three treatment processes can achieve complete removal of opportunistic pathogens from the secondary effluent at the optimal filtration rate.GAC filtration and BAC filtration could enhance the removal of DOC,UV₂₅₄and fluorescent organics（microbial metabolism by-products and humic acid organics）from water by NF.Compared with direct NF and GAC-NF,the combined BAC-NF process is the most effective in the removal of DOC,UV₂₅₄,microbial metabolism by-products and humic acid organics,with the removal rates of 85.2%,74.5%,76.2%and 82.0%,respectively.There is a significant correlation between opportunistic pathogens and E.coli and DOC in water,and the purpose pf improving the reduction of opportunistic pathogens by different combination processes can be achieved by strengthening the removal of E.coli and organic matter in water.Pseudomonas,Aeromonas,Nitrophilus and Acinetobacter accounted for the highest percentage of BAC surface biofilm bacteria,which inhibited the growth and reproduction of opportunistic pathogens.The highest percentage of biofilm eukaryotic microbes were Nematodes and Brachionus,and both eukaryotic microbes were able to achieve reduction of opportunistic pathogens in water by means of capture.Both GAC filtration and BAC filtration pretreatment methods could effectively improve the membrane contamination of NF.BAC-NF has the highest membrane specific flux,the lowest total membrane contamination resistance and membrane contamination index compared to the secondary effluent direct NF and GAC-NF.The membrane contamination characteristic curve showed that the secondary effluent direct NF is filter cake layer contamination,GAC-NF and BAC-NF are first clogged in the middle and then form the filter cake layer contamination.Among them,BAC-NF was mainly contaminated by the middle blockage of membrane pores,and the membrane contamination degree is the lightest.The filter cake layer formed by BAC-NF is thinner and looser,and the cake layer is almost removed after hydraulic flushing,which effectively alleviates membrane contamination.The interaction energy between organic matter in membrane feed water and membrane,and between organic matter in membrane feed water and organic matter on membrane surface was analyzed by x DLVO theory.It was found that the combined BAC-NF process showed the weakest hydrophobicity and the largest zeta potential value on the membrane surface after contamination compared to direct NF and GAC-NF.The total interfacial energy between the organic matter and the membrane surface and between the organic matter on the membrane surface of BAC effluent is the largest,and the repulsive force is the largest.In the early stage of BAC-NF filtration,the organic matter in water does not easily reach the membrane surface,and the membrane contamination is the lightest.In the middle and late stage of filtration,it is difficult to produce adhesion between the organic matter in water and the organic matter on the membrane surface,and the cake layer formed is the loosest.In summary,the combined BAC-NF process can effectively remove opportunistic pathogens from the secondary effluent and can mitigate membrane contamination,thus achieving the goal of ensuring the safety of reclaimed water reuse.