Study On Removal Of Bisphenol A By Coagulation-nanofiltration Combined Process And Membrane Fouling Mitigation Mechanism

The problem of water contamination by micropollutant cannot be ignored and the demand of human beings for drinking water continues to increase.As a membrane separation technology which is safety and can ensure the efficiency of water purification,nanofiltration has become a research hotspot and been used widely.In this paper,effect of nanofiltration on the removal of micropollutant bisphenol A and the factors that affect it were studied,and also coagulation was used as a pretreatment process to apply this combined process to the treatment of the water from micropolluted Songhua River.Then,the analysis focused on two aspects of water purification efficiency and membrane fouling.First,bisphenol A was filtered by nanofiltration,and the removal rate was 60.7%.At the same time,other micropollutant with different molecular weights such as ketoprofen,triclosan,and triclocarban were filtered.Studies have shown that steric hindrance effect dominates the retention mechanism of nanofiltration on micropollutant.The effects of bisphenol A concentration,pH,humic acid,bovine serum albumin four factors on removal of bisphenol A were investigated.Bisphenol A concentration had little effect on the removal rate of bisphenol A.The increase of pH and humic acid concentration increased the removal rate of bisphenol A.Conversely,the addition of bovine serum albumin weakened the removal effect of bisphenol A.Humic acid was easy to chelate with Ca²⁺,which affects the removal rate of bisphenol A.Taking into account the greater attenuation of the membrane flux,coagulation was used as a pretreatment method to reduce the membrane fouling,and the coagulant was hexahydrate aluminum chloride（AlCl₃·6H₂O）.Two coagulation methods of the supernatant after sedimentation and on-line coagulation were compared after selected the corresponding optimum dosage of the coagulant by jar tests.Three processes,nanofiltration and two coagulation methods combined with nanofiltration,were compared,and the key research object was the effect of coagulation on membrane fouling,which included the effect of turbidity on it.The specific flux,characterization of contamination layer,membrane resistance were studied to analyze the situation and mechanism of fouling.Secondly,the rejection rate of bisphenol A in the combined processes was measured.This value was greatly affected by pH and turbidity of raw water.Studies have shown that the removal of bisphenol A by coagulation is mainly through the combination of macromolecules（e.g.humic acid）and the formation of large flocs to precipitate.At pH=7,the removal rate of bisphenol A by coagulation was 40.9%.In order to verify the practical application value of the coagulationnanofiltration process,it was applied to the treatment of the water from micropolluted Songhua River,and the raw water quality index was tested.Specific fluxes and membrane fouling of the three processes were compared,specific flux of the supernatant-nanofiltration declined least,which was 0.33 higher than the nanofiltration.The water purification performance of each process and part was compared.Coagulation had a good effect on the removal of turbidity,and most inorganic cations were difficult to remove in this part.Organic matter,turbidity and inorganic ions were substantially removed by nanofiltration.The problem of residual aluminum can be ignored.After characterization and analysis,the severity of membrane fouling presented a trend of nanofiltration > on-line coagulationnanofiltration > supernatant-nanofiltration.In this paper,the optimization of the coagulation parameters slowed the nanofiltration membrane fouling.Also the removal effects of bisphenol A and other pollutants were analyzed.The coagulation-nanofiltration process was extended from prepared water in laboratory to the real water to demonstrate its actual application value.