| The coagulation-ultrafiltration(C-UF)process can effectively control membrane fouling while efficiently removing contaminants.It is a drinking water purification process with a wide range of applications and stable treatment effects.Adding coagulant aid is necessary means to further enhance the removal of contaminants and improve the performance of the membrane system in a specific process environment.However,coagulant aid as a polymer substance may aggravate membrane fouling,leading to reduction of membrane system operation efficiency if it was used improperly.In our works,it was studied the effect of different types of polymer coagulant aids in the C-UF process,and explored the"coagulation aid-fouling"mechanism between coagulant aids and membrane fouling in the C-UF process.The in-situ Raman spectroscopy and electrochemical impedance spectroscopy(EIS)were used to monitor the membrane fouling during the filtration,in order to reveal the causes of membrane fouling and investigate the contribution of coagulant aids to membrane fouling(especially irreversible fouling)and the negative influence mechanism of coagulant aids.This lays a theoretical foundation for the optimization of the integrated C-UF combined process using polymer coagulant aids.First,a full-flow filtration evaluation pool was designed,and an in-situ membrane fouling monitoring system was established for in-suit simultaneous Raman spectroscopy and EIS.Three types of organic polymer-type coagulant aids,including cationic(CPAM),anionic(APAM),and non-ionic(NPAM)polyacrylamide,and activated silicic acid(ASA)inorganic polymer coagulant aids were selected.Differences in membrane fouling properties formed after the addition of coagulant aids.The Raman spectrum and EIS tests were performed by equivalent circuit fitting and spectral characteristic peak processing respectively.The results showed that EIS could track the evolution of the membrane fouling layer,while Raman spectroscopy explained the cause of fouling according to the characteristic functional groups of contaminants.The combination of the two methods can provide more comprehensive information on the mechanism of early membrane fouling formation.Secondly,the dosage of the coagulant aid,the characteristics of the ultrafiltration membrane retention characteristics and the solution environment were changed,and the cluster analysis method was introduced.As a result,it was found that the coagulant aid would change the continuity and transformation time of the initial,development and stabilization stages of the membrane fouling layer.In particular,membrane retention characteristics had a significant effect on the role of coagulant aids in the coupling process.Finally,the fouling recovery effects of different cleaning methods were compared and analyzed.The surface morphology of the contaminated and cleaned membrane was characterized by scanning electron microscope and atomic force microscope.Partial correlation analysis was performed based on the correspondence between the irreversible pollution index(I_i)and the Raman characteristic peaks of the pollutants,and finally the contribution of the coagulant aid to membrane fouling was explained.The results showed that there were significant differences in the role of different types of coagulant aids in the C-UF.Membrane cleaning method will affect the total fouling resistance and the proportion of foulants in reversible/irreversible fouling,but will not change the leading role of coagulant aid and humic acid(HA)in the formation of membrane fouling.Ultrasound and deionized water had a significant effect on membrane fouling of APAM,sodium hypochlorite had a better effect on removing ASA membrane fouling,and citric acid was not conducive to cleaning membrane fouling of coagulant aids.In addition,coagulant aids can also affect the removal of HA from the membrane surface.When the difference between the molecular weight of the coagulant aid and the cut-off molecular weight of the UF membrane was small,the coagulant aid was the main factor causing the membrane irreversible fouling.The effect of the coagulant aid on membrane fouling is determined by the enhanced coagulation in the pretreatment stage and its complex interaction with the membrane surface during the UF stage. |
