Construction Of Catalytic Ozonation-membrane Separation Coupling System And Study On The Membrane Fouling Mechanism

At present,catalytic ozonation technology has been widely used in the advanced treatment of secondary effluent of refractory wastewater due to the highly efficient degradation of organic pollutants.However,the effective separation of catalyst and effluent is a major problem at present,and membrane filtration technology has excellent separation function.Therefore,combined with their respective advantages,this study constructed a catalytic ozonation-membrane separation coupling system（COMS）.Aiming at the coupling system,the removal ability of quinoline by COMS and membrane fouling were investigated under different process conditions.Meanwhile,cellulose nanocrystals（CNC）were used to modify PVDF membrane,and the modified membrane was applied into COMS to analyze the change of membrane fouling and explore the membrane fouling mechanism.In this study,nano-MgO was successfully prepared and used as ozone catalyst in COMS.The effects of different process conditions and water quality conditions on the removal of quinoline by COMS were investigated,including hydraulic retention time,dosage of catalyst MgO and the type and concentrations of coexisting organic matter.The results showed that when HRT=4 h and MgO dosage was 0.2 g/L,COMS had a better quinoline removal effect（the removal rate reached 85.24%after 240 min stable operation）and the lowest membrane fouling conditions(The reversible resistance growth rate and irreversible resistance growth rate were only 0.22×10¹⁰ m^-1 h^-1 and 2.89×10¹⁰ m^-1 h^-1,respectively).When organic matter coexisted,on the one hand,bovine serum albumin（BSA）and humic acid（HA）could affect the removal of quinoline by COMS,which was manifested as inhibition with high hydraulic residence time（6h）,promotion with low hydraulic residence time（2 h and 4 h）,the better promotion effect at high concentration of BSA（40 mg/L）and low concentration of HA（10 mg/L）.On the other hand,BSA and HA could cause more serious membrane fouling.Compared with COMS without coexisting organic matter,the irreversible resistance growth rate of PVDF membrane could reach 69.61 and 8.26 times,respectively.CNC/PVDF modified membrane was prepared by loading and blending method using CNC as modified material.The effects of the contents of CNC on the morphology,hydrophilicity,permeability and fouling resistance of the modified membrane were investigated.It was found that CNC/PVDF blended membrane（CNC-3）with the addition of 3 wt.%CNC had excellent anti-fouling performance.CNC-3 was applied into COMS to further investigate the removal ability and membrane fouling of COMS＿C（CNC-3）.The results showed that compared with COMS＿P（pure PVDF）,COMS＿C had no significant effect on the removal rate of quinoline in the coexistence of BSA,but slightly decreased in the coexistence of HA.At the same time,the irreversible fouling resistance of COMS＿C was significantly lower than that of COMS＿P,which could be reduced by 91.87%at the highest.The mechanism of membrane fouling in COMS was speculated as follows:（1）Reversible fouling:the adhesion of catalyst nano-MgO on the membrane surface during operation;The small molecule intermediates generated during ozonation of quinoline could cause reversible fouling,while the fluorescence intensity in COMS cleaning solution was significantly lower than that in ozonation-membrane separation system,indicating that the reversible fouling was alleviated.In the coexistence of organic matter,BSA and HA and their degradation products catalyzed by catalytic ozonation could cause reversible fouling,and the degree of membrane fouling in the coexistence of HA was lower than that in the coexistence of BSA.2)Irreversible pollution:The deposition of nano-MgO on the membrane surface,while in COMS of coexisting organic matter,such fouling can be alleviated because nano-MgO was involved in adsorption of organic matter or catalyzing ozone oxidation;Nano-MgO catalyzed ozonation of BSA and degraded it into products with smaller molecular weight and size but difficult to be mineralized,which blocked the membrane pores.The hydrophilic modification CNC-3 could form a hydration layer on the membrane surface,reducing the adsorption of hydrophobic BSA and the blockage of membrane pores,and alleviating the irreversible fouling of the membrane.