Study On Ultrafiltration Membrane Fouling Control Technology Based On Pre-membrane Process Enhancement And Conductive Membrane Preparation

Reuse of municipal sewage is an important way to solve water shortage in China.Among various advanced treatment processes,ultrafiltration process has broad application prospects in the field of reclaimed water reuse because of its advantages of stable effluent quality,environmental friendliness and low investment/operation cost.However,the low removal efficiency of organic matter and membrane fouling restrict its large-scale application.In this paper,the secondary effluent of municipal wastewater treatment plant was taken as the object,and the removal efficiency of pollutants in water by pretreatment process based on potassium ferrate and its influence on subsequent ultrafiltration membrane fouling were investigated.At the same time,the effect of electrochemical in-situ cleaning process on membrane fouling was investigated from optimizing the operating conditions of ultrafiltration.Firstly,the removal efficiency of potassium ferrate pretreatment process for various pollutants in secondary effluent and the mitigation effect on subsequent ultrafiltration membrane pollution were investigated.The results showed that the potassium ferrate pretreatment process can significantly remove UV₂₅₄ and fluorescent organics and reduce phytotoxicity in the secondary effluent,but had limited effect on organic mineralization.In addition,potassium ferrate pretreatment can also significantly reduce the absolute abundance of cell-associated resistance genes,cell-free resistance genes and water-borne pathogenic microorganisms in secondary effluent by reducing the viability and relative abundance of potential hosts of resistance genes.In terms of membrane fouling mitigation,increasing the dose of potassium ferrate could slow down the decline of membrane flux.When the dose of potassium ferrate was 12 mg Fe⁶⁺/L,the reversible membrane fouling resistance decreased by 89.0%,while the irreversible membrane fouling resistance changed little.The mitigation of membrane fouling was mainly attributed to the effective removal of organic macromolecules which could block the membrane pores,the reduction of the interaction between pollutants and membrane,and the transformation of the membrane fouling mechanism at the initial stage of filtration.In addition,potassium ferrate was also compared with two conventional coagulation pretreatment processes（ferric chloride and polyaluminum chloride）.The results showed that the potassium ferrate pretreatment process has more advantages in removing UV₂₅₄,fluorescent organics and cell-free resistance genes in the secondary effluent,while the polyaluminum chloride pretreatment process has a more significant effect on the mitigation of membrane fouling.If these two processes can be effectively coupled,it will help to further remove pollutants and alleviate membrane fouling.On this basis,the pretreatment process of potassium ferrate combined with polyaluminum chloride was studied,and the removal efficiency of pollutants in secondary effluent and the mitigation effect on subsequent ultrafiltration membrane fouling by different combined methods（oxidation before coagulation,coagulation before oxidation,and simultaneous oxidation and coagulation）were systematically investigated.The results showed that compared to the coagulation pretreatment,the combined pretreatment process significantly reduced the concentrations of DOC,UV₂₅₄,protein,fluorescent organics and total phosphorus in the secondary effluent,promoted the reduction of genotoxicity and the removal of trace organic pollutants and resistance genes in the secondary effluent.Among them,coagulation before oxidation method could reduce the genotoxicity by 84.1%,while simultaneous oxidation and coagulation method had the highest removal of trace organic pollutants（59.6%）.In terms of membrane fouling mitigation,the combined pretreatment process significantly reduced the total membrane fouling resistance by 23.4-67.0%,and delayed the transformation of membrane fouling mechanism from pore blocking to cake filtration.The characterization of membrane fouling showed that reversible membrane fouling was mainly caused by humic acids and tryptophan proteins,while irreversible membrane fouling was mainly caused by fulvic acids.The continuous flow test of potassium ferrate enhanced coagulation-densadeg-ultrafiltration process was further carried out.The results showed that the process could effectively ensure the effluent quality and alleviate membrane fouling at the same time.Finally,from the perspective of optimizing the operating conditions of ultrafiltration,a stable and efficient conductive ultrafiltration membrane was prepared.The mitigation effect of electrochemical in-situ cleaning process on membrane fouling was investigated,and its mechanism was explored.The results showed that the in-situ cleaning performance of the conductive ultrafiltration membrane was affected by the type of pollutants,electric field intensity and filtration time interval.Under the filtration mode of filtration for 4 min then in-situ cleaning for 1 min,the optimal in-situ cleaning electric field intensities of humic acid,bovine serum albumin and sodium alginate were 5 V/cm,3 V/cm and 5 V/cm respectively,and the corresponding membrane flux recovery rates were 99.6%,70.9%and 80.0%,respectively.Moreover,electrochemical in-situ cleaning can effectively alleviate the ultrafiltration membrane fouling caused by secondary effluent.Under the filtration mode of filtration for 29 min then in-situ cleaning for 1 min,the irreversible membrane fouling resistance could reduced by 50.6%when the in-situ cleaning electric field intensity was 10 V/cm.Through the characterization of conductive ultrafiltration membrane,it was found that after electrochemical in-situ cleaning,the pollutants on the fouled membrane surface were reduced,the membrane pores were gradually exposed,the hydrophilicity of the membrane surface was enhanced,and the absolute abundance of resistance genes intercepted on the membrane surface was significantly reduced.This was mainly due to the oxidation of pollutants on the membrane surface by free chlorine generated at the anode in the electric field,the electrostatic repulsion of negatively charged pollutants on the membrane surface by the electric field force,and the cleaning effect of hydrogen microbubbles induced on the membrane surface by the cathode membrane surface.The above three factors together destroy the cake layer attached to the membrane surface,thereby delaying the formation of the cake layer in the next filtration cycle.Finally,the process of potassium ferrate combined with polyaluminum chloride-conductive ultrafiltration membrane was studied.It was found that when the dose of potassium ferrate and polyaluminum chloride was 0.1 m M and 60 mg/L,respectively,and the in-situ cleaning electric field intensity was 10 V/cm,the process had the best effect on the removal of organic pollutants in secondary effluent and the mitigation of membrane fouling.The effluent of this process met the standards of“The reuse of recycling water-Water quality standard for scenic environment use”（GB/T 18921-2019）,which had a good application prospect.