Influence Of Iron And Manganese On Gravity-driven Membrane Process And Regulation Principle Using Pre-coated Dynamic Membrane

With the problem of manganese pollution in surface water becoming more and more prominent in drinking water supply,the new idea of using membrane process to treat surface water containing iron and manganese has been paid more and more attention.Gravity-driven membrane process（GDM）has the advantages of low energy consumption,low maintenance,no cleaning,relatively simple equipment and simple operation,so it is an ideal decentralized water purification technology.Therefore,the GDM process is chosen to combine the complementary action between the biofilter cake layer and the ultrafiltration membrane in order to treat the surface water containing iron and manganese.In addition,in order to solve the problem of start-up time in the conventional iron and manganese removal process,this study tried to pre-coat the surface of the membrane module with three kinds of common and easily available manganese oxides to form a dynamic membrane filter layer as a control mode for the treatment of surface water containing iron and manganese by GDM process.GDM process can be used to treat surface water containing iron and manganese,and the flux can reach a steady state during its long-term operation.The conventional GDM process has a good effect on the in-depth removal of iron in raw water.When a certain amount of iron and manganese ions exist in the raw water at the same time,the existence of manganese will not have a negative effect on the iron removal performance of the GDM system.In the long-term operation of the GDM system,the simultaneous existence of iron and manganese pollutants in the raw water has a limited impact on their removal efficiency.When a mature filter cake layer is not formed on the surface of the ultrafiltration membrane in the GDM reactor,the GDM process already has an effective effect of iron removal,and the biological filter cake layer formed on the surface of the ultrafiltration membrane is more beneficial to improve the iron removal efficiency of the system.In addition,the removal efficiency of manganese in GDM system is closely related to the filter cake layer.The chemical catalytic oxidation and biological activity during the operation of the system may participate in and contribute to the deep manganese removal process in the biofilter cake layer of the GDM reactor.In terms of organic matter treatment effect,the conventional GDM process has a good removal effect on UV₂₅₄ and DOC in raw water,and the existence of iron and manganese pollutants in raw water can also improve the removal efficiency of organic matter,which can be inferred that the enhancement effect should be related to the adsorption of newly formed iron oxides and manganese oxides on the filter cake layer during the operation of the GDM system.Pre-coating manganese oxide dynamic membrane on the membrane surface can effectively shorten the start-up time of GDM system to remove iron and manganese pollutants,and can show a good removal effect of iron and manganese ions in raw water at the initial stage of operation,and the flux stability of the system will not be destroyed during operation.In addition,the adsorption process of the enhanced GDM system at the initial stage of operation also accelerated the formation of the filter cake layer in the system,so that the average manganese removal rate of each enhanced group quickly recovered from negative to 95.4%when the manganese removal rate of the control group was 57%.Finally,the time required for the manganese concentration in the effluent of each enhanced group to reach the standard was 4-5 days shorter than that of the control group,among which the backwashing water group had the best treatment effect on iron and manganese pollution.Due to the pre-coating of manganese oxide dynamic membrane on the surface of the membrane,the removal efficiency of organic pollutants in GDM system has been improved to a certain extent,in which the adsorption of iron and manganese oxide may be the main reason for this effect.And due to the adsorption,the formation rate of the filter cake layer of the GDM system is also improved,which promotes the system to have the effect of removing organic matter earlier.The method of pre-coating manganese oxide dynamic membrane on the membrane surface can effectively shorten the start-up time of GDM system to remove iron and manganese pollutants,and can show a good removal effect of iron and manganese ions in raw water at the initial stage of operation,and will not destroy the flux stability of the system during operation,among which backwashing water has the best comprehensive strengthening effect on GDM system.Under the condition of iron and manganese in raw water,the flux stability of GDM system will not be affected,which also proves that GDM process is feasible to treat surface water containing iron and manganese.In the treatment of surface water containing iron and manganese,the stable flux of GDM process is mainly attributed to the coverage and porosity of the biological filter cake layer on the membrane surface,without considering the limited effect of EPS accumulation.The manganese oxide effectively improved the roughness and porosity of the biological filter cake layer,reduced the coverage and base thickness of the biological filter cake layer,and increased the flux by about 18%.However,on the contrary,the presence of iron in the influent will form a thicker and denser filter cake layer,increase the coverage of the biological cake layer,reduce the porosity of the biological cake layer,and lead to a decrease of flux（～17%）.The stable flux level of the GDM system has a good correlation with the coverage and porosity of the biological filter cake layer attached to the membrane surface,while the relative roughness and average thickness have little correlation with the stable flux level.In the aspect of strengthening GDM process,iron-manganese oxide and pre-coated manganese oxide filter layer have the function of adsorbing organic matter and EPS on the surface of the membrane.As a result,the related pollutants will gradually accumulate on the surface of manganese oxide,and a large number of manganese particles and clusters attached to the surface of the film will gradually form a variety of filamentous and flaky white films.This effect will make the pollutants that would have attached to the surface of the ultrafiltration membrane to form a dense"film"can not go deep into the cake layer,but can only be attached to the surface of manganese oxide.This structure not only makes the overall structure of the filter cake layer have better heterogeneity and roughness,but also provides a more suitable living environment for a large number of microorganisms in the filter cake layer,so that they have more frequent activities and multiply.and then effectively improve the stable flux of the system.After research,the treatment of surface water containing iron and manganese by GDM process is an economical and effective strategy for decentralized water supply,and the manganese oxide strengthening mode on this basis is also of great significance to improve the effluent quality and shorten the start-up time of GDM process for the treatment of surface water containing iron and manganese.The research results of this project will provide reference theoretical support for strengthening the application of GDM process in distributed water supply system.