Efficiency And Mechanism Of Pre-oxidation And In-Situ Ferric Salts Coagulation To Control Membrane Fouling During The Algal-Laden Water Treatment By Ultrafiltration

The eutrophication of water bodies leads to the risk of algae outbreak in lakes,reservoirs and other water sources,which seriously threatens the drinking water safety of urban residents.Ultrafiltration technology（UF）due to its high-efficiency solid-liquid separation efficiency and lower energy consumption requirements,has been increasingly used in drinking water treatment.Compared with traditional water treatment methods such as coagulation and precipitation,UF can effectively remove all kinds of algae that are ubiquitous in eutrophic lakes and reservoirs.However,membrane fouling caused by algae cells and organic matter discharged from the cells will affect the popularization of UF in algae-laden water treatment.In this thesis,we carried out ultrafiltration experimental research on the algae-laden water as a mixed system based on algae cells and the organic pollutants represented by extracellular organic matter（EOM）,to clarify the nature of the pollutants and prove the membrane fouling characteristics in the process of filtration treatment;comparative study of the strong oxidation method of Fe（II）activated potassium persulfate and the moderate oxidation method of Fe（II）-potassium permanganate coupling on the membrane fouling mechanism and water purification efficiency during the ultrafiltration treatment of algae-laden water,and explore the impact of the interface effect,so as to deeply understand the effectiveness and mechanism of pre-oxidation and in-situ iron salt coagulation to control membrane fouling during the algae-laden water treatment by ultrafiltration.The main conclusions are as follows:（1）The average particle size of algal cells was 2-4μm,and there was no obvious sedimentation.EOM solution is characterized by small molecules and hydrophilicity.The algal source pollutants can be classified as four parts,the humic acid organic matter is the main organic matter.Ultrafiltration technology can retain most of the microbial matter,but it cannot effectively remove humic acid organic matter.（2）The key mechanism of ultrafiltration membrane fouling is standard blocking,intermediate blocking and cake filtration.At the initial stage of filtration,algal cells and macromolecular organic matter were deposited on the surface of ultrafiltration membrane,and the blockage of membrane pore was mainly（1<n<2）.In the middle and later stage of filtration,the membrane fouling is dominated by the blockage of the filter cake layer（n≈0）.（3）The Fe（Ⅱ）/PMS and Fe（Ⅱ）/KMn O₄ pretreatment modified the pollutants,greatly promote the settlement of the algal cells,enhances the proportion of the 3～100 k Da interval of middle molecules and hydrophilic organics.XDLVO theory further explains the process of pollutant property transformation.High dosage of oxidants can cause large numbers of cells to die.（4）Regardless of it is algae-laden water as a mixed system or EOM solution pollution alone,the process of Fe（Ⅱ）/PMS and Fe（Ⅱ）/KMn O₄ pretreatment combination ultrafiltration can effectively alleviate the membrane fouling;XDLVO theory illustrates that the combined process can improve the affinity between the pollutant and the membrane,and effectively alleviate the membrane fouling.（5）The combined ultrafiltration process of Fe（Ⅱ）/PMS and Fe（Ⅱ）/KMn O₄ pretreatment can effectively alleviate ultrafiltration membrane fouling,which is the result of the combined action of cell ablation,oxidation effect and good coagulation effect produced by the new ecological Fe（Ⅲ）,but will aggravate the hydraulic irreversible membrane fouling.（6）The SEM results show that using Fe（Ⅱ）/PMS and Fe（Ⅱ）/KMn O₄ pretreatment can significantly increase the iron and manganese oxide residues on the membrane surface.In order to overcome other adverse effects,the dosage should be optimized and the strength and frequency of cleaning should be appropriately increased when using the ultrafiltration process by pre-oxidation and in-situ ferric salt coagulation.In conclusion,the results show that the oxidation of potassium persulfate or potassium permanganate and the coagulation pretreatment of ferrous sulfate can effectively alleviate the ultrafiltration membrane fouling caused by algal pollutants,and the combined process has a good application prospect in membrane fouling control in algae-laden water treatment.