Study On Membrane Fouling Characteristics And Its Control During The Treatment Of Algae-rich Water By Ultrafiltration

Under the situations that the drinking water quality standard becomes more andmore stringent，ultrafiltration （UF） technology which is excellent in the removal ofpollutant will be widely applied in drinking water production. However, theeutrophication of natural water cannot be effectively controlled till now, and themembrane fouling accosicated with algae will significantly hinder the application ofUF technology. Therefore, the study on the mechanisms and control of UF membranefouling caused by algae is essential for the further promotion of UF technology. Toaddress this problem, the study on the membrane fouling characteristics during thetreatment of algae-rich water by UF membrane, the study on the influence factors ofmembrane fouling caused by algae, and the study on the membrane fouling control bythe pretreatment of permanganate enhanced coagulation were carried out.Cyanobacterium was chosen for these experiments. Based on these bench scale studies,a pilot study was also performed with the natural seasonal algae-rich reservoir water inthe lower reaches of yellow river.Since algal cells and extracellular organic matter （EOM） were the most importantfoulants in algae-rich water, they were previously separated by the high-speedrefrigerated centrifugation, and their fouling effects were subsequently evaluated bythe nominalized flux decline, reversibility analysis of membrane fouling and massbalance of organic carbon. Moreover, the scanning electron microscope （SEM） and thetotal reflectance-fourier transform infrared spectroscopy （ATR-FTIR） were also used toanalyze the membranes fouled by the algal cells or EOM. The experimental resultsindicated that both algal cells and EOM could lead to serious flux decline andirreversible membrane fouling. Meanwhile, fluorescence properties, molecular weight（MW） distribution and hydrophobicity/hydrophilicity of EOM were also studied. Theresults revealed that EOM was comprised of protein-like, polysaccharide-like andhumic-like substances and was characterized by the high-MW （>100kDa） and stronghydrophilicity. In addition，prefiltration, calcium addition and XAD fractionation wereemployed to change the characteristics of EOM, and then the effects of thesecharacteristics on the fouling effects of EOM were compared. It was found that thefraction of EOM between100kDa and0.45μm contributed a significant portion of the fouling. As EOM was negatively charged under neutral condition, the electrostaticrepulsive between foulant and membrane surface could prevent the organics fromadhering to the membrane. The hydrophobic organics in EOM tended to adhere tomembrane surface causing irreversible fouling, while the cake layer formed by thehydrophilic organics caused greater resistance to water flow, leading to faster fluxdecline during UF. Furthermore, EOM was classified into the dissolved EOM （dEOM）which was released into water and the bound EOM （bEOM） which surrounded thecells, and their characteristics and fouling effects were compared. It was found thatbEOM was mainly composed of protein-like substance and was characterized byhigh-MW and strong hydrophobicity, while that dEOM contained protein-like,polysaccharide-like and humic-like substances and was much more hydrophilic.Results of UF experiments indicated that dEOM caused more severe flux decline thanbEOM while that bEOM led to the membrane fouling with much worse reversibility.Both flux decline and irreversible membrane fouling caused by the cells wereaggravated when the cells were together with EOM. In term of fouling mechanisms,cake layer formation, hydrophobic adhesion and pore plugging were the mainmechanisms for membrane fouling when treating algae-rich water with UF.Study on the effects of solution chemistry on the fouling caused by algae showedthat the alkaline condition had not influence on the fouling effects of both algal cellsand EOM, while that the acid environment could lead to the cell lysis and exacerbatedthe flux decline and irreversible fouling. Moreover, the adhesion of EOM onmembrane could also be intensified in the acid environment. Inorganic particles couldalleviated the flux decline and irreversible fouling caused by algal cells, but they wereinefficient in reducing the membrane fouling related to EOM. Natural humic-likesubstance could aggravate the fouling caused by both algal cells and EOM. Theproperties of membrane were also found to apparently affect the membrane fouling.When the membranes with higher molecular weight cutoff （MWCO） were used to treatalgal cell solution or EOM solution, more severe flux decline but better reversibility offouling could be found. The hydrophilic membrane was found to be better inanti-fouling ability than hydrophobic membrane, and the hydrophobic interaction couldenhance the adhesion of algal cells and EOM to the membrane surface. It wasfavorable to increase the frequency and intensity of backwashing for the sake offouling control. However, the combined backwashing with air and water could not mitigate but aggravate the fouling, because it might caused the lysis of algal cells andthe resuspension of cell aggregates on the membrane surface during hydraulicbackwashing.Effects of different pretreatments such as coagulation, KMnO₄preoxidationenhanced coagulation and hydrated manganese dioxide （δ-MnO₂） enhancedcoagulation on the membrane fouling caused by algal cells or EOM were compared. Itwas found that coagulation pretreatment could reduce the fouling caused by both algalcells and EOM under the favorable coagulant dosage. However, the fouling woulddeteriorate when the coagulant was over dosed. In addition, KMnO₄preoxidationenhanced coagulation could dramatically reduce the fouling caused by algal cells orEOM. Reducing the foulant load and improving the structure of cake layer were themain mechanisms for fouling control. The pilot study of treating natural algae-richwater with the hybrid process of preoxidation enhanced coagulation and UF wascarried out. The results of optimizing experiment of the operational parametersindicated that the optimum dosages of PACl and PPC were4and0.6mg/L,respectively. The results of pilot experiment suggested that PPC preoxidation could notonly improve the quality of water, but also mitigate the membrane fouling during UFof natural algae-rich water.This thesis focused on the identification of foulants associated with algae and theeffects of their characteristics on UF membrane fouling. Based on these results, thehybrid process of premangnate preoxidation enhanced coagulation and UF wasconstructed for the treatment of algae-rich water. This thesis can provide supports forthe running and manage of the waterworks in which UF technology is used, when theseasonal algae bloom occurs.