MICROFILTRATION MEMBRANE FOULING IN WATER TREATMENT: IMPACT OF CHEMICAL ATTACHMENTS
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Membrane fouling is the loss of membrane permeability as a result of the accumulation of aquatic materials on membrane surfaces. It was hypothesized in this study that the origin of MF membrane fouling is the chemical attachment of these materials on membrane surfaces, which is variable with varying solution chemistry. This hypothesis was tested using model simulation and experimental work. A mathematical model was first developed based on analyses of particle attachments in MF and a hydraulic model that relates the extent of membrane fouling to the mass of particles attaching to different areas of the membrane. The model simulation results indicate that depositional attachment is primarily responsible for membrane fouling. However, increase in coagulational attachment can reduce the occurrence of pore blocking type of fouling, thereby lowering the extent of the total fouling. Particle size has secondary effects on membrane fouling and generally affects its extent. Particles with radii in a range of 1/6 ~ 1/2 of membrane pore diameter cause the greatest fouling when they are sticky to membrane surfaces. These findings were subsequently validated using a polyvinylidene fluoride (PVDF) MF membrane and monodisperse polystyrene latex particles with predetermined sizes and chemical stabilities. This model was finally applied to the understanding of natural organic matter (NOM) fouling of the PVDF membrane. The combined results from model simulation, membrane fouling experiments, and various analytical techniques suggests that the model NOM consists of three major components, each with different relevance to membrane iii fouling. Component A has sizes close to membrane pores and can cause substantial fouling regardless of the solution chemistry. Component B has sizes and stabilities that vary with varying solution chemistry, and therefore, can foul the membrane to a different extent. Component C does not directly cause membrane fouling due to its small size. Overall, this study established a mechanistic model useful in the understanding of MF membrane fouling, with potential applications to other low pressure membranes. The results suggest that particle-membrane attachment is the primary reason for irreversible MF membrane fouling, which should be of primary concern in the research, design and operation of MF systems.