Mechanistic understandings of membrane fouling in membrane filtration with particulate foulants

• Main Author: Trinh, Thien An
• Other Authors: Chew Jia Wei
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Chemical engineering; Engineering::Environmental engineering::Water treatment
• Online Access: https://hdl.handle.net/10356/137231
• Institution: Nanyang Technological University

Membrane filtration is an advance filtration technology that has been widely applied in many fields that requires separation. However, during membrane filtration, the process of deposition of components to the membrane that reduce the separation performance, i.e., membrane fouling, is inevitable. Despite many research studies that have been conducted, the full understanding of membrane fouling is still far from perfect due to the complication in the mechanism of the fouling process. In view of this gap of knowledge, a series of research studies were carried out to investigate the underlying mechanisms contributing to membrane fouling and provide with deeper understanding of membrane fouling as well as membrane fouling mitigation, using the advance in-situ and non-invasive three-dimensional (3D) optical coherence tomography (OCT) monitoring and characterization method. The first chapter provides basic background of membrane filtration process and the objective of the studies. The working principle of a 3D OCT was discussed and the applications of OCT and other techniques in the past studied was reviewed. Specifically, compared to other techniques, OCT has the advantages of the full combination of real-time, non-invasive and capable of perform 3D scanning to monitor and characterize membrane fouling, which was used as the main novel method in the studies of this thesis. The literature review and gap of knowledge is discussed in Chapter 2, together with the Objectives of the studies in this Thesis and the outline of the Thesis. Membrane fouling in practice is a phenomenon that involves many complex underlying mechanisms caused by different factors such as the surface charges, the polydispersity and the natural phase (i.e., solid or liquid) of the foulants in the feed. Therefore, the studies in this thesis aimed to provide more understandings of these effects. While Chapters 3 and 4 focused on investigating membrane fouling caused by oil-in-water emulsions, Chapters 5 focuses on membrane fouling caused by solid droplets, emphasizing on internal fouling. The OCT monitoring and characterizing technique was applied in all of the studies with the aid of addition mathematical tools such as membrane fouling modeling or thermodynamic theories. In Chapter 3, internal fouling by oil droplets monitored and analyzed by optical coherence tomography was reported for the first time. Both internal and external fouling by oil droplets were studied in comparison with solid glass particle. In the studied cases, compared with solid foulants, oil droplets tended to deposit onto one another to a greater extent, and capable of causing internal fouling even when the droplet size is of order of magnitude larger than the membrane pores. This was due to the nature of the liquid phase that allowed the droplets to deform to penetrate the pore or to wet the membrane. In Chapter 4, to study the effect of the stabilizing surfactants present in oil-in-water emulsions on fouling, thermodynamic models were employed together with OCT to describe and explain the interactions that lead to oil droplet deposition on the membrane surface. Oil emulsions stabilized by three surfactants with three different charges were studied (namely, positively, non-ionic and negatively charged). The repulsive energy, but not the attractive energy, predicted either by the DLVO or XDLVO model correlated well with the extent of fouling. In terms of surface charges, the oil emulsions stabilized by the non-ionic surfactant had the greatest fouling, characterized by the lowest peak of repulsive total Gibbs energy and the greatest increase of intensity in OCT images. In practice, besides external fouling which accounts for the deposition of foulants on the membrane surface, internal fouling is evitable when the particles are smaller than the membrane pores. Compared to external fouling, studying internal fouling is more challenging due to the location of the deposition which is inside the membrane structure. Using OCT could overcome this issue and a study was conducted and reported as in Chapter 5. Specifically, polystyrene particles smaller than a quarter of the membrane pore was used as foulants, and three types of particles with different surface charges was used. The membrane used had negative surface charged too. In addition, a concept of “internal cake” which accounted for the cake formed inside the pores upon internal fouling was applied and developed into a mathematical model. The OCT results and the model regression showed the least fouling by the most negatively charged particles which was described well by the pore constriction scheme alone, and the greater fouling in the case of the less negatively charged and the positively charged particles which was described well only by the combination of the pore constriction, pore blockage and internal cake fouling schemes. Besides, the parameters obtained from the model regression revealed the progressive cake growth perpendicular to the permeate flow which led to the higher cake resistance in the case of positively charged particles, and the less ordered deposition of the negatively charged particle which led to more pore blockage and more pore constriction but lower cake resistance. These deposition mechanics were attributed to the interaction between the membrane and the particles. Chapter 6 concludes the findings in the Thesis and Chapter 7 compiles a list of publications. Chapter 8 provides additional co-authors works that studied membrane external fouling by solid particles using OCT.