MICROFILTRATION MEMBRANE FOULING IN WATER TREATMENT: IMPACT OF CHEMICAL ATTACHMENTS by Haiou Huang A dissertation submitted to The Johns Hopkins University in conformity with the requirements for the degree of Doctor of Philosophy Baltimore, Maryland September 29, 2006 © 2006 Haiou Huang All right reserved Abstract Microfiltration Membrane Fouling in Water Treatment: Impact of Chemical Attachments by Haiou Huang 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 ii 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. Thesis Advisor: Professor Charles R. O’Melia Readers: Professor Charles R. O’Melia Professor William P. Ball Professor Eugene Shchukin Dr. Joseph G. Jacangelo iii Acknowledgements Despite all the technical terms and scientific jargon used, this study is really about a simple aspect of human life, i.e., affordability. More specifically, it carries out the mission of making a relatively novel technology, microfiltration, affordable to everyone who wants to have enough drinkable water for his/her daily life. This mission fits reasonably well into the basic needs of mankind, but does not perfectly match the language of “pure” science. Therefore, the readers have to accept the author’s apology for covering the core mission under verbose descriptions of theories and experimentations throughout this essay. Before letting the readers rush into the forest (and eventually get lost), the author would like to first express his gratitude to his academic advisor and mentor, Dr. Charles R. O’Melia, for his insightful guidance and incredible patience in allowing a student to spend so many years on toy bricks in his lab and explore the nature of science. CROM’s scholarship and altruism set the standard for the author to follow in his own career. Second, the author would like to thank his family for their long term support to his study, even though it has made their own lives so “unaffordable,” both financially and emotionally. Hopefully, all their sacrifices will be rewarded eventually. Third, the spiritual support from Prof. Long-ping Yuan should be acknowledged. Although he never knows the author (one of the hundreds of audiences listening to his talk in Tsinghua University back in 1998), yet it is his dedication to his career and contribution to his people that convinced the author that the bright part of science does shine over the dark clouds of agonies and personal sufferings. Special thanks also go to Dr. Joseph G. iv Jacangelo and Prof. Eugene Shchukin. Their generous support helped the author to enter a broader area of his discipline and career. The author would like to thank the readers of this thesis, Professors William P. Ball, Eugene Shchkin, and Joseph G. Jacangelo, for their helpful comments and suggestions, which strengthen the scientific value of this work. Other professors and colleagues in DoGEE deserve many, many thanks for making the tough life of a researcher bearable and fruitful. The author will remember Dr. Wolman’s remarks on the conflicts of equality and efficiency in the history of mankind. The author will also remember the discussions on “politics” with Adrian Penisson, who, unfortunately, left us before we could end the talks... Meanwhile, Mr. Rodrigue Spinette gave a lot of useful suggestions during this study, which is crucial in the formation of the conceptual model discussed in this thesis. Finally, the author gratefully acknowledge the support from the industrial partners. US Filter/Memcor kindly provided the hollow fiber membranes; Arkema/Atofina donated the Kynar samples used in the study. US Filter also provided partial funding to this study through the Olivieri fellowship. v Table of Contents Abstract ........................................................................................................................... ii Acknowledgements .......................................................................................................... iv Table of Contents ............................................................................................................. vi List of Figures................................................................................................................... xi List of Tables ................................................................................................................. xxii 1 BACKGROUND ............................................................................................. 1 1.1 Scope and Objectives..................................................................................... 1 1.2 Application of Microfiltration in Water Treatment ....................................... 2 1.2.1 The History................................................................................................. 2 1.2.2 MF Membrane Properties ........................................................................... 4 1.3 Membrane Fouling in Practice....................................................................... 5 1.4 Mechanistic Investigation of Membrane Fouling by Aquatic Contaminants 7 1.4.1 NOM Characteristics.................................................................................. 7 1.4.2 Recent Advances in the Understanding of Membrane Fouling.................. 9 1.5 Mathematical Models for Membrane Fouling ............................................. 13 1.6 Chemical Attachment Relevant to Microfiltration ...................................... 17 1.6.1 Chemical Attachment by Dispersion Interaction...................................... 17 1.6.2 Physiochemical Attachment by “Polar” Interactions................................ 19 1.6.3 Chemical Attachment in the Presence of NOM........................................ 21 1.6.4 Chemical Attachments between Heterogeneous Surfaces........................ 22 vi 2 A CHEMICAL ATTACHMENT BASED MEMBRANE FOULING MODEL ......................................................................................................................... 24 2.1 Introduction.................................................................................................. 24 2.2 Physical Description.................................................................................... 25 2.3 Mathematical Modeling............................................................................... 33 2.3.1 Model Membrane and Aquatic Contaminant............................................ 33 2.3.2 Collision and Attachment: Large Particles ............................................... 36 2.3.3 Collision and Attachment: Small Particles without Pore Constriction..... 40 2.3.4 Collision and Attachment: Small Particles with Pore Constriction.......... 47 2.3.5 Reduction of Membrane Permeability: Large Particles (a ≥ Dm/2) .......... 48 2.3.6 Reduction of Membrane Permeability: Small Particles (a < Dm/6).......... 50 2.3.7 Reduction of Membrane Permeability: Small Particles (Dm/6 < a < Dm/2 ). ................................................................................................................... 52 2.4 Model Simulation......................................................................................... 53 2.4.1 Basic Modeling Conditions....................................................................... 53 2.4.2 Fouling by Large Particles.......................................................................