DETERMINATION OF FOULING MECHANISMS FOR ULTRAFILTRATION OF OILY WASTEWATER A thesis submitted to the Division of Graduate Studies and Research of the University of Cincinnati in partial fulfillment of the requirements of the degree of MASTER OF SCIENCE (M.S.) in the Department of Chemical Engineering of the College of Engineering and Applied Science 2011 By Leila Safazadeh Haghighi BS Chemical Engineering, University Of Tehran, Tehran, Iran, 2008 Thesis Advisor and Committee Chair: Professor Rakesh Govind Abstract The use of Membrane technology is extensively increasing in water and wastewater treatment, food processing, chemical, biotechnological, and pharmaceutical industries because of their versatility, effectiveness, high removal capacity and ability to meet multiple treatment objectives. A common problem with using membranes is fouling, which results in increasing operating costs due to higher operating pressure losses, membrane downtime needed for cleaning, with associated production loss and manpower costs. In the literature, four different mechanisms for membrane fouling have been studied, which are complete pore blocking, internal pore blinding, partial pore bridging and cake filtration. Mathematical models have been developed for each of these fouling mechanisms. The objective of this thesis was to investigate the membrane fouling mechanisms for one porous and one dense membrane, during ultrafiltration of an emulsified industrial oily wastewater. An experimental system was designed, assembled and operated at the Ford Transmission Plant in Sharonville, Ohio, wherein ultrasonic baths were used for cleaning transmission parts before assembly. The oil wastewater, containing emulsified oils and cleaning chemicals was collected in a batch vessel and then pumped through a porous polyethersulfone, monolithic membrane, and through a dense cuproammonium cellulose membrane unit. For the porous membrane, use of a Dupont’s flurosurfactant (FS 63) and backwashing with permeate and for the dense membrane the use of both the flurorosurfactant and sparged air were investigated to reduce membrane fouling. ii For the porous membrane study, it was observed that the permeate flux was strongly dependent on the transmembrane pressure difference, and addition of the flurosurfactant significantly improved the performance of the membrane. The backwashing cleaning efficiency was found to depend on the duration of backwashing and its frequency. An integrated fouling model was developed by combining the individual models for each fouling mechanism, originally published by Hermia [18], and analysis of the experimental data for ultrafiltration of oily emulsion revealed that the primary mechanism for fouling of the porous membrane was cake filtration. With increasing transmembrane pressure, the role of other mechanisms, such as pore blocking and partial pore bridging, increases, although the effect of cake filtration dominates. Hence, for oily emulsions, methods to disrupt the formation of a cake layer at the membrane surface would have the most impact in increasing the water permeation rates through the membrane. For the dense membrane study, permeate flux also increased with increasing transmembrane pressure difference, as in the porous membrane, and the major mechanisms for fouling were found to be concentration polarization gel layer formation on the membrane surface. In this case, the use of both sparged air and fluorosurfactant, increased the water permeation rates, but the permeation rate improvement with sparged air alone was significantly higher than with fluorosurfactant only. A mathematical model was developed to derive the mass transfer coefficients under the various operation conditions. iii Future studies will concentrate on improving membrane performance by reducing the impact of the dominant fouling mechanisms, found in this study, for both porous and dense membranes. iv v DEDICATION THIS THESIS IS DEDICATED TO MY FAMILY vi Acknowledgements I would like to express my sincere gratitude to my adviser, Professor Rakesh Govind. I am deeply indebted to him for giving me the opportunity to work on this project. I’m particularly grateful for his enthusiastic guidance, discussion, understanding, encouragement and numerous hours spent helping me complete this thesis. I’d also like to express my special thanks to the thesis committee, Professor Junhang Dong and Professor Joo-Youp Lee for the efforts to provide valuable comments during my proposal presentation, for the their valuable time to review this thesis and for offering me an opportunity to defend in front of them. My appreciation is also extended to Mr. Lyle Carman and Mr. David Ferguson for helping me with the experimental set-up. I am really grateful to my dear husband for his company, understanding, continuous encouragement and support all the way. Without his help, these accomplishments would not have been possible. vii Table of Contents Abstract ..................................................................................................................…..…...ii Acknowledgement……………………………………………………………….………vii Table of Contents ......................................................................................................…..viii List of Figures ....................................................................................................................xi List of Tables ............................................................................................................…...xiv List of Symbols and Abbreviations………………………………………...…………...xvi Chapter 1: Introduction .......................................................................................................1 1.1 Motivation for Research……........................................................................................1 1.2 Membrane Filtration …………….................................................................................1 1.3 Membrane Fouling and its Mechanisms........................................................................6 1.3.1 Concentration Polarization………………………………………..................7 1.3.2 Cake Formation………………………………………………..................….8 1.3.3 Natural Organic Matter Adsorption……………………………...…..……...8 1.3.4 Calcium, Iron and Manganese Precipitation……………………………..… 9 1.3.5 Fouling Mechanisms………………………..…………………………….... 9 1.4 Factors Affecting Membrane Fouling……………………………….………….……10 1.5 Conventional Membrane Cleaning Methods……………………………………...…11 1.5.1 Backwashing………………………………………………...………….……11 1.5.2 Enhanced Backwashing……………………………………..………….……12 1.5.3 Chemical Cleaning…………………………………………..………………13 1.6 Disadvantages of the Conventional Cleaning Methods………….………..................13 1.7 Prevention and Reduction of Membrane Fouling……………………………………14 1.8 Pretreatment……………………………………………………………………….....14 1.8.1 Physical Disruption of Concentration Polarization………………………….15 1.9 Use of Surfactants……………………………………………………………………16 viii 1.10 Thesis Outline………………………………………………………………………17 Chapter 2: Literature Review ............................................................................................18 2.1 Ultrafiltration of Oily Emulsions.................................................................................19 2.2 Conventional Membrane Cleaning Methods….…………….……………………….22 2.2.1 Backwashing ................................................................................................23 2.2.2 Gas Sparging ................................................................................................24 2.3 Ultrasonic Cleaning of Membranes.............................................................................27 2.3.1. Effect of Sonication on Polymeric Membranes..............................................29 2.4 Fouling Mechanism in Ultrafiltration..........................................................................31 Chapter 3: Thesis Objectives ...........................................................................................34 Chapter 4: Materials and Methods ....................................................................................38 4.1 Selection of Membranes..............................................................................................38 4.2 Experimental Systems .................................................................................................42 4.3 Cleaning Procedure......................................................................................................45 4.4 Theory of Ultrafiltration of Oily Wastewater………….………………………….....48 4.5 Models for Membrane Fouling Mechanism……….………………………………...52 4.5.1. Fouling Mechanisms Involved In UF Using Porous Monolith Polyether Sulfone Membrane…………………………………………………………52 4.5.1.1 Complete Pore Blocking Model (n=2) …………………………….55 4.5.1.2. Internal Pore Blocking Model (n=3/2)…………..…….…………..56 4.5.1.3. Partial pore bridging model (n=1)…………………………..…….56 4.5.1.4. Cake Layer Formation Model (n=0)………………………………57 Chapter 5: Results and Discussions……………………………………………………...59 5.1 Filtration of Oily Wastewater using Porous Monolith Polyether Sulfone Membrane ……………………………………………………………………………………………59 5.1.1Effect of Transmembrane Pressure……………………………….…..……..59 5.1.2Effect of Feed Concentration……………………………………….…….....64 5.1.3Effect of Backwashing on Permeate Flux Recovery…………………...……66 ix 5.1.4Prediction of Permeate Flux by Hermia’s models……………...……………70 5.1.5. Flux Decay Analysis by using a combination of Hermia’s models…...........73 5.1.6. Mass Balance Analysis……………………………………………………..82 5.2 Filtration