Rejection of organic matters by RO NF membrane

REJECTION OF ORGANIC MATTERS BY RO/NF MEMBRANE SHAN JUNHONG NATIONAL UNIVERSITY OF SINGAPORE 2005 Founded 1905 REJECTION OF ORGANIC MATTERS BY RO/NF MEMBRANE BY SHAN JUNHONG (B.Sci. Tsinghua Univ.) A THESIS SUBMITTED FOR THE DEGREE OF PHILOSOPHIAE DOCTOR DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2005 ACKNOWLEDGEMENT The author wishes to express her deepest appreciation and gratitude to her supervisors, Associate Professor Hu Jiangyong and Professor Ong Say Leong for their invaluable guidance and encouragement throughout the entire course of the research project. The author would also like to extend her sincere gratitude to all technicians, staff and students, especially Mr. S.G. Chandrasegaran, Ms. Lee Leng Leng, Ms. Tan Xiaolan at the Environmental Engineering Laboratory of Department of Civil Engineering, National University of Singapore, for their assistance and cooperation in the many ways that made this research study possible. Thanks are also due to the Bedok Water Reclamation Plant for the provision of raw water used in this study. The assistance and cooperation of the staff at the Bedok Water Reclamation Plant are greatly appreciated. The author also appreciates Dr. Wang Rong at the Institute of Environmental Science & Engineering and her student for analyzing the contact angle and zeta potential of the membrane materials. i TABLE OF CONTENTS Pages ACKNOWLEDGEMENT . i TABLE OF CONTENTS ii SUMMARY vii NOMENCLATURE . x LIST OF FIGURES xii LIST OF TABLES xv LIST OF PLATES . xvi CHAPTER ONE INTRODUCTION . 1.1 Background . 1.2 Objective and Scope of Study . 1.3 Outline of Thesis . CHAPTER TWO 2.1 LITERATURE REVIEW Water Reclamation 2.1.1 Possible Solution to Water Resource Shortage 2.1.2 Advanced Technologies for Water Reclamation 11 2.2 Membrane Technology and Its Application in Water Reclamation . 12 2.2.1 Microfiltration and Ultrafiltration 12 2.2.2 Reverse Osmosis and Nanofiltration 15 2.2.3 Application of Membrane Technology in Water Reclamation . 16 2.3 Problems in the Use of Reclaimed Water for Public Consumption 18 2.3.1 Organic Matters in Reclaimed Water . 20 2.3.2 Selection of Water Reclamation Process 22 2.4 Organics Rejection by Membrane 24 ii 2.4.1 Size Exclusion Effect 25 2.4.2 Adsorption Effect . 26 2.4.3 Electric Exclusion Effect 27 2.4.4 Intermolecular Interaction Effect 29 2.4.5 Membrane Transport Model . 31 2.5 Current Status and Research Needs 35 CHAPTER THREE MATERIALS AND METHODS 41 3.1 Introduction . 41 3.2 Experimental Set-up and Configuration . 43 3.2.1 Multi-barrier Dual-Membrane System . 43 3.2.2 Backwash for Microfiltration . 46 3.2.3 Cleaning for RO Membrane . 46 3.2.4 Low-pressure Cross-flow Cell System . 47 3.2.5 Operational Conditions . 49 3.3 Fractionation Process 50 3.4 Sampling and Analysis Methods 52 3.4.1 Water Sampling and Analysis 53 3.4.1.1 Water Sampling . 53 3.4.1.2 TOC and UV254 Analysis 54 3.4.1.3 TDS Analysis 55 3.4.1.4 Molecular Weight Analysis . 55 3.4.1.5 pH and Temperature Analysis . 56 3.4.1.6 Ion Analysis 57 3.4.1.7 Fraction Charge Measurement 58 3.4.1.8 Membrane Surface Analysis . 58 iii 3.5 Experimental Design for Factors Affecting the Rejection of Organic Matters by Membrane 62 3.5.1 Preliminary Study on Rejection of Organics Fractions 63 3.5.2 Adsorption Effect Study . 63 3.5.2.1 Static Adsorption and Desorption . 64 3.5.2.2 Dynamic Membrane Seperation 65 3.5.3 Charge Effect Study 66 3.5.3.1 pH Effect . 66 3.5.3.2 Ionic Strength Effect . 67 3.5.4 Interactions among Fractions 67 CHAPTER FOUR RESULTS AND DISCUSSIONS . 69 4.1 Introduction . 69 4.2 Preliminary Study on Rejection of Organics Fractions 71 4.2.1 Characteristics of the Secondary Effluent 71 4.2.2 Rejection of Organics Fractions by RO Process 73 4.2.3 Removal Efficiency of the RO Process with Respect to each Isolated Fractions . 76 4.2.4 Removal Efficiency of the RO Process with Respect to two Experimental Sequences . 78 4.3 Adsorption Mechanism for Organics Rejection by RO/NF Membrane . 81 4.3.1 Static Adsorption Study 82 4.3.1.1 Adsorption Rate Limiting Mechanisms and Classification of Organics Fractions 83 4.3.1.2 Determination of Surface Coefficient, β in the D-CAM . 85 4.3.1.3 Determination of Overall Adsorption Rate, r, in the R-CAM . 95 iv 4.3.1.4 Verification of the D-CAM . 97 4.3.1.4.1 Base and Neutral Fractions 98 4.3.1.4.2 Discrepancies between experimental data and model simulation based on β 100 4.3.1.5 Verification of the R-CAM . 101 4.3.1.5.1 Acid Fractions . 103 4.3.1.5.2 Discrepancies between experimental data and model simulation based on r 104 4.3.1.6 Adsorption Performance and Membrane Surface Characteristics 104 4.3.2 Dynamic Adsorption Study 106 4.4 4.3.2.1 Factors affecting adsorptive effect 107 4.3.2.2 Membrane rejection for neutral organics 109 4.3.2.3 Proposed Adsorption mechanism on organics rejection by NF/RO 118 Electric Exclusion Mechanism for Organics Rejection by RO/NF 120 4.4.1 Charge Characteristics of Acid and Base Fractions . 121 4.4.2 Zeta Potential of membrane materials 123 4.4.3 Effects of pH . 125 4.4.4 Effects of Ionic Strength . 133 4.5 Effects of Interaction between Fractions on Membrane Rejection Performance 142 4.5.1 Interactions between Acid & Base Fractions 144 4.5.2 Interactions between Neutral & Base Fractions . 146 CHAPTER FIVE CONCLUSIONS AND RECOMMENDATIONS 149 5.1 Conclusions . 149 5.2 Recommendations . 153 v REFERENCES . 155 APPENDIX RATE MODELS DEVELOPMENT 174 A.1 Diffusion-Controlled Adsorption Model 174 A.2 Reaction-Controlled Adsorption Model . 178 vi SUMMARY Water reclamation and reuse have become an increasingly important means for meeting increasing demand for water caused by continued population growth and contamination of water sources. However, the presence of various contaminants in treated effluent presents a challenge to the operation of water reclamation system, especially organics which are of increasing concerns with respect to their potential health effects. Membrane technology such as nanofiltration (NF) and reverse osmosis (RO) are likely to play important roles in removal of those compounds. A lab-scale microfiltration (MF)-RO membrane system was used in the preliminary study. Seven isolated fractions were obtained by using column chromatography fractionation process from a secondary effluent, which contained a total organic carbon (TOC) concentration of 14.3 to 29.4 ppm. Isolated fractions and MF pre-treated secondary effluent were subjected to membrane separation. The results revealed that hydrophobicity, charges of solute molecules and membrane materials, as well as the interactions among complex organic matters were the three major factors that could affect the rejection mechanisms of organics removal by NF and RO membranes. A SEPA cell flat-sheet membrane system was used in the later part of the study to assess the effects of these factors on organics rejection and to investigate the possible mechanisms for their removal. In the study of adsorption effect, two adsorption models, the modified diffusioncontrolled adsorption model (D-CAM) and reaction-controlled adsorption model (Rvii CAM), were modified and applied to describe the rate of organics adsorption. The results of atomic force microscopy (AFM) analysis indicated that adsorption capacity of a membrane tended to increase with its membrane surface roughness. With continuous-flow experiments using neutral fractions, it was found that the membrane rejection performance varied with the significance of adsorption (indicated by σ). More specifically, the larger the σ value, the greater the potential effect of adsorption on membrane rejection performance. In the study of electric exclusion effect, pH and ionic strength were selected as the two most important operating parameters. Rejections of hydrophobic-acid (Hpo-A), hydrophilic-acid (Hpi-A) and hydrophilic-base (Hpi-B) were found to be among the worst at pH 4, better at pH and the best at pH 9. This rejection phenomenon is attributed to a combination of the hydrophobicity of the organics fraction, variation of membrane charge and tightness, and the extent of dissociation of the organics fractions with pH. Ionic strength showed less significant effect. However, it was noted that medium ionic strength was the most favorite condition for rejection of charged organics fractions. 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The rate-limiting step in DCAM is molecular diffusion from the bulk solution to the proximity of the membrane surface. Fick’s 1st Law is given by: J s = − Dsw ∂C ∂x (A.1) and Fick’s 2nd Law is given by: ∂C ∂ 2C = Dsw ∂t ∂x (A.2) where, Js = solute flux, µg/cm2s; Dsw = diffusion coefficient of solute in water, cm2/s; C = bulk concentration of solutes, µg/cm3; x = distance from the membrane surface, cm; and t = time, s. The following assumptions have been made in the derivation: 1. Membrane surface is assumed to be flat and large so that end effects could be neglected. 2. Experience has shown that the flux often remains linear over substantial ranges 174 Appendix of the concentration gradient (Lyklema, 1991). Hence diffusion coefficient is taken to be a constant value. 3. The bulk volume is assumed to be so large that the concentration changes due to the depletion of the thin layer near the membrane surface are negligible (semi-infinite diffusion). This gives rise to two boundary conditions: C ( x,0) = C (A.3) C (−∞, t ) = C (A.4) where, C0 = initial bulk concentration of solutes, µg/cm3. 4. Solute concentration on membrane surface is assumed to be zero as the surface process is fast compared with the supply by diffusion. Therefore: C (0, t ) = (A.5) 5. Langmuir-type and monolayer adsorption is assumed. Linear diffusion in the x-direction perpendicular to the membrane surface where x = 0, was considered. Equation (A.2) was first solved by Laplace transformation, which involves conversion of C into its Laplace transform C as a function of x and s, where s is the Laplace variable that replaces t. Hence, the Laplace transformation of (A.2) is given by: C ( x, s ) = C Dsw d C ( x, s ) + ( ) s s dx (A.6) which is an ordinary differential equation in x, rather than the partial one given by (A.2). The general solution of (A.6) can be expressed as: C ( x, s ) = C0 s 1/ s 1/ + A( s ) exp[− x( ) ] + B ( s ) exp[ x( ) ] s Dsw Dsw 175 (A.7) Appendix where the integration constants A(s) and B(s) were determined using the boundary conditions (A.3) and (A.4). As x → -∞, A( s) = (A.8) and when x = 0, B(s) is expressed in terms of the surface concentration C (0, s ) : B ( s ) = C (0, s ) − C0 s (A.9) Substituting (A.8) and (A.9) into (A.7) lead to: C ( x, s ) = C C0 s 1/ + [C (0, s ) − ] exp[ x( ) ] s Dsw s (A.10) The Laplace transformation of equation (A.1) can then be determined: J s ( x, s ) = Dsw d C ( x, s ) dx (A.11) and the differentiation was carried out using (A.10). This yields: 1/ J s ( x, s ) = s / Dsw [C ( x, s ) − C0 s 1/ ] exp[ x( ) ] s Dsw (A.12) At the membrane surface where x = 0, the solute flux becomes: 1/ J s (0, s ) = s / Dsw [C (0, s ) − C0 ] s (A.13a) or rearranging, C (0, s ) = C J s (0, s ) + 1/ 1/ s Dsw s (A.13b) Equation (A.13b) was back-transformed into the real time domain and by applying the boundary condition (A.5). The result for the solute flux at the membrane surface is: 176 Appendix Js = ( Dsw / ) C0 πt (A.14) Since monolayer adsorption (Langmuir-type adsorption) is assumed, as the adsorption proceeds, the number of sites available for adsorption decreases. To account for this decrease in membrane surface area for adsorption, the term − Γ(t ) / Γe was introduced (Jones and O’Melia, 2000): J s = [1 − Γ(t ) Dsw / ]( ) C0 Γe π t (A.15) where, Γ(t) = mass adsorbed per membrane surface area at time t, µg/cm2; Γe = mass adsorbed per membrane surface area at equilibrium for a given bulk concentration and solution condition, µg/cm2. A surface coefficient, β, was proposed to account for the increase in surface area for adsorption due to roughness and porosity of membrane. Hence, the solute flux adhering to the membrane surface is: J s = β [1 − Γ(t ) Dsw / ]( ) C0 Γe π t (A.16) The adsorption process increases at a rate determined by the diffusion flux so that: Js = dΓ ∂C = Dsw dt ∂x (A.17) x =0 Assuming there is no adsorption at t = 0, we have Γ(0) = Solving (A.16) and (A.17) yields the modified diffusion-controlled adsorption model 177 Appendix which was proposed to describe the adsorbed concentration at the membrane surface: Γ(t ) = Γe{1 − exp[( − 2C β Dsw t / )( ) ]} Γe π (A.18) A.2 Reaction-Controlled Adsorption Model In the reaction-controlled adsorption process, the molecular interaction at the membrane surface is the rate limiting step. Based on this understanding, the R-CAM was derived. The following assumptions have been made in the derivation: 1. There exists a subsurface layer (a thickness of a few molecular diameters immediately adjacent to the membrane surface) within which the adsorption process is studied. 2. The local equilibrium between the subsurface and the membrane surface is assumed to follow Langmuir kinetics. 3. The subsurface concentration is assumed to be equal to the initial bulk concentration since the diffusion process is fast compared to the surface process. Langmuir kinetics is expressed as a difference between adsorption and desorption flows (Peterson and Kwei, 1961; Baret, 1968; Bleys and Joos, 1985): Γ dΓ Γ = k a C s (1 − ) − k d dt Γm Γm (A.19) where, Γ = mass adsorbed per membrane surface area at time t, µg/cm2; t = time, s; 178 Appendix ka = adsorption rate constant, cm/s; kd = desorption rate constant, µg/cm2s; Cs = concentration in the subsurface, µg/cm3; and Γm = maximum mass adsorbed per membrane surface area, µg/cm2. Applying the assumptions, the Langmuir kinetics is transformed to: Γ dΓ Γ = k a C (1 − ) − k d dt Γm Γm (A.20) Defining k a ' = k a * C where ka' represents the adsorption rate constant which takes into account the effect of initial bulk concentration of solutes. This definition is similar to that of kd. Thus, equation (A.20) becomes: dΓ Γ Γ = k a ' (1 − ) − k d dt Γm Γm (A.21) At equilibrium when the rate of adsorption is equal to the rate of desorption from the membrane surface, that is dΓ = , Langmuir isotherm is yielded: dt k a' Γe = Γm k a ' + k d (A.22) Γe = mass adsorbed per membrane surface area at equilibrium, µg/cm2. Substituting (A.22) into (A.21) results: Γ dΓ Γe (k a ' + k d ) − (k a ' + k d ) = dt Γm Γm (A.23a) and hence, 179 Appendix dΓ k a ' + k d = (Γe − Γ) dt Γm (A.23b) Assuming there is no adsorption at t = 0, that is Γ(0) = 0. The general solution of (A.23b) becomes: Γ(t ) = Γe {1 − exp[−( ka' + kd )t ]} Γm (A.24) Defining, r= ka' + k d Γm (A.25) where r represents the overall rate of adsorption. The bigger the r value, the faster and larger the adsorption will be. In addition, Γm from (A.22) can be expressed as: Γm = Γe ( k a' + k d ) ka' (A.26) Substituting (A.25) into (A.24), the reaction-controlled adsorption model is therefore obtained as: Γ(t ) = Γe [1 − exp(− rt )] (A.27) By substituting (A.26) into (A.25), r can be simplified as follows: r= k a' Γe (A.28) 180 [...]... capacity; Membrane roughness & pore size Dynamic rejection profiles; Neutral fractions adopted; Initial organic conc effect Charge properties: Organics fractions; Membrane surface Rejection profiles: Acid & Base fractions adopted; Effect of pH & ionic strength Feasibility of improving the rejection of the most permeable fractions Selection of target fraction based on previous results; Type of added... in hydrophobicity and charge properties Subsequently, study on adsorption of organics fractions, electrostatic interaction between charged fractions and membrane, as well as interactions between organics fractions will be conducted to 4 Chapter One-Introduction provide an in-depth understanding on organics rejections by NF and RO membranes 1.2 Objective and Scope of Study The main objective of this... pH on removals of acid and base fractions by AG membrane 126 Figure 4.25 Effects of pH on removals of acid and base fractions by SG membrane1 27 Figure 4.26 Effects of pH on removals of acid and base fractions by ST-28 membrane xiii 128 Figure 4.27 Effects of pH on removals of acid and base fractions by HL membrane1 29 Figure 4.28 Effects of pH on removals of acid and base... properties when a driving force is applied across the membrane Membranes may be classified by the range of materials separated and the driving forces employed For example, MF and RO are two typical membrane processes that use pressure to transport water across the membrane MF membranes are capable of removing only particulate matter; while RO membranes retain solutes as water permeates through the membrane. .. — Dissolved Organic Matters D-CAM — Diffusion-controlled adsorption model FA — Fulvic Acids HA — Humic Acids Hpi-A — Hydrophilic acids Hpi-B — Hydrophilic bases Hpi-N — Hydrophilic neutrals Hpo-A — Hydrophobic acids Hpo-B — Hydrophobic bases Hpo-N — Hydrophobic neutrals MF — Microfiltration MW — Molecular Weight MWCO — Molecular weight cut-off NF — Nanofiltration NOM — Natural Organic Matters PA —... react with chlorine disinfectants to produce chlorinated disinfection by- products For organics rejection and removal, membrane technology such as reverse osmosis (RO) and nanofiltration (NF) are commonly employed Their usages have also been gaining popularity in water reclamation and reuse systems Mallevialle et al (1996) summarized the following trends in organics rejection by RO: • Rejection increases... In-depth understanding of rejection mechanisms for organics removal at a sub macrolevel by RO/ NF membranes Figure 1.1 Structure of the study 5 Chapter One-Introduction The scope of this research includes: I To conduct a preliminary study on the potential effects of adsorption, electric exclusion and interaction between organics fractions on organics rejections and to postulate appropriate removal mechanisms... limited literature and knowledge on organics 2 Chapter One-Introduction rejection mechanisms by RO and NF A review of literature revealed that solution chemistry, membrane charge, and the presence of inorganics or other organic matters, seem to be the major factors affecting organics rejection by membrane technology (Eisenberg and Middlebrooks, 1986; Van der Bruggen et al., 1998; Braghetta et al., 1994;... interaction study from pure chemicals to groups of organics typically present in reclaimed water In view of the above, it is the aim of this study to investigate the rejection mechanisms of DOMs and their implications to membrane process performance To avoid the shortcomings by taking DOMs as one complex group, as well as to understand the effects of major characteristics of DOMs on their rejection, DOMs... fractions by SP-28 membrane 130 Figure 4.29 Effects of ionic strength on removals of acid and base fractions by AG membrane 135 Figure 4.30 Effects of ionic strength on removals of acid and base fractions by SG membrane 136 Figure 4.31 Effects of ionic strength on removals of acid and base fractions by ST-28 membrane 137 Figure 4.32 Effects of ionic . REJECTION OF ORGANIC MATTERS BY RO/ NF MEMBRANE SHAN JUNHONG NATIONAL UNIVERSITY OF SINGAPORE 2005 Founded 1905 REJECTION OF ORGANIC MATTERS BY RO/ NF MEMBRANE. 4.3.2.2 Membrane rejection for neutral organics 109 4.3.2.3 Proposed Adsorption mechanism on organics rejection by NF /RO1 18 4.4 Electric Exclusion Mechanism for Organics Rejection by RO/ NF 120. 69 4.1 Introduction 69 4.2 Preliminary Study on Rejection of Organics Fractions 71 4.2.1 Characteristics of the Secondary Effluent 71 4.2.2 Rejection of Organics Fractions by RO Process 73