Reverse Osmosis: Industrial Processes and Applications [3 ed.] 1119724740, 9781119724742

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Reverse Osmosis 3rd Edition

Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106 Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Reverse Osmosis 3rd Edition

Jane Kucera

This edition first published 2023 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA © 2023 Scrivener Publishing LLC For more information about Scrivener publications please visit www.scrivenerpublishing.com. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. Wiley Global Headquarters 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Limit of Liability/Disclaimer of Warranty While the publisher and authors have used their best efforts in preparing this work, they make no rep­ resentations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchant-­ ability or fitness for a particular purpose. No warranty may be created or extended by sales representa­ tives, written sales materials, or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further informa­ tion does not mean that the publisher and authors endorse the information or services the organiza­ tion, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Library of Congress Cataloging-in-Publication Data ISBN 9781119724742 Cover images: Left to right Used Water Filters, Iryna Piskova | Dreamstime.com Desalination Plant, Tifonimages | Dreamstime.com Reverse Osmosiss Plant, Tifonimages | Dreamstime.com Earth in Rain Drop, Romolo Tavani | Dreamstime.com Cover design by Kris Hackerott Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines Printed in the USA 10 9 8 7 6 5 4 3 2 1

Dedication For, and in memory of, my dad; he’ll always be O.K.

v

Contents Preface to the 3rd Edition

xxi

Acknowledgements xxiii

Section I: Fundamentals

1

1 Introduction to Reverse Osmosis: History, Challenges, and Future Directions 3 1.1 Introduction 3 1.2 A Brief History of Reverse Osmosis 5 1.2.1 Early Development 5 1.2.2 Advances 1970s–1980s 10 1.2.3 Advances from 1990s through the Early 2000s 12 1.3 Challenges and Prospects 14 1.3.1 Membrane Materials Development 15 1.3.2 Modification of Element Construction for Ultra-High Pressure or High-Temperature Operation 17 1.3.2.1 Ultra-High Pressure Spiral Wound RO 17 1.3.2.2 High-Temperature Elements 18 1.3.3 Optimization of RO Element Feed Channel Spacer 19 1.3.4 Other Advances and Future Requirements 23 1.4 Summary 26 Symbols 26 Nomenclature 27 References 27 2 Principles and Terminology 2.1 Semipermeable Membranes 2.2 Osmosis 2.3 Reverse Osmosis

33 33 33 35

vii

viii  Contents 2.4 Basic Performance Parameters: Recovery, Rejection, and Flux 2.4.1 Recovery and Concentration Factor 2.4.2 Rejection 2.4.3 Flux 2.4.3.1 Water Flux 2.4.3.2 Solute Flux 2.5 Filtration 2.5.1 Dead-End Filtration 2.5.2 Cross-Flow Filtration 2.6 Concentration Polarization Symbols Nomenclature References

35 35 38 41 41 43 43 43 43 45 47 48 48

3 Membranes: Transport Models, Characterization, and Elements 51 3.1 Membrane Transport Models 51 3.1.1 Solution-Diffusion Transport Model 52 3.1.2 Modified Solution-Diffusion Transport Models 55 3.1.2.1 Solution-Diffusion Imperfection Model 55 3.1.2.2 Extended Solution-Diffusion Model 56 3.1.3 Pore-Based Transport Models 56 3.1.4 Models Based on Non-Equilibrium Thermodynamics 57 3.2 Polymeric Membranes 57 3.2.1 Cellulose Acetate 57 3.2.2 Linear Polyamide (Aramids) 61 3.2.3 Fully Aromatic Polyamide Composite Membranes 63 3.2.3.1 NS-100 Membrane 64 3.2.3.2 FT-30 Composite Membrane 67 3.2.4 Characterization of CA and Composite Polyamide Membranes 73 3.2.4.1 Surface Roughness 73 3.2.4.2 Zeta Potential (Surface Charge) 76 3.2.4.3 Hydrophilicity 76 3.2.5 Other Membrane Polymers 78 3.3 Membrane Elements 80 3.3.1 Plate and Frame Elements 81 3.3.2 Tubular Elements 82 3.3.3 Hollow Fine Fiber Elements 83 3.3.4 Spiral Wound Elements 84

Contents  ix 3.4 Specialty Membranes and Elements 3.4.1 Specialty Membranes 3.4.1.1 Dry Membranes 3.4.1.2 Boron-Rejecting Membranes 3.4.2 Specialty Elements 3.4.2.1 Sanitary Elements 3.4.2.2 Disc Tube Elements 3.4.2.3 Vibratory Shear Enhanced Processing (VSEP) Elements and System 3.4.2.4 Ultra-High Pressure and High Temperature Elements Symbols Nomenclature References

Section II: System Design and Engineering

91 91 91 92 93 93 94 94 95 95 96 97

103

4 Basic Design Arrangements and Concentration Polarization Guidelines 105 4.1 Arrays and Stages 105 4.1.1 Recovery per System Array 106 4.1.2 Element-By-Element Flow and Quality Distribution 108 4.1.3 Flux Guidelines 109 4.1.4 Cross-Flow Velocity Guidelines for Array Design 111 4.1.5 Concentrate Recycle 112 4.2 Passes 113 Symbols 115 Nomenclature 115 References 115 5 RO System Design Using Design Software 5.1 RO System Design Guidelines 5.2 Step-by-Step Design—Sample Problem 5.2.1 Step 1—Water Flux 5.2.2 Step 2—Membrane Selection 5.2.3 Step 3—Number of Elements Required 5.2.4 Step 4—System Array 5.3 Design Software 5.3.1 Water Application Value Engine (WAVE)— DuPont Water Solutions 5.3.2 IMSDesign—Hydranautics 5.3.3 Q+ Projection Software LGChem

117 117 118 119 119 119 120 121 123 131 135

x  Contents 5.4 Optimum Design Result for the Sample Problem Symbols Nomenclature References

140 141 141 142

6 Design Considerations 6.1 Feed Water Source and Quality 6.1.1 Feed Water Source 6.1.2 Feed Water Quality and Guidelines 6.1.3 pH 6.1.3.1 pH Profile Through an RO System— Alkalinity Relationships 6.1.3.2 pH and Membrane Scaling Potential 6.1.3.3 pH Effects on Solute Rejection and Water Permeability 6.2 System Operations 6.2.1 Pressure 6.2.2 Compaction 6.2.3 Temperature 6.2.4 Balancing Flows 6.2.5 Designing for Variable Flow Demand 6.3 Existing RO System Design Considerations 6.3.1 Changing Membranes 6.3.1.1 Changing Membrane Area 6.3.1.2 Changing Membrane Types 6.3.1.3 Mixing Membrane Types 6.3.2 Increasing Recovery 6.3.3 Changing Feed Water Sources 6.3.4 Reducing Permeate Flow Symbols Nomenclature References

143 143 143 145 147

7 RO Equipment 7.1 Basic RO Skid Components 7.1.1 Cartridge Filters 7.1.2 High Pressure Feed Pump 7.1.3 Pressure Vessels 7.2 Skid Design Considerations 7.2.1 Piping Materials of Construction 7.2.2 Feed Distribution Headers 7.2.3 Stage-by-Stage Cleaning

163 163 164 172 177 181 181 183 184

148 148 149 149 149 151 155 156 157 157 157 158 158 158 159 160 161 161 161 162

Contents  xi 7.2.4 Sampling and Profiling/Probing Connections 7.2.5 Instrumentation 7.2.6 Controls and Data Acquisition/Analysis 7.2.6.1 System Control 7.2.6.2 Data Acquisition and Analysis 7.2.7 Designs for Variable Permeate Flow Demand 7.3 Energy Recovery Devices (ERDs) 7.3.1 ERD Types 7.3.2 ERD Applications for RO 7.3.2.1 Single-Stage RO 7.3.2.2 Multi-Stage RO 7.4 Clean-In-Place (CIP) Equipment 7.5 Mobile RO Equipment Symbols Nomenclature References

187 188 193 193 194 195 196 196 197 197 197 200 203 205 205 206

Section III: Membrane Deposition and Degradation: Causes, Effects, and Mitigation via Pretreatment and Operations 207 8 Membrane Scaling 8.1 What is Membrane Scale? 8.2 Effects of Scale on Membrane Performance 8.3 Hardness Scales 8.3.1 Types of Hardness Scale 8.3.1.1 Carbonate-Based Hardness Scales 8.3.1.2 Sulfate-Based Hardness Scales 8.3.1.3 Other Calcium Scales: Calcium Phosphate and Calcium Fluoride 8.3.2 Mitigation of Hardness Scales 8.3.2.1 Chemical Pretreatment—Acid and Antiscalant Dosing 8.3.2.2 Non-Chemical Pretreatment—Sodium Softening and Nanofiltration 8.3.2.3 Operational Techniques—Flushing, Reverse Flow, and Closed Circuit Desalination 8.4 Silica Scale 8.4.1 Forms and Reactions of Silica 8.4.2 Factors Affecting Silica Scale Formation 8.4.3 Mitigation of Silica Scale

211 211 212 215 215 215 216 218 219 220 221 225 226 227 228 232

xii  Contents 8.5 Struvite 8.5.1 What is Struvite? 8.5.2 Mitigation of Struvite 8.6 Scaling Mitigation Guidelines—Summary Symbols Nomenclature References

236 236 238 239 240 240 240

9 Generalized Membrane Fouling 249 9.1 What is Membrane Fouling? 249 9.2 Classification and Measurement of Potential Foulants 250 9.2.1 Settleable and Supra-Colloidal Particulates 251 9.2.2 Colloids 252 9.2.2.1 Measurement of Colloids for RO Applications—Silt Density Index (SDI15) 252 9.2.2.2 Measure of Colloids—Modified Fouling Indices 255 9.2.2.3 Summary of Colloidal Fouling Indices 257 9.2.3 Natural Organic Material (NOM) 257 9.2.4 Other Organics 259 9.2.5 Other Foulants: Cationic Coagulants and Surfactants, and Silicone-Based Antifoams 259 9.2.6 Metals: Aluminum, Iron, Manganese, and Sulfur 259 9.2.6.1 Aluminum 259 9.2.6.2 Iron and Manganese 261 9.2.6.3 Hydrogen Sulfide 262 9.3 Effects of Fouling on Membrane Performance 265 9.3.1 Effects of Inorganic Foulants 266 9.3.1.1 Fouling with Larger Settleable and Supra-Colloidal Solids 266 9.3.1.2 Cake Layer Surface Fouling with Colloids 266 9.3.1.3 Feed Channel Fouling 268 9.3.1.4 Summary of Fouling Effects of Inorganic Particulates and Colloids 271 9.3.2 Effects of NOM and Other Organics 273 9.3.2.1 Effects of NOM—Humic Acids 273 9.3.2.2 Effects of Hydrocarbons 276 9.3.2.3 Effects of Cationic Coagulants and Surfactants 278 9.3.2.4 Summary of the Effects of Organic Surfactant and Antifoam Fouling on Membrane Performance 279

Contents  xiii 9.4 Pretreatment to Minimize Membrane Fouling 279 9.4.1 Primary Pretreatment—Clarification for Colloids and Organics (NOM) Removal 280 9.4.1.1 Coagulation 280 9.4.1.2 Flocculation 283 9.4.2 Pressure Filtration: Particles, SDI15, and Organics Removal 283 9.4.2.1 Multimedia Pressure Filters: Suspended Solids Removal 283 9.4.2.2 Catalytic Filters: Soluble Iron, Manganese, and Hydrogen Sulfide Removal 287 9.4.2.3 Carbon Filters: TOC Removal 292 9.4.2.4 Walnut Shell Filters: Hydrocarbon Oil Removal 296 9.4.2.5 Cartridge Filters: What is Their Purpose? 299 9.4.3 Membrane Filtration Turbidity, SDI15, and Metal Hydroxide Removal 300 9.4.3.1 Membrane Materials and Elements 301 9.4.3.2 Membrane Filtration Operations— Polymeric Membranes 306 9.4.3.3 Membrane Filtration as Pretreatment for RO 311 9.4.4 Nanofiltration (NF): Organics and Color Removal 321 9.5 Feed Water Quality Guidelines to Minimize Membrane Fouling 323 Symbols 324 Nomenclature 324 References 326 10 RO Membrane Biofouling 10.1 What is RO Membrane Biofouling? 10.2 Factors Affecting Membrane Biofouling 10.2.1 Polyamide RO Membrane Characteristics 10.2.1.1 Membrane Surface Roughness 10.2.1.2 Surface Charge and Zeta Potential 10.2.1.3 Membrane Hydrophilicity 10.2.2 Feed Water Matrix 10.2.2.1 Concentration of Microorganisms and Nutrients 10.2.2.2 Feed Water Ionic Strength and pH 10.2.2.3 Pretreatment Antiscalants

335 335 339 339 339 339 339 340 340 341 341

xiv  Contents 10.2.2.4 Feed Water Organic Concentration and Fouling 341 10.2.3 RO System Hydrodynamics 341 10.3 Effects of Biofouling on Membrane Performance 342 10.3.1 Scale Formation 342 10.3.2 Hydrodynamic Effects on Performance 342 10.4 Measurement of Biofouling 343 10.4.1 Predictive Techniques 343 10.4.1.1 Assimilable Organic Carbon (AOC) 343 10.4.1.2 Adenosine Triphosphate (ATP) and the Biofilm Formation Rate (BFR) 344 10.4.2 Plate Counts 344 10.4.2.1 Heterotrophic Plate Counts (HPC) 344 10.4.2.2 Total Direct Counts (TDC) 345 10.5 Mitigation Techniques 345 10.5.1 Pretreatment 346 10.5.1.1 Reduction of Feed Water Nutrients and Microorganisms 346 10.5.2 Disinfection 348 10.5.2.1 Physiochemical Disinfection Method— Ultraviolet (UV) Light 348 10.5.2.2 Chemical Disinfection—Oxidizing Biocides 353 10.5.2.3 Chemical Disinfection—Non-Oxidizing Biocide 368 10.5.2.4 Biocides Not Recommended for Use with Polyamide RO Membranes 370 10.5.2.5 Chemical Disinfection—Prospective Biocides for RO 370 10.5.3 Membrane Cleaning for Biofouling Removal 373 10.5.4 Membrane “Sterilization” 375 10.5.5 Biocide Flushing 375 10.6 Biofouling and Mitigation Summary 376 Symbols 378 Nomenclature 378 References 379 11 Membrane Degradation 11.1 Chemical Degradation 11.1.1 Polyamide Layer Degradation—Oxidation 11.1.1.1 Chlorine

387 388 388 388

Contents  xv 11.1.1.2 Chloramine 396 11.1.1.3 Chlorine Dioxide 398 11.1.2 Polysulfone Support Layer Degradation 400 11.1.3 Polyester Fabric Degradation—Hydrolysis 402 11.1.4 Prevention of Chemical Damage 402 11.1.4.1 Removal of Oxidizers 402 11.1.4.2 Protection of Membrane Support Layers 404 11.2 Mechanical Damage 404 11.2.1 Physical Membrane Damage Due to Abrasion 404 11.2.2 Physical Membrane Damage Resulting from Operational Factors 407 Symbols 412 Nomenclature 412 References 412

Section IV: System Monitoring, Normalization, and Troubleshooting 12 Data Collection and Normalization 12.1 Data Collection 12.2 Data Normalization Symbols Subscripts Nomenclature References

417 419 419 422 427 428 428 428

13 Membrane Issues and Troubleshooting 431 13.1 Observed Performance Issues 432 13.1.1 High Permeate Solute Concentration 432 13.1.1.1 Increase in Feed Water Concentration of Ions 433 13.1.1.2 Hardness Scaling 433 13.1.1.3 Membrane Damage 434 13.1.1.4 Temperature Increase/Pressure Decrease 435 13.1.1.5 System Operations and Mechanical Issues 438 13.1.2 Changes in Permeate Flow 439 13.1.3 Changes in Feed Pressure 439 13.1.4 High Differential Pressure 440 13.2 Common Causes of Performance Failures 445

xvi  Contents 13.2.1 Mechanical Failures 13.2.2 RO Equipment Design 13.2.3 Operational Problems 13.2.4 Feed Water Quality Issues 13.2.5 Membrane Issues 13.3 Troubleshooting Techniques 13.3.1 Mechanical Inspection 13.3.2 Cartridge Filter Inspection 13.3.3 Water Analyses 13.3.4 RO Projections 13.3.5 Profiling and Probing 13.3.5.1 Profiling 13.3.5.2 Probing 13.3.6 Normalized Data Analysis 13.3.7 Autopsy 13.3.7.1 Visual Inspection—External 13.3.7.2 Visual Inspection—Internal Symbols Nomenclature References

445 445 446 446 446 447 447 447 448 449 449 449 452 455 457 458 459 471 471 472

Section V: Off-Line Activities: Membrane Cleaning, Flushing, and Layup 475 14 Membrane Cleaning 14.1 When to Clean 14.2 Cleaning Chemicals 14.2.1 High pH Cleaning 14.2.2 Low pH Cleaning 14.3 Cleaning Equipment Design 14.3.1 Design of the RO Skid for Effective Cleaning 14.3.2 Design of the Cleaning Skid 14.3.2.1 Cleaning Tank 14.3.2.2 Cartridge Filters 14.3.2.3 Cleaning Pump 14.4 Cleaning Techniques 14.4.1 Conventional Cleaning 14.4.2 Two-Phase Cleaning 14.4.3 Reverse Cleaning 14.4.4 Preventative Cleaning 14.4.4.1 Extrapolative Preventative Cleaning

477 478 479 480 481 483 483 484 484 486 486 487 487 489 490 490 491

Contents  xvii 14.4.4.2 Direct-Osmosis High-Salinity (DO-HS) On-Line Cleaning Technique 14.5 Determining the Efficacy of Cleaning 14.6 Clean-In-Place (CIP) Versus Offsite Cleaning 14.6.1 CIP 14.6.2 Off-Site Cleaning 14.7 Membrane Disinfection 14.7.1 Hydrogen Peroxide/Peroxyacetic Acid 14.7.2 Non-Oxidizing Biocides 14.7.2.1 DBNPA 14.7.2.2 Isothiazolones—CMIT/MIT 14.7.2.3 Other Non-Oxidizing Biocides Symbols Nomenclature References

491 493 494 494 494 495 495 497 497 499 500 500 500 501

15 Controlling Off-Line Membrane Deposition via Flushing and Layup 15.1 Membrane Flushing 15.1.1 End of Service Flush 15.1.2 Stand-By Flush 15.1.3 Return to Service Flush 15.2 Membrane Layup 15.2.1 Short-Term Layup 15.2.2 Long-Term Layup 15.2.2.1 Sodium Metabisulfite (SMBS) 15.2.2.2 DBNPA 15.2.2.3 CMIT/MIT 15.3 Membrane Preservation Nomenclature References

505 505 506 506 507 508 508 508 508 510 510 510 512 512

Section VI: Sustainability and Future Prospects

515

16 Concentrate Management 16.1 Discharge 16.1.1 Discharge to Surface Waters 16.1.2 Discharge to Sewer 16.1.3 Discharge to On-Site Treatment Facility 16.1.4 Deep Well Injection 16.2 Land Application 16.2.1 Irrigation

517 517 517 518 518 518 519 519

xviii  Contents 16.2.2 Evaporation Ponds 16.3 Reuse 16.3.1 Direct Reuse 16.3.1.1 Wash Down Systems 16.3.1.2 Cooling Tower Make-Up 16.3.2 Treated Concentrate for Reuse—Brine Minimization 16.3.2.1 Recovery RO Systems 16.3.2.2 Zero Liquid Discharge (ZLD) 16.4 Off-Site Disposal 16.5 Emerging Technologies for Concentrate Management 16.5.1 Membrane Distillation (MD) 16.5.2 Forward Osmosis (FO) Symbols Nomenclature References

519 519 520 520 520 520 520 522 523 523 524 526 529 529 529

17 High-Recovery Reverse Osmosis 531 17.1 Single-Step High Recovery Processes 531 17.1.1 Closed Circuit RO (CCRO) 531 17.1.1.1 Managing Scale Formation 533 17.1.1.2 Managing Membrane Fouling 535 17.1.1.3 Energy Savings 536 17.1.2 Osmotically-Assisted RO (OARO) 538 542 17.1.3 Pulse Flow RO (PFRO™) 17.1.4 Feed Flow Reversal (FFR) 545 17.2 Enhanced High Recovery Processes with Interstage Solute Precipitation 548 17.2.1 Intermediate Concentrate Demineralization (ICD) 549 17.2.2 Accelerated Seeded Precipitation (ASP) 551 17.3 Multi-Step High Recovery Membrane Processes 552 17.3.1 Toward Zero Liquid Discharge (ZLD) 552 17.3.2 Challenging Waters and Wastewaters 553 17.3.3 Commercialized Multi-Step, High-Recovery RO Processes 553 17.3.3.1 Optimized Pretreatment and Unique Separation (OPUS®) 554 17.3.3.2 High Efficiency Reverse Osmosis (HERO®) 556

Contents  xix Symbols Nomenclature References

558 558 559

18 New and Alternative Membrane Materials For Sustainability 565 18.1 Specific Requirements to Improve Sustainability 566 18.1.1 Membrane Performance 566 18.1.2 Fouling Resistance 568 18.1.3 Chlorine (Oxidant) Tolerance 570 18.1.4 Energy-Water Nexus 570 18.2 Membrane Materials to Meet RO Demineralization Challenges 571 18.2.1 Modification of Polyamide Interfacial Polymerization (IP) Preparation Chemistries and Techniques 572 18.2.2 Membrane Surface Modifications 575 18.2.3 Nanotechnology and Nanoparticle Membranes 578 18.2.3.1 Carbon Nanotube (CNT) Nanocomposite Membranes 578 18.2.3.2 Thin Film Nanoparticle (TFN) Membranes 584 18.2.4 Graphene Oxide (GO)-Based Membranes 586 18.2.5 Biomimetic Aquaporin Membranes 591 Symbols 594 Nomenclature 594 References 595 Index 601

Preface to the 3rd Edition The use of reverse osmosis (RO) has grown significantly since the first commercial systems were installed in the mid-1960’s. Today, RO is used for a variety of applications from seawater desalination for drinking water, to industrial demineralization for boiler make-up and steam generation, and preparing ultrapure water for semiconductors or pharmaceutical water for injection. The use of RO is currently outpacing both thermal desalination and ion exchange demineralization techniques for such applications. RO offers smaller infrastructure relative to thermal desalination processes and has recently equaled or exceeded energy savings afforded by thermal techniques for seawater desalination. The ability of RO to replace or augment conventional ion exchange saves end users the need to store, handle, and dispose of large amounts of acid, caustic, and regeneration waste, making RO a “greener” technique than ion exchange for demineralization. Overall costs of RO have declined with the introduction of interfacial composite membranes in the early 1980’s and improvements in membrane performance (permeability and selectivity) have driven down the costs of operations. Additionally, improvements in energy recovery devices have reduced energy requirements for the technique, further easing operating costs. These advances have enabled RO to become the leading demineralization technique in the world. As RO has experienced significant growth, knowledge about RO has not kept pace. Personnel are often faced with operating an RO system without understanding the technique or meaningful training. Further, RO system designers often lag in knowledge of best design practices. This has resulted in the perpetuation of misconceptions and “lore” about RO design and operations, often leading to poor RO system performance. Much of the current literature about RO includes lengthy discussions or focuses on niche applications, both of which have their place, but which make it difficult to quickly resolve practical issues associated with more commonplace applications. Hence, the objective of this book is to bring

xxi

xxii  Preface to the 3rd Edition clear, concise, and practical information to engineers, designers, end users (including operators), and consultants regarding the breadth of the “what, how, and why” of RO. The 3rd edition includes expanded details and guidelines along with corresponding rationale. Also included in this edition is a full section regarding sustainability, new innovations, and future prospects for this demineralization technique.

Acknowledgements

Global Acknowledgements: Douglas E. Smith Dr. Drannan Hamby Prof. Julius “Bud” Glater Paul Szustowski

3rd Edition Acknowledgements: Dr. Eric M.V. Hoek, UCLA Dr. Menachem Elimelech, Yale University Dr. Jeffrey McCutcheon, UCONN Wayne Bates, Hydranautics Elke Peirtsegaele, DuPont Water Solutions Lyndsey Wiles, Zwitterco Matthew Flannigan, Nalco Water, an Ecolab Company Donna Murphy, DuPont Water Solutions Peter Metcalf, Toray Membrane Jessica Uy, Toray Membrane Garth Parker Jr., DuPont Water Solutions Eugene Rozenbaoum, LG Chem Tony Fuhrman, NX Filtration Andy Taverna, Mack Pump Christie MacKenzie, Grundfos Alden Whitney, Pentair-XFlow, XFlow Bassem Khoury, Hydrodex Seong Hoon Yoon, Nalco Water, an Ecolab Company Scot Farmer, Nalco Water, an Ecolab Company xxiii

xxiv  Acknowledgements Brendan Kranzmann, Nalco Water, an Ecolab Company Alex Barr, NW Nalco Water, an Ecolab Company Greg Coy, GL Coy & Associates, Inc. Georgine Coy, GL Coy & Associates, Inc. Jeanne Modelski, Nalco Water, an Ecolab Company Dennis Bitter, Atlantium Greg Johnson, New Logic Research Angela Romanoff, Trojan Technologies Emma Anderson, Trojan Technologies Michael Boyd, Gradiant Uzi Kafri, Rotec, Ltd. Malcolm Man, Saltworks Technologies, Inc.

Section I FUNDAMENTALS

1 Introduction to Reverse Osmosis: History, Challenges, and Future Directions

1.1 Introduction Reverse osmosis (RO) is a demineralization technique (also known as a “desalination” technique) used to separate solutes in solution from solvents. As a demineralization technique, the solutes are defined as dissolved ions and organics, while the solvent is usually water. RO relies on a semi-­ permeable membrane that is responsible for the separation. The membrane allows water to pass through it while retaining most of the dissolved solids. The driving force for RO is an applied pressure that forces water through the membrane in the direction opposite of that via the natural process of osmosis (detailed in Chapter 2). Figure 1.1 shows how the separation performance of RO compares to other membrane- and conventionally-based separation/filtration technologies. RO is the finest “filtration” technique currently available, capable of removing monovalent ions from solution to yield demineralized water (as discussed in Chapter 3, RO does not rely on size-exclusion filtration to separate solutes from solution but uses the most-cited Solution-Diffusion Model of separation to describe how solutes pass through a membrane. Reverse osmosis is the leading worldwide technology for demineralization for both industrial and municipal applications today. Figure 1.2 shows that membrane techniques (including RO, electrodialysis, electrodialysis reversal, continuous electrodeionization, membrane distillation, etc.) have been on the rise since 2000 while thermal processes have been on the decline during the same time period (note that world-wide capacity for both types of techniques were about equal just prior to the year 2000) [1]. RO offers several advantages over other demineralization processes. Total energy requirements for RO are lower than that for thermal processes [2]. Further, RO systems have a smaller footprint and are modularized for each of installation, use, and expansion [2]. Jane Kucera. Reverse Osmosis 3rd Edition, (3–32) © 2023 Scrivener Publishing LLC

3

4  Reverse Osmosis 3rd Edition

Approximate size, µm

Ions (1+/-) Ions (2+/-) Colloids Bacteria

Sand

10.0

Media Filtration

0.1

Microfiltration

0.01

Ultrafiltration

0.001

Nanofiltration Reverse Osmosis

0.0001?

New Contracted Capacity (million M3/d)

Figure 1.1  Filtration spectrum comparing various membrane-based technologies (italics) and conventional multimedia filtration (bold) for separation capabilities based on approximate size of removal and nature of the dissolved solute or suspended solid to be removed. 7 Membrane

6

Thermal

5 4 -3 2 1 0

2002

2004

2006

2008

2010

2012

2014

2016

2018

2020

2022

Figure 1.2  New contracted capacity of membrane- and thermal-based desalination techniques from 2002 through June 2022. Courtesy of IDA Desalination and Reuse Handbook, 2022-2023, Page 7 [1].

RO has also replaced ion exchange in many plants for brackish water demineralization to avoid handling acid, caustic and regeneration waste. Today, RO is commonly used to compliment ion exchange by removing the bulk of solutes from water prior to treatment with ion exchange, thereby greatly reducing the chemicals and neutralization needed for the ion exchange polishing. Since commercialization in the mid-1960s, RO has seen developmental strides in selectivity and water permeability, thereby producing better

Introduction to Reverse Osmosis  5 quality water at lower applied pressure. For example, the 1965 pilot test at Coalinga, CA, USA (conducted prior to start-up of the first commercial RO facility at this location) demonstrated solute passage (measured as total dissolved solids (TDS)) through the cellulose acetate membranes of about 9% [3]. Today, TDS passage through commercial, brackish water, polyamide composite membranes is as low as 0.2% [4], and, for commercial, thin film nanocomposite seawater membranes, 0.11% [5]. RO operating pressures have decreased from about 41 bar in the 1970s to less than 16 bar today. Specific energy consumption (SEC) has also decreased from the range of 7.0–9.0 kWh/m3 to about 2.5–3.5 kWh/m3 in 2016 [6], owing to more water-­permeable membranes and efficiencies in pumping equipment.

1.2 A Brief History of Reverse Osmosis 1.2.1 Early Development The first recorded description of osmotic properties of semipermeable membranes occurred in 1748, when Jean-Antoine Nollet observed the phenomenon of osmosis [7]. Wilhelm Pfeffer, in his book, Osmotic Investigations: Studies on Cell Mechanics, published in 1877, describes the osmotic properties observed in plant cell membranes [8]. At about the same time, Moritz Traube developed artificial membranes made of cupric ferrocyanide (carbon, copper, nitrogen, and iron), and demonstrated that these membranes interacted differently with water than with dissolved solutes [8]. In 1948, Dr Gerald Hassler, at the University of California, Los Angeles (UCLA), proposed an “air film” barrier between two cellophane membranes where he surmised that osmosis involves evaporation of water at one membrane, followed by transport through the air film as a vapor, and then the vapor condensed at the other membrane [7]. The timeline to today’s membranes begins in 1955 as shown in Figure 1.3. Professor Charles Reid at the University of Florida together with Ernest Breton, demonstrated a pressure-driven process of reversing osmotic flow through cellulose acetate membranes [10]. They had investigated several materials in a trial-and-error process, including cellophane, rubber hydrochloride, and polystyrene, in addition to cellulose acetate. They focused on available polymers at the time, with hydrophilic groups to facilitate water transport. Some polymers investigated exhibited no product flow or passed 75% of the feed water chloride concentration at up to 55 bar applied pressure, which clearly were not effective [10]. The cellulose acetate membrane prepared by Reid and Breton from DuPont (88 CA-43 (E.I. du Pont de Nemours, Wilmington, DE USA)) exhibited chloride passage of

6  Reverse Osmosis 3rd Edition

A 1940

1950

B C

E D

H J F G I

1960

M

1970

Q

O N

L K

1980

1990

P

R

2000

S

T

2010

2020

A. 1948 - Hassler studies osmotic properties of cellophane at UCLA

J. 1968 - First multi-leaf spiral wound module developed by Fluid Systems

B. 1955 - First reported use of the term “reverse osmosis”

K. 1971 - Richter-Hoehn at DuPont patents aromatic polyamide membrane

C. 1955 - Reid begins study of membranes of demineralization at University of Florida

L. 1972 - Cadotte develops interfacial composite membrane

D. 1959 - Breton and Reid demonstrate desalination capability of cellulose acetate film at the University of Florida E. 1960 - Loeb and Sourirajan develop asymmetric cellulose acetate membrane at UCLA

M. 1974 - First commercial seawater RO facility at Bermuda N. 1994 - TriSep introduces first “low fouling” membrane O. 1995 - Hydranautics introduces first “energy saving” plymide membrane P. 2002 - Koch Membrane Systems introduces first 18-inch “MegaMagnum” module

F. 1963 - First practical spiral wound module developed by General Atomics

Q. 2003 - High Efficiency RO (HERO) patented

G. 1965 - First commercial brackish water pilot RO facility at Coalinga, CA

R. 2006 - Thin-film nanocomposite membrane developed at UCLA

H. 1965 - Solution Diffusion transport model described by Lonsdale, et .al

S. 2009 - Cotinuous Closed Circuit Desalination (CCRO) developed

I. 1967 - First commercially successful hollow fiber module developed by DuPont

T. 2013 - Graphine oxide composite membrane patented

Figure 1.3  Milestones in the history of RO development.

Introduction to Reverse Osmosis  7 less than 4% at applied pressures of only 27.5 bar [10]. Water throughput ranged from 0.08 m3/m2-d (m/d) for a 22-μm thick membrane up to 0.56 m/d for a 3.7-μm thick membrane tested at 41 bar on a 0.1  M sodium chloride solution [10]. Reid and Breton concluded that their cellulose acetate membranes exhibited requisite semipermeable properties for practical

(a)

(b)

Figure 1.4  Sidney Loeb’s “big dripper”, cellulose acetate flat sheet membrane equipment. Courtesy of Julius “Bud” Glater. (a) disassembled module and (b) completely assembled module.

8  Reverse Osmosis 3rd Edition applications, but improvements in durability and throughput were required [10]. The breakthrough resulting in commercially-viable membranes for “reverse osmosis” (a term first used in a 1956 UCLA Engineering Report by Hassler [9, 11]) was achieved by Sidney Loeb and Srinivasa Sourirajan over the years of 1958–1960 while working in Professor Samuel Yuster’s UCLA lab [9]. After months of work, Loeb and Sourirajan developed a suitable cellulose acetate membrane with higher throughput and lower solute passage than the Reid and Breton membranes [12]. The membranes were initially hand-cast and characterized as a homogeneous material with a physically-asymmetric structure [13]. Figure 1.4 shows Loeb and the flatsheet membrane equipment dubbed the “big dripper”. Later, tubular configurations of the membrane were achieved. Figure 1.5 shows a schematic of the tubular casting system [14], while Figure 1.6 shows permeating water from the tubular membrane. Figure 1.7 shows the capped, in-floor immersion preserved in Boelter Hall at UCLA.

2.29 cm Inner Diameter

Casting Tube Rough Guide for Casting Tube

0.01

Casting Solution

2.20 cm Diameter Membrane Tube Formation

Casting Bob 30 cm

Membrane Tube Immersion

Floor Level Immersion Tank Ice Water (0-4°C)

(a)

(b)

Figure 1.5  Schematic of the tubular cellulose acetate membrane casting device used at UCLA. Courtesy of Julius “Bud” Glater.

Introduction to Reverse Osmosis  9

Figure 1.6  Productivity of tubular cellulose acetate RO membrane. Courtesy of Julius “Bud” Glater.

Figure 1.7  Capped immersion tube used in the tubular membrane casting shown in Figure 1.4 at UCLA, 2008.

In 1961, the first company to apply RO was Havens Industrials in Southern California, USA [9]; details of this first application were not found in other historical references. However, on June 4, 1965, the City of Coalinga, CA USA, began a pilot test of the tubular, Loeb-Sourirajan

10  Reverse Osmosis 3rd Edition

Figure 1.8  Sidney Loeb (left), UCLA professor Joseph McCutchan (right) and other team members at the Raintree RO pilot test facility at Coalinga, CA USA, cir. 1965. Courtesy of Julius “Bud” Glater.

cellulose acetate membranes, under the direction of Loeb and Professor Joseph McCutchan of UCLA (see Figure 1.8) [15]. The 3-year pilot dubbed “Raintree” generated 5,000 gallons per day of drinking water for the city [14].

1.2.2 Advances 1970s–1980s Cellulose acetate membranes exhibited good throughput with low solute passage, but they had some serious operational design limitations. Limitations included high operating pressure requirements (24–31 bar), narrow operating pH range (4–6), and a low maximum temperature limit of 35°C. And, although solute passage was less than 5%, even lower solute passage was desired. Hence, for RO to truly grow in application, membranes that could achieve performance beyond the limitations of cellulose acetate membranes were needed.

Introduction to Reverse Osmosis  11 In 1967 E. I. du Pont de Nemours & Company (DuPont) developed and later, in 1971, patented a linear polyamide (aramid) membrane in hollow fine fiber form (Richter, Square, Hoehn, US patent Number 3,567,623, assigned to DuPont, 1971 [16]). The membranes were commercialized as the brackish water Peramsep™ B-9 (1969) and seawater B-10 (1974) Permeators (Permasep is a registered trademark of DuPont Company, Inc. Wilmington, DE, USA). While the patent claimed various preparations of organic, nitrogen-linked aromatic polymers of the formula —LR—, where L is the nitrogen linkage such as an amide, and R is an aromatic linkage such as phenylene, a 100% —CONH— membrane exhibited chloride passage of 0.5% when operating on a 35,000 ppm sodium chloride solution at 103 bar at 30°C [16]. In practice, however, the rated solute passage for the permeators was about 10%, with most operational plants observing closer to 5% passage, on a par with cellulose acetate membranes [17]. Further, the productivity of these membranes was lower than that of cellulose acetate membranes, ranging from about 0.04–0.08 m/d [18]. However, the equivalent salt passage and much higher packing density of the hollow fiber membranes, were advantages over flat sheet and tubular cellulose acetate membrane configurations. Other aramid advantages included greater stability over a broader pH range and tolerance to higher temperature (up to 49°C) [19, 20]. Research work continued to try to increase productivity without sacrificing low solute passage [18]. The thinking was that by independently preparing an ultrathin film on top of a microporous, highly permeable support (rather than relying on the singular dense region defined by cellulose acetate membranes), greater water permeability could be achievable [18]. Such a “composite” membrane offered several degrees of freedom in preparation, including independent selection and optimization of the film and support layers, more control in yielding reproducible thin-film thickness, and, perhaps greater control over the permeability of solutes through the thin film [18]. Out of these efforts came John Cadotte’s cross-linked polyetherimide NS-100 membrane (US patent Number 4,039,440, assigned to the US Dept. of the Interior, 1977 [21]. This membrane demonstrated water throughput of 0.8 m/d at 103.4 bar, with solute passage of 0.5% [22]. Further, this membrane exhibited stability over a wide pH range. Soon after the NS-100 membrane, came the Cadotte’s salient “344” patent, “Interfacially Synthesized Reverse Osmosis Membrane,” (US Patent Number 4,277,344, assigned to FilmTec Corporation, 1981 [23]). This membrane became known as the “FT-30” membrane. The interfacially prepared membrane involved the reaction of phenylene diamine with

12  Reverse Osmosis 3rd Edition trimesoyl chlorine to create a fully aromatic thin film atop a microporous polysulfone support. The resultant thin film composite or “TFC” polyamide membrane exhibited throughput of 1.44 m/d and salute passage of 0.5% at 69 bar and 25°C when operating on seawater [23]. This work paved the way for the commercially-available polymeric membranes today, which are all based on the basic FT-30 chemistry and preparation technique [24]. Other noteworthy events in the historical development of RO include the construction of the spiral wound membrane element (also called a “module”) and creation of the solution-diffusion model of membrane transport. The spiral wound element developed at Gulf General Atomic, Inc., in 1963 and followed up by a patent, “Reverse Osmosis Purification Apparatus” (Bray, US Patent Number 3,417,870, assigned to Gulf General Atomic Incorporated, 1968 [25]), resulted in a multi-leaf, spiral element with greater packing density and lower pressure drop than the flat-sheet and tubular devices of the time, but not the hollow fine fibers used in the DuPont Permasep permeators. However, the spiral configuration was easier to clean in-situ than the hollow fine fiber, and, hence, become a compromise element configuration. The Solution-diffusion model of membrane transport was proposed by Harry Lonsdale, U. Merten, and Robert Riley in 1965 [26]. The model’s premise was that transport through RO membranes is governed by the ability of a solute or solvent to dissolve in the membrane polymer matrix and diffuse through it; the model assumes a defect-free membrane. While the model experienced considerable debate since its proposal, general consensus today is that this model most accurately predicts membrane performance [24].

1.2.3 Advances from 1990s through the Early 2000s Much of the work in the 1990s and early 2000s focused on improving thin film composite membrane performance with respect to selectivity and permeability while reducing the pressure requirements of the RO system. This resulted in a host of new membrane classifications, where membranes can essentially be “tuned” for the specific application type. Table 1.1 lists some of these classifications and membrane characteristics. Other advances during this period include improving energy efficiency of the RO process. Improvements in pump efficiencies, membrane permeability to water, and the use of energy recovery devices (ERDs), such as isobaric and centrifugal devices, reduced the energy requirements of RO systems. In high-pressure seawater systems, there is enough energy remaining in the RO waste stream that can be captured via an ERD and applied to assist in pressurizing the incoming, low-pressure seawater. ERD devices can reduce the

Introduction to Reverse Osmosis  13

Table 1.1  Classification and current performance at 25°C of various polyamide RO membranes originally developed through the early 2000s. RO membrane classification

Test pressure (bar)

Test solution (ppm sodium chloride)

Solute passage (%)

Water permeability (m/d-bar)

Reference

Seawater

55

32,000

0.2

0.017

[27]

Brackish Water

15.5

1,500

0.2

0.081

[4]

Low Energy

10.3

2,000

0.7

0.113

[28]

Fouling Resistant

15.5

2,000

0.35

0.075

[29]

14  Reverse Osmosis 3rd Edition 20

4 3

2.5

2

2.0

1 10

0 2000 2004 2008

5

SEC (kWh/m3)

SEC (kWh/m3)

15

45 g/L NaCI 35 g/L NaCI 25 g/L NaCI

1.5 1.0 .5

0

0 1970 1980 1990 2000 2004 2008 Year (a)

0 10 20 30 40 50 60 70 80 Percent Recovery (b)

Figure 1.9  (a) SEC requirements for 35,000 ppm TDS seawater RO showing decrease since 1970 (dotted line indicates theoretical thermodynamic minimum SEC of 1.06 kWh/ m3); and (b) minimum SEC requirements as a function of feed water sodium chlorine concentration and system recovery, typical recovery of 45%–55% for most seawater systems highlighted in gray [32].

energy requirements of a seawater (or other high TDS RO systems) by up to 60% [30]. The average Specific Energy Consumption (SEC) for seawater RO plants operating on 33,500 TDS Pacific Ocean water was 3.1 kWh/m3 in 2018, with a range of 2.5 kWh/m3 for well managed systems to a high of 4.0 kWh/m3 [31]. These SEC values cover the requirements of the RO system only, and it is estimated that this energy requirement represents about 65% to 80% of the entire demineralization plant requirement [31]. The theoretical minimum SEC for an RO system operating on 35,000 ppm TDS seawater at 50% recovery is 1.06 kWh/m3 [32]. However, since RO systems are finite and do not operate as reversible thermodynamic processes, the actual minimum SEC is about 1.56 kWh/m3 [33]. The minimum SEC requirement depends on several factors, including feed water TDS and system recovery. Figure 1.9a demonstrates how the SEC for RO has declined since 1970 and Figure 1.9b shows the minimum SEC as a function of feed water concentration and system recovery [32].

1.3 Challenges and Prospects Despite the advances since the 1960s in RO as a demineralization technique, there are still challenges to overcome. These include minimizing deposition of foulants and scale on the membranes, particularly at higher

Introduction to Reverse Osmosis  15 water recoveries; sufficient biocontrol strategies; managing or eliminating membrane susceptibility to degradation when exposed to oxidants that are used for biocontrol; managing the RO waste stream; and reducing energy requirements. These challenges are magnified under conditions of water scarcity. The need for high recovery of feed water and use of wastewater and other impaired make-up sources shine the light on the limitations of RO. Strides have been made to at least partially address these issues. Researchers have investigated incorporating engineered nanomaterials (ENM) into polyamide membranes and the polysulfone microporous support layer, modifications in element construction, new chemical treatments, and unique operating and cleaning techniques. Some research has led to commercialized products, but more work needs to be done to allow RO to fully and successfully address future water challenges.

1.3.1 Membrane Materials Development ENM such as carbon nanotubes, nanoparticles (e.g., zeolites, silver, titanium oxide, etc.), and graphene oxide (GO) have been and continue to be studied to achieve improved membrane performance. While most investigations focus on employing hydrophilic ENM to increase water permeability (thereby reducing pressure and energy requirements), other objectives include increased selectivity, resistance to fouling, particularly biofouling, and oxidant tolerance [34]. The ENMs are generally imbedded into the polyamide polymer matrix or used as a coating either on the polysulfone support layer or on the polyamide layer. The resultant mixed matrix membranes (MMM) have shown variable results with respect to permeability and selectivity in laboratory and pilot studies. However, a thin film nanocomposite (TFN) membrane developed in 2006 by Eric M.V. Hoek et al. [35] exhibited higher water permeability, 5.5 × 10-4 m/bar-d, than a TFC membrane, 3.0 × 10-4 m/bar-d, for handcast membranes; sodium chlorine passage was 6.1% and 6.6%, respectively [36]. The TFN was prepared by dispersing 0.004–0.4 w/v% zeolite A nanoparticles (hydrophilic, aluminosilicate molecular sieves) in the trimesoyl chlorine/hexane organic phase used to prepare conventional TFC membranes. Figure 1.10a shows conceptual characterizations of the structures of a TFC and the TFN membrane discussed here [36]. Figure 1.10b shows actual TFC and TFN membranes, with the zeolite nanoparticle clearly visible in the TFN membrane. This membrane was commercialized in 2010 by the start-up company Nano H2O (acquired by LG Chem in 2014) and Marketed under the brand name QuantumFlux. Commercialization of this membrane represented the first breakthrough

16  Reverse Osmosis 3rd Edition in membrane innovation in a quarter century. Today, a seawater version of the QuantumFlux TFN membrane exhibits 0.11% solute passage (the lowest of any c­ ommercially-available RO membrane), and water permeability of 0.011 m/bar-d [5]. Graphene oxide (GO) and multi-walled carbon nanotube (MWCNT) membranes have been studied for use in RO membranes since the early 2010s for resistance to biofouling and oxidation with chlorine, but not so much for permeability and solute passage improvements [34, 37]. Both of these TFN membrane types exhibited improved resistance to biofouling when charged with exposure to bovine serum albumin (BSA), but not complete resistance [34, 38]. Hence, both would still require treatment with an oxidant to minimize biofouling. Park et al. [39] demonstrated good chlorine tolerance for MWCNT membranes, as did GO membranes, in studies by Shao et al., Chae et al., and Kim et al. [40–42]. Again, however, resistance to chlorine as the oxidant was not 100%. Work with these membranes is still in its infancy, so improvements in biofouling and oxidant resistance, as well as permeability and solute passage, may be on the horizon.

Polymide

100 nm

Polysulfone

Polysulfone Polyamide

100 nm

Zeolite

Nanoparticle Polysulfone

Polyamide

(a)

(b)

Figure 1.10  (a) Conceptual characterization of polyamide TFC and TFN membranes. (b) actual transmission electron micrographs of the same TFC and TFN membranes with the TFN zeolite nanoparticle clearly visible [36].

Introduction to Reverse Osmosis  17

1.3.2 Modification of Element Construction for Ultra-High Pressure or High-Temperature Operation Ultra-high pressure and high-temperature operations are becoming necessary as more challenging water sources and higher water recovery are required of RO systems. While TFC membranes can tolerate higher temperature and/or pressure, accommodations in the area of element construction are required. Jorgen Wagner introduced the Wagner unit (pressure, in bar, times temperature, in degrees C) to help describe the combined effects that temperature and pressure have on element materials [43]. Figure 1.11 shows Wagner limits as used to describe different element constructions [44]. Wanger notes that membrane compaction is possible when the Wagner unit exceed 1200 (bar-°C) and can be severe when the Wagner unit exceeds 2000 (bar-°C) [43].

1.3.2.1 Ultra-High Pressure Spiral Wound RO A very recent development (cir. 2018) is the emerging commercialization of ultra-high pressure RO using spiral wound elements. Its development is to meet high-recovery needs in areas of water scarcity. The objective is to use ultra-high pressure RO (UHPRO) in minimal liquid discharge (MLD) 90 80

Temperature (°C)

70

High Wagner Units: Compaction Possible

High Temperature

60 50 40 30

Standard Elements

20

Standard Seawater

Ultra-High Pressure

10 10

20

30

40

50

60

70

80

90

100

Pressure (bar)

Figure 1.11  Membrane construction as a function of operating temperature and applied pressure in terms of Wagner units. Courtesy of Engineers’ Society of Western Pennsylvania [44].

18  Reverse Osmosis 3rd Edition and zero liquid discharge (ZLD) systems, where RO is currently limited to about 59 bar (70,000 ppm TDS). By going beyond this limit with UHPRO, the size of the subsequent thermal brine concentrators that are both capital and energy intensive and which follow RO in ZLD systems, can be reduced or even eliminated. UHPRO could theoretically operate at pressures up to 300 bar, corresponding to a TDS of about 250,000 ppm [45]. Commercially-available UHPRO today is limited to 120 bar (130,000 ppm TDS as sodium chloride and 150,000 ppm TDS as sodium sulfate). However, this does represent a nearly 50% reduction in RO waste volume over conventional seawater RO operations. It is noteworthy that UHPRO membranes are still performance tested at standard seawater conditions (32,000 ppm sodium chloride at 55 bar). A study by McGovern et al. [46] showed that the membrane permeability decreases at ultra-high pressure from that at 55 bar, a result of membrane compaction. Hence, the rated permeabilities for UHPRO membranes may not be valid when they operated at the higher pressures. Permeability at 172 bar was found to be less than half of that at 34.5 bar; observed permeabilities were about 0.014 m/bar-d and 0.034 m/bar-d, respectively [46]. This study also showed that the lower permeability at ultra-high pressure was somewhat reversible [46]. Solute passage also decreased at higher pressures, as would be expected during a compaction event. Conclusions of this study indicate that UHPRO would require more membrane area and, thus, larger footprint than conventional seawater RO systems, but that the potential benefits of ultra-high-pressure operation over thermal processes may balance this issue [46]. UHPRO elements use conventional seawater RO membranes but with modified a permeate collection tube and permeate spacer to prevent collapse under the high-pressures these elements experience. The permeate spacers and permeate tubes are thicker and so more robust to withstand ultra-high pressures [47]. Because of this, each UHPRO element contains less membrane area than standard elements, approximately 30.6 m2 versus 37.2 m2, respectively. Note that just recently (2021) have ERDs been developed handle the pressure that UHPRO elements operate at [48].

1.3.2.2 High-Temperature Elements Studied as early as the 1980s [49], high-temperature RO is now coming into its own for water and wastewater applications due to water scarcity and energy savings issues. Although used for high-temperature process applications in the past, the need to recycle industrial wastewater, some of which can be at elevated temperatures, and yield high-quality water for

Introduction to Reverse Osmosis  19 reuse is a driver for high-temperature RO use today. The ability to recover water by using RO at high temperature will save energy by eliminating the need to cool the water prior to the membrane unit and then reheat for reuse. High-temperature RO elements employ conventional RO membranes, but use more robust element materials of construction. Conventional materials of construction would suffer damage at high temperatures. Likely damage includes intrusion of the membrane into the permeate spacer, feed spacer migration (telescoping), and deformation of materials of construction (e.g., the membrane polyester backing (particularly at higher pH), permeate spacer, permeate tube, and adhesives) [44]. Again, as with UHPRO, the membrane area per element is lower at about 24.2 - 31.2 m2 than for a standard element [47, 50]; feed spacer thickness can be up to 1.2 mm (48-mil) [50].

1.3.3 Optimization of RO Element Feed Channel Spacer Feed channel spacers, also called netting, mesh, or Vexar™ (Cowed Plastics LLC, Minneapolis, MN USA), are used in spiral elements to provide channels for the feed water to flow through (see Figure 1.12). The geometric design and thickness of the spacer is predicated on the following: • promoting turbulence near the membrane surface without increasing pressure drop (see Figure 1.13 and Table 1.2. While the overall flow regime in a spiral element is laminar (Reynold’s Number, Re, is less than 300), the spacer geometry can increase the Re number across the open spaces [51].

Figure 1.12  RO feed channel spacer.

20  Reverse Osmosis 3rd Edition

L Feed flow

λ

d

h

Figure 1.13  Geometric parameters of feed channel spacers [53].

Table 1.2  Pressure drop in bar/m as a function of feed spacer L/d ratio and λ (as shown in Figure 1.13), and Re number [MPDI Garcia—ref 22] [54]. λ = 90°

λ = 105°

λ = 120°

L/d = 6

2.25Re-0.31

2.15Re-0.23

3.72Re-0.18

L/d = 8

0.78Re-0.19

0.88Re-0.15

1.18Re-0.14

L/d = 12

1.47Re-0.40

1.08Re-0.31

0.69Re-0.19

A 1 bar of pressure drop through RO elements is equivalent to about 0.025 kWh/m3 energy loss [52]. • minimizing resistance to flow both during “normal” and fouled conditions. • maximize exposed membrane area to enhance productivity (minimize spacer/membrane touch points). • providing a channel large enough to minimize fouling and enable more efficient membrane cleaning. Conventional RO feed spacers employ a bi-planar net, typically with rhomboid-shaped openings or “void spaces” [51]. This “diamond” configuration is shown in Figures 1.13 and 1.14. Feed water flows at a 90° angle to the diamond and around the bi-planar (or “zig zag”) filaments that provide a tortuous path to promote turbulence. As shown in Figure 1.15a,

Introduction to Reverse Osmosis  21

Figure 1.14  Scanning electron micrograph (SEM) of a Hydranautics low-differential pressure (LD) feed spacer in the diamond configuration with bi-planar filaments. (Scale = 2 mm.) [47]. Particles

Feed Spacer

Feed Channel

Feed Flow Paths

(a)

Membrane

Bacteria

(b)

EPS Layer

Figure 1.15  Characterization of larger particulate (a) and smaller materials such as bacteria and biofilm (EPS) (b) fouling on or near the feed spacer filaments [47].

particulates not removed during pretreatment can get caught in between the spacer filaments and the membrane, and smaller materials, such as scale or bacteria and its extra cellular polymers (EPS, also known informally as bacterial “slime”), can build up in the low- to no-flow regions next to the filament/membrane touch points (Figure 1.15b). Figure 1.16 shows a feed spacer fouled with microbes and EPS. Investigations into optimizing feed channel spacers have considered several mechanical aspects of the spacer, including filament cross-section, filament torsion, location of transverse filament, angles between cross filaments, spacer orientation, and among other factors [51]. The objectives

22  Reverse Osmosis 3rd Edition

Figure 1.16  Biofouled feed spacer. Courtesy of The Engineers’ Society of Western Pennsylvania [55].

were to minimize pressure drop and minimize low-flow areas near the spacer/membrane touch points [52]. An alternating spacer design (ASP) patented by Alexander Ktowell has demonstrated lower pressure drop than conventional bi-planar diamond-­ net spacers [56]. This ASD spacer is constructed using alternating thick and thin strands. The Ktowell patent [56] claims up to a 30% decrease in pressure drop with the ASD technology, while Sharpe [52] found a 5% decrease in overall system power consumption during a pilot test of the ASD technology at a full-scale RO plant. Early feed spacers had a thickness of 0.71 mm (28-mil) such that 37.3 m2 of membrane area could be packed into standard elements. With the advent to automatic element construction, glue lines were more precise and, hence, could be narrower and closer to the membrane edges. This allowed for high membrane area elements with thicker feed spacers. Today,

Figure 1.17  Cross-sections of membrane elements with (right to left): 28-mil (0.71 mm), 34-mil (0.86 mm), and 50-mil (1.26 mm) thicknesses.

Differential Pressure of 1st Stage (bar)

Introduction to Reverse Osmosis  23 5 28 mil

4

CIP CIP

3

CIP

CIP

CIP

2

31 mil CIP

1

34 mil

0 0

30

60

90 120 150 180 Operation Period (days)

210

240

270

Figure 1.18  Comparison of pressure drop through the 1st stage of an RO skid, 6 RO elements in series, as functions of time for standard 28-mil (0.71mm), 31-mil (0.78 mm) and 34-mil (0.86 mm) thick feed spacers. Cleaning events are noted at CIP (clean in place) [57].

0.78 mm (31-mil) and 0.86 mm (34-mil) spacers are available in 37.2 m2 elements. For elements with the highest membrane area, 40.9 m2, 0.71 mm spacers are still required to achieve the high packing density. Figure 1.17 shows cross sections of 3 membrane elements (described in detail in Chapter 3) that have various thickness of feed spacer. Figure 1.18 compares the pressure drop and cleaning frequency (CIP) required for elements of 3 different feed spacer thicknesses operating on highly fouling-prone (suspended solids) feed water [47].

1.3.4 Other Advances and Future Requirements Minimizing membrane fouling/scaling, removal of deposits off of the membranes, and carbon footprint are other concerns regarding RO as a demineralization technique. As RO is charged with treating more challenging feed water sources, these concerns will need to be abated. Deposit control, particularly for silica and microorganisms, and removal of these and other deposits once formed on the membranes are critical for future operations. Silica deposition is complicated and not well understood [58, 59], as over 20 species of silica in water have been identified [60]. Silica is present as one form or another in most surface and well waters, as well as in some wastewaters. Many feed water conditions affect silica deposition, including pH, temperature, and the presence of other solutes, such as sodium, calcium, magnesium, aluminum, and iron. Currently available silica antiscalants are limited to about 200–250 ppm in the RO waste stream

24  Reverse Osmosis 3rd Edition due to these factors. This correlates to a limit of less than 65 ppm silica in the RO feed stream if a reasonable system recovery is to be achieved. Biofouling is ubiquitous and controlling it on RO membranes is hampered by the sensitivity of the polyamide membrane to oxidizers. There are a few chemistries under investigation for direct membrane application for biofouling control. These include nitric oxide donor compounds, 1-Bromo-3-Chloro-5,5-Dimethylhydantoin (BCDMH), and dichloroisocyanurate (DCC) [34]. Chlorine dioxide, which is an excellent biocide, is also being investigated, but current data indicate that damage of the polyamide polymer is possible [34]. Despite pretreatment to minimize membrane deposition, it still occurs, and cleaning of the membranes is inevitable. Conventional cleaning relies on afterthe-fact removal of the deposits. Preventative cleaning, such as direct osmosis-­ high salinity (DO-HS), has shown some efficacy for minimizing the degree of membrane fouling, particularly with respect to biofouling [34, 61]. DO-HS involves on-line injections of up to 25% sodium chlorine solution at 194 bar into the feed stream. The resulting high osmotic back pressure of the “spiked” feed stream causes the membrane permeate to flow through the membrane via direct osmosis. The osmotic flow can lift materials off of the feed side of the membrane surface, which is then swept away by the feed-side flow. Further, the high salinity causes microorganisms to experience osmotic shock resulting in death of the organism [34]. DO-HS needs to be conducted on a frequent basis to limit the accumulation of deposits on the membrane [62]. Reducing energy requirements of RO is necessary to allow RO to be a sustainable technique going forward. Energy requirements can be reduced in several ways: increasing water permeability of the membrane, improving efficiency of the feed spacer, improving efficiency of pumping devices and ERDs, and using renewable energy sources (RES) to power RO. RO plants have historically been powered by fossil fuels, as these are the most commonly used energy sources in the world today (see Figure 1.19 [63]). Greenhouse gas (GHG) emissions are a side effect of using fossil fuels. In 2019, Jia et al. [64] estimated the following specific carbon dioxide emissions for RO based on the nature of the feed water: • Seawater: 0.4–6.7 kg CO2 eq/m3 • Brackish water: 0.4–2.5 kg CO2 eq/m3 • Reuse water: 0.1–2.4 kg CO2 eq/m3 Research by Liu et al. [65] found that in the United Arab Emirates (UAE), carbon emission for thermal and RO demineralization techniques were as follows:

World net electricity generation (trillion kWh)

Introduction to Reverse Osmosis  25 45 40 35 30 25 20 15 10 5 0 1990

history

Other RES Solar

projection

Wind Hydro-electric All Other Fuels

2000

2010

2020 Year

2030

2040

2050

Figure 1.19  Historical and predicted global net energy generation by energy source [63].

• Multi-stage flash (MSF—thermal): 2.988 kg CO2 eq/m3 • RO: 2.562 kg CO2 eq/m3 • Multi-effect distillation (MED—thermal): 1.280 kg CO2 eq/m3 By comparison, moving water from water rich areas to water starved areas (another common method to supply water due to water stress [66]) “costs” only 0.179 kg CO2 eq/m3 in China’s south-to-north water diversion project [65]. The carbon footprint of RO does depend on the energy source. A study by Statista [67] in 2016, developed a more detailed look at various energy sources for seawater RO and the resultant carbon footprint (Figure 1.20). For RO, wind and solar PV appear to be most promising RES to reduce GHG emissions [68]. In some cases, hybrid solar PV/wind energy RO plants are constructed to take advantage of each energy source’s strengths [69]. Solar energy (converted into electricity) powers the low and high pressures pumps, while the wind turbine provides shaft energy that can directly power the high-pressure RO feed pump [69]. The major drawback to wind and solar energies is that they are dependent on weather and climate. Hence, power storage via batteries are required [70]. But batteries have their own drawbacks, namely low storage life, high maintenance costs, and environmental impacts [70]. Ali et al. [69] have cited works where power and product flow regulation has been proposed, to smooth the energy demand. More work needs to be conducted in this area.

4 3.5

3.58

3

2.729

2.5

2.121

2 1.5 1

0.233

al

V

he

rm

rP la So

as s

Bi om

ot Ge

Na

tu

ra

lG

as

Oi l

Co al

0.109

0.089

0.049

Nu cle ar

0.248

oe le ct ric

0.3

0

W in d

0.5

Hy dr

Carbon Emission kg CO2 eq/m3

26  Reverse Osmosis 3rd Edition

Figure 1.20  Carbon footprints for seawater RO plants operating on various energy sources, 2016 [67].

1.4 Summary RO is a viable demineralization technique that has seen significant performance improvements since the early days in the 1960s. Membranes today are highly selective. Energy improvements via greater water-permeable membranes and the employment of ERDs were realized. However, greater selectivity, lower energy consumption and carbon footprint, operation at temperature and pressure extremes, and membrane deposition mitigation and removal are still some of the challenges facing the technique and will be imperative to address as more impaired feed sources are being treated. Work is being conducted to improve upon current performance and address these shortcomings to improve the sustainability of RO.

Symbols β = Contact angle of intersecting feed spacer filaments d = feed spacer filament diameter h = feed spacer channel height L = distance between parallel feed spacer filaments Re = Reynolds number

Introduction to Reverse Osmosis  27

Nomenclature ASD = alternating spacer design BCDMH = 1-bromo-3-chloro-5.5-dimethylhydantoin BSA = bovine serum albumin DCC = dichloroisocyanurate DO-HS = direct osmosis-high salinity ENM = engineered nanomaterials EPS = extra cellular polymers GO = graphene oxide LD = low differential pressure MED = multi-effect distillation MLD = minimum liquid discharge MSF = multi-stage flash MWCNT = multi-walled carbon nanotubes RES = renewable energy sources RO = reverse osmosis SEC = specific energy consumption, kWh/m3 SEM = scanning electron micrograph TDS = total dissolved solids TFC = thin film composite TFN = thin film nanocomposite UCLA = University of California, Los Angeles UHPRO = ultra-high pressure RO ZLD = zero liquid discharge

References 1. Birch, H., Executive Summary, IDA Desalination and Reuse Handbook, 2022–2023, p. 7, Media Analytics, Ltd., Oxford, United Kingdom, 2022. 2. Kucera, J., Reverse osmosis, in: Kirk-Othmer Encyclopedia of Chemical Technology, Wiley & Sons, Hoboken, NJ USA, 2017, doi: 10.1002/047123896 1.1805220501080120.a01.pub3. 3. Loeb, S., Collected Papers of Sidney Loeb, 1917–2008, Balaban Desalination Publications, Hopkinton, MA, USA, 2018. 4. Hydranautics, CPA7-LD specification sheet, 2019. https://membranes.com/ wp-content/uploads/Documents/Element-Specification-Sheets/RO/CPA/ CPA7-LD.pdf, accessed 5-2-2020. 5. LG Water Solutions, LG SW 400 SR G2 specification sheet, 2019. http://www. lgwatersolutions.com/en/product/seawater-ro, accessed 5-2-2020.

28  Reverse Osmosis 3rd Edition 6. American Membrane Technology Association, Membrane desalination power usage put in perspective, 2016. white paper, https://www.amtaorg. com/wp-content/uploads/07_Membrane_Desalination_Power_Usage_Put_ In_Perspective.pdf, accessed 5-2-2020. 7. Cheryan, M., Ultrafiltration and Microfiltration Handbook, 2nd Ed., CRC Press, Boca Raton, FL, USA, 1998. 8. Pfeffer, W., Osmotic investigations: Studies on cell mechanics, in: Embryo Project Encyclopedia, (20170-05-09), ISSN: 1940-5030, 1877, https://embryo. asu.edu/pages/osmotic-investigations-studies-cell-mechanics-1877-­ wilhelm-pfeffer, accessed 5-8-2020. 9. Glater, J., The early history of reverse osmosis membrane development. Desalination, 117, 297–309, 1998. 10. Reid, C.E. and Breton, E.J., Water and ion flow across cellulosic membranes. J. Appl. Polym. Sci., 1, 2, 133–143, 1959. 11. McCutchan, J.W. and Beorse, B., UCLA Department of Engineering Report 56-42, August 1956. 12. Loeb, S. and Sourirajan, S., UCLA Department of Engineering Report 60-60, July 1960. 13. Riley, R.L., Merten, U., Gardner, J.O., Replication electronic microscopy of cellulose acetate osmotic membranes. Desalination, 1, 1, 30–34, 1966. 14. Glater, J., UCLA, Personal Communication, 10-8-2008. 15. Wiles, L. and Peirtsegaele, E., Reverse osmosis: A history and explanation of the technology and how it became so important for desalination, paper no. IWC 18-49. 79th Annual International Water Conference, Scottsdale, AZ USA, Nov. 4-8, 2018. 16. J.W. Richter, K. Square, H.H. Hoehn, Permselective, aromatic, nitrogen-­ containing polymeric membranes. US Patent 3,567,632 assigned to E.I. du Pont de Nemours and Company, Wilmington, DE, USA, 1971. 17. Shields, C.P., Five year’s experience with reverse osmosis systems using du pont “permasep” permeators. Desalination, 28, 157–179, 1979. 18. Dupont, R.R., Eisenberg, T.N., Middlebrooks, E.J., Reverse Osmosis in the Treatment of Drinking Water, 1982, Reports paper 505, https:// digitalcommons.usu.edu/water_rep/505. Accessed 5-8-2020. 19. Bruns, and Roe, Reverse Osmosis Technical Manual, NTIS PB 80-186950, Washington, DC, USA, 1975. 20. Kosarek, L., Purifying water by reverse osmosis. Plant Eng., 8, 103–106, 1979. 21. J.E. Cadotte, Reverse osmosis membrane. US Patent 4,039,440, assigned to USA Secretary of the Interior, Washington DC, USA, 1977. 22. Porter, M.C., What, why, and when of membranes MF, UF, and RO. AIChE Symp. Ser., 171, 73, 83–103, 1978. 23. J.E. Cadotte, Interfacially synthesized reverse osmosis membrane. US Patent 4,277,344, assigned to FilmTec Corp, Minnetonka, MN, USA, 1981. 24. Baker, R., Membrane Technology and Applications, 2nd Ed., John Wiley & Sons, Ltd., Chichester, West Sussex, England, 2004.

Introduction to Reverse Osmosis  29 25. Bray, D.T. and USA, C.A., Reverse osmosis purification apparatus, US Patent 3,417,870, assigned to Gulf General Atomic Corp, San Diego, 1968. 26. Lonsdale, H.K., Merten, U., Riley, R.L., Transport properties of cellulose acetate osmotic membranes. J. Appl. Polym. Sci., 9, 1341–1362, 1965. 27. Hydranautics, SWC5-LD specification sheet, 2019. https://membranes.com/ wp-content/uploads/Documents/Element-Specification-Sheets/RO/SWC/ SWC5-LD.pdf, accessed 5-2-2020. 28. DuPont FilmTec, BW30-XFRLE-400/34 specification sheet, 2020. https:// www.dupont.com/content/dam/dupont/amer/us/en/water-solutions/ public/documents/en/45-D01719-en.pdf, accessed 5-2-2020. 29. DuPont FilmTec, BW30-XFR-400/34 specification sheet, 2020. https://www. dupont.com/content/dam/dupont/amer/us/en/water-solutions/public/ documents/en/45-D01707-en.pdf, accessed 5-2-2020. 30. Stover, R.L., Seawater reverse osmosis with isobaric energy recovery devices. Desalination, 203, 168–175, 2007. 31. Voutchkov, N., Energy use for membrane seawater desalination—Current status and trends. Desalination, 431, 2–14, 2018. 32. Elimelech, M., The future of seawater desalination: Energy, technology, and the environment. Science, 333, 712–717, 2011. doi: 10.1126/science.1200488. 33. Antonyan, M., Energy Footprint of Water Desalination, Master’s Thesis University of Twente, The Netherlands, 2019, https://www.utwente.nl/en/et/ wem/education/msc-thesis/2019/antonyan.pdf, accessed 5-9-2020. 34. Kucera, J., Biofouling of polyamide membranes: Fouling mechanisms, current mitigation and cleaning strategies, and future prospects. Membranes, 9, 111-192, 2019, doi: 10.3390/membranes9090111. 35. E.M.V. Hoek, Y. Yan, B.-H. Jeong, Nanocomposite membranes and method of making and using same. US Patent 10,618,013, assigned to The Regents of the University of California, USA, 2020. 36. Jeong, B.-H., Hoek, E.M.V., Yan, Y., Subramani, A., Huang, X., Hurwitz, G., Ghosh, A., Jawor, A., Interfacial polymerization of thin film nanocomposites: A new concept for reverse osmosis membranes. J. Membr. Sci., 294, 1–7, 2007. 37. Lau, W.J., Gray, S., Matsuura, T., Emadzadeh, D., Chen, J.P., Ismail, A.F., A Review on polyamide thin film nanocomposite (TFN) membranes: history, applications, challenges and approaches. Water Res., 80, 306–324, 2015. 38. Zhao, H., Qiu, S., Wu, L., Zhang, L., Chen, H., Gao, C., Improving the performance of polyamide reverse osmosis membranes by incorporation of modified multi-walled carbon nanotubes. J. Membr. Sci., 450, 249–256, 2014. 39. Park, J., Choi, W., Kim, S.H., Chun, B.H., Bang, J., Lee, K.B., Enhancement of chlorine resistance in carbon nanotube-based nanocomposite reverse osmosis membranes. Desalin. Water Treat., 15, 198–204, 2010. 40. Shao, F., Dong, L., Dong, H., Zhang, Q., Zhao, M., Yu, L., Pang, B., Chen, Y., Graphene oxide modified polyamide reverse osmosis membranes with enhanced chlorine resistance. J. Membr. Sci., 525, 9–17, 2017.

30  Reverse Osmosis 3rd Edition 41. Chae, H.R., Lee, J., Lee, C.H., Kim, I.C., Park, P.K., Graphene oxide-­imbedded thin-film composite reverse osmosis membrane with high flux, anti-biofouling and chlorine resistance. J. Membr. Sci., 483, 128–135, 2015. 42. Kim, S.G., Hyeon, D.H., Chun, J.H., Chun, B.H., Kim, S.H., Novel thin nanocomposite RO membranes for chlorien resistance. Desalin. Water Treat., 51, 6338–6345, 2013. 43. Wagner, J., Membrane Filtration Handbook Practical Tips and Hints, 2nd Ed, Osmonics Corporation, Minnetonka, MN, USA, 2001, http://www.soulwaterfilter.com/images/pro6/pdf3/MORE%INFO.pfd., accessed 5-11-2020. 44. Peirtsegaele, E., Innovative spiral wound elements for high temperature desalination applications, paper no. 19-36. 80th Annual International Water Conference, Orlando, FL, USA, Nov. 10-14, 2019. 45. Davenport, D.M., Deshmukh, A., Werber, J.R., Elimelech, M., High-pressure reverse osmosis for energy-efficient hypersaline brine desalination: Current status, design considerations, and research needs. Environ. Sci. Technol. Lett., 5, 467–475, 2018. 46. McGovern, R.K., McConnon, D., Lienhard, J.H., The effect of very high hydraulic pressure on the permeability and salt rejection of reverse osmosis membranes, paper no. IDAWC15-McGovern. International Desalination Associated World Congress on Desalination and Water Reuse, Sand Diego, CA, USA, 2015. 47. Bates, W., Hydranautics, Personal Communication, 5-11-2020. 48. Pinto, J.M., Personal Communications, Energy Recovery Inc., 5-19-2020. 49. Chu, H.C., Campbell, J.S., Light, W.G., High-temperature reverse osmosis membrane element. Desalination, 70, 65–76, 1988. 50. DuPont, FilmTec XUS120308 and XUS120304 specification sheet. https:// www.dupont.com/content/dam/dupont/amer/us/en/water-solutions/ public/documents/en/45-D01737-en.pdf, accessed 5-3-2020. 51. Haidari, A.H., Heijman, S.G.J., van der Meer, W.G.J., Optimal design of spacers in reverse osmosis. Sep. Purif. Technol., 192, 441–456, 2018. 52. Schellenber, C., Lehmann, S., Sharpe, A.D., New ASD feed spacer geometry reduces power consumption and bioaccumulations, paper no. 16-ReseveRO02. 77th Annual International Water Conference, Orlando, FL, USA, Nov 6-9, 2016. 53. Ruiz-Garcia, A. and de la Nuez Pestana, I., Feed spacer geometries and permeability coefficients. Effect on the performance in BWRO spiral-wound membranes modules. Water, 11, 1–13, 2019. 54. Koutsou, C., Yiantsios, S., Karabelas, A., Direct numerical simulation of the flow in spacer-filled channels: Effect of spacer geometrical characteristics. J. Membr. Sci., 291, 53–69, 2007. 55. Gilabert-Oriol, G., Niewersch, C., Massons, G., Tsoutsoura, A., Johnson, J., Cheng, Y., Arrowood, T., Garcia-Molina, V., Improving the fouling resistance

Introduction to Reverse Osmosis  31 of reverse osmosis elements, paper no. 16-13. 77th Annual International Water Conference, Orlando, FL, USA, Nov. 6-9, 2016. 56. A.J. Ktowell, Membrane filtration using low energy feed spacer. World Patent Application No. 2104/004142 A1, assigned to Cowed Plastic LLC, Minneapolis, MN, USA, 2014. 57. Franks, R., Bartels, C., Anit, A., Demonstrating improved RO system performance with new low differential (LD) technology, 2015. White Paper, https:// membranes.com/wp-content/uploads/Documents/Technical-Papers/ Product%20line/RO/Demonstrating-Improved-RO-System-Performancewith-New-Low-Differential-LD-Technology.pdf, accessed 5-13-2020. 58. Matin, A., Rahman, F., Shafi, H.Z., Zubair, S., Scaling of reverse osmosis membranes used in water desalination: Phenomenon, impact, and control; Future directions. Desalination, 455, 135–157, 2019. 59. Semiat, R., Sutzover, I., Hasson, D., Scaling of RO membranes from silica supersaturated solutions. Desalination, 157, 169–191, 2003. 60. Milne, N.A., O’Reilly, T., Sanciolo, P., Ostarcevic, E., Beighton, M., Taylor, K., Mullett, M., Tarquin, A.J., Gray, S.R., Chemistry of silica scale mitigation for RO desalination with particular reference to remote operations. Water Res., 65, 107–133, 2014. 61. Liberman, B., Reverse osmosis/direct osmosis cleaning using high concentration of NaCl. Int. Desalination Water Reuse, 2005. http://www8.zetatalk. com/docs/reverse_osmosis/direct_osmosis_cleaning_using_high_concentration_of_NaCl_2007.pdf, accessed 7-1-2016. 62. Qin, J.J., Oo, M.H., Kekre, K.A., Seah, H., Optimization of direct o ­ smosis-high salinity cleaning for RO fouling control in water. Water Supply, 10, 800–805, 2010. 63. US Energy Information Administration, EIA projects that renewables will provide nearly half of world electricity by 2050. Today Energy, 2019. https:// www.eia.gov/todayinenergy/detail.php?id=41533 accessed 5-16-2020. 64. Jia, X., Klemes, J.J., Varbanov, P.S., Alwi, S.R.W., Analyzing the energy consumption, GHG emission, and cost of seawater desalination in China. Energies, 12, 2019. doi: 10.3390/en12030463. 65. Liu, J., Chen, S., Wang, H., Chen, Z., Calculation of carbon footprints for water diversion and desalination projects. Energy Proc., 75, 2483–2494, 2015. 66. Kucera, J., Introduction to desalination, in: Desalination: Water from Water, 2nd Ed, J. Kucera (Ed.), Scrivener Publishing, Beverly, MA, USA, 2019. 67. Statista, Carbon footprint for powering a reverse osmosis desalination plant in 2016, by energy source, 2016. https://www.statista.com/statistics/802422/ carbon-footprint-of-powering-desalination-plant-by-energy-source/, accessed 5-10-2020. 68. Al-Jabr, A.H. and Ben-Mansour, R., Optimum selection of renewable energy powered desalination systems. Proceedings, 2, 2018. doi: 10.3390/ proceedings2110612.

32  Reverse Osmosis 3rd Edition 69. Ali, A., Tufa, R.A., Macedonio, F., Curio, E., Drioli, E., Membrane technology in renewable-energy-driven desalination. Renew. Sust. Energ. Rev., 81, 1–21, 2018. 70. Esmaeilion, F., Hybrid renewable energy systems for desalination. Appl. Water Sci., 10, 34, 2020. doi: 10.1007/s13201-020-1168-5.

2 Principles and Terminology 2.1 Semipermeable Membranes Reverse osmosis (RO) relies on semipermeable membranes to achieve separation of solutes from solvents. A semipermeable membrane, as used in RO, allows the solvent to pass while retaining a portion of the solutes; the degree to which a solute is retained is a function of the specific membrane chemistry and the nature of the solute (the actual transport models for water and solutes through semipermeable membranes is discussed in Chapter 3). Virtually all RO membranes used today are based on polyamide chemistry as developed by John Cadotte in the early 1980s [1, 2]. These membranes offer good performance over a wide range of feed water conditions and are durable, with the exception of degradation upon exposure to oxidants. These membranes are described in detail in Chapter 3.

2.2 Osmosis Osmosis is a natural process where water, as the solvent, flows through a semipermeable membrane from a solution with a low concentration of solutes into a solution with a high concentration of solutes. The objective is to equalize the concentration of solutes of both sides of the membrane. Image a container divided into 2 compartments by a semipermeable membrane, and an equal volume of solution on each side of the membrane. One side of the membrane has a solution with a high concentration of solutes and the other compartment has a low concentration of solutes. During osmosis, water will naturally move from the low-concentration compartment through the membrane into the high-concentration compartment, as shown in Figure 2.1 in the “osmosis” panel. When the concentration of solute is the same on both sides of the membrane, the system is at equilibrium. At equilibrium, there is no more net flow between the compartments. Jane Kucera. Reverse Osmosis 3rd Edition, (33–50) © 2023 Scrivener Publishing LLC

33

34  Reverse Osmosis 3rd Edition Semipermeable Membrane

Osmosis

Equilibrium

Natural Osmotic Pressure

Low Solute Concentration

Externally Applied Pressure

Reverse Osmosis High Solute Concentration

Figure 2.1  Depiction of “pure” water flow during osmosis, equilibrium, and reverse osmosis.

Figure 2.1 center panel shows that at equilibrium, there will a greater volume of water in the compartment that once contained the high-­concentration solution due to the osmotic flow into that compartment. The difference in height between the two compartments corresponds to the osmotic pressure head (generally represented by pi, π) of the solution at the equilibrium concentration. The osmotic pressure is a direct function of the type and concentration of solutes in the solution (see Equation 2.1). To generalize, π ranges from 0.04 to 0.075 bar for every 100 ppm total dissolved solids (TDS) of concentration.

= π



nRT = MM RT V

(2.1)

Where: n = number of moles of solute present R = Ideal Gas Law constant T = temperature in degrees Kelvin V = volume of the solution MM = molar mass of the solute So, for example, a 1,500 ppm TDS brackish water solution would have an osmotic pressure of roughly 1.02 bar. A 32,000 ppm TDS seawater solution would have an approximate osmotic pressure of 21.8 bar.

Principles and Terminology  35

2.3 Reverse Osmosis For RO to occur, water must move in the direction reverse to that of osmosis, that is, from a concentrated compartment into a more dilute compartment. For this to happen, external pressure must be applied to force water to move in the reverse direction (see Figure 2.1 “reverse osmosis” panel). The applied pressure must be great than the osmotic pressure at a minimum. Due to the resistance of the membrane itself, pressure loses in an RO system, and any fouling or scaling that has occurred on the membrane, the required applied pressure is considerably greater than the pressure to overcome osmotic pressure for reverse flow to occur. For example, brackish water RO systems operate at around 15.5 bar (or higher) while the osmotic pressure for a 2000 ppm TDS brackish water solution is less than 2 bar. For seawater at 32,000 ppm TDS with an osmotic pressure of about 22 bar, applied operating pressure is typically 55 bar. Since water is the primary constituent passing through the membrane, with solutes largely unable to pass, the compartment experiencing the loss of water will become more concentrated and the other compartment will become more dilute. The dilute compartment is “demineralized,” and the other concentrated compartment is “dewatered” or concentrated. The water passing through the membrane is known as the permeate (sometimes called the product), while the water and solutes not passing through the membrane is known as the concentrate (also known as the reject or waste).

2.4 Basic Performance Parameters: Recovery, Rejection, and Flux 2.4.1 Recovery and Concentration Factor Recovery (also known as “conversion”) describes the amount of water in percent of influent water that becomes, or is “recovered”, as permeate. A recovery of 75% means that for every 100 m3/m2-d of influent water, the permeate flow is 75 m/d. Recovery is calculated as follows:

 permeate flow  Recovery % =   ∗100 influent flow  

(2.2)

Recovery for an individual RO membrane element (defined in Chapter 3), ranges from about 10% to 15% in performance testing. RO system

36  Reverse Osmosis 3rd Edition recoveries can range from 40–90% depending on the nature (concentration) of the influent water. The most commonly operated system recovery is 75%. The recovery for a system is controlled via the concentrate flow control (throttling) valve. Figure 2.2 shows a simplified block diagram of an RO system showing the feed, permeate, and concentrate, along with the concentrate flow control valve. The diagonal line represents the RO membrane. The concentrate flow control valve exerts pressure on the concentrate stream such that the influent water is forced to flow through the membrane rather than by-passing the membrane when the flow control valve is open, as shown in Figure 2.3. This figure shows what would happen without the concentrate flow control value or when this valve is wide open, as during flushing or cleaning (in practice, there is a by-pass valve for flushing or cleaning such that the concentrate flow control valve does not need to open completely, as shown in the figure). Recovery is increased by closing the concentrate flow control valve, forcing more water through the membrane and keeping less water on the feed side of the membrane; the tighter the valve, the higher the recovery. For a system with 75% recovery, the permeate stream would be 75% of the influent stream and the concentrate stream would be 1/4 that of the influent volume flow. If it is assumed that ALL of solutes in the influent remain in the concentrate steam, the concentration of solutes in the concentrate would be 4 times that of the original influent concentration. The concentration factor would be 4. Since not all of the salutes remain in the concentrate, this concentration is only an estimate. But for generalizations,

FEED

PERMEATE

14 bar

1 bar 11.5 bar

FCV atmospheric

CONCENTRATE Concentrate Flush Valve Closed

Figure 2.2  RO system flows with concentrate flow control valve (FCV) particially closed.

Principles and Terminology  37 FEED

PERMEATE

7 bar

1 bar Atmospheric

CONCENTRATE Concentrate Flush Valve Closed

FCV Atmospheric

Figure 2.3  Flow configuration with the concentrate FCV open.

it can provide good information regarding the concentrate for purposes of disposing of it or for estimating scaling tendencies. The concentration factor is a function of the recovery of the system. Higher recovery yields a smaller concentrate volume and a higher concentrate concentration. Figure 2.4 shows how the concentration factor as a function of RO system recovery. Higher recovery generates more permeate and less concentrate from the system. This is an important consideration in areas with water scarcity: more permeate per water treated and less concentrate to dispose of. 12

Concentration Factor

10 8 6 4 2 0

0

10

20

30

40

50 60 Recovery %

70

80

90

100

Figure 2.4  Concentration factor used to approximate concentrate concentration as a function of RO system recovery.

38  Reverse Osmosis 3rd Edition However, there is a tread off between high recovery and still achieving high permeate quality. As the recovery increases, the permeate quality decreases. For example, as 75% recovery system can have up to 30% higher permeate TDS concentration that a 50% recovery system (see discussions in Chapters 4.1.1 and 4.1.2).

2.4.2 Rejection Rejection defines what percentage of an influent solute a membrane retains. Rejection is calculated as follows:

 Ci − Cp  Rejection % =   ∗100  Ci 



(2.3)

where: Ci = influent concentration Cp = permeate concentration For precise calculation, the concentrations used in in Equation 2.3 should be natural log-mean concentrations. Sometimes it is easier to consider solute passage rather than rejection. Solute passage is calculated simply as:



Solute passage % = 100 − Rejection %

(2.4)



  Cp   Solute passage  1 −    ∗ 100   Ci  

(2.5)

Therefore, a membrane exhibiting 98% rejection would have a solute passage of 2%. It should be noted that a membrane exhibiting 98% rejection will pass twice as much solute as a membrane with 99% rejection. Rejection is a function of the specific membrane as well as the nature of the influent solution and the specific solute. Some of the conditions influencing rejection include the following. • Valence: solutes with a higher valence, e.g., calcium with a 2+ charge, are rejection to a greater degree than monovalent solute, e.g., sodium with a 1+ charge.

Principles and Terminology  39 • Degree of hydration: the more hydrated the ion, the greater the rejection, e.g., chloride is rejected to a greater degree than nitrate. • Degree of dissociation: solutes that are in a more dissociate state, such as weak acids at high pH and weak bases at low pH, are rejected to a greater degree. • Molecular weight: in general, higher molecular weight solutes are rejected to a greater degree than lower molecular weight solutes. For example, rejection of calcium is slightly higher than that of magnesium. • Polarity of the solute: non-polar solute are not rejected well. For example, benzene, a relatively large but non-polar solute is rejected at about 25% by RO membranes. • State of the solute: gases are not rejected by RO membranes. For example ammonia gas is not rejected, but at lower pH, where ammonium ion predominates, this ion is rejected. • Degree of molecular ranching: the greater the branching, the higher the rejection, e.g., isopropanol is rejected to a greater degree than normal propanol. Other factor influencing rejection include feed water conditions, such as pH, ionic strength, and hardness, along with membrane characteristics, such as surface charge (via zeta potential), hydrophilicity, and surface morphology [3]. Generalizations can be made about rejections of specific solutes, as shown in Table 2.1. Actual rejections, again, depend on many factors as described previously. For more exact rejection performance for critical applications, lab scale testing can provide some rejection information, but a pilot test on site in actual conditions is recommended. The fact that RO membranes do not reject gasses indicates that the permeate, concentrate, and influent concentrations of a given gas are the same. This can have implications for some system that polish permeate from the RO. For example, carbon dioxide passing through an RO membrane would put a demand on any anion exchange polishing resin downstream of the RO. However, adding caustic to pH 8.3 to 8.4 prior to the RO membranes converts all the carbon dioxide gas to bicarbonate ion, which is well rejected by the RO membrane (see discussion in Chapter 4.2). Another gas to be concerned about is ammonia. Ammonia in known to swell RO membranes, resulting in a reversible loss of TDS rejection [4]. This occurs with increasing pH starting at about 7 (refer to Figure 2.5). This factor is an important consideration when chloramines are present

40  Reverse Osmosis 3rd Edition Table 2.1  General rejections of inorganic and organic solutes and gases by typical polyamide RO membranes at 25°C. Molecular weights of the inorganic and organic species are provided to demonstrate that molecular weight of a solute is not the singular determinator of its rejection. Inorganics (molecular weight)

Organics (molecular Rejection weight) (%) Gases

Rejection (%)

Aluminum (27) ≤ 98

Acetone (58)

≤ 71

Carbon dioxide

0

Alkalinity ≤ 96 (bicarbonate) (61)

Benzene (78)

≤ 25

Oxygen

0

Ammonia* 0

Rejection (%)

Ammonium* (18)

≤ 90

Biological Oxygen Demand (NA)

≤ 95

Calcium (40.1)

≤ 98

Chemical Oxygen Demand (NA)

≤ 97

Chloride (35.5)

≤ 95

Ethanol (45)

≤ 85

Fluoride (19)

≤ 95

Glucose (150) ≤ 99.6

Iron (55.8)

≤ 98

Haloacetic acids (NA)

Magnesium (24.3)

≤ 97

Methanol (32) ≤ 25

Manganese (54.9)

≤ 98

Phenol (94)

OrthoPhosphate (95)

≤ 98

Selenium (78.96)

≤ 97

Silica (60.1)

≤ 95

Sodium (23)

≤ 98

≤ 99.0

≤ 65

*pKa = 9.26 at 25°C (pH at which ammonium and ammonia are present in equal concentrations) NA = not applicable.

Relative Concentration

Principles and Terminology  41 110 100 90 80 70 60 50 40 30 20 10 0 -10

Ammonium Ion NH4+

Ammonia Gas NH3

0

1

2

3

4

5

6

7 pH

8

9

10

11

12

13

14

Figure 2.5  Relative concentrations of ammonia gas and ammonium ion as a function of pH in a low ionic-strength solution at 25°C.

or breakpoint chlorination is conducted (see Chapter 10.5.2.2.2). Solute rejections return to nominal once the pH is lowered to about 7, thereby favoring ammonium ion over ammonia gas.

2.4.3 Flux Flux is defined as the amount of a substance passing through a given area during a given time. For RO, water and solute flux are both considered.

2.4.3.1 Water Flux Water flux is defined as the cubic meters of water passing through a square meter of membrane area in a day (m3/m3-d or m/d) or liters per square meter per hour (l/m2-h or lmh). In US units, water flux is the gallons of water per square foot of membrane area per day (g/ft2-d or gfd). The Solution-Diffusion Model of membrane transport describes water flux as follows [5] (this transport model is discussed in Chapter 3.1.1):

Jw = A (ΔP − Δπ)

where:

Jw = water flux A = water permeability coefficient

(2.6)

42  Reverse Osmosis 3rd Edition ΔP = pressure difference across the membrane Δπ = osmotic pressure difference across the membrane Note that water flux depends directly on the pressure and concentration via the osmotic pressure. Also, the water transport coefficient is unique to each specific membrane. Specific flux (or more correctly, permeability) should be used to compare the productivity of different membranes, since not all membranes are tested at the same pressure:

water permeability = water flux/applied pressure



(2.7)

Table 2.2 shows the permeability for some commercially-available membranes. In general, as the water permeability decreases, the solute rejection tends to increase. This mutually-exclusive feature of RO membranes, permeability and selectivity, is another feature of the Solution-Diffusion Model; high solute rejection and high water permeability are difficult to achieve with polymeric membranes [6]. This is clearly seen for the SWC5-LD, CPA5-LD, and ESPA2-LD membranes; as the permeability increases, the solute rejection decreases. The outlier in these data is the CPA7-LD membrane. This is a next generation membrane introduced in the late 2010s by Hydranautics (a Nitto Company, Oceanside, CA, USA). Changes in some of the aqueous and/or organic phase reactants as well as

Table 2.2  Permeability of various hydranautics membranes at 25°C [8]. Membrane type

Model number

Flux (m/d)

Test pressure (bar)

Permeability (m/bar-d)

Solute rejection (%)

Seawater

SWC5-LD

0.919

55

0.0167

99.8

Brackish Water*

CPA7-LD

1.17

15.5

0.0757

99.8

Brackish Water

CPA5-LD

1.11

15.5

0.0722

99.7

Brackish Low Energy

ESPA2-LD

1.02

10.3

0.0989

99.6

*Next generation polymeric membrane.

Principles and Terminology  43 manufacturing techniques can be used to engineer a membrane to achieve improved performance. For the CPA7-LD, the base membrane remains polyamide, but the specific modification(s) in preparation of the membrane is not publicly available [7].

2.4.3.2 Solute Flux Solute flux is defined as the grams of the solute per square cm per second (g/cm2-s), and per the Solution-Diffusion Model:

Js = B (Csb − Csp)



(2.8)

where: Js = flux of solute “s” B = salute permeability coefficient Csb = concentration of the solute “s” at the membrane surface Csp = concentration of solute “s” in the permeate Solute flux is independent of pressure. It depends only on the concentration difference between that of the solute at the membrane surface (which differs from that in the bulk, as discussed in Chapter 2.6) and the concentration in the permeate. As with the water transport coefficient, the solute transport coefficient is unique to each specific membrane.

2.5 Filtration 2.5.1 Dead-End Filtration Dead-end filtration (also known as “end flow” or “direct flow” filtration) is a batch filtration process, as shown in Figure 2.6. The influent solution contacts the filter in a perpendicular manner and permeate water (with some solutes and suspended solids) passes through. This is a batch process as flows continues until the filter plugs, at which point the filtration system is taken offline and the filter is either cleaned or replaced. There is no waste stream from this type of filtration process.

2.5.2 Cross-Flow Filtration With cross-flow filtration, influent water flows tangentially to the membrane surface, as shown in Figure 2.7. Permeate water and a few of the

44  Reverse Osmosis 3rd Edition Feed

Effluent

Figure 2.6  Dead-end filtration iwth one influent stream and one permeate or product stream; this is a batch operation.

Permeate

Permeate

MEMBRANE Feed

Concentrate

Figure 2.7  Cross-flow filtration showing one feed stream and 2 effluent streams: one containing relatively pure permeate and one containing water and solutes that did not pass through the membrane (known as “concentrate”).

solutes permeate through the membrane while the majority of solutes and some of the water remain on the influent side of the membrane. Hence, cross-flow filtration yields 2 effluent streams: the permeate stream and the concentrate stream that remains on the influent side of the membrane. The theory behind cross-flow filtration is that the influent/concentrate flows along the membrane surface essentially “scours” the surface to keep it free of deposits of suspended and scale. In practice, however, the scouring action in typically not effective enough to completely prevent deposits from collecting on the membrane. Membranes eventually need to be removed from service for cleaning or replacement. A consequence of cross-flow filtration is the increase in feed and permeate concentrations through the system, as shown in Figure 2.8.

Principles and Terminology  45 More concentrated

Permeate

MEMBRANE Feed

More concentrated

Concentrate

Figure 2.8  Cross-flow filtration showing increasing concentrations of both the permeate and concentrate streams, as more permeate is removed from the feed stream through the system.

This phenomenon is described in detail in Chapter 4.1.2. At this point in the discussion, suffice it to say that as more water is removed from the feed stream, the solutes remaining behind in the concentrate become more concentrated. And, as the feed becomes more concentrated, the permeate will become more concentrated.

2.6 Concentration Polarization Concentration polarization describes the concentration gradient within the membrane element that is created as a consequence of cross-flow filtration. Specifically, there is a concentration gradient of solutes between the bulk solution and those at the membrane surface, as shown in Figure 2.9. Concentration polarization is influenced by axial and transverse hydrodynamics within the membrane element. As water flows transverse to the membrane surface to permeate it, the water brings along solutes (known as “permeate drag”). The solute flow through the membrane is greatly inhibited due to rejection by the membrane, such that as the water permeates, the solutes form permeate drag remain behind at the membrane surface. Due to the low or no shear rate at the membrane surface (see Figure 2.10), the solutes remain near the membrane in the “boundary layer,” with only diffusive flow away from the membrane. Increasing the axial cross flow velocity can minimize the thickness of this boundary layer, and, hence the time solutes spend in the layer. Feed channel spacers help promote higher cross flow across the membrane, but do not generate turbulent flow and suffer from “dead” areas near the filaments. Minimizing permeate drag of solutes to the boundary layer by reducing water flux will minimize the concentration of solutes in the boundary

46  Reverse Osmosis 3rd Edition RO Membrane CP layer Feed solution

Polyamide Layer Polysulfone/Fabric Layer

Cm

Js Cb

Permeate solution δcp D∞

Cp Jw

x=0

x

δm

Figure 2.9  Concentration polarization. Cb = bulk concentration, Cm = concentration at the membrane surface, CP = permeate concentration, δcp = thickness of concentration polarization layer, δm = membrane thickness, Jw = water flux, Js = solute flux, D∞ = back diffusion. Adapted from [X.25 deposits].

Boundary Layer Bulk Feed Flow

Cbulk

Convective Region Boundary Layer Back Diffusion

Cmembrane

Permeate Drag Water Flux

Figure 2.10  Flow pattern between filaments of the spacer/membrane touch points, as shown in Figure 1.16, demonstrating concentration polarization.

layer. Both of these factors (cross-flow velocity (axial flow) and water flux (transverse flow)) are important to minimize scaling of the membranes. Scaling is enhanced by greater concentration of solutes at the membrane surface and by longer time spent in the boundary layer.

Principles and Terminology  47 The ratio of Cm/Cb is called the concentration polarization modulus or beta (β). For all RO systems, β > 1.0, meaning the concentration of solutes at the membrane surface is always greater than in the bulk solution. Beta is also related to water flux and boundary layer thickness (a function of crossflow velocity) as follows:



  δ cp    Cm   β =   =  exp  Jw   Cb     Ds  

(2.9)

where Ds is the solute diffusion coefficient in the feed solution. For our purposes here, Ds can be assumed to be constant for a given system. Equation 2.9 demonstrates that increasing water flux (Jw) will result in β  >> 1.0. And, decreasing the cross flow velocity will increase boundary layer thickness, δcp, thereby also resulting in β >> 1.0. A greater β value increases the potential for deposits collecting on and perhaps scaling the membrane. Hence, water flux and cross-flow velocity are important considerations in system design and operations as factors under control of the system designer and operator to minimize potential for membrane deposits. The other effect of a greater β value is that the flux of solute(s) increases due to the increase in Cm (see Equation 2.8). Thus, system hydrodynamics in an RO element and indeed, in the RO system, directly affects the potential for solute deposition and scaling as well as permeate water quality. Design considerations to minimize β are discussed in Chapter 4.1.1.

Symbols A = water permeability coefficient β = concentration polarization modulus B = solute permeability coefficient Cb = bulk solute concentration Ci = influent solute concentration Cm = solute concentration at the membrane surface CP = permeate solute concentration Csb = concentration of solute “s” in the bulk solution Csp = concentration of solute “s” in the permeate δcp = concentration polarization boundary layer thickness δm = membrane thickness

48  Reverse Osmosis 3rd Edition Ds = solute diffusion coefficient in the feed solution  dCs  Ds  = D∞ = solute back diffusion  dx  D∞ = solute back diffusion γ0 = wall shear rate H = axial flow channel height Jw = water flux Js = solute flux Ka = acid dissociation constant MM = molar mass n = number of moles of solute present π = osmotic pressure Δπ = differential osmotic pressure from the feed side to the permeate side of the membrane ΔP = differential pressure from the feed side to permeate side of the membrane pKa = negative log of the acid dissociation constant R = Ideal Gas Law constant Re = Reynolds number T = temperature in degrees Kelvin U0 = axial flow velocity V = volume of the solution v = water flux

Nomenclature RO = reverse osmosis TDS = total dissolved solids

References 1. Baker, R., Membrane Technology and Applications, 2nd Ed., John Wiley & Sons, Ltd., Chichester, West Sussex, England, 2004. 2. J.E. Cadotte, Interfacially synthesized reverse osmosis membrane. US Patent 4,277,344, assigned to FilmTec Corp, Minnetonka, MN, USA, 1981. 3. Bellona, C., Drewes, J.E., Xu, P., Amy, G., Factors affecting the rejection of organic solutes during NF/RO treatment—A literature review. J. Water Res., 12, 2795–2809, 2004. doi: 10.1016/j.waters.2004.03.034.

Principles and Terminology  49 4. DuPont Answer Center. https://water.custhelp.com/app/answers/detail/a_ id/433/related/1, accessed May 21, 2020. 5. Lonsdale, H.K., Merten, U., Riley, R.L., Transport properties of cellulose acetate osmotic membranes. J. Appl. Polym. Sci., 9, 1341–1362, 1965. 6. Lonsdale, H.K., Recent advances in reverse osmosis membranes. Desalination, 13, 317–332, 1973. 7. Bates, W., Hydranautics, Personal Communication, May 22, 2020. 8. Hydranautics, 2020. https://membranes.com/solutions/products/ro/, accessed May 22, 2020.

3 Membranes: Transport Models, Characterization, and Elements 3.1 Membrane Transport Models The objective of a membrane transport model is to use it to prepare new and improved membranes. Understanding how solutes and solvents pass through membranes is critical to the development of new membranes with higher selectivity and higher water permeability. There are several membrane transport models in the literature [1–9]. The nature of these models range from assuming defect-free membranes [3, 6, 10] to pore flow [8] and even transport in terms of non-equilibrium thermodynamics [11–13]. The debate over which model best represents actual solute/solvent transport through reverse osmosis (RO) membranes began with the preparation of the Loeb-Sourirajan membrane in the early 1960s and continues today [1]. At the heart of the debate is the existence or nonexistence of pores in the dense, permselective layer of the membrane [1]. Researchers have attempted to use finer, more sophisticated analytical techniques to determine whether pores exist or not. Small-angle neutron scattering (SANS), atomic force microscopy (AFM) and positron annihilation lifetime spectroscopy (PALS) are three analytical techniques that have been used to study RO membrane surfaces. While pore structures have not yet been conclusively identified in RO membranes, the SANS and PALS have shown that the RO membrane surface is heterogeneous and not uniform [1]. This non-uniformity could be interpreted as a bimodal pore size distribution [1]. In a review of transport models developed since the mid-1960s, Ismail and Matsuura [1] conclude that as characterization techniques become more detailed, that more features of membrane structure can be identified, with the ultimate goal of preparing improved membranes.

Jane Kucera. Reverse Osmosis 3rd Edition, (51–102) © 2023 Scrivener Publishing LLC

51

52  Reverse Osmosis 3rd Edition

3.1.1 Solution-Diffusion Transport Model The Solution-Diffusion (S-D) model was first proposed by Lonsdale [3], and Lonsdale et al. [10] in the mid-1960s. It is the most mainstream and cited model today [1, 14]. It assumes that the permselective membrane layer is homogeneous and uniform. This model is essentially mute on the existence of pores in the membrane surface structure. The model is based on the chemical potential gradient across the membrane, starting with Fick’s Law of Diffusion (Equation 3.1) for steady state under ideal conditions.

 dc  J = −D    dx 



(3.1)

The chemical potential of water drives water to flow through the membrane in the direction reverse that of osmosis when the hydrostatic pressure on across the membrane is greater than the osmotic pressure differential across the membrane. The water flux, then, is described by Equation 2.6 and preproduced here as Equation 3.2:

Jw = A (ΔP − Δπ)



(3.2)

where: Jw = water flux A = water permeability coefficient ΔP = pressure difference across the membrane Δπ = osmotic pressure difference across the membrane The water permeability coefficient can be defined as:

A=



DwSV RTl

where: Dw = diffusivity of water in the membrane S = water solubility in the membrane V = molar volume of water R = Ideal Gas Law constant T = temperature l = thickness of the rejecting layer of the membrane

(3.3)

Membranes Transport Models  53 The S-D model assumes that the transport of solvent (water) and the solute(s) are independent of each other, such that solute transport is a function of the chemical potential gradient (concentration difference) across the membrane, as noted in Equation 2.8 and reproduced here as Equation 3.4:

Js = B (Csm − Csp)



(3.4)

where: Js = flux of solute “s” B = salute permeability coefficient Csm = concentration of the solute “s” at the membrane surface Csp = concentration of solute “s” in the permeate The salt permeability coefficient can be defined as:

B = (DbKb)/l



(3.5)

where: Db = diffusivity of the solute through the membrane Kb = partition coefficient of the solute between the solution and the membrane surface The S-D model can simply be visualized as shown in Figure 3.1. The impacts of the S-D model are demonstrated in Figure 3.2. The figure shows the effects of pressure on the flux of water and sodium chloride with a seawater membrane operating on a 35,000 ppm total dissolved solids (TDS) solution. As pressure is applied no permeate is generated until Membrane

Absorption

Desorption

Diffusion

Figure 3.1  Simplistic visualization of the S-D model. Transport through the membrane depends on the ability of the species to adsorb (dissolve) into the membrane matrix and then desorb on the permeate side [1].

54  Reverse Osmosis 3rd Edition 30

Water flux

20 15

5 4

10 Osmotic pressure ∆π

5

Salt flux

0 100

3 2 1 0

Salt flux (g/m2•h)

Water flux (L/m2•h)

25

Salt rejection (%)

80 60 40 20

0

0

13.7

27.6 41.4 55.1 68.9 Applied pressure (bar)

82.7

Figure 3.2  Solution-diffusion model presented by data from a seawater FilmTec FT-30 membrane operating at 35,000 ppm total dissolved solids (approximately 24 bar osmotic pressure, Δπ) solidum chloride solution [15].

the applied pressure exceeds the osmotic pressure of the solution (23.8 bar) plus the resistance of the membrane. From then on, the water flux increases linearly with applied pressure, as predicted by Equation 3.2. Solute flux remaines constant regardless of applied pressure, as predicted by Equation 3.4. This result has important implications for membrane performance. For example, at greater applied pressure, more water passes through the membrane relative to solute, and the permeate concentration of solute therefore decreases. The reverse is true as the applied pressure decreases. However, work by Paul [14] points out that Equation 3.2 indicates that as the pressure approaches infinity so would the water flux. Paul argues that this is true for a pore-flow transport model, but not for a strict S-D process. Paul  [14]

Membranes Transport Models  55 also discusses other inconsistencies and suggests modifications to the S-D model, but these are beyond the scope of this book. Because Lonsdale considered pores to be defects in the permselective membrane layer and not an integral part of this layer, the S-D model became synonymous with describing a non-porous, defect-free membrane [1]. However, as Ismail and Matsuura [1] point out, the S-D model is essentially mute on the existence of pores.

3.1.2 Modified Solution-Diffusion Transport Models 3.1.2.1 Solution-Diffusion Imperfection Model The S-D imperfection (SDI) model, derived by Sherwood in 1967 [8], seeks to account for defects during membrane preparation that are presumed to be present in the permselective membrane layer. The SDI model assumes that transport of water and solute through the membrane is by the parallel process of solution-diffusion and pore flow. In addition to S-D flow, flow through pores, where feed water passes through the pores without a change in solute concentration also occurs. This implies then, that since flow through pores does not reduce the solute concentration, that the total flow through the pores must be a small component of the total flow through the membrane for RO to show adequate selective to be a useful demineralization technique [8]. The SDI model explains performance of membranes that exhibit lower selectivity of solutes than predicted by the S-D model. The equations representing the SDI model for total water and solute fluxes, respectively, are as follows:

Nw = A (ΔP − Δπ) + KMwCwΔP

(3.6)

Ns = B (ΔCsb − Csp) + KMsCsΔP

(3.7)

where: Nw = total water flux Ns = total solute flux of solute “s” K = membrane coefficient for pore flow Mw = molecular weight of water Ms = molecular weight of solute “s” Cw = concentration of water at the membrane surface Cs = concentration of solute “s” at the membrane surface

56  Reverse Osmosis 3rd Edition Water flux Flux

Flux

Water flux

Salt flux Osmosis

Reverse osmosis

Salt flux Osmosis

Pressure (a)

Reverse osmosis

Pressure (b)

Figure 3.3  Comparison of the predicted water and solute fluxes as function of applied pressure for the S-D model (a) and the SDI model (b). Adapted from [16].

Equations 3.6 and 3.7 represent S-D model Equations 3.2 and 3.4 for water and solute respectively, with terms added for pore flow. Expected SDI model performance is shown in Figure 3.3 as compared to that for the S-D model [16].

3.1.2.2 Extended Solution-Diffusion Model The Extended Solution-Diffusion (ESD) model as described by Jonsson [17], addresses the inconsistencies with the S-D model as ΔP approaches infinity, as described earlier by Paul [14]. In such a case, maximum rejection would reach 100%. As this is clearly never the case, the ESD model includes two terms regarding a given solute, that were assumed to be negligible in the S-D model: a term which includes the ratio of solute to water uptake by the membrane, and a kinetic term to cover solute drag and a pressure diffusion factor [17]. Detail regarding the ESD model can be found in Jonsson [17].

3.1.3 Pore-Based Transport Models Pore-based models make assumptions regarding the nature of the pore structure in the membrane barrier layer. Below are some porous models and their basic assumptions. • Kimura-Sourirajan Analysis (KSA): transport is through pores only. Water is forced through the pores by applied pressure and solutes diffuse through the water in the pores.

Membranes Transport Models  57 The water and solute flux equations are essentially the same as Equations 3.2 and 3.4, respectively, with the exception of the diffusion coefficients and lengths correspond to that for the pore rather than the membrane polymer [18]. • Finely Porous Model (FPM): as developed by Merten [19], this model is based on a one-dimensional pore. Flow of both water and solute considers both applied and frictional forces through the pore. • Surface Force-Pore Flow (SF-PF) Model: this model is based on two-dimensional pore flow and is considerably more complex that other models, but more accurately describes transport in a porous membrane [18]. • Sieve-Model: in 1998, Singh et al. [20] proposed that solute transport through pores is sieve based, such that rejection is either 0% or 100%. Rejection depends on the size of the solute relative to the diameter of the pore.

3.1.4 Models Based on Non-Equilibrium Thermodynamics These models can be developed without any knowledge of the membrane structure (porous or non-porous) and without any specific transport mechanisms [1, 17]. They are based on the fact that if a system is divided into small enough subsystems in which local equilibrium exists, thermodynamic equations can be written for the subsystems [11–13]. Refer to [11–13] for more detail on these models.

3.2 Polymeric Membranes 3.2.1 Cellulose Acetate As described in Chapter 1, cellulose acetate (CA) (polysaccharide) membranes as prepared by Loeb and Sourirajan in the early- to mid-1960s, were the first commercially-viable RO membranes developed. These membranes were prepared using a non-solvent phase separation or “phase inversion” method [21, 22]. This method has 4 basic steps, as described by Strathmann et al. [22] and Lonsdale [3]: 1. A polymer (cellulose acetate)/solvent (acetone) solution is cast as a thin film on a flat glass plate.

58  Reverse Osmosis 3rd Edition 2. The film is exposed to air for a specified time, allowing some of the solvent to evaporate. During this step, the concentration of polymer increases in the film surface region, as solvent is evaporated. 3. The film is then immersed in cold water at about 0°C. Water is a non-solvent for the polymer, but is miscible with the solvent. During this step, water diffuses into the film, and since the polymer is not soluble in water, a gel layer occurs at the surface of the film. 4. The film is then annealed in hot water at 70–90°C. This step sets the surface, shrinking the pores and thereby generating the dense rejecting layer of the membrane. The thickness of the resulting rejecting layer was about 0.2 μm, while the thickness of the entire membrane was about 100 μm [23]. The membrane is homogeneous (made of all the same polymer) and asymmetric in cross section. The support section of the CA membrane beneath the dense rejecting layer is micro porous and can be described as “spongy”. The Loeb-Sourirajan membrane was developed empirically [22]. Cellulose acetate was dissolved in acetone to create the casting solution. The addition of the appropriate swelling agent, formamide, resulted in membranes which exhibited variable degrees of water and salt permeabilities, depending on the amount of formamide added (see Figure 3.4) [22, 24]. The addition of formamide at 30% with 45% acetone and 25% cellulose acetate yielding the best-performing membrane. Scanning electron microscopy (SEM) images of the cross section of a CA membrane are provided in Figure 3.5. The membrane is homogenous (made out of the same material throughout) yet asymmetric (variable structure through the cross section). The rejecting layer at the top of the figure is shown with the porous structure underneath. This porous structure serves to minimize restriction to permeate flow once water passes through the rejecting layer. As membrane casting methods improved, membrane were cast on a fabric backing which provided support during handling of the flat sheet membrane. Figure 3.6 shows the chemical structure of the CA membrane. The acetyl (CH3OR) and hydroxyl (OH-) end groups do not dissociate at the pH range for CA membranes (4–6) [26]. Hence, the base membrane has little surface charge, but, as current CA membranes are constructed, post-­treatment with additives containing acidic functional groups appear probable (the proprietary nature of membrane preparation precludes confirmation from membrane manufacturers) [26]. This posttreatment render the membrane

Membranes Transport Models  59

80 60 40

PRODUCT WATER FLUX

5

5

4

4

3

3 50

2

2

1

1 10

20 30 % FORMAMIDE

40

SALT FLUX (g/cm2 sec x 103)

100

(cm/sec x103)

20 (gal/ft2day)

SALT REJECTION (%)

100

50

Figure 3.4  Performance of CA membrane as a function of amount of formamide (swelling agent) added. Formamide at 30% plus acetone at 45% and cellulose acetate at 25% yielded the optimum membrane. Preparation conditions: evaporation, 1 minute at 25°C and heat cure, 2 minutes at 75°C l [22].

a net negative surface charge at pH above the resultant membrane isoelectric point of about 3.5; implications of this are discussed in Chapter 3.2.4.2. A significant advantage of the CA membrane is that it can tolerate up to 1 ppm free chlorine on a continuous basis. This is helpful to minimize biofouling of the membrane. It is also critical for operations, as bacteria can degrade the membrane polymer. Limitations include susceptibility to hydrolysis at pH extremes; operating pH range is limited to a range of 4–6 (see Figure 3.7). The rate of hydrolysis is also affected by temperature, with higher temperature increasing the reaction rate. Further, brackish water CA membranes require relatively high operating pressure, up to 27 bar, and are limited to maximum temperature of 35°C. CA membranes also suffer from greater than polyamide membranes discussed in Chapter 3.2.3 compaction, where the membrane “collapses” upon itself, with the “spongy” microporous portion of the membrane becoming denser resulting in a gradual decline in permeate flow with time [27].

60  Reverse Osmosis 3rd Edition CA membrane layer

Fabric backing

100 μm

20 kV

0001

× 400

(a)

Asymmetric cellulosic membrane

Polyester (PET) fabric support

Acc.V Spot Magn 10.0kV 3.0 269x

Det WD Exp SE 4.6 1

100 µm

(b)

Figure 3.5  Cross sections of a homogeneous, asymmetric CA membrane. (a) courtesy of Mark Wilf (Scale = 100 µm); (b) courtesy of Jeffrey McCutcheon (Scale = 100 µm) [25], reprinted with permission of Elsevier BV.

H

CH2OR

O

H

RO H

H O H

CH2OR

O

Figure 3.6  Chemical structure of a CA membrane.

O

H

OR H

O H OR

H

H

n

Membranes Transport Models  61 10000

Membrane Lifetime (days)

1000 100 10 1 0.1

0

2

4

6

8

10

12

14

0.01 0.001 0.0001 pH

Figure 3.7  Membrane life due to hydrolysis of CA membranes as a function of pH at 25°C. Operating range for CA membranes is between pH 4 and 6 for maximum membrane life.

The original CA membranes were cast as flat sheet or tubular membranes. Today, commercially-available CA membranes are prepared as flat sheet and hollow fine fibers (see Chapter 3.3.2) using cellulose diacetate and triacetate (CTA) blends. Some of these blends exhibit even greater tolerance to free chlorine than CA membranes and are somewhat more resistant to hydrolysis [28]. For example, Toyobo’s (Toyobo Co. LTD, Osaka, Japan) CTA membranes claim free chlorine tolerance of up to 5 ppm [29]. The operating pH range for the Microdyne-Nadir (Microdyne-Nadir GmbH, (a Mann + Hummel GmbH Company), Wiesbaden, Germany) CTA membranes has increased from a range of 4–6 to a slightly wider range of 4–7 [30].

3.2.2 Linear Polyamide (Aramids) In 1967, E.I. du Pont de Nemours & Company (DuPont) commercialized the Permasep™ B-9 brackish water membrane followed by the Permasep B-10 seawater membrane in 1973 [31, 32]. Figure 3.8 shows some of the options in preparing the Permasep linear polyamide (also known as aramid) membranes. The B-9 and B-10 membranes were two of the first non-cellulosic (or non-polysaccharide) membranes commercialized. (At the same time, other similar membranes were developed by Toray [33] and Monsanto [34] but they did not achieve the commercial success of the Permasep membranes [35].) The aramid membranes offered high solute

62  Reverse Osmosis 3rd Edition

or

H N

O or

C

O

H

C

N

or

R

R = ionic group

Figure 3.8  Options in preparing DuPont Permasep membranes. R = ionic group such as sulfonate, phosphate, ammonium, and phosphonium, for example [36]. Reprinted with permission of Elsevier B.V.

rejection (up to 99.5%) but suffered low flux compared to CA membranes, from 0.048 to 0.144 m/d for the aramids to about 0.86 m/d for CA membranes [10, 35]. To address low permeability DuPont introduced the B-15 seawater membrane that included sulfonic acid groups (-SO3-) (Option C in Figure 3.8) to make the membrane more hydrophilic (i.e., wettable (see Chapter 3.2.4.3); structure of the B-15 membrane is shown in see Figure 3.9. Another method of addressing lower permeability was for DuPont to prepare the Permasep membrane in the hollow fine fiber form (see Figure 3.10) [37]. This membrane configuration yielded elements with high membrane area in a relatively small volume, ranging from about 500 to 5000 m2/ m3 (see Chapter 3.3.3). The tubular CA membranes had packing densities much lower, ranging from about 20 to 374 m2/m3. By packing more membrane area into a smaller volume than the tubular CAs, high productivity could be achieved for the Permasep elements. In addition to the permeability limitation of these aramid membranes, Kucera-Gienger [38] and Glater et al. [39] discuss the sensitivity of these membranes to attack by chlorine and other oxidants. Kucera-Gienger [38] demonstrated chlorine attack to the amide (-CONH2-) bonds followed by OC

CONH

NH

OC

X

CONH

NH

SO3Na Y

Figure 3.9  Permasep B-15 structure with a few sulfonic acid groups added to the polymer matrix to make membrane more hydrophilic (increased permeability) than the B-9 and B-10 membranes.

Membranes Transport Models  63

42 µm

Porous

85 µm Outside Diameter Thin Skin 0.1 - 1 µm thick

Figure 3.10  Dimensions of Permasep HFF membranes. Feed flow is external to the fiber while permeate flows through fiber inner diameter (lumen) [37].

Orton Rearrangement to ring chlorination, similar to chlorine attack on polyamide composites (chapter 3.2.3.2.1). Glater et al. [39] demonstrated attack of the amide linkages by oxidation, but also that not all mechanisms of attack are similar to that of chlorine. Oxidant intolerance was a troublesome limitation of the aramids compared to the CA membrane. DuPont ended production of the Permasep membranes in the year 2000 [35].

3.2.3 Fully Aromatic Polyamide Composite Membranes Composite membranes, also known as thin-film composites or TFC membranes, are comprised of a thin-film of an interfacially-polymerized barrier layer that is formed in-situ on and into the surface pores of a microporous polymeric support. Figure 3.11 shows the basic steps to the prepare of a composite membrane via interfacial polymerization; details are as follows. • Position 1 corresponds to a microporous support, typically polysulfone, prepared using the Loeb-Sourirajan preparation method but without the high-temperature annealing step so surface pores are retained (see Figures 3.12a and 3.12b). Position 1 in Figure 3.11 shows only the surface pores of the polysulfone support layer. • The polysulfone is immersed in a aqueous solution of monomer A, which results in a coating of solution A atop of and into the surface pores (Position 2 in Figure 3.11). • The coated polysulfone is then immersed in an organic solution, with hexane as the typical solvent, and reactant B the monomer. The reaction of A and B result in a cross-linked layer atop of and into the surface pores of the polysulfone support, thereby anchoring the thin film layer to the support

64  Reverse Osmosis 3rd Edition

TDI IN HEXANE AND HEAT CURE

PEI IN WATER

SURFACE OF POLYSULFONE SUPPORT FILM

PEI COATING

CROSSLINKED

PEI-TDI REACTED ZONE

Figure 3.11  Process of preparing an interfacially-polymerized, thin-film composite membrane. Step 1 involves a Monomer “A” in aqueous solution coating the polysulfone microporous support. Step 2 involves Monomer “B” in organic solution (such as hexane) which crosslinks with Monomer A in a self-limiting, interfacial reaction.

(Position 3 in Figure 3.11), which is an advantage over just a topical membrane coating. This cross-linked layer becomes the barrier rejecting layer of the membrane. The interfacial polymerization reaction is self-limiting [41], resulting in a membrane layer that is extremely thin, typically 0.21 μm or less [42]. The polysulfone layer serves two purposes: support the extremely thin membrane layer while at the same time minimizing resistance to permeate flow. The ultimate performance of the membrane can be tuned by altering the nature of monomers A and B, as well as the polysulfone support [43], an advantage over the homogeneous CA membrane.

3.2.3.1 NS-100 Membrane The NS-100 (NS standing for “non-polysaccharide”) membrane was the first generation polyamide membrane developed to address some of the limitations of the CA and aramid membranes, namely relatively low water permeability and selectivity [42]. The NS-100 membrane was able to achieve water permeability of about 0.0074 m/d-bar (flux of 0.75 m/d) and rejections of 99.5% while operating at 102 bar and 25°C on a 35,000 ppm sodium chloride feed solution.

Membranes Transport Models  65

100 nm (a) 100 nm

(b)

Figure 3.12  Top surface (a) and cross section (b) of microporous polysulfone support layer. (Scale = 100 nm) [40].

Rozelle et al. [42] describe the NS-100 membrane preparation using the interfacial polymerization procedure outlined in Chapter 3.2.3. Monomer A in the aqueous solution was polyethyleneimine (PEI), and tolylene (also known as toluene) 2,4-diisocyanate (TDI) was used as monomer B in the organic phase (hexane). The cross-linked membrane was heat cured at 115°C under a nitrogen blanket for 15 minutes to achieve the final membrane [44]. The reaction scheme and resultant membrane structure is shown in shown in Figures 3.13 and 3.14 respectively. The NS-100 membrane was developed in the early 1970s and patented by John Cadotte in 1977 while working at North Star Research under contract with the United States Office of Saline Water (OSW) (US patent Number

66  Reverse Osmosis 3rd Edition CH3

CH2CH2NH CH2CH2N

CH2CH2NH

CH2CH2N

+

CH2CH2 NH2

N C O

N C O

CH2CH2NH CH2CH2N CH2CH2N

CH2CH2N

O CH2CH2NHCNH

C O NH

NHCHN O

CH3

NHCHN

CH3

O

Figure 3.13  Reaction scheme for the preparation of the NS-100 membrane.

N

NHC

CH

3

O

N

O NHC

NH

NH

NH

N

N NH

N NH

CH3

NH

NH NH

N

NH

N

CH3 C

NH

NH

N

N

NH

NH

O

N

C O

NH

N

NH

NH

C O

NH

NH

NH2

NH

N

N NH

NH

NH

HN C

NH

O CH2 CH2 GROUPS REPRESENTED BY ++

Figure 3.14  Structure of the NS-100 membrane prepared from PEI cross-linked with TDI. (CH2—CH2 groups represented by +–+).

4,039,440, assigned to the US Dept. of the Interior) [41]. The membrane did not suffer from compaction or biodegradation that CA membrane were susceptible to. Further, the rejection of inorganic and organic compounds was superior to that of CA membranes. Optimized NS-100 membranes exhibited water permeabilities of up to 0.011 m/d-bar (flux of 1.12 m/d)

Membranes Transport Models  67 and rejection of 99.8% at 35,000 ppm sodium chloride, 102 bar, and 25°C [45]. The other advantage claimed by the membrane is resistance to chlorine attack as opposed to aramids which demonstrated virtually no resistance to chlorine degradation [41, 42, 45]. (Peterson [44] contradicts these claims, finding that patent applications commonly overstate chlorine tolerance such that claims of chlorine tolerance were probably inaccurate. Later work has demonstrated that this membrane is very sensitive to chlorine attack [35].)

3.2.3.2 FT-30 Composite Membrane The development of the FT-30 (FT stands for FilmTec Corporation, Minnetonka, MN USA)) polyamide (metaphenylene trimesamide) membrane has been hailed a watershed moment in RO technology. While the proof of concept in the method of an interfacially-polymerized membrane was achieved earlier with the NS-100 membrane which made possible today’s membranes and high performance [44], the final FT-30 membrane itself set the stage for today’s RO performance successes. The FT-30 membrane, developed by Cadotte at FilmTec in the late 1970s and patented in 1981 [46], was essentially an improvement on the NS-100 membrane. It bridged the gap between relatively low permeability/ selectivity CA and aramid membranes and high-performance. The FT-30 membrane exhibited high water permeability and high selectivity at lower applied pressures, did not suffer compaction or biological degradation, and could operate at higher temperature and over a much broader pH range than CA membranes could. Virtually all commercially-available polymeric membranes today are based on the FT-30 membrane chemistry [35]. The original FT-30 membrane developed by Cadotte exhibited water permeability of about 0.0072 m/d-bar (flux of 0.72 m/d) and sodium chloride rejections of 99.0% while operating on 35,000 ppm TDS at 100 bar [35]. Today, the FT-30-type membranes can exceed water permeabilities of 0.034 m/d-bar (flux of 1.19 m/d) with rejections up to 99.8 at the same feed water concentration but at lower, 35 bar, pressures [35]. Today’s membranes exhibit roughly five times the permeability at five times lower solute passage than the original FT-30 membrane. And, they has about two times the permeability of CA membranes with half the solute passage [35]. Preparation of the FT-30 membrane is similar to that of the NS-100 membrane. Referring to Figure 3.11, the monomer A used in the aqueous solution is m-phenylenediamine (MPD) and monomer B in the organic phase is trimesoyl chloride (TMC). Figure 3.15 shows the chemicals involved in FT-30 preparation, and Figure 13.6 shows the final chemical

68  Reverse Osmosis 3rd Edition CIOC

H2N

COCI

O C

NH2

+

O H C N C O NH

COCI

+

TMC

MPD

O C

H N

O H C N COOH

NH

1-n

n

Polyamide

Figure 3.15  Preparation of FT-30 polyamide membrane using MPD aqueous phase and TMC organic phase monomers.

CO

NH NHCO

CONH

NH

CO NH

CONH

CO

CO

NH

Figure 3.16  Chemical structure of FT-30 membrane as prepared by Cadotte.

structure of the membrane. (An SEM of the composite membrane is shown in Figure 3.17). The resultant polyamide membrane exhibited higher rejection (99.7%) and permeabilities of 0.034 m/d-bar (flux of 1.2 m/d) at 35 bar on a 35,000 ppm sodium chloride solution [35]. Today’s polyamide membranes exhibit rejections of 99.8% (33% lower solute passage) for seawater and brackish water membranes and permeabilities of 0.081 m/d-bar (flux of 1.26 m/d) when operating on 1,500 ppm TDS solution at 15.5 bar [42]. The operating

Membranes Transport Models  69 PA membrane surface Polymeric support Fabric backing

Figure 3.17  SEM cross-section of a polyamide composite membrane, showing the polyamide thin film atop the microporous polysulfone polymeric support, which is all prepared on a non-woven polyester fabric backing for ease of handling. Scale = 100 μm. Courtesy of Mark Wilf.

pH range for most polyamide membranes ranges from about 2–10.5, with allowable cleaning pH from 1 to 13. Note that rejection and water flux vary with pH, as shown in Figure 3.18. The maximum temperature is for the polyamide membrane is 45°C for a standard element construction.

6.0

100

5.0

99

Flux (gfd) 4.0

98

3.0

0.5% NaCI 500 psig, 25°C 2

4

6

8

1.0

Salt rej. (%)

97

1.2

pH

Figure 3.18  Water flux (circles) and sodium chloride rejection (triangles) as a function of pH. Conditions: 5,000 ppm sodium chloride at 34.4 bar and 25°C [44]. Reprinted with permission of Elsevier B.V.

70  Reverse Osmosis 3rd Edition While the FT-30 membrane exhibits excellent demineralization performance and stability over a wide pH range, the membrane suffers from some significant limitations. The most serious of these limitations concerns oxidative damage to the polyamide barrier layer. The polysulfone layer is also prone to damage, as is the polyester fabric backing.

3.2.3.2.1 Oxidation of the Polyamide Barrier Layer

Oxidation of the thin polyamide membrane is a serious problem for RO systems, as this hampers membrane biofouling control; direct usage of oxidizing biocides on the membrane is not possible. Common biocides, such as chlorine, chloramine, ozone, peroxide, and chlorine dioxide, all negatively affect the membrane to some extent (see Chapters 10 and 11). The degree to which oxidation impacts the polyamide is a function of several variables, including temperature, pH, concentration of the oxidant, and exposure time [47]. Chlorine is the most commonly-used biocide in RO pretreatment. Free chlorine (from here on referred to as simply “chlorine”) attacks the weak amide-nitrogen in the polyamide membrane [48]. Figure 3.19 shows a simplification of the generally accepted interaction of chlorine with amide-­ nitrogen and how this sets off a chain reaction leading to damage of the polyamide membrane [49]. Figure 3.19a (step A) shows the intact membrane with hydrogen bonding off of the amide functional group [42, 43, 50]. Chlorine attacks these hydrogen bonds and substitutes onto the nitrogen (Step B, Figure 3.19b). Subsequent ring substitution then occurs via Orton Rearrangement (Step 3, Figure 3.19c). Following the ring substitution, the likely ultimate failure mechanisms for the FT-30 polyamide membrane is partial or total depolymerization [49, 51]. Two possible depolymerization pathways were identified by Koo et al. [51], as shown in Figure 3.10. The method of chlorine attack is important to understand as this knowledge can guide researches to develop chlorine-resistant membranes. pH has an impact on degradation with chlorine. pH influences which path of degradation is followed and the degree to which the membrane is

O

O N H

O N CI

N H

CI

Figure 3.19  Simplified chlorine interaction with polyamide membranes; damage via hydrogen boding destruction and subsequent ring substitution via Orton Rearrangement.

Membranes Transport Models  71 damaged. At less than neutral pH, N-chlorination dominates, with lower pH resulting in more Cl for H substitution on the N; N-Cl bonds dominate (Path 2 in Figure 3.20) with eventual hydrolysis [48]. At higher pH, direct hydrolysis dominates (Path 1 in Figure 3.20) [48]. The degree of damage is also pH related. Given the relative oxidative potentials for hypochlorous acid, HOCl, and hypochlorite ion, OCl- (hypochlorous acid with the higher potential, +1.49, vs. +0.94 for the ion), and since the acid form exists at pH below 7.5, operations at lower pH results in greater membrane damage due to chlorine than higher pH [39]. Polyamide membrane exposure limits to chlorine are given in terms of ppm of chlorine and hours of exposure (ppm-hrs). Most polyamide membranes can tolerate 200–1000 ppm-hrs of exposure before solute or TDS passage doubles [52]. This means that exposure to 1 ppm of chlorine would result in a doubling of solute passage after 200–1000 hours; 0.5 ppm of chlorine exposure for 400 to 2000 hours would have the same result on solute passage. The presence of metal catalysts, such as iron and manganese,

C 0 N C1

COOH O HN

HN

O

O C

O C

C 0 N C1

NaOC1

C 0 NH HN

HN

HN

O

COOH

C 0 N C1

NaOC1

C 0 NH

O C1

C1 HN

COOH O

HN

Figure 3.20  Hydrolysis, N-chlorination.

C1 HN

O

72  Reverse Osmosis 3rd Edition and to a less extent, aluminum, calcium, magnesium, sodium, potassium, and barium, accelerate chlorine damage by catalyzing N-hydrolysis [48]. In other words, metals can significantly reduce the 200–1000 ppm-hrs exposure to chlorine and significant membrane damage can occur earlier than predicted. Many municipalities are now using chloramine in their distribution systems, so RO systems operating on such feed water need also be aware of the damage to the polyamide membrane with chloramine. Chloramines are generated with by adding ammonia to chlorine in solution or by adding chlorine to ammonia already present in water such as wastewater being treated for reuse. Chloramine, typically present as monochloramine, has about the same oxidation potential as hypochlorous acid, but kinetics of reaction are slower [53]. Hence, exposure limits to monochloramine are greater than that for free chlorine, typically 150,000 to 300,000 ppm-hrs [52, 54]. But, just as with chlorine, the presence of metals acting as catalysis can significantly reduce this exposure limit [52].

3.2.3.2.2 Stability of Polysulfone in Organic Solvents

The 2 primary purposes of the polysulfone layer of the membrane are to provide support for the ultra-thin polyamide layer and to minimize resistance to permeate flow through the membrane as a whole. The asymmetric, microporous structure of polysulfone supports has pore sizes ranging from 0.02 μm at the polyamide interface and 4 μm at the fabric backing interface [27]. Polysulfone is resistant to compaction and biological degradation making it a good support material [27]. However, while resistant to most chemicals, including hydrochloric acid, caustic, and peroxide, it is sensitive to organic solvents. Solvents such as benzene, toluene, methyl ethyl ketone, methylene chloride, and dimethyl formamide can destabilize the polysulfone, making it more susceptible to compaction, or they can even dissolve the polysulfone layer [55].

3.2.3.2.3 Hydrolysis of Polyester Fabric

The polyester fabric support layer for the composite polyamide/polysulfone membrane is stable in many liquid environments containing organics and oxidants but hydrolyses (de-polymerizes) at pH extremes. The polyester reverts back to the original acid (benzene-1,4-dicaroxylic, also known as terephthalic acid) and alcohol (ethane-1,2-diol) used to prepare the polyester fabric (see Figure 3.21). Thus, the material loses strength and the

Membranes Transport Models  73 Acid or Base (pH extremes) OC

CHOOCH2CH2O

OC

COOH

+ HOOCH2CH2O

Figure 3.21  Hydrolysis of the polyester non-woven fabric backing into original monomers, terephthalic acid and ethane-1,2-diol alcohol.

composite membrane can collapse and/or crack. Hydrolysis is exacerbated at high temperature in addition to pH extremes.

3.2.4 Characterization of CA and Composite Polyamide Membranes RO membranes are characterized based on surface properties and performance (flux (or permeability) and rejection). Surface properties are important to define as these can impact performance as well as the nature of the membrane to collect deposits that may be present in the feed water. The three surface characteristics measured are roughness, zeta-potential (an indicator of surface charge), and contact angle (a measure of the wettability or hydrophilicity (water preference) of the membrane). Looking at properties of surface roughness, surface charge, and hydrophilicity individually, however, will not give a good indication of performance, such as flux, rejection, and fouling potential; a more complex and comprehensive view is required that is beyond the scope of this book [56, 57].

3.2.4.1 Surface Roughness The roughness of a membrane surface has a significant effect on the collection and deposition of solids on the membrane. Additionally, a rough structure shields the deposits from cross-flow shear forces designed to sweep them away. Figure 3.22 shows SEMs and a transmission electron microscopy (TEM) images of the surfaces of a polyamide and a CA membrane [58, 59]. As the figure shows, the surface of the CA membrane is smooth in contrast to the rough polyamide membrane. The “ridge and valley” structure of the polyamide membrane is quantified in Figure 3.23 via an AFM [58] as compared to an AFM of a smooth CTA membrane [51]. Petersen et al. [44] reported ridge peaks of 0.26 μm (polyamide thickness) and valley thicknesses of 0.04 μm. Several studies describe how the rougher surface of a polyamide membrane leads to greater membrane deposition and less efficient cleaning as compared to the smooth CA and CTA membranes [60–64].

74  Reverse Osmosis 3rd Edition

(a) 200 nm

Polyamide

Polysulfone (b)

SEM picture of surface of cellulose acetate membrane

(c)

Figure 3.22  SEM of the surface of (a) a polyamide membrane (Scale = 2.00 μm, courtesy of Eric M.V. Hoek [58] reprinted with permission of Elsevier BV.), (b) TEM of the polyamide surface (Scale = 2 nm, courtesy of Eric M.V. Hoek [59] reprinted with permission of Elsevier BV.), (c) SEM of a CA membrane (Scale = 10 μm, courtesy of Mark Wilf).

Membranes Transport Models  75 Digital Instruments NanoScope Scan size 10.00 μm Scan rate 1.001 Hz Number of samples 512 Image Data Height Data scale 500.0 nm

2 4 6 8

x 2.000 μm/div z 500.000 nm/div

µm (a)

xle-3.22.08-001

µm 0.8 0.6 0.4 0.2 (b)

Figure 3.23  AFMs of the surface of (a) a polyamide composite membrane, Z-axis scale = 0.5 μm. Courtesy of Eric M.V. Hoek [58]; Reprinted with permission of Elsevier BV., and (b) a CTA membrane, Z-axis scale not provided [51]; Used with permission of John Wiley & Sons.

76  Reverse Osmosis 3rd Edition

3.2.4.2 Zeta Potential (Surface Charge) The surface charge of a membrane depends on several factors including pH, the type of membrane (specifically the degree of water permeability), and the nature of the feed solution chemistry [58, 65]. Zeta (or streaming) potential is typically used to measure the surface charge under specific conditions. Figure 3.24 compares the zeta potentials of a polyamide membrane to that of a CA membrane as a function of pH when tested on a 0.01 M sodium chloride solution [61]. Both the polyamide and CA membranes are shown to be amphoteric, with isoelectric points of 5.2 and 3.5, respectively. Hence, at higher pH both membranes are negatively charged and at lower pH, both membranes are positively charged, at these given feed solution conditions. The impact of surface charge on membrane deposition is discussed in Chapters 8–10. Suffice it to say at this point that positively-­ charged species will adhere to membranes at a higher pH where the charge is negative and vice-versa.

3.2.4.3 Hydrophilicity Hydrophilicity is defined as the wettability of a surface. A hydrophilic membrane prefers association with water and eschews solids (known as non-cohesiveness), whereas a hydrophobic membrane is cohesive with solids and “avoids” water. RO membranes that are more hydrophilic tend

10

Zeta potential (m/V)

5 0 -5 -10 -15

Composite Cellulose acetate

-20 2

3

4

5

6

7

8

9

pH

Figure 3.24  Zeta-potential as a function of pH for a polyamide composite membrane and a cellulose acetate membrane in a 0.01 M solution of sodium chloride [61]. Reprinted with permission of Elsevier BV.

Membranes Transport Models  77 to exhibit greater water permeability. Additionally, the hydrophilicity of a membrane affects the interaction of potential deposits (e.g., organics and inorganics) with the membrane, either enhancing or discouraging actual deposition. Hydrophilicity can be generalized in part by measuring contact angle between a liquid and a surface. There are several methods used to determine contact angle; Figure 3.25 shows contact angle via the immersion method [56]. Table 3.1 shows the measured contact angle for the FT-30 membrane and three versions of CA membranes [56]. Childress and Brandt [56] note that the break point between hydrophilic and hydrophobic membranes occurs at a contact angle of about 50°. However, they also indicate that other factors (such as surface tension and free energy) contribute to the hydrophilicity of a membrane surface [56] (Table 3.1 also shows the Gibb’s free energy for each membrane and classification based on that; lower free energy corresponds to greater hydrophobicity).

θ

Water Droplet Membrane

Figure 3.25  Immersion method of determining hydrophilicity via contact angle; contact angle, ϴ, is measure through the liquid phase. Smaller ϴ is equivalent to a more hydrophilic membrane.

Table 3.1  Comparison of hydrophilicity using Gibb’s free energy and hydrophilicity using contact angle for a commercial FT-30 membrane and 3 commercial CA membranes. Adapted from [56]. Membrane

Gibb’s free energy, ΔG (mJ/m2)

Contact angle (water as fluid) (°)

FT-30

4.72

Hydrophobic

52.7

Slightly hydrophobic

CA—A

1.22

Slightly hydrophobic

48.4

Slightly hydrophilic

CA—B

+0.18

Slightly hydrophilic

52.8

Slightly hydrophobic

CA—C

9.14

Hydrophobic

60.2

Hydrophobic

78  Reverse Osmosis 3rd Edition As the Table 3.1 shows, the hydrophilicity based on free energy and contact angle are not exactly in agreement. In general, a more hydrophilic membrane is more permeable to water [56, 66]. And, a more permeable membrane typically exhibits higher flux and greater rejection. However, other factors, such as surface charge and surface roughness, contribute to the final, overall performance observed for a given membrane [66].

3.2.5 Other Membrane Polymers The search for membranes usually focuses on improving permeability, selectivity, deposit resistance, or oxidant tolerance. First, consider permeability and selectivity. Lonsdale [67] describes research in the early 1970s that investigated water permeability vs. sodium chloride permeability for several polymeric membrane families. Figure 3.26 shows the summary of this work [67]. Lonsdale [67] discusses how all families of polymer types investigated demonstrated increasing solute permeability with increasing water permeability. In fact, the slopes for all polymers is positive but less than +1, indicating that increasing water permeability is accompanied by decreasing selectivity. In other words, high water productivity and high solute rejection are mutually exclusive for polymeric membranes. (Chapter 18.2.3 discusses the development of thin film nanocomposite membranes (TFN) and other membranes prepared with engineered nanomaterials (ENM) which are beginning to show promise of higher permeability with higher selectivity.) Although Lonsdale [67] reported on work that was conducted for polymeric membrane materials in the late 1960s and early 1970s, this relationship of water and solute permeabilities for polymeric membranes is still valid today. Figure 3.27 shows water and solute permeability for three currently-­available commercial membranes: DuPont HRSW, a “tight” seawater membrane with high rejection and lower water permeability; DuPont XLEBW, a higher-permeability brackish water membrane with somewhat higher solute passage; and a DuPont NF90 membrane (NF is a nanofiltration membrane which can be described as a high water permeable, lower rejecting membrane. Refer to Figure 1.1, Chapter 1). These three membranes are, essentially, in the same FT-30 polymeric family and exhibit a positive slope less than +1, confirming Lonsdale’s findings. However, the slope for this specific membrane family is steeper than for those membrane families shown in Figure 3.26, indicating that rejection (selectivity) has increased over the years relative to water permeability. A few of the other polymeric membranes that have been investigated over the years are listed in Table 3.2 [35, 44], some of which were commercialized. An excellent review of membranes developed prior to 1993 is given in Petersen [44].

Water Permeability, D1c1 (g/cm-sec)

Membranes Transport Models  79 10-5

Polyvinylptarolidone – Polyisocyanate Interpolymers

Solute Rejection = 99% at Δp - Δπ = 50 atm (50.7 bar)

10-6

Cellulose Acetates

10-7

10-8 10-12

Nylons Ethyl Cellulose-Polyacrylic-Acid Interpolymers

10-11

10-10

10-9

NaCI Permeability, D2

10-8

10-7

10-6

K (cm2/sec)

Figure 3.26  Log of water permeability vs. salt permeability for various polymer systems. D1c1 = A*(RTΔx)/V, where A is the water transport coefficient for the S-D model, and D2K = B*Δx, where B is the solute transport coefficient in the S-D model [67]. Reprinted with permission of Elsevier BV.

2.5E-11

Dow SWHR

NF

Dow XLE-BW

Dow NF90

2.0E-11 1.5E-11

Solution-diffusion TX model JWATER = A (ΔP - Δπ) JSALT = B (CA2 – CA3)

1.0E-11 5.0E-12

Co me nve mb nti ra ona ne l p s oly me ric

Water permeability, A (m/Pa-s)

3.0E-11

LEBW

HRSW

0.0E+00 1.0E-08

1.0E-07

1.0E-06 1.0E-05 Salt permeability, B (m/s)

1.0E-04

Figure 3.27  Water permeability as a function of salt permeability for three conventional polyamide membranes: DuPont (Dow) NF90 (NF); DuPont (Dow) XLE-BW (LEBW); and DuPont (Dow) SWHR (HRSW), where NF = nanofiltration, LEBW = low energy brackish water, and HRSW = high rejection seawater membranes. Courtesy of Eric M.V. Hoek.

Secondly, research has been conducted into improving membrane resistance to deposition (primarily for biofouling control) and chlorine damage. Several works have described modification to polyamide membranes or

80  Reverse Osmosis 3rd Edition Table 3.2  Other membrane materials investigated since the 1960s. Membrane

Developer

Comments

RC-100

Fluid Systems

Epamine polymeric membrane used in the first large-scale RO plant in Jeddah, Saudi Arabia.

NTR 2750

Nitto Denko

Piperazine cross-linked with trimesoyl chloride brackish water membrane with better chlorine tolerance.

NS-200

North Star Research

Furfuryl alcohol reacted with sulfuric acid yields a black membrane that is very sensitive to oxidants and even dissolved oxygen.

PEC 1000

Toray

Cross-linked polyether also extremely sensitive to oxidants.

NTR-7100 series

Nitto Denko

Polyurea barrier layer

NS-300

North Star Research

Poly (piperazine amide)

alternate monomers for polymeric membranes that exhibit good chlorine tolerance but suffer from lower permeability compared to polyamide membranes [68–71]. The introduction of ENM, such as graphene oxide (GO), into polymeric membranes has also produced membrane with improved chlorine tolerance and resistance to biofouling [72–77]. However, while permeability of these GO membranes was on a par with polyamides, the solute rejection was somewhat lower [72]. Chapter 18 discusses improvements in membrane performance by using ENMs.

3.3 Membrane Elements RO membranes are modularized for ease for use. The objective is to place a relatively large area of membrane into a small, compact container known as an element (or “module”). The elements allow for a smaller system footprint and for easy replacement when necessary; single elements may be replaced rather than replacing an entire system. There are four basic RO element configurations: plate and frame (using flat sheets); tubular; hollow fine fiber; and spiral wound (using flat sheets). Table 3.3 summarizes the features of these four element configurations [78].

Membranes Transport Models  81 Table 3.3  Relative characteristics of the 4 common RO membrane elements configurations. Adapted from [78]. Property

Plate-frame

Tubular

Hollow fine fiber

Spiral wound

Approximate Packing Density (m2/m3)

150–500

20–375

500–5000

500–1250

Fouling Potential

Moderate

Low

Very High

High

Ease of Cleaning

Fair

Excellent

Poor

Poor

Manufacturing Cost/ Membrane Area

High

High

Low

Moderate

3.3.1 Plate and Frame Elements Plate and frame elements were the original elements developed by Loeb for RO applications in the early 1960s (see Figure 1.4). Today, they are used primarily for electrodialysis, continuous electrodeionization, and

Permeate Tension Feed Nutth

Retentate

Pressure Tube

Support Plate Membrane Envelope

Tension Rod O-Ring Seal Permeate Channel End Plate

Figure 3.28  Example of a plate-and-frame RO element.

82  Reverse Osmosis 3rd Edition pervaporation applications; RO applications are limited to source waters with a high potential for deposition with suspended solids and organics (e.g. food processing and some wastewater applications). Figure 3.28 shows the general construction of a plate and frame element. The element actually has its origins in filter presses and the material of construction of the housing was stainless steel. Packing density is low and much hardware is required, so cost of the element per membrane area is high. Cleaning in-situ is difficult unless cleaning is frequent, which is typically the case in food processing applications. Otherwise, foulant can hide out in “dead” areas on the membrane surface where shear is lower and become irreversibly lodged in the element or on the membrane. Elements can be disassembled for hand cleaning of the membrane sheets.

3.3.2 Tubular Elements Tubular RO elements are also typically used for high-solids applications, such as food or wastewater processing. Figure 3.29 shows an older-style tubular element constructed of stainless steel. Current elements use more plastics for element components such as for the membrane sleeves. The elements operate similarly to shell-and-tube exchanges, with the shell-side as permeate, and the tube-side as feed. Membrane tubes are generally 12–25 mm in diameter and are inserted end-to-end into perforated sleeves. Compared to plate and frame elements, cleaning is relatively easy and little pretreatment for suspended solids removal is required. But, just like the plate and frame elements, a lot of hardware is required for a relatively small area of membrane and element cost is high. Permeate RO Tubular Membrane Concentrate (in series)

Permeate Collection Shroud

Perforated Stainless Steel Support Tubes

Feed Inlet

Figure 3.29  Tubular RO membrane element resembling a shell-in-tube heat exchanger. Feed water enters the element tube-side and the permeate exits the element shell-side.

Membranes Transport Models  83

3.3.3 Hollow Fine Fiber Elements Hollow fine fibers have a very small diameter compared to tubular membrane. Figure 3.10 shows the cross-section of the DuPont hollow fine fiber membrane. Figure 3.30 shows a bundle of such fibers, looking like strands of human hair. Fiber bundles are taken as shown in the Figure 3.30 and placed into a housing as shown in Figure 3.31.

Figure 3.30  DuPont hollow fine fiber membrane strands.

Epoxy Block

Module Outside Shell

Hollow Fine Fiber Membrane

Feed Water IN Permeate OUT Concentrate OUT

Epoxy Tube Sheet

Figure 3.31  Simplified conceptual drawing of a hollow-fine fiber element.

84  Reverse Osmosis 3rd Edition Advantages of this element configuration is that the packing density is extremely high, so construction costs per membrane area are low. The limitation of this element is that it has dead zones in “corners” and among the fibers where flow does not scour the membrane well, allowing deposits to readily accumulate and making it difficult to clean them away.

3.3.4 Spiral Wound Elements The spiral wound configuration is the most commonly-used element for RO in the world. It is a compromise among factors of cost, ease of manufacture, packing density, potential for membrane deposition, and cleanability. Figures 3.32 and 3.33 show an intact spiral element (standard size, 20.3 cm diameter by 101.6 cm long) and a conceptual deconstructed spiral element, respectively. The spiral element consists of sheets of membrane, feed channel spacers, and permeate spacers. Assembly of elements was once a manual process, but has been automated to improve tolerances, such as the glue lines, to ensure maximum available membrane area and reproducibility. For assembly, membrane sheets are paced back to back (fabric backs sides together) with a permeate spacer in between. Three edges of this “leaf ” are glued such that permeate collecting in the permeate spacer can only exit the leaf on one side. Several leaves are then assembled with a feed channel spacer between the polyamide feed side of the membranes

Figure 3.32  Standard spiral wound RO element.

Membranes Transport Models  85 Concentrate Permeate Perforated collection tube Concentrate Feedwater Membranes Feedwater Feedwater and concentrate spacer Permeate carrier

Permeate flow toward collection tube Covering and bypass spacer

Figure 3.33  Deconstructed spiral wound element.

(see  Figure 3.34). The entire assembly is then rolled up like a jelly roll around the perforated permeate collection tube with the open edges of the leaves against this tube. Feed water enters the element to flow axially along the direction of the permeate tube. Permeate flows through the membranes into the channel created by permeate spacer within the membrane leaf. Since the three sides of the leaf are glued, the permeate must spiral in through the permeate spacer channel into the permeate collection tube, to exit out of this tube (see Figure 3.35). Concentrate exists the element on the

Permeate Collection Tube

Feed spacer

Permeate spacer Membrane leaf

Figure 3.34  Cross section of a spiral element showing placement of feed and permeate spacers, membrane leaves, and permeate collection tube.

86  Reverse Osmosis 3rd Edition

Figure 3.35  Path of permeate in a spiral element.

side opposite of the feed. There are anywhere from about 17 to 27 membrane leaves in a standard 20.3 cm diameter element. The length of the leaves is critical to the permeate-side pressure drop, as a longer leaf creates a longer path for the permeate to travel to reach the permeate collection tube. Leaf lengths can range from about 87.0 cm to 104.5 cm. The feed spacers are not anchored within the element, and therefore, are mobile. If a substantial collection of deposits accumulate within the element, the spacer can “telescope” out of the element, as shown in Figure 3.36. Telescoping results in a couple of serious issues. First, the spacer moves away from the membrane feed side such that the channel between the membrane leaves disappears resulting in loss of usable membrane area. This increases the flux through the remaining available area, leading to enhanced potential for deposition on the membranes. Secondly, as the spacer moves across the membrane, any deposits already on the membrane

Spacer Protruding from Outlet Face

(a)

Figure 3.36  Mild (a) and severe (b) telescoping of feed spacers.

(b)

Membranes Transport Models  87 can be “ground” into the membrane surface thereby damaging the polyamide layer of the membrane. To minimize telescoping, the ends of elements are capped with anti-telescoping devices (ATDs). There are several configuration of ATDs, some of which are shown in Figure 3.37. The standard sizes of spiral wound elements are 10.16 cm (4 inches) and 20.3 cm (8 inches) in diameter and 101.6 cm long (40 inches). Each 20.3 cm diameter element contains anywhere from 33.9 m2 to 40.9 m2 depending, in part, on the thickness of the feed channel spacer, as described in

Figure 3.37  Various anti-telescoping, element end-cap device configurations.

88  Reverse Osmosis 3rd Edition Chapter 1.3.3. Productivity per element can reach up to about 45.5 m3/d. Large-scale elements of up to 45.7 cm in diameter and 152.4 cm long are available but have been slow to realize in practice due to the size and weight of each element. A single 40.6 cm diameter element can produce between 240 m3/d and 480 m3/day. Most spiral wound elements are encased in a fiberglass shell as was shown in Figure 3.32. Because the fiberglass shell is not a pressure vessel, elements must be housed in pressure vessels during operation. Pressure vessel housing are generally made of fiber reinforced plastic (FRP) with pressure ratings of up to 1800 psi for seawater applications. Figure 3.38 shows a cut away of a spiral element (ADT clearly visible) in a cut away of an FRP pressure vessel. Stainless steel vessels sometimes used for food and beverage or pharmaceutical applications. The rationale is to allow for high-temperature cleaning to minimize biofouling, but the FRP vessels are also capable of such cleaning conditions. Elements are loaded into the pressure vessels in series, with up to eight, 101.6 cm long elements in a single vessel. Each element is connected to the  next using permeate interconnectors adapters, as shown in Figure 3.39. The O-rings on the interconnectors are essentially what keeps the feed on the outside of the elements from mixing with the permeate flowing through permeate tubes and interconnectors. They are very susceptible to “rolling” out of position usually during assembly. Care should be exercised during assembly to minimize rolling the O-rings; lubrication with glycerin (or any non-petroleum-based lubricant) can aide in assembly. Several years ago, Dow Water and Process Solution (Now DuPont Water Solutions, Edina, MN USA) introduced the interlocking end cap (iLEC™) which eliminates the interconnectors between elements. The feed end of the element has an integral O-ring which fits into the tail end of another

Figure 3.38  Cut-away of an FRP pressure vessel housing containing cut-aways of spiral wound elements.

Membranes Transport Models  89

(a)

(b)

Figure 3.39  Element interconnectors lose (a) and installed in a membrane element (b).

element in a pressure vessel (see Figure 3.40). The advantage of such an arrangement is that there are essentially zero O-rings that can be rolled since there are no interconnectors. This minimizes leaks due to rolled O-rings, which is especially important for very large-scale installations, where hunting for leaking O-rings can be a challenge. End cap adapters are used at the ends of the pressure vessels to secure the elements in place. Figures 3.41 a and b show the feed-side and concentrate-­ side end caps, respectively. Note the “thrust cone” on the concentrate-side end cap. This thrust cone serves to protect the end cap from axial pressure the elements exert on the end cap during pressurized operation. The thrust cone comes in various configurations and always is installed on the concentrate end of the pressure vessel. Sometimes, elements in the pressure vessels are not perfectly fit into the length of the vessel. To prevent elements from moving back and forth

90  Reverse Osmosis 3rd Edition

(a)

(b)

Figure 3.40  DuPont Water Solutions (Edina, MN USA) interlocking End Cap “iLEC™” with an integral O-ring on side (b) which locks with the grove on side (a). (b) also shows the feed-end brine seal around the circumference of the element (see Figure 3.43).

(a)

(b)

Figure 3.41  Feed-side pressure vessel end cap (a) and concentrated-side end cap showing thrust cone on the left and permeate sample point on the right (b).

during pressurization and depressurization, shims are inserted between the feed-side end cap and the lead element, as shown in Figure 3.42. Movement can be detrimental to the integrity of various the O-rings and can lead to movement of the unsecured feed-channel spacer leading to abrasion with subsequent leakage of feed solution into the permeate. At the feed end of an element is a brine seal, as seen on the left side of the element in Figure 3.32. The purpose of the brine seal is to prevent feed water from by-passing the elements in the pressure vessel. Hence, elements should be placed in the pressure vessels such that the brine seal is at the feed end of the vessel. So, for example, the flow in the element shown in Figure 3.32 would be left to right. The brine seal itself is a U-cup design, with the “U” facing the feed end of the element (see Figure 3.43).

Membranes Transport Models  91 Pressure Vessel End Cap

Membrane Module Shims

End Cap Adapter

Figure 3.42  Shimming at the feed end of the pressure vessel between the end cap adapter and the pressure vessel end cap to minimize movement of the elements in the vessel due to hard starts and water hammer.

DIRECTION OF FLOW Brine Seal

Module Membrane

Figure 3.43  “U” cup brine seal to prevent by-passing of the feed solution around the membrane element.

3.4 Specialty Membranes and Elements 3.4.1 Specialty Membranes 3.4.1.1 Dry Membranes In general, membranes are shipped wet, following wet testing at the factory. Dry elements are available that have not been tested but rely on the

92  Reverse Osmosis 3rd Edition average of tested membrane performance for specifications. The advantages of membrane shipped dry is that the storage life is longer and the risk of biofouling during storage prior to initial use is reduced. The limitations are that the specific element is not performance tested and it can take some time following start-up before the membranes truly wetted and yield specific performance.

3.4.1.2 Boron-Rejecting Membranes Boron is naturally found in seawater at average concentration of 4.6 ppm while less than 0.1 ppm in most ground waters [79]. With a pKa of 9.23, boron is in the acid form (non-ionized) in naturally-occurring waters (exists as boric acid). Boron is important for human health but also a concern for human and animal health. There is a fine line between supportive and unhealthy concentrations of boron. Limits for boron vary from country to country, while the World Health Organization (WHO) in 2006 established a drinking water limit of 2.4 ppm [79]. Since boron is non-ionized at the typical pH of most waters (7–9), rejection by RO membranes is limited. Fritzmann [80] found that the rejection

Boron Rejection (%)

100 99

Seawater Element Comparison (3.2% NaCI + 5 ppm B, 800 psi, 10% rec, pH = 7)

98 97 96 95 94 93 92

SWC5

SWC4+ SW30HR

SWC3+

91 90 TM820-400 89 5,500 6,500

SW30HR LE

7,500

8,500

9,500

Permeate Flow (gpd)

Figure 3.44  Boron rejection and permeability for various commercial seawater membrane elements. Conditions: TDS = 32,000 ppm, boron = 5 ppm, pH = 7, temperature 25°C, pressure = 55 bar, recovery = 10% [81].

Membranes Transport Models  93 of boric acid by seawater RO membranes ranged from 89–91% in laboratory settings and are expected to be lower than this in actually settings [79]. Figure 3.44 shows the boron rejection of some commercially-available seawater RO membranes [81]. Rejections depend on several factors, including pH, temperature, ionic strength, and the nature of the specific membrane used [80]. Boron-rejecting RO membranes have been developed to slightly improve upon the performance of seawater RO membranes. These membranes offer rejections ranging from 94–96% at standard seawater conditions [81, 82]. To enhance rejection, operation at high pH, greater than 9.5–10 where boric acid is ionized, is recommended.

3.4.2 Specialty Elements 3.4.2.1 Sanitary Elements Sanitary elements (sometimes referred to as “full fit”) differ from standard elements in that the outer wrap is made of netting rather than the fiberglass shell (see Figure 3.45). These elements also lack a brine seal. They are designed to operate with a percentage of feed by-pass around the element to minimize the accumulation of deposits and bacteria that could hide out in the shell of a standard element. This effect can be critical for food and beverage and pharmaceutical applications. Highcross flow velocities are used to minimize the potential for deposition of solids and bacteria.

Figure 3.45  Sanitary or “full fit” spiral element with a netting outer wrap instead of the standard fiberglass outer wrap. These elements lack a brine seal.

94  Reverse Osmosis 3rd Edition

Hydraulic disc

Lip seal Tie rod Membrane cushion Concentrate outlet

Cover

Permeate outlet Feed Inlet

Figure 3.46  Expanded view of a disc tube element.

3.4.2.2 Disc Tube Elements Disc tube (DT) elements are used for high-concentration wastewater treatment and recovery shows the applications, particularly for landfill leachate. Figure 3.46 shows a deconstructed representative DT element. The element consists of back to back membrane discs sometimes called a “cushion.” Standard RO membrane flat-sheet stock is used. The cushions are placed alternately with hydraulic discs. The discs have raised patterns on them that act as turbulence promotors to minimize concentration polarization. The short cross-flow length across each membrane cushion further minimizes concentration polarization. Minimizing concentration polarization allows for the use of this technology for treating high-strength wastewaters typically at ultra-high pressures.

3.4.2.3 Vibratory Shear Enhanced Processing (VSEP) Elements and System VSEP developed by New Logic Research, Inc. (Minden, NV, USA) is another method of minimizing the concentration polarization layer at the membrane surface thereby minimizing deposition on the membrane. This factor, as with the DT technology, allows VSEP to treat highly contaminated waters.

Membranes Transport Models  95

Figure 3.47  VSEP configuration with flow rating of 5.7 m3/h for RO and up to 13.6 m3/h for UF operations. Courtesy of Greg Johnson, CEO and founder [83].

A VSEP filter pack is similar to the DT with alternating membrane packets and disc spacers. The filter pack is oscillated over a 1.9–3.2 cm range to provide shear waves to the membrane surface. This generates approximately 50 HZ (an equivalent gravitational force of about 200 Gs) at the membrane surface, disrupting the concentration polarization layer and discouraging deposits from collecting on the membrane [83]. Figure 3.47 shows a 34 m3/h VSEP system in an RO configuration.

3.4.2.4 Ultra-High Pressure and High Temperature Elements Ultra-high pressure RO (UHPRO) and high temperature RO elements are becoming more commercially requested due to the need to treat more challenging feed sources. Refer to Chapters 1.3.2.1 and 1.3.2.2 for specific information for UHPRO and higher temperature RO, respectively.

Symbols A = water permeability coefficient B = solute permeability coefficient c = ideal concentration in Fick’s Law of Diffusion Cs = concentration of solute “s” at the membrane surface

96  Reverse Osmosis 3rd Edition Cw = concentration of water at the membrane surface Csb = concentration of solute “s” at the membrane surface Csp = concentration of solute “s” in the permeate D = diffusion coefficient D1c1 = water permeability D2K = solute permeability Db = diffusion coefficient of solute in the membrane Dw = diffusion coefficient of water in the membrane ΔG = Gibb’s free energy J = flux Js = solute flux Jw = water flux K = membrane coefficient for pore flow Kb = partition coefficient of the solute between the solution and the membrane surface l = thickness of the rejecting layer of the membrane M = molar concentration Ms = molecular weight of solute “s” Mw = molecular weight of water Ns = total flux of solute “s” Nw = total water flux Δπ = osmotic pressure difference across the membrane ΔP = pressure difference across the membrane R = Ideal Gas Law constant S = water solubility in the membrane ϴ = contact angle T = temperature UHPRO = ultra-high pressure reverse osmosis V = molar volume of water x = position in Fick’s Law of Diffusion Δx = membrane thickness

Nomenclature AFM = atomic force microscopy ATD = anti-telescoping device CA = cellulose acetate CTA = cellulose triacetate DT = disc tube ENM = engineered nanomaterials

Membranes Transport Models  97 ESD = extended solution-diffusion FPM = finely porous model GO = graphene oxide KSA = Kimura-Sourirajan analysis MPD = m-phenylenediamine NF = nanofiltration OSW = Office of Saline Water PALS = positron annihilation lifetime spectroscopy PEI = polyethyleneimine RO = reverse osmosis SANS = Small-angle neutron scattering S-D = solution-diffusion SDI = solution-diffusion imperfection SEM = scanning electron microscopy SF-PF = surface force-pore flow TDI = 2,4-diisocyanate TDS = total dissolved solids TEM = transmission electron microscopy TFC = thin film composite TFN = thin film nanocomposite TMC = trimesoyl chloride VSEP = vibratory shear enhanced processing WHO = World Health Organization

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98  Reverse Osmosis 3rd Edition 8. Sherwood, T.K., Brian, P.L.T., Fisher, R.E., Desalination by reverse osmosis. Ind. Eng. Chem. Fundam., 6, 2–12, 1967. 9. Glueckauf, E., On the mechanisms of osmotic desalting with porous membranes. Proceedings, First International Symposium of Water Desalination, vol. 1, US Department of the Interior, Office of Saline Water, Washington DC, USA, pp. 143–156. 10. Lonsdale, H.K., Merten, U., Riley, R.L., Transport properties of cellulose acetate osmotic membranes. J. Appl. Polym. Sci., 9, 1341–1362, 1965. 11. Jagur-Grodzinski, J. and Kedem, O., Transport coefficient and salt in uncharged hyperfiltration membranes. Desalination, 1, 327–341, 1966. 12. Johnson Jr., J.S., Dresner, I., Kraus, K.A., Hyperfiltration (reverse osmosis), in: Principles of Desalination, Spiegler (Ed.), Academic Press, New York, NY USA, 1966. 13. Spiegler, K.S. and Kedem, O., Thermodynamics of hyperfiltration (reverse osmosis): Criteria for efficient membranes. Desalination, 1, 311–326, 1966. 14. Paul, D.R., Reformulation of the solution-diffusion theory of reverse osmosis. J. Membr. Sci., 241, 371–386, 2004. 15. Wijmans, J.G. and Baker, R.W., The solution-diffusion model: A review. J. Membr. Sci., 107, 1–21, 1995. 16. Applegate, L.E. and Antonson, C.R., The phenomenological characterization of DP-q membranes, in: Reverse Osmosis and Membrane Research, Lonsdale and Podall (Eds.), Plenum Press, New York, NY, USA, 1972. 17. Jonsson, G., Overview of theories for water and solute transport in UF/RO membranes. Desalination, 35, 21–38, 1980. 18. Mehdizadeh, H., Modeling of Transport Phenomena in Reverse Osmosis Membranes, PhD. Thesis, McMaster University, Canada, 1990. 19. Merten, U., Transport properties of osmotic membranes, in: Desalination by Reverse Osmosis, Merten (Ed.), MIT Press, Cambridge, MA, USA, 1966. 20. Singh, S., Khulbe, K.C., Matsuura, T., Ramamurthy, P., Membrane characterization by solute transport and atomic force microscopy. J. Membr. Sci., 142, 111–127, 1998. 21. Lonsdale, H.K., Reverse osmosis, in: Synthetic Membranes: Science, Engineering, and Applications, Lonsdale and de Pinho (Eds.), D. Reidel Publishing Company, Dordrecht, Holland, 1986. 22. Strathmann, H., Scheible, P., Baker, R.W., A rationale for the preparation of Loeb-Sourirajan-type cellulose acetate membranes. J. Appl. Polym. Sci., 15, 811–828, 1971. 23. Taylor, J.S. and Jacobs, E.P., Reverse osmosis and nanofiltration, in: Water Treatment: Membrane Processes, Mallevialle, Odendaal, Wiesner (Eds.), McGraw-Hill Publishers, New York, NY, USA, 2006. 24. Glater, J., UCLA, Personal Communication, February 24, 2009. 25. McCutcheon, J.R. and Elimelech, M., Influence of membrane support layer hydrophobicity on water flux in osmotically driven membrane processes. J. Membr. Sci., 318, 458–466, 2008.

Membranes Transport Models  99 26. Childress, A.E. and Elimelech, M., Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes. J. Membr. Sci., 119, 253–268, 1996. 27. Kumano, A. and Fujiwara, N., Cellulose triacetate membranes for reverse osmosis, in: Advanced Membrane Technology and Applications, Li, Fane, Ho, Matsuura (Eds.), John Wiley & Sons, New York, NY, USA, 2008. 28. Toyobo Hollosep® Membranes,. https://www.toyobo-global.com/seihin/ro/ tokucho.htm, accessed May 26, 2020. 29. Microdyne-Nadir, Membrane chemistry: Cellulose acetate vs thin-film composite, form TB-017, dated 2020. 30. Cadotte, J.E., Petersen, R.J., Larson, R.E., Erickson, E.E., A new thin-film composite seawater reverse osmosis membrane. Desalination, 32, 25–31, 1980. 31. J.W. Richter, K. Square, H.H. Hoehn, Permselective, aromatic, nitrogen-­ containing polymeric membranes. US Patent 3,567,632 assigned to E.I. Du Pont de Nemours and Company, Wilmington, DE, USA, 1971. 32. Shields, C.P., Five years’ experience with reverse osmosis systems using Du Pont “permasep” permeators. Desalination, 28, 3, 157–179, 1979. 33. Endoh, R., Tanaka, T., Kurihara, M., Ikeda, K., New polymeric materials for reverse osmosis membranes. Desalination, 21, 1, 35–48, 1977. 34. McKinney, R. and Rhodes, J.H., Aromatic membranes for reverse osmosis separations. Macromolecules, 4, 5, 633–637, 1971. 35. Baker, R.W., Membrane Technology and Applications, 3rd Ed., John Wiley & Sons, LTD., Chichester, West Sussex, United Kingdom, 2012. 36. Beasley, J.K., The evaluation and selection of polymeric materials for reverse osmosis membranes. Desalination, 22, 181–189, 1977. 37. E.I. du Pont de Nemours & Co., Permasep Engineering Manual, 1982. 38. Kucera-Gienger, J., Studies on Halogen Interactions with Polyamide-Type Reverse Osmosis Membranes, MS Thesis, University of California, Los Angeles, USA, 1984. 39. Glater, J., Zachariah, M.R., McCray, S.B., McCutchan, J.W., Reverse osmosis membrane sensitivity to ozone and halogen disinfectants. Desalination, 48, 1–16, 1983. 40. Xie, W., Geise, G.M., Freeman, B.D., Lee, H.-S., Byun, G., McGrath, J.E., Polyamide interfacial composite membranes prepared form m-phenylene diamine, trimesoyl chloride, and a new disulfonated diamine. J. Membr. Sci., 403-404, 153–161, 2012. 41. J.E. Cadotte, Reverse osmosis membrane. US Patent 4,039,440, assigned to USA Secretary of the Interior, Washington DC, USA, 1977. 42. Rozelle, L.T., Kopp Jr., C.V., Cadotte, J.E., Cobain, K.E., NS-100 membranes for reverse osmosis applications. J. Eng. Ind., 97, 1, 220–223, 1975. 43. Fang, H.H.P. and Chian, E.S.K., Optimization of NS-100 membrane for reverse osmosis. J. Appl. Polym. Sci., 20, 303–314, 1976.

100  Reverse Osmosis 3rd Edition 44. Petersen, R.J., Composite reverse osmosis and nanofiltration membranes. J. Membrane Sci., 83, 81–150, 1993. 45. United States Department of the Interior, Office of Saline Water, Catalog of Research Projects, 1974, https://books.google.com/books?id=VN9U_LzCFbE C&pg=PA2&lpg=PA2&dq=NS-100+membrane&source=bl&ots=m CediKASNB&sig=ACfU3U2CCFooL3j3w84LY1G-7lQWhlaCUg&h l=en&sa=X&ved=2ahUKEwjAqOftiNTpAhXPGs0KHV4lC8gQ6AEwCHoECAoQAQ#v=onepage&q=NS-100%20membrane&f=false accessed May 27, 2020. 46. J.E. Cadotte, Interfacially synthesized reverse osmosis membrane. US Patent 4,277,344, assigned to FilmTec Corp, Minnetonka, MN, USA, 1981. 47. Hydranautics, CPA7-LD specification sheet, 2019. https://membranes.com/ wp-content/uploads/Documents/Element-Specification-Sheets/RO/CPA/ CPA7-LD.pdf, accessed May 2, 2020. 48. Verbeke, R., Gomez, V., Vankelcom, I.F.J., Chlorine-resistance of reverse osmosis (RO) polyamide membranes. Prog. Polym. Sci., 72, 1–15, 2017. 49. Glater, J., Hong, S.-K., Elimelech, M., The search for a chlorine-resistance reverse osmosis membrane. Desalination, 95, 325–345, 1994. 50. Kwan, Y.N. and Lecke, J.O., Hypochlorite degradations of crosslinked polyamide membranes—changes in hydrogen bonding behavior and performance. J. Membr. Sci., 282, 456–464, 2006. 51. Koo, J.-Y., Petersen, R.J., Cadotte, J.E., ESCA characterization of chlorine-­ damaged polyamide reverse osmosis membrane. Polym. Prepr., ACS Paper Presented at Anaheim CA USA Meeting, 391–392, 1986. 52. Bates, W., Reducing the fouling rate of surface and waste water RO systems, paper IWC-98-08, in: Proceedings of the 59th International Water Conference, Pittsburgh, PA, USA, October 19–21, 1998. 53. Kucera, J., Biofouling of polyamide membranes: Fouling mechanisms, current mitigation and cleaning strategies, and future prospects. Membranes, 9, 9, 111–192, 2019, doi: 10.3390/membranes9090111. 54. Microdyn-Nadir, Pretreatment: Chloramine, Technical Service Guide, 2017. 55. Lencki, R.W. and Williams, S., Effect of nonaqueous solvents on the flux behavior of ultrafiltration membranes. J. Membr. Sci., 101, 1-2, 43–51, 1995. 56. Childress, A. and Brandt, J., Characterization of the hydrophobicity of polymeric reverse osmosis and nanofiltration membranes: Implications to membrane fouling, in: Desalination and Water Purification Research and Development Program Report No. 57, U.S. Department of the Interior, Bureau of Reclamation, Denver, CO, USA, 2000, https://www.usbr.gov/research/ dwpr/reportpdfs/report57.pdf, accessed May 29, 2020. 57. Zhang, Y., Wan, Y., Guo, M., Pan, G., Shi, H., Yao, X., Liu, Y., Surface modification on thin-film composite reverse osmosis membrane by cation complexation for antifouling. J. Polym. Res., 26, 68, 2019.

Membranes Transport Models  101 58. Hurwitz, G., Guillen, G., Hoek, E.M.V., Probing polyamide membrane surface charge, zeta potential, wettability, and hydrophilicity with contact angle measurements. J. Membr. Sci., 349, 349–357, 2010. 59. Ghosh, A.K., Jeong, B.-H., Huang, X., Hoek, E.M.V., Impacts of reaction and curing conditions on polyamide composite reverse osmosis membrane properties. J. Membr. Sci., 311, 34–45, 2008. 60. Ramon, G.Z. and Hoek, E.M.V., Transport through composite membranes, part 2: Impacts of roughness on permeability and fouling. J. Membr. Sci., 425426, 141–148, 2013. 61. Elimelech, M., Zhu, X., Childress, A.E., Hong, S., Role of membrane surface morphology in colloidal fouling of cellulose acetate and composite aromatic reverse osmosis membranes. J. Membr. Sci., 127, 101–109, 1997. 62. Zhu, X. and Elimelech, M., Colloidal fouling of reverse osmosis membranes: Measurement and fouling mechanisms. Environ. Sci. Technol., 31, 3654– 3662, 1997. 63. Yrijenhoek, E.M., Hong, S., Elimelech, M., Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration membranes. J. Membr. Sci., 188, 115–128, 2001. 64. Hoek, E.M.V., Bhattacharjee, S., Elimelech, M., Effect of membrane surface roughness on colloidal-membrane DLVO interactions. Langmuir, 19, 4836– 4847, 2003. 65. Deshmukh, S.S. and Childress, A.E., Zeta potential of commercial RO membranes: Influence of source water type and chemistry. Desalination, 140, 87–95, 2001. 66. Singh, P.S., Rao, A.P., Ray, P., Bhattacharya, A., Singh, K., Saha, N.K., Reddy, A.V.R., Techniques for characterization of polyamide thin film composite membranes. Desalination, 282, 78–86, 2011. 67. Lonsdale, H.K., Recent advances in reverse osmosis membranes. Desalination, 13, 317–332, 1973. 68. Riley, R.L., Lin, S.W., Murphy, A., Wiater-Protas, I., Ridgeway, H.F., Development of a New Chlorine and Biofouling Resistant Polyamide Membrane, US Department of the Army, Army Research Office, Report no. 201370, 2002. 69. Singh, R., Characteristics of a chlorine-resistant reverse osmosis membrane. Desalination, 95, 27–37, 1994. 70. Liu, T., Chen, D., Yang, F., Chen, J., Cao, Y., Xiang, M., Kang, J., Xu, R., Enhancing the permeability and anti-fouling properties of a polyamide thinfilm composite reverse osmosis membrane via surface grafting of L-lysine. RCS Adv., 9, 20044–20052, 2019. 71. Huang, H., Lin, S., Hou, L., Chlorine-resistant polyamide reverse osmosis membrane with monitorable and regenerative sacrificial layers. ACS Appl. Mater. Interfaces, 9, 11, 10214–10223, 2017.

102  Reverse Osmosis 3rd Edition 72. Ali, M.E., Wang, L., Wang, X., Feng, X., Thin film composite membranes embedded with graphene oxide for water desalination. Desalination, 386, 67–76, 2016. 73. Mokkapati, V.R.S.S., Koseoglu-Lmer, D.Y., Yilmax-Deveci, N., Majakovix, I., Koyuncu, I., Membrane properties and anti-bacterial/anti-biofouling activity of polysulfone-graphene oxide composite membrane phase inversed in graphene oxide nonsolvent. RSC Adv., 7, 4378–4386, 2017. 74. Shao, F., Dong, L., Dong, H., Zhang, Q., Zhao, M., Yu, L., Pang, B., Chan, Y., Graphene oxide modified polyamide reverse osmosis membranes with enhanced chlorine resistance. J. Membr. Sci., 525, 9–17, 2017. 75. Huang, X., Marsh, K.L., McVerry, B.T., Hoek, E.M.V., Kaner, R.B., Lowfouling antibacterial reverse osmosis membranes via surface grafting of graphene oxide. ACS Appl. Mater. Interfaces, 8, 14334–14338, 2016. 76. Lawler, J., Incorporation of graphene-related carbon nanosheets in membrane fabrication for water treatment: A review. Membranes, 6, 57, 2016. 77. Chae, H.R., Lee, J., Lee, C.H., Kim, I.C., Park, P.K., Graphene oxide-­ imbedded thin film composite reverse osmosis membrane with high flux, anti-fouling, and chlorine resistance. J. Membr. Sci., 483, 128–135, 2015. 78. Singh, R., Hybrid Membrane System for Water Purification, Elsevier, Amsterdam, 2006. 79. Tu, K.L., Nghiem, I.D., Chivas, A., Coupling effects of feed solution pH and ionic strength on the rejection of boron by NF/RO membranes. Chem. Eng. J., 168, 2, 700–706, 2011. 80. Fritzmann, C., Lowenberg, J., Wintgens, T., Melin, T., State of the art of reverse osmosis desalination. Desalination, 216, 1–76, 2007. 81. Hydranautics, Boron Removal by hydranautics RO membranes, in: Technical Application Bulletin, TAB 113, 2005. 82. Taniguchi, M., Fusaoka, Y., Nishikawa, T., Kurihara, M., Boron removal in RO seawater desalination. Desalination, 167, 419–426, 2004. 83. New Logic Research, Inc. VSEP, 2020. https://www.vsep.com/technology/ accessed June 5, 2020.

Section II SYSTEM DESIGN AND ENGINEERING

4 Basic Design Arrangements and Concentration Polarization Guidelines 4.1 Arrays and Stages For spiral wound elements, an RO skid consists of an array of pressure vessels (each containing elements) arranged in a specific layout; the “array” is used to describe the specific layout. Figure 4.1 shows a layout for an sample RO skid. In this figure, there are four pressure vessels in parallel followed in series by two pressure vessels in parallel. Each set of pressure vessels in parallel is called a stage. Hence, the RO skid shown in Figure 4.1 is described as having a two-stage array. Further, since feed water enters the skid through the four pressure vessels on the left (as shown in the figure) and passes through the system via the last two pressure vessels on the right, the specific configuration is a 4:2 array. The array, then, describes that there are two stages and that the first stage has four pressure vessels in parallel and the second stage has two pressure vessels in parallel. To complete the description, the number of elements in each pressure vessel is added to the array, such as 4:2-6M, where “6M” means there are six elements in each pressure vessel (virtually all RO skids contain the same number of elements in each pressure vessel included in that specific skid). The feed flow to each of the pressure vessels in the first stage of the 4:26M array shown in Figure 4.1 is generally equivalent (exceptions would include if fouling were occurring in elements in some pressure vessels but not in others, such as if new elements were installed in single but not all pressure vessels). The feed water passes through each of the elements, in series, through the first stage. Permeate is removed from the feed as it passes through each element, such that the concentrate flow out of each subsequent element is lower than the feed flow to it. Ergo, the total concentrate flow out of the pressure vessels in the first stage, is less than the feed flow to that stage. While the total permeate from the first stage is removed from the from that stage, the concentrate flow from the first stage is sent to the second Jane Kucera. Reverse Osmosis 3rd Edition, (105–116) © 2023 Scrivener Publishing LLC

105

106  Reverse Osmosis 3rd Edition

FEED STREAM

PERMEATE

CONCENTRATE

Figure 4.1  Drawing showing a 4:2 array.

stage as the feed to that stage. The same feed, concentrate, and permeate hydraulics occur in the second stage as the first, where permeate from each element is removed and the concentrate flow at the outlet from the second stage is less than the feed flow to it each subsequent stage has fewer pressure vessels to maintain the equivalent hydraulics for each stage. Finally, the permeate flow from each stage is combined to become the overall permeate from the system. The concentrate from the final stage is the overall concentrate from the system.

4.1.1 Recovery per System Array System recovery is a function of the number of stages, the degree or “evenness” of the taper, and number of elements per pressure vessel in each stage. In most cases for an even taper, a six-element long pressure vessel will recover about 50% of the feed flow to the vessel as permeate, leaving 50% of the remaining initial feed flow as concentrate. For a two-stage system, this concentrate becomes the feed to the second stage, as describe previously. Hence, the number of pressure vessels required in the second stage is about half that of the vessels in the first stage to maintain the same, high cross-flow velocity. This is called a tapered array, where, given “N” pressure vessels in the first stage, “N/2” would be the number of pressure vessels in the second stage. Examples of tapered or “tree” arrays include 2:1, 6:3, 8:4, 10:5, and 12:6:3 (a three-stage array which is used when higher overall permeate recovery is required). Sometimes the array design may not result in an “even” taper; tapers such as 5:3, or 3:2:1 may be necessary due to the hydraulics required to minimize concentration polarization for given feed and permeate flow rates and desired recovery. A 75% recovery system would typically include two stages with six elements per pressure vessel. The first stage recovers 50% of the feed water while the second stage recovers 50% of its feed leading to 75% overall recovery, as shown in Figure 4.2. In other words, given 100 l/m feed flow

Basic Design Arrangements  107 50 l/m

FEED STREAM

50 l/m

25 l/m

PERMEATE 75 L/M

100 l/m CONCENTRATE 25 l/m

Figure 4.2  Approximate flow distribution through a two-stage, 75% recovery RO unit.

to the RO unit, 50 l/m would be recovered as permeate from the first stage. The feed to the second stage would be 50 l/m. A recovery of 50% through the second stage would yield 25 l/m permeate. Combining the permeate from both stages yields an overall permeate flow of 75 l/m, or 75% recovery of the initial feed to the unit. The concentrate from this 75% recovery unit would be 25 l/m. While a two-stage system with six elements per pressure vessel typically recovers about 75% of the feed water as permeate, 80% recovery may be achievable with two stages, but concentration polarization effects might become significant. Recoveries higher than 80% generally require three stages to minimize concentration polarization. Arrays with less than six elements per pressure vessel will recover less than 50% of the feed per stage; the fewer elements per stage, the lower the recovery per stage and the lower the overall recovery for the system. Figure 4.3 shows the approximate water quality through a two-stage, 75% recovery system (water quality is approximated using the concentration

50 l/m 2 ppm

FEED STREAM 100 l/m 100 ppm

50 l/m

25 l/m 4 ppm

PERMEATE 75 l/m 2.67 ppm

200 ppm CONCENTRATE 25 l/m 400 ppm

Figure 4.3  Approximate water quality profile through at two-stage, 75% recovery system, assuming a membrane rejection of 98%.

108  Reverse Osmosis 3rd Edition Table 4.1  Impact of recovery on approximate concentrate flow and permeate quality for the system described in Figure 4.3. Recovery (%)

Concentrate flow (l/m)

Permeate TDS (ppm)

50

50

2

75

25

2.67

factor described in Figure 2.4 for the concentrate and assumed rejection of 98% for the permeate). Key issues to note are summarized in Table 4.1 and described as follows: • Each additional stage increases recovery and decreases concentrate flow from the system. Concentrate is often considered a waste stream and so higher recovery minimizes waste from the system. • The product quality from each successive stage is lower (higher total dissolved solids (TDS)) than the previous one. This is because the feed concentrate to the successive stage is higher than the feed to the previous stage. Hence, there is a critical design challenge so to speak between higher recovery (potentially less wastewater generated) and better permeate quality. System design requires consideration of this trade-off between the amount of waste generated and permeate quality.

4.1.2 Element-By-Element Flow and Quality Distribution A more detailed flow and quality distribution is provided in Figure 4.4. In this example, recovery per element is assumed to be 11% and membrane rejection is assumed to be 98%. This element-by-element description is more accurate than the stage-wise examples in Figures 4.2 and 4.3. Figure 4.5 shows the results from an actual system six-element stage designed to recover 50% of 200 l/m of feed flow at a membrane solute rejection of 99.8%. In practice, the recovery per each subsequent element decreases through the stage as does the permeate flow per element. Note that the concentration polarization modulus, β, also increases with each subsequent element in the stage ranging from 1.16 to 1.19. Recall from the discussion in Chapter 2.6 that β (as described by Equation 2.9) represents the ratio of the solute concentration at the membrane surface to that of the bulk feed stream. Higher β results in higher concentration polarization, and

Basic Design Arrangements  109 TOTAL PRODUCT 2 ppm

2.12

2.22

2.35

2.48

2.61

2.5 m3/h

4.9

6.9

8.6

10.2

11.4

22.7 m3/h

20.2

17.8

15.8

14.1

12.5

11.3

100 ppm

112

127

142

160

179

200

FEED - CONCENTRATE

Figure 4.4  Element-by-element flow and quality distribution for a single, six-element long stage. Assumes 11% recovery per element and 98% solute rejection. TOTAL PRODUCT 0.6 ppm

0.85 4.9

2.5 m3 /h 1.16

1.17

22.7 m3/h

20.2

100 ppm

112

6.9 1.17

17.8

8.6 1.17

15.8

10.2 1.18

14.1

12.5

11.4 1.19 11.3 199.2

FEED - CONCENTRATE

Figure 4.5  Element-by-element flow and quality distribution for an actual 50% recovery, six-element stage with a membrane solute rejection of 99.8%. Beta ranges from 1.16 to 1.19 for the lead to the last element.

subsequent membrane deposition. This phenomenon is particularly critical for last stage elements, where the feed concentration is the highest and membrane deposition via scaling is probable (see discussion in Chapter 8). This detailed analysis for flow and permeate quality is critical to the designer when considering the array design necessary to maximize performance and minimize β and concentration polarization.

4.1.3 Flux Guidelines The total number of pressure vessels required on a skid depends on the required permeate flow and the appropriate water flux to minimize concentration polarization. Recall that flux defines the flow rate of a substance, in this case, water, through a given area, in this case, membrane area, and is

110  Reverse Osmosis 3rd Edition usually represented as m3/m2-day and often shortened to m/d. As described in Chapter 2.6, higher flux leads to higher β, thereby increasing the potential for membrane deposition. However, higher flux also denotes that less membrane area (fewer elements and a smaller RO system) is required to generate the desired permeate flow. Hence, there is a trade-off between system size and membrane fouling/scaling potential. Guidelines have been developed empirically over the years to balance this trade-off. Without running RO design projections, the approximate number of total elements required to produce a given permeate flow can be estimated via these guidelines. Table 4.2 lists the generally-accepted flux guidelines as a function of feed water source and quality (as measured by the 15-minute silt density index (SDI15) discussed in Chapter 9; higher SDI15 is indicative of greater membrane fouling potential). To use Table 4.2, assume, for example, a system is required to deliver 60 m3/hr permeate flow for a surface water feed source with an SDI15 of