Reverse Osmosis Seawater Desalination Volume 2: Planning, Process Design and Engineering – A Manual for Study and Practice 3030819264, 9783030819262

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Heinz Ludwig

Reverse Osmosis Seawater Desalination Volume 2 Planning, Process Design and Engineering – A Manual for Study and Practice

Reverse Osmosis Seawater Desalination Volume 2

Heinz Ludwig

Reverse Osmosis Seawater Desalination Volume 2 Planning, Process Design and Engineering – A Manual for Study and Practice

Heinz Ludwig Nufringen, Baden-Württemberg, Germany

ISBN 978-3-030-81926-2 ISBN 978-3-030-81927-9 https://doi.org/10.1007/978-3-030-81927-9

(eBook)

# The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface and Acknowledgement

During the more than five decades in which I have now been active in water, wastewater, and environmental process engineering, from the 1970s onwards, membrane technology, evolving from a novel and innovative process for separating solids and dissolved salts from water, whose potential and applications were initially perceived only as a possible complement to conventional solids separation, such as filtration, sedimentation, and flotation as well as desalination by ion exchange and evaporative technologies has since advanced to become a dominant technology for separating suspended and colloidal substances from water as well as for desalination. Following the initial phase of investigating the technical possibilities and evaluating its economic potential in comparison with conventional treatment technologies, the next phase in its development was quickly reached with diverse applications in the industrial sector for treating process water, wastewater, and process solutions. After the establishment of membrane technology within the range of separation and desalination processes available and suitable for the treatment of water and solutions, the next development step was its use in seawater and brackish water desalination, initially in plants with a lesser product water capacity, but then for large-scale plants alongside the thermal multistage flash (MSF) and multi-effect distillation (MED) technologies that were already well established for this application. In the past two decades, increasingly it has found application in stand-alone installations as an alternative to or in competition with evaporative desalination technology, even to the extent of supplanting this altogether. A particular attraction of working in a specific sector over many years is having the opportunity to witness and participate in technological developments right from their inception and up to the attainment of fully fledged maturity. In the case of membrane technology, it is particularly its use in seawater desalination that has developed during my professional career from the more industrially oriented smalland medium-sized plants with a few 100 m³/day of treated water output to today’s large-scale facilities for supplying drinking water with capacities of several 100,000 m³/day. It presented a professional challenge and was a particular motivation for me to play a part in introducing and establishing this technology for securing the water supply of many cities and regions and even for parts of an entire continent like Australia.

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Preface and Acknowledgement

What has always fascinated me during my professional career in projects for planning and realizing large seawater desalination plants is the diversity of professions that come together in this sector in order to achieve the goal, namely, the design, erection, and operation of such a plant. This starts with the variety of technical disciplines involved in the design and construction of its process engineering components. However, right from the project development stage, when decisions have to be taken on strategies for the procurement, project implementation, and contractual regulation of plant operation, economists, lawyers, and experts in contract law play important roles. In addition, for major state-owned desalination projects, media experts are oftentimes called in at this stage to help gain public acceptance for the plant and its location. The success of a seawater desalination project and the timing of its implementation are also greatly influenced by planning and executing the procedures for obtaining permits under environmental regulations and thus the involvement of ecologists with coordination of the various disciplines that work with them as part of the plant’s planning and design teams. This multidisciplinary approach to seawater desalination projects is therefore also reflected in the structure and content of this book as described in Chap. 1. The catalyst for writing this book was the presentation of the Jim Stewart Lifetime Achievement Award to me by Global Water Intelligence (GWI) at the Global Waters Summit 2011 in Berlin, which honored my over 40 years of professional involvement in the field of seawater desalination. As a follow-up to this acknowledgement of my professional work, together with Fichtner GmbH & Co, KG, the engineering consultancy company for which I was active for a large number of seawater desalination projects over this entire period in a managerial capacity, as an expert and as a technical advisor, the idea emerged of authoring a manual based on the experience I had gained in this field, in which the state of the art of seawater desalination on the basis of reverse osmosis technology is comprehensively presented, starting from its physical, chemical, and process engineering fundamentals up to the planning and design of these RO desalination plants. The reference book that is the outcome of this collaboration is not an academic textbook, but rather it is intended to serve as a practice-oriented introduction to the planning processes and the approach for the design and realization of plants for the application of membrane processes to seawater desalination and also to introduce the relevant design and operating parameters. However, theoretical principles and the findings of academic research that serve to provide an understanding of the calculations involved in dimensioning the systems and components of such installations are presented in detail and also clarified by means of the associated mathematical algorithms. This book sets out not only my own experiences and the knowledge that I have acquired from my engineering and design activities but, especially in its sections describing the design of the treatment processes of an SWRO plant as regards their dimensioning and their operating characteristics, it is additionally based on voluminous technical information from desalination plant constructors as well as from the suppliers of membranes, conditioning chemicals, and the various process components, together with results from calculation software provided by these

Preface and Acknowledgement

vii

manufacturers for the application of their products. In addition, there are insights gained from the exchange of information between the various disciplines within the planning teams of an SWRO plant as well as during the phases for developing and realizing such projects in dialogue with the various project participants. Further contributions to the subject matter of this book come from selected publications in diverse technical journals as well as from presentations at international congresses on desalination and membrane technology held under the aegis of the International Desalination Association (IDA) and the European Desalination Society (EDS) as listed in the references for each chapter of the book. The technical know-how that is brought together and described in this book thus comes from a wealth of resources, while many experts contribute to the development of this range of expertise as needed for the construction of reverse osmosis seawater desalination plants. For this reason, only a general word of thanks can be addressed to the many experts from the various disciplines who are involved in planning, constructing, and optimizing the efficiency of these plants, which are of great consequence for the water supply of humankind. I am particularly grateful to Fichtner’s management for their technical and financial support, and for giving me access to the company’s many resources during the preparation of this book, to the staff of the Fichtner Seawater Desalination Division for the manifold exchanges of experience and, together with the other divisions of the company, for the fruitful cooperation while tackling a large number of projects. My thanks also go to Peter Billard for translating the German parts of the manuscript into English and for his critical review of my English texts. Last but not least, I would like to especially thank my wife for her tolerance and patience during the time I worked on the book and for the support she gave the family and me during my active professional life which involved much travelling and frequent, sometimes prolonged, absences. Nufringen, Germany May 2021

Heinz Ludwig

Contents

1

Introduction and Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

2

Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1 Foulants in Seawater . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Kinds of Fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3 Methods for Fouling Potential Testing . . . . . . . . . . . . . . . 2.2 Pretreatment Measures, Options, and Configurations . . . . . . . . . . . 2.2.1 Disinfection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1.1 Disinfectants’ Chemistry . . . . . . . . . . . . . . . . . . 2.2.1.2 Efficacy of Disinfectants . . . . . . . . . . . . . . . . . . 2.2.1.3 Chlorine and Hypochlorite Dosing Systems . . . . . 2.2.1.4 Chlorine Disinfection: Mode of Dosing, Dosing Rates, Dosing System Sizing, Chlorine Consumption, and Power Demand . . . . . . . . . . . 2.2.2 Pretreatment Process Options for Solids and Colloidals Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2.1 Selection of Pretreatment Process Configurations . 2.3 Pretreatment Processes Unit Operations . . . . . . . . . . . . . . . . . . . . 2.3.1 Coagulation and Flocculation . . . . . . . . . . . . . . . . . . . . . . 2.3.1.1 Coagulant and Flocculant Chemistry . . . . . . . . . . 2.3.1.2 Basic Principles of Coagulation and Flocculation Process Design . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1.3 Mixing and Flocculation Devices . . . . . . . . . . . . 2.3.1.4 Mixing and Flocculation Process Design . . . . . . . 2.3.2 Sedimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2.1 Sedimentation Basics . . . . . . . . . . . . . . . . . . . . . 2.3.2.2 Sedimentation Devices . . . . . . . . . . . . . . . . . . . . 2.3.2.3 Sedimentation Process Design . . . . . . . . . . . . . . 2.3.3 Flotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3.1 Flotation Basics . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3.2 Flotation Devices . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3.3 Flotation Process Design . . . . . . . . . . . . . . . . . .

19 20 20 21 23 26 27 28 42 46

56 67 68 76 76 78 88 91 93 102 102 106 111 118 118 121 123 ix

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2.3.4

3

Granular Media Filtration . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4.1 Granular Media Filtration Basics . . . . . . . . . . . . 2.3.4.2 Granular Media Filtration Devices . . . . . . . . . . . 2.3.4.3 Granular Media Filtration Filter Design . . . . . . . . 2.3.5 Membrane Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5.1 Membrane Filtration Basics . . . . . . . . . . . . . . . . 2.3.5.2 Membrane Filtration Devices and Systems . . . . . 2.3.5.3 Membrane Process Design . . . . . . . . . . . . . . . . . Annexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.A1 Static Mixer Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.A2 Mixing and Coagulation Basin Calculation . . . . . . . . . . . . . 2.A3 Inclined Plate and Sedimentation System Design . . . . . . . . . 2.A4 Dissolved Air Flotation Design . . . . . . . . . . . . . . . . . . . . . . 2.A5 Open Gravity Filter Floc Filtration Design . . . . . . . . . . . . . . 2.A6 Membrane Filtration Design . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

132 132 144 150 166 166 177 182 220 220 221 224 226 228 230 234

Post-Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Seawater RO Permeate Quality and Factors of Influence . . . . . . . . 3.2 Determination of SWRO Product Water Composition Values . . . . 3.2.1 International and National Drinking Water Guidelines . . . . 3.2.2 Irrigation Water Guidelines and Recommendations . . . . . . 3.2.3 Calcium Carbonate Saturation Indexes, Corrosion Indexes, and Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3.1 Calcium Carbonate Saturation Indexes . . . . . . . . 3.2.3.2 Corrosion Indexes and Guidelines . . . . . . . . . . . . 3.3 Product Water Target Values and Guidelines . . . . . . . . . . . . . . . . 3.3.1 Product Water Guidelines and Practised Range of Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Post-Treatment Configuration and Treatment Systems . . . . . . . . . 3.4.1 Post-Desalination: Function and Design . . . . . . . . . . . . . . 3.4.2 Remineralization/Alkalinization: Possible Processes and Process Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2.1 Carbon Dioxide Supply and Production . . . . . . . 3.4.2.2 Lime/CO2, Limestone/CO2, Limestone/H2SO4, and Dolomite/CO2 Processes: Systems, Devices, and Process Design . . . . . . . . . . . . . . . . . . . . . . 3.4.2.3 Composition and Properties of Post-Desalinated and Remineralized RO Product Water . . . . . . . . . 3.4.2.4 Remineralization Process Modifications for Increase of Magnesium Content in SWRO Product Water . . . . . . . . . . . . . . . . . . . . . . . . . .

237 239 240 241 243 246 247 258 262 262 265 267 275 277

281 314

319

Contents

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3.4.3

Conditioning: Dosing of Corrosion Inhibitors and Magnesium Compounds and Fluoridation . . . . . . . . . . . . . 3.4.3.1 Corrosion Inhibitor Dosing . . . . . . . . . . . . . . . . . 3.4.3.2 Magnesium Compound Dosing . . . . . . . . . . . . . . 3.4.3.3 Fluoride Dosing . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.4 Disinfection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.4.1 Disinfectant Properties and Selection . . . . . . . . . 3.4.4.2 Disinfection Process Design . . . . . . . . . . . . . . . . 3.5 Power Demand of Post-Treatment Systems and Drinking Water Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Post-Treatment Power Demand . . . . . . . . . . . . . . . . . . . . 3.5.1.1 Post-Desalination Power Demand . . . . . . . . . . . . 3.5.1.2 Remineralization/Alkalinization Power Demand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1.3 Conditioning and Disinfection Power Demand . . . 3.5.2 Design and Power Demand of Product Water Supply . . . . . Annexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.A1 Post-Treatment: Remineralization/Alkalinization—Lime/CO2 Process Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.A2 Post-Treatment: Remineralization/Alkalinization—Limestone/ CO2 Process Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.A3 Post-Treatment: Disinfection Process Design—Primary and Secondary Disinfection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Seawater Extraction and Supply and Concentrate Discharge . . . . . 4.1 Seawater Extraction and Supply . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Surface Water (Direct) Intakes . . . . . . . . . . . . . . . . . . . . 4.1.1.1 Open Intake Channel or Lagoon Type . . . . . . . . 4.1.1.2 Submerged Pipe/Tunnel Active Screening Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1.3 Submerged Pipe Passive Screening Type . . . . . . 4.1.2 Indirect (Soil Filtration) Intakes . . . . . . . . . . . . . . . . . . . 4.1.2.1 Vertical Intakes (Beach Wells) . . . . . . . . . . . . . 4.1.2.2 Horizontal Intakes . . . . . . . . . . . . . . . . . . . . . . 4.1.2.3 Beach Galleries and Seabed Filters . . . . . . . . . . 4.1.3 Assessment and Selection of Intake Withdrawal Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.4 SWRO: Seawater Supply Pumping Facilities and Intake Power Demand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.4.1 Seawater Supply Pumping . . . . . . . . . . . . . . . . 4.1.4.2 Power Demand of Intake Systems . . . . . . . . . . . 4.2 Concentrate and Wastewater Discharge . . . . . . . . . . . . . . . . . . . 4.2.1 Discharge Flow of SWRO . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Salt Content of SWRO Discharge . . . . . . . . . . . . . . . . . .

322 322 324 326 329 330 339 349 349 349 351 352 354 356 356 358 360 361

. . . .

365 365 370 370

. . . . . .

374 381 385 389 393 398

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400

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405 405 409 410 410 413

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Contents

4.2.3

Discharge Diffuser Systems and Their Design . . . . . . . . . 4.2.3.1 Software Systems for Modelling of Near-Field and Far-Field Zone Conditions . . . . . . . . . . . . . Annexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.A1 SWRO Outfall Discharge: Diffuser System Conceptual Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

.

415

. .

421 425

. .

425 427

SWRO Effluents and Residues: Composition, Environmental Impacts, Discharge and Disposal Regulations, and Treatments Measures . . . . 5.1 Overview of SWRO Plant Effluents and Solid Residues . . . . . . . . 5.1.1 SWRO Processing Stages and Wastes Produced During Their Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 SWRO Effluents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Composition of Main Discharges . . . . . . . . . . . . . . . . . . . 5.2.1.1 Reverse Osmosis Concentrate . . . . . . . . . . . . . . . 5.2.1.2 Backwash Water of Pretreatment Filtration Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1.3 Cleaning-in-Place (CIP) . . . . . . . . . . . . . . . . . . . 5.2.1.4 Membrane Preservation . . . . . . . . . . . . . . . . . . . 5.2.1.5 Posttreatment Discharges . . . . . . . . . . . . . . . . . . 5.2.1.6 Sludge of Sedimentation/Flotation Systems in Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Environmental Impacts of Constituents of Discharges and Regulations for Discharge . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2.1 Environmental Impacts and Their Mitigation . . . . 5.2.2.2 Regulations for Discharge of Concentrate and Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 SWRO Wastewater Treatment Installations and Their Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3.1 Wastewater Treatment Plant Configuration and Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3.2 SWRO Wastewater Treatment Plant Design . . . . 5.2.3.3 SWRO Wastewater Treatment Plant Power Demand and Energy Consumption . . . . . . . . . . . 5.3 SWRO Solid Residues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Regulations for Disposal of Solid Residues and Wastes . . . 5.3.2 Types of SWRO Residues, Their Composition, and Waste Management Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2.1 Seawater Screening Debris . . . . . . . . . . . . . . . . . 5.3.2.2 Residues from Pretreatment Solids Separation and Posttreatment . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2.3 Spent Membrane Elements from Membrane Replacement in Membrane Filtration and RO . . .

429 430 430 432 432 434 435 437 439 439 440 441 444 451 457 457 464 485 486 486 488 488 489 491

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xiii

5.3.2.4 5.3.2.5

6

7

8

Spent Cartridge Filter Elements . . . . . . . . . . . . . Loss and Replacement of Filter Material of Granular Media Filters . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

497

Materials for SWRO Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Part-Streams of SWRO Plant: Medium, Flow, Pressure, and Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Selection of Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Stainless Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Unprotected, Coated, and Ebonite-Lined Mild Steel . . . . . 6.2.3 FRP/GRP and Other Plastics . . . . . . . . . . . . . . . . . . . . . 6.2.4 Concrete with or Without Protective Coating . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

501

. . . . . . .

501 506 506 517 518 522 523

SWRO Plant Operation Organization, Monitoring, and Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Tasks and Functions of an SWRO Operational I&C System . . . . . 7.2 Structure of a DCS/SCADA Operation I&C System . . . . . . . . . . . 7.3 Instrumentation and Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 Online Instrumentation and Monitoring . . . . . . . . . . . . . . . 7.3.2 Process Monitoring by Water Sampling and Laboratory Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2.1 Seawater, RO Feed, Concentrate, and Product Water Analyses . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2.2 SWRO Product Water/Drinking Water Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2.3 SWRO Waste Water and Outfall Analyses . . . . . 7.4 Mode and Measures for SWRO Process Water Quality Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Energy Consumption of an SWRO Plant . . . . . . . . . . . . . . . . . . . . 8.1 Energy Consuming Components of an SWRO Plant . . . . . . . . . . 8.2 Energy Consumption Calculation and Optimization Possibilities in Process Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Reverse Osmosis Tract . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1.1 Pressure and Salt Rejection Conditions and Membrane Design . . . . . . . . . . . . . . . . . . . . . . 8.2.1.2 RO System Configuration: Pump Systems and RO Arrays . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 Pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.3 SWRO Wastewater Treatment and Posttreatment . . . . . . . 8.2.3.1 Wastewater Treatment Plant . . . . . . . . . . . . . . . 8.2.3.2 Posttreatment . . . . . . . . . . . . . . . . . . . . . . . . .

498 499

525 525 531 534 534 538 540 542 544 545 547

. .

549 549

. .

551 552

.

554

. . . . .

572 575 579 579 580

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8.3

Specific SWRO Energy Consumption: Plant Configuration Modelling and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 SWRO Plant Configuration Modelling . . . . . . . . . . . . . . 8.3.1.1 Design Data for Input into Plant Modelling . . . . 8.3.1.2 SWRO Plant Modelling Approach . . . . . . . . . . 8.3.1.3 SWRO Plant Modelling Results . . . . . . . . . . . . 8.3.1.4 SWRO Plant Modelling Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9

Economics, Life Cycle Cost, and Water Production Cost . . . . . . . . 9.1 Capital Cost: CAPEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Operation and Management Cost: OPEX . . . . . . . . . . . . . . . . . . 9.2.1 Variable OPEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.1.1 Energy Cost . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.1.2 Cost of Chemicals . . . . . . . . . . . . . . . . . . . . . . 9.2.1.3 Membrane, Cartridge, and Filter Medium Replacement . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.1.4 Waste and Residues Disposal . . . . . . . . . . . . . . 9.2.2 Fixed OPEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Determination of Capital and O&M Costs during the Various Phases of SWRO Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 SWRO Contractual Approaches, Life Cycle Cost, and Specific Water Production Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.1 Life Cycle Cost and Levelized and Escalated Specific Water Production Cost . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.2 SWRO Contractual Approaches and Specific Water Production Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.2.1 Contractual Approaches . . . . . . . . . . . . . . . . . . 9.4.2.2 Specific Water Production Costs . . . . . . . . . . . . Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . .

581 582 582 585 587

. .

593 594

. . . . . .

595 595 598 599 601 603

. . .

608 610 614

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615

.

616

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617

. . . .

622 622 628 633

1

Introduction and Overview

The subject matter and scope of this book are the conceptual design and advanced planning as well as the engineering of plants for the desalination of seawater by applying reverse osmosis membrane technology together with the associated processes for pretreatment of seawater and for post-treatment of the product water. Students and teachers may find this manual to be informative and useful as an introduction to reverse osmosis seawater desalination, but it is also intended as a working tool for engineers, technicians, economists, and ecologists involved in the planning, design, and operation of such desalination plants. It may also be consulted by the staff of environmental agencies when considering issuing licenses for the erection and operation of membrane desalination plants as well as by interested laypersons as a source of information on the technical and ecological aspects of such plants. In Volume 2 of the book, namely, in Chaps. 2, 3, and 4, further design- and engineering-oriented issues of the additional treatment processes of an SWRO plant, such as seawater pretreatment, product water post-treatment, seawater extraction, and the discharge into the sea of the SWRO concentrate, are described in terms of their process sequences and components. The process options available in each case and their possible configurations are explained, and the physical, physico-chemical, and basic process engineering principles of the treatment technologies are presented together with their associated algorithms, and the application of these fundamentals for the practical design and dimensioning of the installations and equipment, including calculation methods, is described. In particular, design aspects specific to the dimensioning of process equipment for seawater treatment are clarified. These parts are followed by information concerning the nature of SWRO waste products, their quantities and compositions, the potential environmental impacts of their constituents, the disposal of waste and residual products in accordance with sitespecific environmental regulations, the treatment processes and facilities needed to comply with the relevant environmental stipulations, and the design of these techniques.

# The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 H. Ludwig, Reverse Osmosis Seawater Desalination Volume 2, https://doi.org/10.1007/978-3-030-81927-9_1

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2

1

Introduction and Overview

Further topics of Volume 2 are the selection of suitable materials for the process components of an SWRO, the description of its I&C systems and their interaction with the plant monitoring by manual sampling and laboratory analysis, the determination of the total energy consumption of an SWRO and the influences of design and operation on it and the economic aspects of planning and realization of SWRO plants, and their dependence on contractual structures and ownership as well as calculation of the water production costs and their proportionate CAPEX and OPEX components. In the following, a more detailed overview is presented of the subject matter in Chaps. 2–9 of Volume 2 of this book. Chapter 2 describes the process steps in the SWRO pretreatment stage for preparing the extracted seawater so as to minimize the risks of fouling the RO membranes with biological and inorganic matter. In this connection, membrane manufacturers lay down specifications for the quality of the RO feedwater and thus effectively for pretreatment efficiency. These manufacturers’ specifications for the target values and permissible maximum values of various aggregate parameters characterizing the fouling potential and for a range of water constituents are compiled in a table. The various fouling substances present in seawater and analytical methods for determining the fouling potential are described, and the different types of membrane fouling produced by these foulants are explained. For membrane fouling, essentially a distinction is made between: • Biological fouling due to the deposition and growth of biologically active substances. • Particulate and colloidal fouling due to deposition of biologically inert inorganic and organic fouling substances. To supress biological fouling, the seawater feed is disinfected by adding biocides, for which mainly oxidative inorganic biocides such as chlorine, hypochlorite, chlorine dioxide, or chloramine are used. The resulting chemical reactions for disinfecting seawater and, in particular, the influence of the seawater’s bromide content and its pH on the efficacy of the disinfection process are described. In addition, the basics of the kinetics of disinfection are explained, and the influence of the concentrations of the disinfectants and the reaction time, i.e. the so-called CT value, on the inactivation rate of the aforementioned disinfectants for different types of microorganisms are described. For dosing chlorine and hypochlorite solution, the storage and dosing of these chemicals, including the necessary equipment and its design, are described together with the calculations of the respective chemical demands of the commercially available products as well as the energy consumption of the dosing stations, and the relevant calculation equations are provided. At large SWRO plants, hypochlorite solution is also generated by electrolysis from seawater within the plant itself. The design of such seawater electrochlorination units, their process steps as well as dosing of the generated hypochlorite solution, how its dosing rate and energy consumption are calculated, and dosing equipment are presented.

1

Introduction and Overview

3

The seawater’s particulate and colloidal contents are separated in the SWRO pretreatment stage after its flocculation by means of sedimentation and/or flotation as a pre-separation process and for separation of fine-grained particles and colloids, followed by filtration by being routed through granular media or membrane filters. The choice of the type and configuration of the separation processes depends on the quality of the seawater to be treated. In the section of Chap. 2 dealing with the selection of treatment processes for an SWRO plant’s pretreatment stage, how the types and configurations of these are assigned to various seawater quality classes is depicted in a table. The quality of the seawater is characterized according to its turbidity, the silt density index (SDI), and the value of the total organic carbon content (TOC), with pretreatment process configuration options being assigned to these classes. This allows an initial appraisal of the required pretreatment configuration for a plant site, which then has to be finalized or modified for the design of the plant following further investigations. The sections of Chap. 2 describing the various treatment processes and explaining their design are broken down into: • The theoretical foundations of each process. • A description of the process equipment technology. • The technical design of the process type used in the SWRO pretreatment stage, including dimensioning of the process components and calculation of its energy consumption, its recovery rate for the production of clarified water or filtrate, and the quantities of wastewater and sludge generated during operation. The section dealing with flocculation and coagulation also includes the chemistry of dosing ferric compounds as flocculants and the calculation for dimensioning the dosing equipment. The description in this section of the technical design includes the calculation of the mixing and flocculation equipment in the form of static mixers or dimensioning of the basins of the mixing and flocculation trains using the velocity gradient G for determining the Camp Number G  τ, with calculation of the energy consumption. For sedimentation of particles present in seawater or in RO concentrate, the terminal sedimentation rate is largely governed by the density of these waters and thus by their salinity. In such cases, conventional sedimentation units require extensive sedimentation zones due to the significantly lower sedimentation rate compared to their application for fresh water treatment. If a sedimentation step is included in the SWRO pretreatment stage or for its wastewater treatment, lamella or tube bundle assemblies are used as so-called inclined plate sedimentation units, which have a much smaller surface area and thus take up less space. In addition to outlining the application of conventional sedimentation units, the sedimentation section of Chap. 2 explains these compact sedimentation installations in detail and describes their design. Dissolved air flotation (DAF) is the most widely used process in SWRO plants for pre-separating solids from seawater. These flotation units are operated continuously in the pretreatment stage together with the downstream filtration step, or they serve

4

1

Introduction and Overview

as backup and are put into operation when there is a rise in particulate contamination in the seawater, for example, in the event of an algal bloom. The flotation process is likewise influenced by the seawater’s salinity and temperature, with respect to both the particles’ flotation speed and the solubility of air in the plant’s recirculating part stream. In order to achieve the same air-to-solids ratio when using DAF equipment for seawater treatment as is specified for fresh water treatment, i.e. for water of low salinity, it must be operated either at a higher pressure when injecting air or at a higher flow of the recycling part stream due to the lower solubility of air in water with a higher salinity. In the section of Chap. 2 describing the flotation process, its theoretical principles as well as the basis for the practical design of such installations for treating seawater are presented together with their calculation algorithms. The section of Chap. 2 that covers granular media filtration starts with the presentation of the basics of filtration using such media, like calculation of the pressure drop across a filter bed, bed expansion during backwashing, and the required backwash velocity as well as the dependency of these parameters on particle size distribution and structure and type of filter media together with seawater temperature and salinity. This section provides equations for calculating these data for both single media and multi-media filters. If the filters are pressurized, in addition to its temperature, the salinity of the seawater in particular significantly influences the pressure that has to be applied as well as the required backwashing rate in order to attain a specified bed expansion. Gravity filters and pressure filters of various designs are used for filtering seawater through granular media. In the section dealing with granular media filtration, the different types of these filters are presented and their designs and dimensioning explained together with the respective algorithms and filter design data. In one of the tables in this section, various filter material configurations are summarized as they find practical application for seawater filtration with regard to media material, their particle size distribution, and the layer heights for pressure filters and gravity filters. Another table provides figures for the range of filtration rates for different methods of seawater filtration, namely, flocculation-filtration, after pre-separation of solids in a second filtration step or when seawater is supplied from indirect extraction. For the design of the filter backwashing process, the required values for single media and multi-media filters, such as backwashing speed for air and water, and the bandwidths for its duration are shown in a further table. For membrane filter installations used for seawater pretreatment in SWRO plants, ultrafiltration (UF) membranes predominate. A range of organic polymer compounds are used as membrane materials, and those that are used most widely for the UF membranes are listed in a table in the membrane filter section of Chap. 2, stating their tolerance ranges for chlorine concentration and pH as well as the maximum permissible operating temperatures. The ultrafiltration membranes are mainly used in their configuration as hollow fibre bundle membranes and find application in the dead-end filtration operating mode, in which the solids to be separated are deposited on the membrane surface and removed by backwashing when a maximum value of the transmembrane pressure (TMP) is reached. These hollow fibre membranes contain one or more capillaries. The seawater to be filtered

1

Introduction and Overview

5

can be fed in either through the fibres’ capillaries with the filtrate drawn off the outside membrane surface, or alternatively it can be fed from the outside membrane surface to flow through the capillaries and then be removed. As a result of these different operating modes, membrane filtration installations also differ regarding their design details and plant construction. Another difference results from the filtration operating mode and that is whether the membrane filtration is designed as a pressurized system or immersed as a negative pressure system. With a pressurized system, the membrane elements or membrane modules can be arranged either vertically or horizontally. Following the presentation of the fundamentals of dead-end membrane filtration and the basic algorithms for its calculation, the membrane filter section describes the various constructions and operating modes of these filtration installations for treating seawater and also explains the various membrane cleaning processes, such as chemically enhanced backwashing (CEB) and chemical membrane cleaning as cleaning-in-place (CIP). Then in the next chapter section dealing with the design of membrane filter installations, an overview of the range of properties of UF membrane modules available for seawater filtration is presented in a table as a design basis for dimensioning, stating parameter bandwidths for the design of membrane filters for this application. To provide a basis for the design of the chemical membrane treatment facilities, other tables in this section cover: • A list of the steps for physically and chemically enhanced membrane backwashing, stating the respective backwashing speed and duration. • The chemicals used for CEB treatment, the dosing rate bandwidth, empirical values for intervals between CEB treatments and guidance on operating conditions during treatment. Similar design and operating values are also compiled in two tables for CIP membrane treatment regarding the chemicals proposed by membrane manufacturers for removing different types of fouling and scaling as well as the operating conditions during cleaning. The sequence of cleaning steps is also shown in these, and guide values are given for their duration. Illustrative design calculations for the various seawater treatment options in the pretreatment stage of the reference SWRO with a product water output of 100,000 m3/day are included in the appendix to this chapter for the respective processes and for a desalination plant. The permeate produced in an SWRO plant’s seawater RO stage must be subsequently treated in a post-treatment stage to meet consumer requirements for the composition of the SWRO product water. This may be used: • As drinking water. • For irrigating crops or for livestock farming. • As process water in industry or for power plants.

6

1

Introduction and Overview

Chapter 3 sets out the criteria to be met with regard to the composition of the product water, the influencing factors to be taken into account, the treatment steps to be installed in the post-treatment stage, and how these are dimensioned and calculated. Whereas the focus of post-treatment for the use of the SWRO product water as industrial process water is on a further reduction of the salinity of the permeate from seawater desalination, a number of additional quality criteria and influencing factors must be taken into account if it is to be used as drinking water and/or in agriculture. These are: • National and international drinking water regulations applicable at the desalination plant’s location. • Regulations governing the composition of water for agricultural irrigation and livestock farming. • Guidelines for mitigation of the product water’s corrosive action on the materials used in the water distribution network. Also to be taken into account is the make-up of existing water resources into which the SWRO product water is fed. Chapter 3 compares the key provisions of the WHO drinking water guidelines with national regulations and also lists and explains irrigation water directives with regard to the relevant constituents and parameters for assessing the quality of this water. Evaluation parameters for the corrosive properties of the product water are the calcium carbonate saturation indices (SI) and the Langelier saturation index (LSI); the calcium carbonate precipitation potential (CCPP); the corrosion indicator Ryznar index (RI); the Puckorius scaling index (PSI); and the guidelines and indices set forth in DIN EN 12502. How these indices are calculated and how the composition of product water is evaluated with these parameters with regard to its corrosiveness are explained. The SWRO post-treatment stage comprises post-desalination, alkalinization/ remineralization, conditioning, and primary and secondary disinfection of the product water. In the production of drinking water, the target values set for boron and bromide in the post-desalination stage are in most cases the dominating factor for dimensioning its treatment capacity and for plant dimensioning. The design of the post-treatment stage under the aspect of reducing the boron content of the permeate of the seawater desalination stage as well as post-desalination configuration options is presented and explained. If the product water is to have a very low boron content, this can be achieved by a hybrid configuration of post-desalination and selective ion exchange. Options for this treatment approach are shown in a process schematic. In the post-treatment alkalinization/remineralization step, the alkalinity and the calcium content of the product water are raised to meet the specifications of the drinking water guidelines applicable to the SWRO product water. Conceivable treatment processes in this post-treatment step are compiled in a table together with their chemical equations, the stoichiometry of the reactants per mole rise in

1

Introduction and Overview

7

HCO3 alkalinity, and the resulting increases in cation and anion content and in TDS. Of the treatments listed there, the most commonly used are the lime/CO2 process as well as limestone/CO2 and limestone/H2SO4 filtration. The utilization of semicalcined dolomite together with CO2 is also possible with this filtration method. For these processes, the process sequence is shown in schematics, and the design and dimensioning of their key components are explained. Further, calculation of the hourly chemicals demand for each process combination together with the quantities of generated wastewater and solids is shown. In addition to raising the calcium content, national drinking water regulations sometimes additionally stipulate a specific magnesium concentration for the product water. One option for achieving this is application of the semi-calcined dolomite/ CO2 process. If the potential increase in magnesium content is still not sufficient, an expansion of alkalinization with additional process steps will be necessary. This can be done by adding a filtration step for which the filter is filled with magnesium oxide pellets or an additional dedicated ion exchanger charged with selective chelate resins that exchange calcium ions for magnesium ions and is regenerated using seawater or RO concentrate. Another possibility is dosing with magnesium compounds such as magnesium chloride or magnesium sulphate in the post-treatment conditioning step. In this step, magnesium can be dosed in accordance with the reference value for the magnesium content of the product water as specified by the drinking water regulations. Shown are the calculations of the respective dosing rates of magnesium chloride or magnesium sulphate and the hourly demands of the two chemicals as well as the dosing flow at the dosing point. Dosing of fluoride may also be necessary at this stage depending on the requirements of national or local drinking water regulations. Various fluorine compounds can be used as dosing chemicals for which the calculation of the respective dosing rate, the hourly chemical requirement, and the dosing flow is also described for a specified fluoride content of the product water. If corrosion inhibitors are to be dosed for corrosion protection and to create artificial protective layers on the surfaces of the piping materials in the water supply network, this is also done in the post-treatment conditioning step. Corrosion inhibitors that find application in practice are listed, their mechanisms are described, an empirical value for the dosing rate of each inhibitor type is stated, and how these dosing systems are calculated is shown. The RO tract and upstream pretreatment steps of an SWRO plant present multiple barriers which to a very large extent intercept the microbiological constituents of seawater, so only minuscule quantities get as far as the SWRO product water. Nevertheless, if the product water is to be used as drinking water, it must be sterilized by dosing disinfectants both prior to its buffer storage, referred to as primary disinfection, and before it is fed into the water distribution network, referred to as secondary disinfection. In the section of Chap. 3 which describes this post-treatment process, the suitability and use of the disinfectants chlorine, monochloramine, chlorine dioxide, and ozone for these two sterilization steps are described, and the chemical mechanisms for their use are explained. In a table, the chemical properties and operating conditions of these compounds are compared, and in another table, the

8

1

Introduction and Overview

possibilities and conditions under which these products can be used for primary and secondary disinfection alone or in combination are presented. Then, on the basis of the CT value model, i.e. the effective concentration of the disinfectant C and the reaction time during the disinfection process T with empirical values for CT, the dosing rate for the respective biocide is calculated under the contact conditions prevailing in the respective reaction zones. With the dosing rate thus obtained for the compounds concerned, the quantity of primary substances needed for their production is determined for chlorine dioxide and monochloramine, and then the capacity of the dosing equipment needed for dosing the various disinfectants and the hourly demand for each one, or the chemicals required for their production, are calculated for the usual commercial concentration. Chapter 3 concludes with descriptions for calculating the power take-up of the post-treatment stage as a whole and its constituent process steps. Further, the influence of a range of operating conditions on the SWRO plant’s delivery arrangements for the water distribution network during water supply, matching of the delivery pumping station to these conditions, and the impact of fluctuating water supply take-off and pressure conditions on the effective and average power demand of the delivery equipment are described. For an SWRO plant with a product water capacity of 100,000 m3/day, illustrative design calculations for the lime/CO2 and the limestone/CO2 processes as well as for primary and secondary disinfection of the product water with chlorine as the primary disinfectant and chloramine from chlorine and ammonium hydroxide solution for secondary disinfection are included in the appendix to Chap. 3. Seawater is supplied to an SWRO plant by means of extraction and supply installations that match the operational requirements. The concentrate produced during desalination in the SWRO plant as well as the wastewater that is also generated is returned to the sea via an outfall structure. These two waste streams are the subject of Chap. 4 with regard to process options as well as to their associated installations and components. Their design and dimensioning are described, and how their power demand is calculated is explained. The seawater withdrawal flow rate of the SWRO extraction section and outfall discharge flow are largely determined by the configuration of the pretreatment stage and the RO tract and in particular by their respective recovery rates. How these parameters influence the plant’s extraction and outfall design is described at the beginning of Chap. 4, together with the respective calculation algorithms. An overview is then given of the seawater extraction options, distinguishing between direct extraction of water from the sea and its extraction after it has passed through seabed or shore structures (indirect extraction). Direct extraction can in turn be further classified into arrangements in which seawater is extracted via a channel or lagoon or from greater depths via submerged pipelines or tunnel structures. In the case of submerged extraction, depending on the design of the screening equipment, a distinction is again to be made between active equipment with trash racks and fine screens with a moving screen surface, comprising travelling belt screens or rotating drum screens, or passive/static equipment with intake heads at the extraction point that are constructed as screening units and where the solids are intercepted directly at

1

Introduction and Overview

9

the extraction point. The dimensioning of the intake heads is explained and demonstrated by means of an example of a submerged extraction configuration. In the case of indirect seawater extraction, the many extraction options such as vertical or horizontal beach wells, engineered beach galleries, and seabed infiltration galleries as well as horizontal wells produced by horizontally directed drilling are described, the basis for their dimensioning is presented, and this is illustrated by a calculation example for vertical wells. The selection of the most suitable extraction arrangement at a particular location depends on the SWRO plant’s feed flow, on the specific characteristics of the potential extraction configurations, and also, to a large extent, on the conditions at the extraction site. In addition, there are ecological aspects regarding the protection of flora and fauna during construction and operation of the extraction system, both onshore and offshore, as well as socio-economic and regional planning factors to be considered at the site. A number of selection criteria that have to be considered under planning, technical, operational, and ecological aspects are tabulated, and for each of these criteria, a general comparative evaluation symbol is assigned for each of the various extraction options. The design of the intake pumping station required to deliver the extracted seawater to the desalination plant is described. The key aspects and dimensioning criteria for their feed pumps are outlined, and the calculation of the power take-up of the entire SWRO plant’s intake facility is described. The section of Chap. 4 dealing with the discharge into the sea of concentrate and effluent via the SWRO plant’s outfall structure begins with calculation of the effluent flow on which the outfall design is based and determination of the effluent’s salinity. Next, the design of the diffusers that provide for mixing of the effluent with the seawater and the fundamentals of their calculation are described. With the help of an Excel calculation tool created by Bleninger et al., the conceptual design of a multiport diffuser is calculated and shown in the annex to this chapter. The more detailed determination of the mixing and concentration ratios in the near-field and far-field zones around the outfall discharge point are key factors for positioning of an SWRO outfall in relation to its intake in order to prevent re-entrainment of the discharge flow and thus contamination of the extracted seawater at the desalination plant’s extraction point. However, this requires very complex and laborious calculations, for which relevant on-site oceanographic data have to be collected as input. Descriptions are provided of the software systems needed for both the near- and the far-field zones as well as how these are coupled for modelling the conditions in these two zones. Depending on its design and configuration, operation of an SWRO plant generates fluctuating quantities and compositions of waste in the form of concentrates, wastewaters, and sludges as well as solids. The nature of these waste products, their quantities and compositions, the potential environmental impacts of their constituents, the disposal of waste and residual products in accordance with site-specific environmental regulations, and the treatment processes and facilities needed to comply with the relevant environmental stipulations are described and explained in Chap. 5.

10

1

Introduction and Overview

The chapter commences by tabulating the solid and liquid waste products generated in the various SWRO process steps. This is followed by more detailed compilations, likewise in tabular form, of the waste constituents and their concentrations for each treatment process, together with notes on the specific operating conditions under which the waste is generated or which influence its volume and composition. Since at the present status of technology a major share of the wastewater from the SWRO processes is discharged into the sea together with the concentrate from the plant, the concentrations of the wastewater constituents at the outfall, with or without its prior treatment, are critical with regard to the environmental impact on the marine fauna and flora at the point of discharge. For this reason, the determination of the concentrations of the wastewater constituents in the outfall after mixing the wastewater part stream with the concentrate flow and the dependence of these concentrations on the pretreatment and RO stage recovery rates are described, and the associated calculation equations are provided. It is shown how environmental impacts can be mitigated or completely eliminated for the share of the wastewater that enters the sea via the outfall through appropriate design of the operating conditions in the SWRO, through wastewater pretreatment, and also through the selection of environment friendly “green” dosing chemicals. The wastewater that is generated during chemical flushing, i.e. the cleaning-in-place (CIP) treatment of the SWRO membranes, should not, if avoidable, be discharged into the sea, even after appropriate pretreatment, but should rather be directed to the sewer network for processing in a sewage treatment plant. The bandwidth of limit values for the maximum concentrations of the wastewater constituents as prescribed by the environmental protection agencies on a national level for discharge of wastewater into the sea, both for the outfall of SWRO plants and for their mixing zone, is compiled in a table. Officially imposed maximum concentration limits for selected wastewater parameters when discharging wastewater into the public sewer network, as set by the environmental authorities of various cities and city states, and the resulting range of these discharge conditions are given in another table. In order to comply with these environmental regulations and to ensure that they are also adhered to during the operation of the desalination plant, appropriate wastewater treatment measures have to be in place, depending on the site-specific requirements. The design of such a SWRO wastewater treatment plant and the configuration of its treatment units as required to meet such environmental regulations are shown in a process flow schematic, and its process technology is described. The sludge produced in the wastewater treatment plant must be dewatered prior to its disposal, and examples of such sludge dewatering equipment with mechanical dewatering devices such as centrifuges and filter presses are shown with their function explained. The treatment capacity of an SWRO wastewater treatment plant is dimensioned to cater for an average influent wastewater flow to the plant, but measures must also be taken to avoid the wastewater treatment plant becoming a bottleneck under the worst-case operating conditions of all the SWRO processes, i.e. pretreatment, the RO tract, and post-treatment, so that in this situation the SWRO product water performance will not be jeopardized. In order to maintain this average inflow

1

Introduction and Overview

11

capacity for wastewater treatment, SWRO wastewater part streams that are generated in batches are intercepted and buffered in adequately sized wastewater basins. These basins also serve for wastewater pretreatment, such as neutralization, sulphite oxidation, and chlorine reduction, before being piped to the sedimentation step of the solids separation equipment. In Chap. 5, the calculation procedures are shown, and the necessary algorithms are provided for determining the average values for wastewater generation from the SWRO treatment processes and for establishing the basin capacities required for buffering the wastewater so that the feed flow to the wastewater treatment plant is kept as uniform as possible. For wastewater pretreatment, the chemical reactions for sulphite oxidation using air and hydrogen peroxide as well as the reduction of chlorine with bisulphite and hydrogen peroxide are described together with the calculation of the chemicals required for this. The section of Chap. 5 describing the treatment of SWRO wastewater concludes with explanations of how to calculate the amount of sludge generated in the sludge dewatering step of the wastewater plant and how to determine the wastewater plant’s power consumption from the power take-up of its various installations and components. In addition to the dewatered sludge from the wastewater plant, the solid wastes and residues generated during the operation of an SWRO plant include, in particular, the discarded elements when membranes are replaced in the RO tract and also those from a membrane filtration step, if this type of filtration is installed in the SWRO plant’s pretreatment stage. Likewise adding to the solid waste are the discarded cartridge filter elements from the safety filtration in the feed line to the RO tract and of the cartridge filter of the cleaning-in-place (CIP) station as well as the seawater screening debris from the active screening equipment that is installed for surface seawater extraction. These solid wastes have to be either disposed of or, if possible, recycled in accordance with the national or regional waste legislation in force at the plant’s location. Guidance on waste management strategies and disposal regulations is provided in this section of Chap. 5. With regard to reducing the amount of membrane element waste, reference is made to the possibilities for this during operation of the desalination plant as well as to the findings of investigations into the direct reuse of discarded RO membrane elements or the possibility of their recycling for membrane filtration after appropriate chemical treatment. Recycling the plastic materials of the membrane elements is problematic due to their relatively complex structure incorporating diverse membrane materials. This is more straightforward in the case of cartridge filter elements, which are already being recycled to some extent. It is also possible to reuse in part the replaced filter material from granular media filters after its cleaning. The selection of materials for the processing components of an SWRO plant together with their interconnecting pumps, pipelines and valves, etc. plays a crucial role for its future trouble-free and reliable operation. This applies especially to those parts of the plant that are in direct contact with seawater or concentrate. Chapter 6 starts with a table listing the various mass flows of an SWRO plant, the media concerned, and the magnitudes of flow, pressure, composition, and temperature in the respective part streams. This is followed by discussions on the possibility of

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1

Introduction and Overview

using a number of non-metallic and metallic materials with an evaluation of their suitability and for their applicability to the respective SWRO part streams. In particular, the use of stainless steels in the SWRO plant’s high-pressure section is gone into in detail. In this connection, suitable steels are compiled in a table, stating their European and American standard designations and respective alloying substances together with the descriptors that characterize their corrosion resistance. These are the pitting resistance equivalent PREN, the critical pitting temperature CPT, and the critical crevice temperature CCT. How the PREN is calculated from the percentage of alloying elements of the high-grade steel and how CPT and CCT are then determined from this are shown. As a further aid for selecting the most suitable steels, two graphs showing the risk of pitting and crevice corrosion of various steel grades as a function of chloride concentration and temperature of the medium with which they are in contact are included in this chapter. In another table, the PREN range selected in practice to inhibit corrosion and the best high-alloy steels for this purpose are listed for the various media flowing in the SWRO part streams. A table shows the assignment of these materials to the SWRO part streams and their components, and this is explained in more detail. In other tables, the use of mild steel that is either unprotected or protected with rubber linings or other coatings as well as the use of various plastic materials such as glass fibre-reinforced plastics (FRP/GRP), polyvinyl chloride (PVC), polypropylene (PP), polyethylene, or fluoroplastics such as PVDF, etc. is assigned to the systems or components in the SWRO plant’s low- and medium-pressure sections, and their main areas of application and how they are used are explained. Concrete is used in the SWRO plant for storage, treatment, and buffer basins and also at the seawater intake for the intake heads and extraction piping. Where, in unprotected form or with a protective coating, it finds application for the various SWRO part streams is likewise compiled in a table, also stating the components and where it is used. With regard to the long-term durability of unprotected concrete that is in contact with seawater or concentrate, its composition is stated with references to practical experience with regard to the influence of additives. Chapter 7 describes the tasks and functions as well as the characteristics and structure of the I&C systems of a reverse osmosis seawater desalination plant. Further, how these systems and their online measurement readings are used in combination with monitoring of the SWRO plant’s treatment processes by manual sampling of product water, wastewater, and process chemicals followed by their laboratory analysis is explained. The chapter commences with a tabulation of the range of information and data exchanged within an SWRO plant, stating what the information comprises, its source and origin, the type of data, and the nature of the information content, with details of the various ways by which data is exchanged. This is followed by a description and graphical representation of a possible structure of the process I&C system of a large-scale SWRO. The I&C system is made up of the programmable logic controllers (PLCs) for the SWRO plant’s various processing installations; the distributed control system (DCS) with the operator workstations located in the plant’s central control room from which the plant is monitored and controlled; the

1

Introduction and Overview

13

supervisory control and data acquisition (SCADA) system; and the communication network that connects all I&C components to the SWRO process installations for purposes of data exchange. All data from the online metering stations as well as the analysis results from manual plant monitoring are fed into the SCADA system, where they are stored, visualized, and displayed to the operator in the form of tables and graphics. The key operating parameters that are supplied by the plant’s online instrumentation to its control system as well as the alarm setpoints that require action by the plant operator when they are triggered are compiled in a further table, with comments. Those parameters whose values are to be determined by manual sampling and lab analyses are also assigned in a table to the SWRO part stream for which sampling is to be done, and the sampling frequency customary in operational practice is stated. For each sampling point, the table lists the constituents and parameters that are usually the target of analysis during SWRO operation. In addition to the sampling and analysis tables, this section of Chap. 7 also contains passages explaining target analysis values and how these data are related to the desalination plant’s operating conditions as well as references to other sections of the book that deal with the various analysis standards and instructions on how to calculate aggregate parameters and indices from the analysis data. Running checks of all parameters that influence the SWRO plant’s product water quality and specifying the operational measures to ensure that this remains at an acceptable level as well as organizing and managing these actions are the key elements of a water quality management plan (WQMP), which is an indispensable component of the operational and maintenance documentation of an SWRO facility. In addition to online and lab analyses, this also includes testing the quality of the chemicals used in the plant. An organization chart should also be included in the WQMP to illustrate the organization and management of the quality assurance (QA) activities, the responsibility for their implementation, and the management and organizational structures within the QA system. An example for the possible structure and contents of a WQMP is shown and explained at the close of Chap. 7. Chapter 8 explains how the total energy consumption of a reverse osmosis seawater desalination plant is determined over a given operating period from the energy consumptions of its various treatment processes and infrastructure facilities and how the SWRO’s specific energy consumption (SEC), referred to the volume of product water generated during this period, is calculated from these figures. Also described is how the results of modelling the SWRO design with the influence of the pretreatment stage configuration and the process design of the RO tract, in particular the choice of energy recovery devices (ERDs) in the seawater desalination stage, affect the SWRO plant’s energy consumption. The energy consumptions of the treatment processes on an SWRO plant and its other installations are derived from their specific power consumptions and the durations of their operating cycles during the desalination plant’s operating time. The calculation of power consumption data is described in detail in each of the book’s chapters that deal with the design of the SWRO plant’s treatment processes. The influence of the design, configuration, and mode of operation of each process

14

1

Introduction and Overview

step on its energy consumption is also presented in these passages. In Chap. 8 these influencing parameters are summarized and explained, especially for the seawater RO and the post-desalination stages, and algorithms for their calculation are presented. For various optimizations of the design conditions of the RO tract, such as varying the average membrane flux, the number of membrane elements per membrane module, or the average membrane lifetime (AMLT) as well as application of the split partial mode, the resulting operating data of the RO tract that influence its power consumption are plotted in graphs. Whether the reverse osmosis arrays are in a feed centre, three centre or train configuration can also significantly influence the power consumption of the RO tract. Similarly, the choice and configuration of the pretreatment processes impact the SEC of an SWRO plant. The SEC values for different process configurations for the seawater pretreatment stage referred to their output flow are shown in a table. However, in order to determine the share of pretreatment in the total specific energy consumption of the RO desalination process, its own specific energy consumption has to be stated with reference to the quantity of product water generated by the entire SWRO plant during a given operating period. But then the SEC of the pretreatment stage is dependent on the overall SWRO recovery rate, and the dependence of the pretreatment SEC on this is plotted in a graph. Other SWRO plant components that influence its total energy consumption are post-treatment of the product water, in particular selection of the alkalinization process, the wastewater treatment plant, and the water supply train. Detailed designs of the various treatment processes and components are required in order to determine the SWRO plant’s total specific energy consumption. After aggregating the individual power demand values to determine the plant’s total power demand, the overall specific energy consumption can be calculated for a given operating period from the net product water output generated over this time. During the SWRO design phase, for the orientating determination by the plant planners of the SEC, SWRO plant models are used in part, which include the various possible configurations of the pretreatment processes and the RO tract as well as the options for product water post-treatment and SWRO wastewater treatment to a level of design detailing that enables calculation of the plant’s specific energy consumption with reasonable accuracy. The input data for membrane design of the RO tract are generated by the membrane manufacturers’ calculation programs, and, as far as the SWRO plant model contains no or only limited algorithms for energy recovery, additional input data derived from the software tools of the suppliers of the energy recovery devices are incorporated. Chapter 8 includes a table in which for an SWRO plant with a total recovery rate of 41–42% with a number of pretreatment configurations as well as the energy recovery options of work exchanger, turbocharger, and Pelton turbine, and for seawater with a salinity, i.e. total dissolved solids (TDS), of 30,000–45,000 mg/l at a temperature of 15–35  C, the range of specific energy consumption for the whole plant is shown for the RO1 seawater desalination stage, the RO1 and RO2 reverse osmosis tract, the pretreatment and post-treatment stages, and for the SWRO wastewater treatment plant. The tabulated data are calculated using a software tool that can

1

Introduction and Overview

15

be used to model the design and operation of all the SWRO plant’s treatment processes. The calculation of the SWRO configurations was prepared on the basis of data derived from a membrane manufacturer’s design software. The procedure for plant modelling is explained, and the input data needed for the calculation software for modelling the design and operation of the entire plant, as well as the input data for the membrane design tool, are compiled in a series of tables. The results of this modelling and the dependencies of specific energy consumption on the configuration of the pretreatment stage and the selection of the energy recovery devices in the seawater desalination stage that thereby become apparent, together with the dependency on the operating conditions resulting from plant design as well as the seawater temperature and salinity, are shown in a series of graphs. Chapter 9 covers the financial aspects considered during the planning and implementation phases of SWRO plants, describes possible contractual structures of ownership during their construction and subsequent operation as well as over the entire plant lifetimes, and outlines the influence of different contractual ownership and operating relationships on water production costs. The costs of generating the product water to be supplied by the SWRO plant result as the sum of: – Its capital cost CAPEX, comprising the expenditures for project development, planning, and construction as well as for management and monitoring of these phases. – Its operation and maintenance cost OPEX, comprising the energy expenditure for plant operation; the costs for consumables like chemicals, membranes, etc.; the labour and staff costs for plant operation, quality control and maintenance; and the costs for the disposal of wastewater and solid residues. Both of these categories are made up of a number of cost components, each of which in turn consists of a number of cost elements. These cost parameters and their structures are compiled in tables, one for the CAPEX share and another for the OPEX components, and these are explained and commented on in subsequent passages. The CAPEX and the OPEX, as calculated from their respective cost components and cost elements, are the basis for determining the production costs for the desalinated and treated product water of an SWRO desalination plant over a defined operating period, for which the CAPEX portion corresponds to the repayments to be made over the amortization period of the investment. The specific water production cost, SWPC, is then calculated from the volume of water produced by the plant during the cost accounting period. During the planning and development phases of an SWRO plant, the life cycle cost calculation method is usually applied for financial comparison of offers from plant suppliers but also for cost comparisons when selecting specific installations and components. The life cycle cost, LCC, or net present cost, NPC, is calculated from the CAPEX and the present value of the annual OPEX. With the LCC calculation, however, the price escalation of the OPEX components greatly

16

1

Introduction and Overview

influences the financial evaluation of the configuration of SWRO installations and components. The anticipated developments of the specific electricity costs and the future course of membrane costs in particular significantly influence the economic performances of the options for the SWRO plants’ treatment processes. To take into account the differing expected escalation rates of the OPEX components, the total OPEX cash value is calculated from the sum of the cash values of each of these components. Sensitivity analyses with conceivable bandwidths of interest rates and price escalations reveal the impact of changes in these parameters on the economic efficiency of a plant configuration, so it is then possible to estimate the financial risks when comparing the various process options of a desalination plant. In addition to the determination of life cycle costs, the annual levelized cost method also finds application for calculating the seawater desalination production costs. In this approach, the annual water production costs are determined on the basis of the respective CAPEX and OPEX figures, and from this the discounted specific water production costs SWPClev are calculated, for which the CAPEX component of the production costs is converted into annual payments for debt service. This is done under the simplifying assumption that the SWRO plant is financed entirely through borrowed capital and that repayment is made as an annuity with equal annual payments at a constant interest rate over the entire repayment period. An annuity factor is then applied to determine an annual payment of this annuity from the total CAPEX. The discounted specific water production costs SWPClev are calculated from the annual production costs determined in this way and the annual volume of product water produced, while the escalated specific costs SWPClev,esc are calculated from the escalated LCCesc costs. A detailed description of how to carry out these economic efficiency calculations is provided in Chap. 9, including the calculation equations. By way of example, for an SWRO plant with a daily net product water capacity of 100,000 m3/day, a seawater salinity of 35,000 mg/l and a temperature of 20  C, a product water recovery rate of 43%, a membrane age of four years, a seawater pretreatment stage with flocculation-filtration, and product water post-treatment comprising the hydrated lime/CO2 process and calculations of its life cycle costs LCC and LCCesc as well as the specific water production costs SWPClev and SWPClev,esc are presented in a table. SWRO projects can be realized and operated through a number of contractual structures. The nature of such contractual arrangements agreed between the project parties will regulate the ownership of the desalination plant and the product water produced by it during the plant construction phase and then, after its commissioning and performance verification, continue to do so during the SWRO plant’s operation phase. Some of these project delivery methods as applied in the realization of SWRO projects are illustrated graphically in Chap. 9, showing the phases in which they proceed and identifying the respective project participants together with the nature of the services to be provided by the contracting parties. Related text passages explain the various contractual structures in terms of the categories and number of contractual partners, the scope and the nature of the respective project processes together

1

Introduction and Overview

17

with the responsibilities of the contractual parties, and thus also the range of acceptance of project risks during the successive project phases. The approach to contracting a SWRO project significantly impacts its financing and the cost of the desalination project and that of its product water. As the contractual arrangements become more complex with more contractual parties, the plant’s CAPEX and OPEX rise, while project financing becomes more diversified, with the result that modelling of cash flows, determining the water production costs, and setting of the water tariffs likewise increase in complexity. In particular, realization of SWRO plants as public-private partnership (PPP) projects leads in some cases to the emergence of such complex contractual structures and elaborate cost modelling. Whereas the specific design SWRO water production costs SWPCdes as taken as the basis for the design are calculated for the plant at full load at its average availability, the operational water production costs SWPC(op) are matched to the water production costs as they arise for a desalination plant in practical operation when it produces and delivers a net volume of water over a given operating period under the operating conditions then prevailing and with a certain plant performance, which means that at times its effective net output is also less than its design performance. The operational water production costs are made up of, firstly, the capacity charges for the provision of the facilities and the desalination capacity and, secondly, the output charges, which depend on the volume of product water delivered. The capacity charges therefore cover all costs independent of the production capacity of the plant, that is, the CAPEX component together with the fixed OPEX share that is independent of output, while the output charges cover the outputdependent OPEX share of the total plant costs. Since the capacity charges are a constant component of the operational water production costs and are included in its calculation independently of the plant output, the amount of the SWPC(op) is a function of the SWRO plant’s actual net output flow, and thus it fluctuates with the respective capacity-dependent output charges, which means that in practice it tracks the effective net SWRO plant output flow. For the 100,000 m3/day reference SWRO, this dependence of the SWPC(op) on its net product water output is depicted in a graph plotted over the range of 20–100% of the plant’s design product water output capacity. The value of the specific water production costs, SWPC, of an SWRO plant depends on a variety of economic factors as set out in Chap. 9 and also on the nature of the contractual structure for project realization. In addition, there are other technical and ecological influences, some of which are site-specific, that also impact the plant’s CAPEX and OPEX. As a consequence, an evaluative comparison between differing SWRO plant designs on the basis of the SWPC has very limited informative value, even if more detailed information relating to the plants’ design and operation is known.

2

Pretreatment

Alongside the scalants alkaline earth carbonates, alkaline earth sulphates, and calcium fluoride, seawater contains other constituents, referred to as foulants which, if they were allowed to penetrate the membrane system of an RO seawater desalination plant, would cause fouling and could thus result in a significant loss of performance that would manifest itself as a loss of membrane permeability, an elevated operating pressure, and a decline in permeate quality (see Sect. 5.3 in [1]). Within an RO seawater desalination plant, the task of the pretreatment stage upstream of the RO membrane system is therefore to prepare the feed stream that the risk of fouling in the flow channels and of the membrane surfaces is minimized. To this end, membrane manufacturers specify the quality of the feed flow to the RO membranes, this amounting to a stipulation for the effectiveness of pretreatment of the seawater. If this is complied with, there is only a slight reduction in membrane system performance due to fouling which can be controlled by membrane cleaning. Table 2.1 lists the principal parameters as quoted by the various membrane manufacturers as quality specifications for the feed to an RO system for the substances in question. The principal quality criteria for assessing the efficiency of separation of particles and colloids in the pretreatment stage are the target and maximum values for the silt density index SDI15 and for the modified fouling index MFI0.45 as well as, depending on manufacturer, the turbidity value of the treated seawater. In addition, to protect the integrity of the membranes, upper limits are quoted for free chlorine, oil, and hydrocarbon content as well as for the metals iron, manganese, and aluminium. As limit value for the permissible organic loading, normally the total organic carbon (TOC) is specified and sometimes also the chemical oxygen demand (COD), although it is difficult to measure this in seawater due to its high chlorine content. An indication of the potential for biological fouling of the seawater following pretreatment is provided by measuring its content of assimilable organic carbon (AOC). This value is a measure of the organic carbon that can be degraded by

# The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 H. Ludwig, Reverse Osmosis Seawater Desalination Volume 2, https://doi.org/10.1007/978-3-030-81927-9_2

19

20

2

Pretreatment

Table 2.1 RO feed quality parameters RO feed parameter Symbol Unit Operation parameters Silt density index SDI15 %/min Modified fouling index MFI0.45 s/l2 Turbidity NTU Free chlorine Cl2 mg/l Oil and grease (hydrocarbons) mg/l Ferric iron Fe3+ mg/l Manganese Mn mg/l Aluminium Al mg/l Total organic carbon TOC mg C/l Chemical oxygen demand COD mg O2/l Advanced and scientific parameters to identify biological fouling potential Assimilable organic carbon AOC μg/l Ac-Ca Biofilm formation rate BFR pg/cm2,day ATPb a

RO feed values Target Max. 2–3 1 [–] When dimensioning the filtration tract, it must be ensured that the sulphuric acid feed concentration cH2 SO4 ,feed,kin to the filters does not attain a value at which the solubility of calcium sulphate is exceeded; that would result in precipitation in the filter bed. As Eq. (3.33a) shows, the H2SO4 feed concentration is dependent on the increase in alkalinity ΔmAC,target and the capacity factor fC, Filtr of the filtration tract. The share of the feed flow for remineralization routed via the filtration tract to increase alkalinity should therefore not be selected too low [23]. From the stoichiometric and effective sulphuric acid feed concentration cH2 SO4 ,feed,stoich and cH2 SO4 ,feed,kin respectively, the stoichiometric requirement is calculated using Eq. (3.33b) and the effective kinetic requirement using Eq. (3.33c). DH2 SO4,req,stoich

  F Pr,RO  ΔmAC,target  sd,H2 SO4 ,rem ¼ 1000   F Pr,RO  ΔmAC,target  49 ¼ 1000

DH2 SO4,req,eff,kin ¼ DH2 SO4,req,stoich  f kin

ð3:33bÞ ð3:33cÞ

DH2 SO4,req,stoich ¼ stoichiometric demand of H2SO4 for alkalinity increase and CO2 supply [kg/h] DH2 SO4,req,eff,kin ¼ effective demand of H2SO4 for alkalinity increase and CO2 supply with kinetics surplus demand [kg/h] The dosing flow of sulphuric acid solution F H2 SO4,req,eff , that is, the effective requirement of sulphuric acid solution DH2 SO4,req,eff,sol is derived from the effective kinetic demand DH2 SO4,req,eff,kin with Eqs. (3.33d) and (3.33e).

300

3 Post-Treatment

F H2 SO4,req,eff ¼

DH2 SO4,req,eff,kin  100 ¼ DH2 SO4,req,eff,sol %H2 SO4,sol  ρH2 SO4,sol

F H2 SO4,dos,eff ¼ 4:9 

F Pr,RO  ΔmAC,target  f kin %H2 SO4,sol  ρH2 SO4,sol

ð3:33dÞ ð3:33eÞ

F H2 SO4,dos,eff ¼ dosing flow of sulphuric acid solution [l/h] DH2 SO4,req,eff,sol ¼ effective demand of H2SO4 solution with kinetics surplus demand [l/h] %H2 SO4,sol ¼ concentration of sulphuric acid solution [%] ρH2 SO4,sol ¼ density of sulphuric acid solution [kg/l] With this process option, the limestone consumption cCaCO3 ,feed,stoich is split between the proportion for increasing alkalinity and that for CO2 generation. The increased CO2 concentration in the alkalinization filters as required by the reaction kinetics therefore results in a higher limestone consumption cCaCO3 ,feed,kin . The stoichiometric limestone consumption is calculated with Eq. (3.33f) and the value taking reaction kinetics into account with Eq. (3.33g). cCaCO3 ,feed,stoich ¼ ΔmAC,Filtr  sd,CO2 ,rem ¼ ΔmAC,Filtr  100:1 cCaCO3 ,feed,kin ¼ ΔmAC,Filtr  100:1  f kin ¼

ΔmAC,target  100:1  f kin f C,Filtr

ð3:33fÞ ð3:33gÞ

cCaCO3 ,feed,stoich ¼ stoichiometric CaCO3 demand concentration for alkalinity increase and CO2 supply [mg/l] cCaCO3 ,feed,kin ¼ CaCO3 demand concentration for alkalinity increase and CO2 supply with kinetics surplus [mg/l] The stoichiometric limestone demand of this alkalinization process is given by Eq. (3.33h) and the demand taking into account the reaction kinetics by Eq. (3.33i). DCaCO3,req,stoich

  F Pr,RO  ΔmAC,target  sd,CaCO3 ,rem ¼ 1000   F Pr,RO  ΔmAC,target  100:1 ¼ 1000

DCaCO3,req,eff,kin ¼ DCaCO3,req,stoich  f kin

ð3:33hÞ ð3:33iÞ

DCaCO3,req,stoich ¼ stoichiometric hourly demand of CaCO3 for alkalinity increase and CO2 supply [kg/h]

3.4 Post-Treatment Configuration and Treatment Systems

301

DCaCO3,req,eff,kin ¼ effective hourly demand of CaCO3 for alkalinity increase and CO2 supply with kinetics surplus demand [kg/h] If semi-calcined or half-burned dolomite is used as filter material for alkalinization, the empirical bed contact time EBCT can be determined using Eq. (3.34) [24]. The calculation is done iteratively by entering values of the CO2 feed concentration cCO2,feed and the CO2 concentration in the filtrate cCO2,filtr until the desired EBCT value is reached.

EBCT ¼ 60 

ð10 þ CHfeed Þ 

pffiffiffiffiffiffiffiffiffiffiffiffiffiffi cCO2,feed  ln 10 

k 00act



cCO2,feed cCO2,filtr





CaHfeed MgHfeed

0:1

 dp,eff

e0:05tF,filter

ð3:34Þ

CHfeed ¼ carbonate hardness feed [mg/l CaO] ¼ mC [mmol/l] * 28.04 ¼ cCO2 cHCO 3 3  28:04 61 þ 2  60 Ca  Hfeed ¼ calcium hardness feed [mg/l CaO] ¼ cCa2þ  1:399 Mg  Hfeed ¼ magnesium hardness feed [mg/l CaO] ¼ cMg2þ  2:307 dp, eff ¼ effective particle diameter [mm] k00act ¼ reactivity coefficient for half-burned dolomite [h1] ¼ 120–240 tF, filter ¼ temperature of filter feed [
C] cCO2,filtr is calculated from cCO2,feed using Eqs. (3.34a) and (3.34b). cCO2,filtr ¼ cCO2,feed  cCO2,stoich cCO2,filtr ¼ cCO2,feed 

ΔmAC,target  33 f C,Filtr

ð3:34aÞ ð3:34bÞ

Semi-calcined/half-burned dolomite is considerably more reactive than limestone, and thus not nearly as much of this material is needed to produce a defined degree of alkalinization. Thanks to its better reaction kinetics, for conversion of the material with CO2, the excess of CO2 needed in the alkalinization feed is also lower compared to alkalinization with limestone. The graph in Fig. 3.18 shows for semicalcined dolomite, similar to Fig. 3.17 for limestone, with an EBCT of 10 min, the influence of increasing alkalinity and temperature in the alkalinization filters on the ratio of the CO2 feed concentration to the stoichiometric CO2 concentration. However, the curves of the graph in Fig. 3.18 are calculated for a grain size of the semicalcined dolomite of 4 mm instead of 2 mm as for the limestone graph. Despite the larger grain size of the semi-calcined dolomite, when comparing the two graphs, it can be seen that to achieve the prescribed EBCT value with semicalcined dolomite, a much smaller CO2 excess over the stoichiometric value of the reaction is required than with limestone. Figure 3.18 also shows the marked influence of temperature on the degree of reactivity of the material.

302

3 Post-Treatment Feed CO2 to stoichiometric CO2 ratio 1.4

1.3

Half burned dolomite MgO*CaCO3 filtration Particle diameter effective: 4 mm Temperature [°C] Empty bed contact time: 10 min 10

1.2

20 1.1

30 1.0 1.0

1.5

2.0

2.5

3.0

3.5

Alkalinity increase ∆mAC

4.0

4.5

5.0

5.5

[mol/m3]

Fig. 3.18 Semi-calcined dolomite filtration: dependence of feed CO2 to stoichiometric CO2 concentration ratio on alkalinity increase and temperature

For a temperature of 30
C and up to an alkalinity increase of 2 mol/m3, the stoichiometric CO2 concentration is almost sufficient to achieve an EBCT of 10. This has the consequence, though, that the CO2 concentration in the filtrate is lower than its CO2 equilibrium concentration, i.e. the pH of the filtrate is in the alkaline range, and it has a positive saturation index and reacts by precipitating CaCO3. Thus, to avoid deposits in the beds of the alkalinization filters, at this temperature, either a lower EBCT has to be selected when designing the filters or filter material with larger grain sizes has to be used. When dimensioning alkalinization filters with semi-calcined dolomite, care must therefore be taken to ensure that the grain size of the material and the EBCT are specified in such a way that, in the temperature range in which the filters are operated, their filtrate does not get into the zone of the calcium carbonates/CO2 equilibrium where CaCO3 precipitates. These material properties also limit the partload operating range of this type of alkalinization filtration. If the design EBCT is significantly exceeded due to operation at part load, the reactivity of the filter bed is increased, thus resulting in excessive alkalization of the filtrate. For this reason, the filters should not be operated at less than 70–80% of their design capacity. With the CaCO3/CO2 process, physical CO2 degassing of the alkalinisated product water is mostly carried out to reduce the amount of NaOH to be dosed into the SWRO product water to attain the target SITH and/or CCPP values. Due to the reduced excess of CO2 in the filtrate of a semi-calcined dolomite filtration, such

3.4 Post-Treatment Configuration and Treatment Systems

303

CO2 degassing of the alkalinisated product water is usually not necessary with this type of alkalinization process. In addition to its higher reactivity compared to limestone and the consequent reduced consumptions of reactant material and CO2, semi-calcined dolomite also has the advantage that, in addition to calcium, magnesium is also added to the RO product water during the alkalinization reaction. The stoichiometric hourly CO2 demand for alkalinization with semi-calcined dolomite is calculated by Eq. (3.35) and the effective demand by Eq. (3.35a). DCO2,req,stoich

  F Pr,RO  ΔmAC,target  sd,CO2 ,rem ¼ 1000   F Pr,RO  ΔmAC,target  33 ¼ 1000

DCO2,req,eff ¼

f C,Filtr  F Pr,RO  cCO2,feed f purCO  f adsCO  1000 2

ð3:35Þ ð3:35aÞ

2

f adsCO2 ¼ CO2 adsorption rate factor [–]  0.9–0.95 The stoichiometric and effective hourly demand for semi-calcined dolomite result from Eqs. (3.35b) and (3.35c) respectively.   F Pr,RO  ΔmAC,target  sd,MgOCaCO3 ,rem 1000   F Pr,RO  ΔmAC,target  35:1 ¼ 1000

DMgOCaCO3,req,stoich ¼

DMgOCaCO3,req,eff

  F Pr,RO  ΔmAC,target  35:1 ¼ 1000  f purMgOCaCO

ð3:35bÞ

ð3:35cÞ

3

DMgOCaCO3,req,stoich ¼ stoichiometric demand of half-burned dolomite for alkalinity increase [kg/h] DMgOCaCO3,req,eff ¼ Effective demand of half-burned dolomite for alkalinity increase [kg/h] f purMgOCaCO ¼ Purity factor of half-burned dolomite [–] 3

The reactivity of the semi-calcined dolomitic material may vary from one supplier to another and particularly depending on the degree of calcination. The detailed design of an alkalinization filtration installation should therefore be based, as far as this is available, on the technical data provided by the material’s supplier regarding

304

3 Post-Treatment

the dependence of the reactivity and EBCT of his product on its particle size distribution, alkalinity increase, and temperature. After determining the empty bed contact time (EBCT), the filter units of the alkalinization filtration can be dimensioned. The filtration velocity vfilt is calculated from the height of the filter bed hfb, full and the EBCT (Eq. 3.36), the feed flow to the alkalinization filter system FFiltr from the total flow FPr, RO to the alkalinization installation and the capacity factor fC, Filtr (Eq. 3.36a), and the flow FF, f, u to each of the filter units from FFiltr and the number Nf, u in the filtration system (Eq. 3.36b). From the feed flow to a filter unit FF, f, u and the filtration velocity vfilt, there results the filtration surface area As, f, u of the filter unit with Eq. (3.36c), and for vertical round filters, the internal diameter di, filt is determined with Eq. (3.36d). The filtration velocity vfilt for pressure filtration, which is most common for alkalinization filtration, is in the range of 10–20 m/h for limestone filters and up to 25 m/h for filters with semi-calcined dolomite. vfilt ¼

hfb,full  60 EBCT

ð3:36Þ

F Filtr ¼ f C,Filtr  F Pr,RO

ð3:36aÞ

f C,Filtr  F Pr,RO N f,u

ð3:36bÞ

F F,f,u ¼

F F,f,u vfilt rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4  F F,f,u ¼ vfilt  π

As,f,u ¼ di,filt

ð3:36cÞ ð3:36dÞ

vfilt ¼ filtration velocity [m3/m2,h] [m/h] FFiltr ¼ feed flow of filter system [m3/h] fC, Filtr ¼ capacity factor of filter system [–] hfb, full ¼ filter bed height full [m] FF, f, u ¼ feed flow of filter unit [m3/h] Nf, u ¼ number of filter units of the filter system [–] As, f, u ¼ filtration surface area of filter unit [m2] di, filt ¼ inner diameter of filter unit [m] The volume of the filter bed Vfb, full, u in each filter unit is obtained using Eq. (3.37) and the corresponding mass of the filter bed Mfb, full, u with Eq. (3.37). V fb,full,u ¼ hfb,full 

F F,f,u vfilt

ð3:37Þ

3.4 Post-Treatment Configuration and Treatment Systems

M fb,full ¼ ρb,M  V fb,full ¼ ρb,M 

d2i,filt  π  hfb,full 4

305

ð3:37aÞ

Vfb, full, u ¼ volume of filter bed full of filter unit [m3] Mfb, full, u ¼ mass of filter bed full of filter unit [kg] ρb, M ¼ bulk density of filter medium [kg/m3] If the alkalinization filters are refilled discontinuously in batches, the volume of the filter bed Vfb, full, u is reduced by the volume change ΔVop, u (Eq. 3.37b) due to bed material consumption by the alkalinization reaction. The permissible reduction in bed volume until it can be replenished from the resulting minimum volume Vfb, min , u back to the full volume Vfb, full, u is determined by the filter bed volume change factor fop, red (Eqs. 3.37c and 3.37d). The volume reduction at which the filter bed is usually refilled is 10–20% of the volume of the full bed ( fop, red ¼ 0.1–0.2). V fb, min ,u ¼ V fb,full,u  ΔV op,u F F,f,u vfilt

ð3:37cÞ

EBCT  F F,f,u 60

ð3:37dÞ

ΔV op,u ¼ f op,red  hfb,full  ΔV op,u ¼ f op,red 

ð3:37bÞ

Vfb, min , u ¼ volume of filter bed minimum of filter unit [m3] ΔVop, u ¼ filter bed volume change during operation of filter unit [m3] fop, red ¼ filter bed volume change factor ¼ 0.1–0.2 [–] The mass loss of the filter bed ΔMop, u corresponding to the volume reduction ΔVop, u, is calculated from the material’s bulk density ρb, M with Eqs. (3.37e) and (3.37f). ΔM op,u ¼ ΔV op,u  ρb,M ΔM op,u ¼ ρb,M  f op,red 

EBCT  F F,f,u 60

ð3:37eÞ ð3:37fÞ

ΔMop, u ¼ filter bed mass change during operation cycle of filter unit [kg] From the bed mass loss ΔMop, u during a certain operating time interval of the filter unit and the stoichiometric mass requirement of the alkalinization material Dmat, requ, stoich, u used for the specified increase in alkalinity, the time Δτ op required between refills of the filter for the alkalinization filtration processes limestone/CO2 and semi-calcined dolomite/CO2 can be determined using Eqs. (3.38)–(3.38b).

306

3 Post-Treatment

Δτop ¼

ð3:38Þ

F F,f,u  ΔmAC,Filtr  sd,material,rem 1000

ð3:38aÞ

ρb,M  f C,Filtr  f op,red  f purmaterial  EBCT  1000 60  ΔmAC,target  sd,material,rem

ð3:38bÞ

Dmat,requ,stoich,u ¼ Δτop ¼

ΔM op,u  f purmaterial Dmaterial,requ,stoich,u

Δτop ¼ time interval between filter top-up [h] Dmat, requ, stoich, u ¼ stoichiometric demand of material for alkalinity increase in filter unit [kg/h] sd, mat, rem ¼ specific material demand of remineralization process per alkalinity increase according to Table 3.13 [g material/mol] f purmaterial ¼ purity factor of alkalinization material [–] In the CaCO3/H2SO4 process, both the stoichiometric consumption required for the increase in alkalinity and the amount of CaCO3 needed to produce both the stoichiometric amount of CO2 and the excess resulting from reaction kinetics must be taken into account when calculating the time interval Δτop between refills (Eq. 3.38c). Using the calcium carbonate requirement DCaCO3,req,kin,u calculated with Eq. (3.38d), Δτop is then calculated in accordance with Eq. (3.38e). ΔM op,u  f purmaterial DCaCO3,req,kin,u   F F,f,u  ΔmAC,Filtr  100:1 ¼  f kin 1000

τop ¼ DCaCO3,req,kin,u Δτop ¼

ρb,M  f C,Filtr  f op,red  f purmaterial  EBCT  1000 60  ΔmAC,target  100:1  f kin

ð3:38cÞ ð3:38dÞ ð3:38eÞ

DCaCO3,req,kin,u ¼ demand of CaCO3 for alkalinity increase and CO2 supply including kinetic excess in filter unit of CaCO3/H2SO4 process [kg/h] An empirical value determined in trials or based on the experience of the designer is taken for the kinetic factor fkin. After every replenishment of a filter, it is backwashed. This backwashing is done as combined air/water flushing, normally with RO product water. The backwashing process with its different sequences, their duration, and the backwashing rates of air and water are shown in Table 3.14. At the backwashing velocities quoted in Table 3.14 for combined backwashing vbw, air,water and backwashing with water vbw, water, the freeboard height hf above the

3.4 Post-Treatment Configuration and Treatment Systems Table 3.14 Limestone and half-burned dolomite filtration—Backwash sequence and conditions

Backwash sequence Drain down Duration τdrain Air scour Duration τair Air velocity vbw,air Air/water backwash Duration τbw,air,water Air velocity vbw,air Water velocity vbw, air,water,10
C Water backwash Duration τbw,water Water velocity vbw, water,10
C Infiltration Duration τbw,inf at vfilt

307

Unit

Value

min

5–10

min m/h

~5 60

min m/h m/h

~5 60 8–12

min m/h

~5–10 20–25

min

5–10

filter bed, including a safety margin, should be around 300–500 mm (see Sects. 2.3. 4.3 and 2.3.4.3.1, Eqs. 2.90 and 2.90a). The backwashing velocity required to obtain a defined filter bed expansion depends on the temperature of the backwash water. The values quoted in Table 3.14 or the backwashing velocities specified by the supplier of the alkalinization material in his technical documentation refer to a specific reference temperature and must therefore be adapted to the range of operating temperatures of the plant (Eqs. 3.39–3.39b) (also see Sects. 2.3.4.3 and 2.3.4.3.1). vbw,water,comb,t ¼ vbw,air,water,t,r  f bw,t

ð3:39Þ

vbw,water,t ¼ vbw,water,t,r  f bw,t

ð3:39aÞ

 !0:8333 ν0:8 bw,t,r ρbw,t,r  ρg,M  ρbw,t,t    ν0:8 bw,t,t ρbw,t,t  ρg,M  ρbw,t,r

ð3:39bÞ

f bw,t ¼

vbw, water, comb, t ¼ backwash velocity water in air/water backwash at temperature t
C [m/h] vbw, air, water, t, r ¼ backwash velocity water air/water backwash at reference temperature t
C [m/h] fbw, t ¼ temperature correction factor of backwash velocity [–] νbw, t, r ¼ kinematic viscosity of backwash water at reference temperature t [m2/s] νbw, t, t ¼ kinematic viscosity of backwash water at temperature t [m2/s] ρg, M ¼ density of grains of alkalinization material [kg/m3] ρbw, t, r ¼ density of backwash water at reference temperature t [kg/m3] ρbw, t, t ¼ density of backwash water at temperature t [kg/m3]

308

3 Post-Treatment

The pumps and blowers of the backwash station of the plant are dimensioned depending on the filtration surface area As, f, u of the filter units of the alkalinization system and the specific backwashing velocity for each backwash sequence. Infiltration of the filters is done at the same filtration velocity vfilt at which they are operated (Eqs. 3.40–3.40c). F bw,water,comb ¼ vbw,water,comb  As,f,u

ð3:40Þ

F bw,water ¼ vbw,water  As,f,u

ð3:40aÞ

F bwW,air ¼ vbw,air  As,f,u

ð3:40bÞ

F bw, inf ¼ vfilt  As,f,u

ð3:40cÞ

Fbw, water, comb ¼ water flow air/water backwash of unit [m3/h] Fbw, water ¼ water backwash flow of unit [m3/h] Fbw, air ¼ air backwash flow of unit [m3/h] Fbw, inf ¼ backwash infiltration water flow of unit [m3/h] The backwash water pressure pbw,fb for a defined degree of expansion of the filter bed can be calculated with Eq. (2.85) of Sect. 2.3.4.1. Likewise to be found in this section of Chap. 2 is the Kozeny-Carman equation (Eq. 2.80), with which the pressure loss of the filter bed Δpfb,cl can be determined as a function of the filtration velocity vfilt. The flow of RO product water FF, fs, bw, ∅ needed on average for backwashing the alkalinization filters, i.e. their own water consumption, is made up of the average flow for backwashing FF, bw, ∅ and the average flow for infiltrating the filters FF, inf , ∅ after backwashing (Eq. 3.40d). F F,fs,bw,∅ ¼ F F,bw,∅ þ F F, inf ,∅

ð3:40dÞ

FF, fs, bw, ∅ ¼ average RO product water feed flow for backwash and infiltration of remineralization system ¼ internal water consumption [m3/h] FF, bw, ∅ ¼ average RO product water feed flow for backwash of system [m3/h] FF, inf , ∅ ¼ average RO product water feed flow for infiltration of system [m3/h] The average backwash water flow FF, bw, ∅ also corresponds to the average flow of wastewater FWW, remin, ∅ that is to be treated to separate out solids (Eq. 3.40e), if the alkalinization plant is not equipped with its own recycling system for recovering the backwash water. The water flow for infiltration of the filters is calculated with Eq. (3.40f).

3.4 Post-Treatment Configuration and Treatment Systems

309

F F,bw,∅ ¼ F WW,remin,∅ ¼

As,f,unbw,h  ðvbw,water,comb  τbw,air,water þ vbw,water  τbw,water Þ 60 F F, inf ,∅ ¼

As,f,u  nbw,h  vfilt  τbw, inf 60

ð3:40eÞ ð3:40fÞ

FWW, remin, ∅ ¼ average wastewater flow from remineralization backwash for treatment [m3/h] nbw, h ¼ number of backwash/top-up [h1] τbw, air ¼ backwash time air [min] τbw, air, water ¼ backwash time air/water backwash [min] τbw, water ¼ backwash time water backwash [min] τbw, inf ¼ backwash infiltration time [min] The alkalinization filters’ own water consumption FF, fs, bw, ∅ as well as the average wastewater flow FWW, remin, ∅ generated when they are backwashed depends on how often the filters are backwashed or replenished. The number of these operations is defined by the parameter nbw, h as calculated by Eq. (3.41). If the filters are refilled discontinuously after an operating period Δτop and are only backwashed at this time, when calculating nbw, h, the time interval Δτop and the number of filters Nf, u are considered in the calculation. nbw,h ¼ N f,u 

1 Δτop

ð3:41Þ

Nf, u ¼ number of filter units [–] If the filters are continuously filled with alkalinization material, nbw, h is calculated as shown by Eq. (3.41a) from a predefined interval τfc between backwashing operations. Backwashing is then done after a specified time interval, but at least after a period of about 5–7 days of operation. nbw,h ¼ N f,u 

1 τfc

ð3:41aÞ

τfc ¼ duration between backwashing[h] When backwashing the filters, some 5–10% of the effective demand Dmat,req,eff for alkalinization material is swept out by the backwash wastewater. This corresponds to a range of values for the backwash loss factor fmat, bw, loss of from 0.05 to 0.1. The quantity of material removed as, bw, mat that then has to be disposed

310

3 Post-Treatment

of depends on the purity of the filter material and the fine grain content of its particle size distribution. This loss of mass as, bw, mat must be taken into account when calculating the effective requirement for alkalinization material Dmat,req,eff and the refill batch quantity ΔMrefill, u for discontinuous replenishment of the filters. This mass loss as, bw, mat also corresponds to the amount of solid matter arising in the alkalinization plant that has to be handled as solid matter in an SWRO wastewater treatment plant. The effective demand for alkalizing material including backwash losses Dmatreq,eff,bw is calculated for the alkalinization processes CaCO3/CO2 and MgO*CaCO3/CO2 by Eq. (3.42) and for the sulphuric acid process by Eq. (3.42a) with the empirical kinetic factor fkin.  F Pr,RO  ΔmAC,target  sd,mat,rem   1 þ f mat,bw,loss ð3:42Þ 1000  f pur,mat     F Pr,RO  ΔmAC,target  100:1 ¼  f kin  1 þ f mat,bw,loss ð3:42aÞ 1000  f purCaCO

Dmatreq,eff,bw ¼ DCaCO3,req,eff,kin,bw

3

Dmat,req,eff,bw ¼ effective demand of material including backwash losses for alkalinity increase [kg/h] DCaCO3,req,eff,kin,bw ¼ effective demand of limestone for CaCO3/H2SO4 process including backwash losses for alkalinity increase [kg/h] fmat, bw, loss ¼ backwash material loss factor  0.05–0.1 [–] For the CaCO3/CO2 and MgO*CaCO3/CO2 processes, the refill batch quantity per filter unit ΔMrefill, u for discontinuous refilling is determined from the effective material demand with backwash mass loss of the alkalinization system Dmat,req,eff,bw , the operating time of the filter units between refills Δτop, and the number of filter units Nf, u according to Eq. (3.42b). For the CaCO3/H2SO4 process, the value of DCaCO3,req,eff,kin,bw according to Eq. (3.42a) is used instead of Dmatreq,eff,bw for the effective material demand. ΔM refill,u ¼

Dmatreq,eff,bw  Δτop N f,u

ð3:42bÞ

ΔMrefill, u ¼ refill amount of material per unit [kg] The mass loss as, bw, mat and thus also the quantity of solid waste to be disposed of from alkalinization filtration are calculated with Eq. (3.42c) using the effective material demand Dmatreq,eff for the CaCO3/CO2 and MgO*CaCO3/CO2 processes and Dmatreq,eff,kin for the CaCO3/H2SO4 process.

3.4 Post-Treatment Configuration and Treatment Systems

as,bw,mat ¼ Dmatreq,eff  f mat,bw,loss as,

311

ð3:42cÞ

bw, mat ¼ average amount of solids loss from material of filter bed during backwash [kg/h]

The product water of the remineralization stage is a mixture of the filtrate from alkalinization filtration and that part stream of RO product water that bypasses the filtration tract. The CO2 content of the resulting blended water cCO2,prod,remin is calculated from the CO2 content of the filtrate cCO2,filtr and that of the bypass water cCO2,Pr,RO with Eq. (3.43).   cCO2,prod,remin ¼ 1  f C,Filtr  cCO2,Pr,RO þ f C,Filtr  cCO2,filtr

ð3:43Þ

cCO2,prod,remin ¼ CO2 concentration in remineralization product water [mg/l] cCO2,Pr,RO ¼ CO2 concentration in RO product water [mg/l] cCO2,filtr ¼ CO2 concentration in filtrate of alkalinization filters [mg/l] If the product water from remineralization is then passed through an atmospheric CO2 degassing unit to reduce the amount of NaOH needed to adjust the pH in the SWRO product water for attaining the target values for the saturation index SITh and the calcium carbonate precipitation potential (CCPP), the CO2 concentration in this remineralization product water cCO2,prod,remin is lowered to a value cCO2,prod,remin,f that corresponds to the degassing efficiency ΔcCO2,degas (Eq. 3.43a). The amount of carbon dioxide to be neutralized by NaOH is calculated using Eqs. (3.43b) and (3.43c) from the free carbonic acid still present in the product water cCO2,prod,remin,f and the CO2 content in the SWRO product water cCO2,prod,SWRO,target needed to attain the target values SITh and CCPP (Eq. 3.43b). cCO2,prod,remin,f ¼ cCO2,prod,remin  ΔcCO2,degas

ð3:43aÞ

ΔcCO2,NaOH ¼ cCO2,prod,remin,f  cCO2,prod,SWRO,target

ð3:43bÞ

ΔmCO2,NaOH ¼

ΔcCO2,NaOH 44

ð3:43cÞ

cCO2,prod,remin,f ¼ CO2 concentration final in remineralization product water [mg/l] ΔcCO2,degas ¼ CO2 concentration removed in CO2 degasifier [mg/l] cCO2,prod,SWRO,target ¼ CO2 target concentration in SWRO product water [mg/l] ΔcCO2,NaOH ¼ CO2 concentration to be neutralized by NaOH [mg/l] ΔmCO2,NaOH ¼ molar CO2 concentration to be neutralized by NaOH [mmol/l]

312

3 Post-Treatment

An approximate value for the molar concentration of free CO2 in the SWRO product water mCO2,prod,SWRO can be calculated with Eq. (3.43d) from the equilibrium pH pHs,prod,SWRO in the SWRO product water and the target value of the saturation index SITh, target from Eqs. (3.43d)–(3.43f). The equilibrium pH in the SWRO product water is determined according to Sect. 3.2.3.1.1, Eqs. (3.4h)–(3.4n), while the first thermodynamic dissociation constant K 01 is calculated with Eq. (3.29a). However, in this calculation mode, neither the CO32 nor the H+ and OH ion concentrations are taken into account, and ion pair formation is not considered in the calculation. pHprod,SWRO,target ¼ SITh,target  pHs,prod,SWRO pHprod,SWRO,target ffi pK 01 þ log mCO2,prod,SWRO ffi 10

log mHCO

3,prod,SWRO

mHCO3,prod,SWRO mCO2,prod,SWRO þpK 01 pHprod,SWRO,target

ð3:43dÞ ð3:43eÞ ð3:43fÞ

pHprod,SWRO,target ¼ pH of SWRO product water at target SITh SITh, target ¼ target saturation index in SWRO product water [–] pHs,prod,SWRO ¼ saturation (equilibrium)- pH of SWRO product water mHCO3,prod,SWRO ¼ HCO3 concentration in SWRO product water [mol/l] mCO2,prod,SWRO ¼ CO2 concentration in SWRO product water [mol/l] From the approximate value for the molar concentration mHCO3,prod,SWRO thus obtained, cCO2,prod,SWRO,target is calculated according to Eq. (3.43g). cCO2,prod,SWRO,target ffi mCO2,prod,SWRO  44  103

ð3:43gÞ

The free CO2 in the SWRO product water can be determined more precisely using one of the software products listed in Table 3.7 under Sect. 3.2.3.1.4. Such a calculation is necessary in particular if, in addition to or instead of the saturation index SITh, a target value for the calcium carbonate precipitation potential CCPP is specified for the SWRO product water. This requires an iterative calculation approach. An analysis of the SWRO product water for which the target value for alkalinity and the concentration of calcium Ca2+, magnesium Mg2+, or sulphate SO42+ resulting from alkalinization depending on the alkalinization material selected, according to Table 3.11, Eqs. (3.19a)–(3.19e), are input and with the input pH to be varied until the values for SITh and/or CCPP thus calculated correspond to the target values. The concentration of free CO2 output by the software provides the basis for calculating ΔmCO2,NaOH with Eq. (3.43c). If the targets for SITh and CCPP are in the positive, i.e. the calcium carbonate precipitation range, caustic soda solution is needed not only to bind the CO2 but also to increase pH and promote the CaCO3 precipitation reaction, so NaOH consumption is correspondingly higher.

3.4 Post-Treatment Configuration and Treatment Systems

313

In this case the required quantity of caustic soda solution has to be determined iteratively, i.e. NaOH is added to the SWRO product water in a subroutine until the SITh and CCPP targets are attained. As starting value for input to the iteration routine, the amount of NaOH can be taken that is equivalent to the CO2 content cCO2,prod,remin,f in the remineralization product water. The hourly NaOH demand is calculated from the NaOH consumption output by the software. The stoichiometric and effective NaOH demand needed to bind the free carbonic acid is calculated from the parameters ΔcCO2,NaOH and ΔmCO2,NaOH with Eqs. (3.43h) and (3.43i), with the effective demand determined iteratively using software according to Eq. (3.43j). DNaOH:req,stoich ¼

F Pr,SWRO  ΔmCO2  40 F Pr,SWRO  ΔcCO2,NaOH  40 ¼ 1000 1000  44 DNaOH:req,eff ¼

ð3:43hÞ

F Pr,SWRO  ΔcCO2,NaOH  40 1000  44  f purNaOH

ð3:43iÞ

F Pr,SWRO  RNaOH 100% 1000  f purNaOH

ð3:43jÞ

DNaOH:req,eff,it ¼

DNaOH. req, stoich ¼ stoichiometric demand of NaOH for neutralization of remineralization product water [kg/h] DNaOH. req, eff ¼ effective demand of NaOH for neutralization of remineralization product water [kg/h] f purNaOH ¼ purity factor of sodium hydroxide [–] DNaOH. req, eff, it ¼ iteratively determined demand of NaOH [kg/h] RNaOH 100% ¼ dosing rate of NaOH 100% [mg/l] [g/m3] Alkalinity is likewise generated by dosing NaOH into the remineralization product water (Table 3.11, Eq. 3.19g). If the specified target value for alkalinity in the SWRO product water mAC,SWRO pw,tv is not to be exceeded as a result of this, the target value for the increase of alkalinity ΔmAC,target during alkalinization filtration can be reduced by an amount ΔmAC,target,rev to match the increase in alkalinity ΔmAC,NaOH due to NaOH dosing (Eq. 3.44). This must then be taken into account accordingly when determining the empty bed contact time (EBCT), for dimensioning the filter units, and in calculating the requirement for alkalinization material. ΔmAC,target,rev ¼ ΔmAC,target  ΔmAC,NaOH ¼ ΔmAC,target  ΔmCO2,NaOH ¼ ΔmAC,target 

RNaOH 100% 40

ð3:44Þ

314

3 Post-Treatment

ΔmAC,target,rev ¼ revised target value for alkalinity increase due to NaOH addition [mol/m3] [mmol/l] ΔmAC,target ¼ target value for alkalinity increase in SWRO product water [mol/m3] [mmol/l] ΔmAC,NaOH ¼ alkalinity increase due to NaOH dosing [mol/m3] [mmol/l] A design calculation corresponding to the calculation algorithms described above for the process of remineralization with alkalinization filtration using limestone and CO2 (CaCO3/CO2 process) for an SWRO capacity of 100,000 m3/day or 4167 m3/ h is provided in Table Annex 3.A2.

3.4.2.3 Composition and Properties of Post-Desalinated and Remineralized RO Product Water In the remineralization stage of an SWRO post-treatment plant, the alkalinity of the RO product water is usually adjusted to values in the range of 0.8–2.2 meq/l, which corresponds to 40–110 ppm CaCO3. Depending on the composition of locally available water resources with which the post-treated product water of the desalination plant is blended, the target value of the SWRO product water for alkalinity mAC,SWRO pw,tv and the total salinity TDSSWRO pw,tv resulting from remineralization is adjusted to take account of the local situation. Furthermore, as described in detail above under Sect. 3.2, when specifying the composition of SWRO product water, the relevant national or international potable water guidelines as well as minimizing the corrosive influence of the water on the materials of the water distribution system and aspects of the suitability of the water for agricultural irrigation must be taken into account. Table 3.15 shows the composition of the feed to the post-desalination stage, i.e. in the feed line to the second RO pass, together with the analysis of the feedwater to the post-remineralization stage, and these analyses are compared with the composition of the post-treated SWRO product water for the different remineralization process options. The alkalinity and the calcium carbonate precipitation potential (CCPP) in the post-treated SWRO product water are the same for the different remineralization options and are set to values for mAC,SWRO pw,tv of 1.5 mmol/l for alkalinity and for CCPPSWRO,pw,tv of 3.0 for CCPP. The calculation of the feed to the post-treatment stage is based on a seawater salinity of 36,000 mg/l and a recovery rate of the RO seawater desalination stage, YRO1, of 45%. Post-desalination is operated at a recovery rate, YRO2, of 90% and a capacity factor, fC,RO2, of 0.8. The membranes of both these RO stages have an age of 3 years and the water temperature is 25
C. For the CaCO3/CO2 and CaCO3/H2SO4 processes, before the specified CCPP target value is set in the SWRO product water by dosing sodium hydroxide solution, the excess carbon dioxide in their product water is reduced to a residual content of 5 mg/l CO2 by physical deacidification in a CO2 degasser to minimize NaOH consumption. With the semi-calcined dolomite/CO2 process, this is not necessary because of its product water’s lower CO2 excess.

SITh pHs pH Ca Mg Na K HCO3 CO3 CO2 Cl SO4 NO3 Br B TDS Ryznar index Puckorius index Larson–Skold index

Parameter Alkalinity target CCPP target

– mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l

Unit mmol/l mg/l CaCO3

Post remineralisation feed – – 3.46 9.61 6.15 0.23 0.71 24.50 1.30 0.55 – 0.62 39.80 1.70 0.01 0.14 0.92 69.8 13.1 15.2 128.4

Post desalination feed (RO second pass) – –

0.65 9.65 9.00 0.99 3.10 102.70 5.14 8.11 0.50 0.01 162.40 7.20 0.03 0.56 1.21 292.0 10.3 13.5 31.6 0.26 7.97 8.23 29.63 0.71 24.50 1.30 89.43 0.48 0.79 39.80 1.70 0.01 0.14 0.92 192.0 7.7 8.7 0.8

0.26 8.09 8.35 23.50 0.71 32.40 1.30 87.23 1.08 0.62 39.80 1.70 0.01 0.14 0.92 190.0 7.8 8.9 0.8

Post remineralisation product CaCO3/ Ca(OH)2/ CO2— CO2—process process 1.5 3.0 0.22 8.28 8.50 15.30 9.83 26.50 1.30 90.46 1.56 0.44 39.80 1.70 0.01 0.14 0.92 189.0 8.1 9.3 0.8

Hb dolomite/ CO2—process

(continued)

0.24 7.78 8.02 59.60 0.71 25.03 1.30 87.60 0.49 1.31 39.80 82.39 0.01 0.14 0.92 311.9 7.5 8.3 2.0

CaCO3/ H2SO4— process

Table 3.15 Type of remineralization processes—impact on SWRO product composition and properties (data source: Toray DS2 & aqion PRO)

3.4 Post-Treatment Configuration and Treatment Systems 315

Parameter DIN EN 12502— Indices S S1 S2 Sodium adsorption ratio SAR

Unit

Table 3.15 (continued)

1.8 35.6 2.5 11.5

Post desalination feed (RO second pass)

0.5 128.5 1.9 5.7

Post remineralisation feed

82.8 0.8 1.9 1.2

80.8 0.8 1.9 1.8

Post remineralisation product CaCO3/ Ca(OH)2/ CO2— CO2—process process

83.8 0.8 1.9 1.3

Hb dolomite/ CO2—process

1.7 2.0 4.6 0.9

CaCO3/ H2SO4— process

316 3 Post-Treatment

3.4 Post-Treatment Configuration and Treatment Systems

317

Due to the increase in alkalinity to the target value of 1.5 mmol/l, the salinity of the post-desalination product water increases from 69.8 mg/l to values in the range of 189–192 mg/l in the effluent of the Ca(OH)2/CO2, CaCO3/CO2, and semi-calcined dolomite/CO2 processes and in the CaCO3/H2SO4 process up to 312 mg/l. If the corrosion indices described in Sect. 3.2.3.2 and the corrosion guidelines of DIN EN 12502 are used to assess the corrosion behaviour of the product waters of the four remineralization processes and their ability to form natural protective CaCO3 scale, the following picture emerges: • Corrosion indices The Ryznar index (RI) has values in the range of 7.5–8.1 for the product water of the four remineralization processes, i.e. according to the table for the assignment of the RI value range to the water properties in Sect. 3.2.3.2.1, Eq. (3.9); this corresponds to a strong dissolving effect on CaCO3 scale and corrosion. The Puckorius index (PSI) calculated with Eqs. (3.10)–(3.10b) with values from 8.3 to 8.9 is also in this range for the Ca(OH)2/CO2, CaCO3/CO2, and CaCO3/H2SO4 processes, while for the semi-calcined dolomite/CO2 process, with a PSI of up to 9.3, its product water is assigned a very strong tendency to dissolve the protective CaCO3 scale and thus to cause corrosion. For the Larson-Skold index (L&SkI), when calculating this with Eq. (3.11), the ratio of the sum of the concentrations in meq/l of the chloride Cl and sulphate SO42 anions to the sum of bicarbonate HCO3 and CO32 is used to evaluate the corrosiveness of the water. For the Ca(OH)2/CO2, CaCO3/CO2, and semicalcined dolomite/CO2 processes, the L&SkI is 0.8, which means that the corrosion potential of these product waters is barely in a range for which the chloride and sulphate concentration has little or no influence on the formation of CaCO3 scale. For the CaCO3/H2SO4 process, the L&SkI is calculated as 2.0 due to the higher sulphate content in its product water compared to the other remineralization processes, leading to the expectation of high corrosion. • DIN EN 12502 Guidelines (Table 3.8) According to DIN EN 12502 Part 1, homogeneous surface corrosion is not to be expected for copper and copper alloys in contact with cold water with a pH of greater than 7.5. To avoid pitting corrosion, the pH in hot water should be >7.0, while the bicarbonate concentration mHCO3 should be >1.5 mmol/l. Although both these conditions are fulfilled by the remineralization process product waters, the guideline value for hot water of a corrosion index S for pitting corrosion of 2.0 mmol/l ≙ >122 mg/l HCO 3 (see Table 3.8) as set forth in DIN EN 12502 Parts 3 and 5 for the formation of a protective layer is not, however, achieved by any of the four alkalinization processes at the selected target value for alkalinity of 1.5 mmol/l. For this, the alkalinity would have to be raised to over 2 mmol/l. This is not the case for the calcium content mCa2þ stated in DIN EN 12502 Part 3 for the formation of deposits on hot-dip galvanized materials of >0.5 mmol/l ≙ 20 mg/l Ca2+. With the exception of the semi-calcined dolomite/ CO2 process, this value is matched or significantly exceeded by the other three remineralization processes. It is therefore to be expected that, under the conditions investigated above, the formation of a protective layer with the dolomite process will be significantly reduced for these materials compared to the other remineralization processes. The concentration of calcium mCa2þ of >1.0 mmol/l ≙ >40 mg/l Ca2+ as stated in DIN EN 12502 Part 5 to be necessary for the formation of deposits on cast iron, mild steel, or low-alloy steel is attained or exceeded only by the CaCO3/H2SO4 process but lacking the concentration of bicarbonate required for the formation of a CaCO3 protective layer. To assess the materials’ potential for pitting corrosion in accordance with DIN EN 12502 Part 5, the corrosion index S1, as defined in Part 3 of the DIN EN guidelines, can be used. The probability of occurrence of this type of corrosion increases for the materials of Part 5 of the standard with greater concentrations of chloride, sulphate, and nitrate and is reduced as the concentration of bicarbonate rises. The probability of pitting and crevice corrosion of stainless steels depends on the chloride content of the product water. For ferritic and austenitic steels which do not contain molybdenum as an alloy component, DIN EN 12502 Part 4 gives a

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319

guide limit chloride concentration for mCl of 6 mmol/l  200 mg Cl/l, above which pitting corrosion is to be expected. With a chloride concentration cCl of 39.8 mg/l as obtained by post-desalination of the RO systems product water of Table 3.14, all post-treated product waters listed there are clearly below this corrosion guide limit value. Evaluation of the product waters of the four remineralization processes under the criteria described above shows that the original corrosiveness of the RO product water can be significantly reduced by post-treatment. However, despite target values for alkalinity of 1.5 mmol/l and CCPP of 3 mg CaCO3/l, the product waters after post-treatment when in contact with hot-dip galvanized, mild steel and low-alloy steel still have a corrosion potential and, according to DIN EN 12502 Parts 3 and 5, a reduced ability to form natural protective layers. Through increased alkalinization to over 2.0 mmol/l, both the indices S and S1 are improved, and the values for mHCO3 and mCa2þ are brought closer to the requirements for the formation of a protective layer as set out in DIN EN 12502. Increasing the pH of the product water results in increases in the Ryznar index (RI), the Puckorius index (PSI), and the CCPP value. Both of these measures can reduce the corrosion potential as well as improve the formation of protective layers. In both cases, though, the consumption of chemicals and thus also the operating costs of remineralization increase. The setting of a higher CCPP value and the associated increase in pH also has the consequence that this can lead to an increased build-up of deposits and scaling in the water distribution system and the effectiveness of chlorine is reduced if this is used as a disinfectant. A further option for reducing the corrosion potential is dosing of corrosion inhibitors in the form of ortho-phosphates, polyphosphates, and silicate compounds, which form artificial protective layers on the materials of the water distribution system or reinforce natural protective layers and can additionally be used to reduce scaling (see Sect. 3.4.3.1). Dosing of such inhibitors is also advantageous if it is not intended to achieve calcium carbonate oversaturation of the product water in the post-treatment stage, but rather to deliver it to water consumers with negative values for CCPP and the saturation index. In this case, natural protective layers only form to a small extent, but by adding corrosion inhibitors, the formation of artificial protective coatings is possible. If SWRO product water is used for irrigation in agriculture, the value of the sodium adsorption ratio (SAR) (see Sect. 3.2.2, Eq. 3.1, and Table 3.4) is important. Under the operating conditions of Table 3.14, the SAR of the RO product water is reduced from 11.5 by post-desalination to 5.7 and by remineralization to a range of 0.9–1.8.

3.4.2.4 Remineralization Process Modifications for Increase of Magnesium Content in SWRO Product Water In addition to the ingestion of calcium and magnesium from food, the supply of these two elements to the human body takes place mainly through drinking water. Therefore, the concentration of these hardness components in drinking water must

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also be oriented to health aspects and must be equal to or exceed defined minimum values. Epidemiological studies provide indications of or reveal a correlation between the concentrations of calcium and magnesium in drinking water and their preventive effect on various types of disease. For magnesium, this is specifically its influence on the frequency of cardiovascular diseases and its protective effect with regard to these types of diseases [25]. On the basis of these studies, the World Health Organization, WHO, notes in [26] that statistically significant protective influences can be determined if drinking water has a magnesium content of at least 10 mg/l, and, although the causality of these relationships has not yet been fully established, this corresponds to known influences of magnesium on the cardiovascular system. However, this statement of the WHO is not to be understood as setting a guideline value. For calcium, a minimum concentration of 20–50 mg/l Ca2+ is recommended for health reasons based on epidemiological studies. However, as a result of the measures taken to reduce the corrosion potential of RO product water in the SWRO post-treatment stage, this calcium concentration is usually achieved and even exceeded. If a certain magnesium content in the product water of an SWRO plant is to be set in accordance with health aspects, of the remineralization processes described above for alkalinization, in addition to calcium, it is only the semi-calcined dolomite/CO2 process that adds magnesium into the treated water. When the bicarbonate alkalinity is increased by 1 mmol/l, with this alkalinization process the concentrations of these two constituents each increase by 0.25 mmol/l to 10 mg/l Ca2+ and 6.1 mg/l Mg2+. To achieve a target value for the magnesium concentration cMg2+ of 10 mg/l, in this case the HCO3 alkalinity has to therefore increase by approximately 1.64 mmol/l. If in the consumer water distribution system the SWRO product water is mixed with drinking water from other sources having a higher magnesium content and if a desired magnesium target value can be achieved in the mixed water as a result, no additional measures for adding magnesium during remineralization or conditioning in the SWRO post-treatment stage are necessary. If there is no possibility of achieving the Mg target value by mixing the SWRO product water with other water sources, the post-treatment stage of the SWRO plant has to be modified accordingly. In the post-treatment stage, there are following possibilities for introducing magnesium or raising its concentration: • Dosing of magnesium compounds, like MgCl2 or MgSO4, in the conditioning stage (see Sect. 3.4.3.2): when this is done, though, the chloride and sulphate contents of the water and thus its salt content and corrosion potential increase in step with the rise in magnesium concentration (see Sect. 3.2.3.2.1, Eq. 3.11, and Sect. 3.2.3.2.2, Table 3.8) • Extension of remineralization by a filter step which is connected upstream of the lime/CO2, limestone/CO2, or limestone/H2SO4 process and in which the RO product water mixed with CO2 and acidified is reacted with magnesium oxide

3.4 Post-Treatment Configuration and Treatment Systems

321

Fig. 3.19 Limestone/ion exchange process for magnesium content increase

pellets, such as the Akdolit Hydrolit-MG product from Rheinkalk Akdolit, according to the following equation [27]: MgO þ 2CO2 þ H2 O⇄MgðHCO3 Þ2 • The magnesium oxide filtration stage can also be arranged upstream of the semicalcined dolomite/CO2 process if, in the SWRO product water, an increased proportion of magnesium is desired in the Ca/Mg molar ratio. • Equipping the remineralization step of the post-treatment stage with an additional ion exchange system, with the help of which the magnesium content of the seawater is used to attain a set target value of the magnesium concentration in the SWRO product water (Fig. 3.19) [28–30]. This ion exchanger stage is installed downstream of the limestone filters of the remineralization step. It is filled with a chelate ion exchange resin with aminomethylphosphonic functional groups (e.g. Amberlite IRC747 from DuPont, LEWATIT® MonoPlus TP 260 from LANXESS, or Purolite S940) that has a higher affinity for the divalent ions Mg2+ and Ca2+ over monovalent ions such as Na+. The resin is loaded with seawater withdrawn from the SWRO plant after pretreatment or with concentrate from the first pass of the RO system. Since both seawater and the concentrate have higher concentrations of magnesium than of calcium, after loading the active groups of the resin contain a higher proportion of magnesium than of calcium. After loading, the resin is washed with the low saline concentrate of the postdesalination pass of the post-treatment stage, the loaded and washed column is drained, and then the limestone filter effluent is fed to it. During the subsequent ion exchange phase, calcium is absorbed by the resin, and in return magnesium is released into the water, at a higher rate at the beginning of the exchange phase and decreasing towards the end. Compared to the water’s quality at the outlet of the

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limestone filters, in the ion exchangers the concentration of calcium is reduced and that of magnesium increased. The ion exchange filter columns are operated alternately in the loading and in the exchange phase. The effluents of the columns being loaded are merged, and due to the differing loading volumes of each column, the various magnesium and calcium concentrations of these effluents are homogenized when they are mixed. Subsequently in the blended water, the CCPP or saturation index is adjusted to the desired value.

3.4.3

Conditioning: Dosing of Corrosion Inhibitors and Magnesium Compounds and Fluoridation

Conditioning options as follows are possible: • Dosing of corrosion inhibitors • Dosing of magnesium compounds • Fluoridation by dosing of fluorine compounds Which of these measures is used in the conditioning step of the post-treatment as well as their respective design and operating conditions depends on the local water supply infrastructure at the SWRO plant’s location, the relevant local or national drinking water regulations, and the technical specifications for conditioning based on the experience of the plant operator and the water supply company into whose supply network the SWRO plant feeds. If the water supply is provided solely by the SWRO plant, the selection, design, and operation of the conditioning measures are dictated by the composition of the SWRO product water. However, if the SWRO product is fed into a water supply system and is there blended with water from other sources, the conditioning step of the SWRO post-treatment must be adapted to the mixed water situation in the existing supply system and the conditioning measures already applied for this, with regard to both the concentration of each added dosing component and the chemicals used for the SWRO product water part stream.

3.4.3.1 Corrosion Inhibitor Dosing By dosing corrosion inhibitors, artificial protective layers form on the internal surfaces of the pipeline materials in the water supply system into which the SWRO product water is fed, or existing or developing natural protective layers are reinforced and compacted. The following find application as corrosion inhibitors: • • • •

Phosphate compounds in the form of ortho-phosphates and polyphosphates Silicate compounds Blends of ortho-phosphates and polyphosphonates Blends of phosphates and silicates

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323

The different phosphate compounds can be characterized by the chemical formula ðnþ2Þ Pn O3nþ1 . Phosphates for which n is 1 are ortho-phosphates, for example, PO3 4 , like the compounds monosodium hydrogen phosphate NaH2PO4 or trisodium phosphate Na3PO4. Polyphosphates are phosphate compounds with n greater than 1, such 5 as dipolyphosphate P2 O4 7 and tripolyphosphate P3 O10 for which n equals 2 and 3, respectively. Hexametaphosphate frequently used for water treatment consists of a mixture of long-chain polyphosphates where n is 5–22. Ortho-phosphates and polyphosphates differ in the way they are efficacious. With ortho-phosphates the inhibiting, i.e. protective, layer-forming property predominates, while polyphosphates have a mainly sequestering effect, i.e. they bind and mask dissolved or undissolved corrosion products. These differing actions of the two types of compounds are exploited in blends, in which the ortho and poly compounds are present in a defined ratio. However, it must be taken into account that the polyphosphates hydrolyse to ortho-phosphates over time, especially at higher temperatures, and then the inhibiting and coating-forming effect of the blended product predominates. Polyphosphates are mainly used when unwanted deposits of corrosion products that have already formed are to be degraded and discolouration of drinking water is to be suppressed or removed by sequestering dissolved metallic ions. However, in order to reduce the still existing corrosion potential of SWRO product water after post-treatment, the formation of dense corrosion protection layers is necessary, so that conditioning of the water with a corrosion inhibitor based on ortho-phosphates is given priority. Ortho-phosphate can be dosed either in the form of phosphoric acid, as a sodium or potassium salt, or as a zinc compound. To some extent, zinc supports the inhibiting action of ortho-phosphate. Sodium silicates with a higher molecular weight having the formula Na2O*(SiO2) *n where n is 3 or 4 also form corrosion-inhibiting protective films and coatings. Due to the alkalinity of the silicate compounds, an increase in pH of the conditioned water is achieved at the same time. These substances are used on their own or mixed with ortho-phosphate and/or polyphosphates. Many manufacturers supply corrosion inhibitors in the form of ortho-phosphates, polyphosphates, and mixed products with varying proportions of the two types of phosphate, as well as blends of silicates and silicate/phosphate. The most suitable type of corrosion inhibitor for a particular application and its required dosage rate should be determined either on the basis of the inhibitor supplier’s experience with similar applications or by laboratory or field trials. The formation of a corrosion protection layer requires a start-up phase for which the dosage rate of the inhibitor is around two to three times higher than what it will be after the formation of the protective layer, and this must be taken into account when planning the dosing systems. Experience shows that the dosing rate of mono-compounds for corrosion inhibition with ortho-phosphate is in the range of 0.5–2 mg/l P. Ortho-phosphates are most effective in the pH range of 7.3–8.0. For the formation of protective layers by dosing

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sodium silicates, the dosing rate is between 5 and 30 mg/l SiO2. Due to the alkaline reaction of the silicate products, the pH of the conditioned water is increased depending on the amount added. The hourly requirement for the corrosion inhibitor DCi, pr that is dosed at a rate of RCi, pr is calculated from the SWRO product water flow using Eq. (3.45). DCi,pr ¼

F Pr,SWRO  RCi,pr 1000

ð3:45Þ

DCi, pr ¼ corrosion inhibitor product demand [kg/h] RCi, pr ¼ dosing rate of corrosion inhibitor product [mg/l] If the dosing concentration cph, Ci is stated in mg P/l, the dosing rate of the inhibitor RCi, pr is calculated from its phosphorus content %ph, pr according to Eq. (3.45a). RCi,pr ¼

cph,Ci  100 %ph,pr

ð3:45aÞ

cph, Ci ¼ phosphorus P dosing concentration [mgP/l] %ph, pr ¼ phosphorus P content of corrosion inhibitor product [%] If the guidelines for the use of chemicals in drinking water treatment in force at the SWRO plant’s location postulate a maximum permissible phosphorus or phosphate concentration or a maximum use level (MUL) resulting from the dosing of the corrosion inhibitor in the treated drinking water, Eq. (3.45b) or (3.45c) can be used to calculate which phosphorus or phosphate concentration cph, Ci or cPO4 ,Ci in the SWRO product water corresponds to a certain dosing rate RCi, pr of the corrosion inhibitor. cph,Ci ¼ cPO4 ,Ci ¼

RCi,pr  %ph,pr 100

94:97 RCi,pr  %ph,pr  ¼ 3:067  cph,Ci 30:97 100

ð3:45bÞ ð3:45cÞ

cPO4 ,Ci ¼ phosphate PO4 dosing concentration [mg PO4/l]

3.4.3.2 Magnesium Compound Dosing If a target value for the magnesium concentration cMg2þ is set for the SWRO SWRO pw,tv product water and this cannot be met by the selected remineralization process, this concentration can be adjusted by adding magnesium compounds such as magnesium chloride MgCl2 or magnesium sulphate MgSO4 in the conditioning step. The

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325

required increase of the magnesium concentration ΔcMg2þ results from the magnetarget sium content of the RO product water cMg2þ

RO pw

SWRO product water cMg2þ

SWRO pw,tv

and the target value specified for the

(Eq. 3.46). The dosing rate of a 100% magnesium

chloride solution RMgCl2 ,100% needed for this is calculated according to Eq. (3.46a). The dosing rate calculated in this way also corresponds to the increase in salinity ΔTDSSWRO pw resulting from dosing of the magnesium component. The chloride concentration of the SWRO product water ΔcClSWRO pw increases accordingly (Eq. 3.46b). ΔcMg2þ ¼ cMg2þ target

SWRO pw,tv

 RMgCl2 ,100% ¼ ΔcMg2þ target

SWRO pw,tv

cMg2þ

RO pw

ð3:46Þ

RO pw

95:21 ¼ 3:917  ΔcMg2þ ¼ ΔTDSSWRO pw target 24:31

ΔcClSWRO pw ¼ RMgCl2 ,100%  cMg2þ

 cMg2þ

2  35:45 ¼ 0:7447  RMgCl2 ,100% 95:21

ð3:46aÞ ð3:46bÞ

¼ magnesium target concentration value in SWRO product water [mg/l]

¼ magnesium concentration in RO product water [mg/l]

¼ magnesium concentration increase for SWRO target value [mg/l] ΔcMg2þ target

RMgCl2 ,100% ¼ dosing rate of magnesium chloride 100% [mg/l] ΔTDSSWRO pw ¼ increase of total dissolved solids concentration in SWRO product water [mg/l] ΔcClSWRO pw ¼ chloride concentration increase in SWRO product water [mg/l] The hourly effective magnesium chloride demand DMgCl2 ,eff depends on the SWRO product water flow FPr, SWRO, the dosing rate of the compound RMgCl2 ,100% , and its purity f pur,MgCl2 (Eq. 3.46c). DMgCl2 ,eff ¼

F Pr,SWRO  RMgCl2 ,100% 1000  f pur,MgCl2

ð3:46cÞ

DMgCl2 ,eff ¼ effective hourly magnesium chloride demand of x % [kg/h] f pur,MgCl2 ¼ purity factor of magnesium chloride [–] For dosing of magnesium sulphate, the corresponding parameters are calculated as shown in Eqs. (3.47)–(3.47b). RMgSO4,100% ¼ ΔcMg2þ  target

120:37 ¼ ΔTDSSWRO pw ¼ 4:9515  ΔcMg2þ target 24:31

ð3:47Þ

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ΔcSO4,SWRO pw ¼ RMgSO4,100%  DMgSO4,eff ¼

96:06 ¼ 0:798  RMgSO4,100% 120:37

ð3:47aÞ

F Pr,SWRO  RMgSO4,100% 1000  f pur,MgSO4

ð3:47bÞ

RMgSO4,100% ¼ dosing rate of magnesium sulphate 100% [mg/l] ΔcSO4,SWRO pw ¼ sulphate concentration increase in SWRO product water [mg/l] DMgSO4,eff ¼ effective hourly magnesium sulphate demand of x % [kg/h] f pur,MgSO4 ¼ purity factor of magnesium sulphate [–] The dosing flow of the magnesium compound solution FD, Mgc, sol is calculated from its effective hourly demand DMgc, eff, the concentration of the solution %Mgc, sol, and its density ρMgc, sol with Eq. (3.48). F D,Mgc,sol ¼

DMgc,eff  100 %Mgc,sol  ρMgc,sol

ð3:48Þ

FD, Mgc, sol ¼ dosing flow of magnesium compound solution [l/h] DMgc, eff ¼ effective hourly demand of magnesium compound [kg/h] %Mgc, sol ¼ percentage of magnesium compound solution [%] ρMgc, sol ¼ density of solution of magnesium compound [kg/l]

3.4.3.3 Fluoride Dosing Fluoride can be added to SWRO product water to protect against tooth decay. A number of extensive epidemiological studies involving populations in many countries with drinking water containing natural and added fluoride have shown that this positive preventive effect occurs in a range of fluoride concentrations from 0.5 to 1.0 mg/l, although this depends on the volume of drinking water consumed per person and the intake of fluorine from other sources. The specific drinking water consumption per capita is particularly influenced by the mean annual temperature, that is, by the seasonal and climatic conditions in a region. At elevated mean temperatures, the specific drinking water consumption also rises and thus the necessary effective fluoride concentration in drinking water drops. Awareness of dental hygiene and the conscientiousness of a population for taking care of their teeth in a given region, such as toothbrushing frequency with fluoride toothpastes, the use of other fluorinated products, and the level of dental care provision, also have significant impacts. If the fluoride content exceeds 1.5–2.0 mg/l, discoloration and mottling of the teeth can occur, and if 4.0 mg/l is exceeded, with prolonged contact, there is a risk of fluorosis, which adversely affects the tooth and bone structure of the human body.

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327

In the WHO guidelines for drinking water quality,6 a recommended maximum of 1.5 mg/l fluoride F is proposed but noting that this value is linked to a specific drinking water consumption of 2 l/day. National standards must therefore be adapted accordingly to take account of local or regional conditions [31]. The fluoride concentration limit of many national standards for drinking water quality is in line with the WHO guidelines, except for USEPA,7 which specifies an MCL value of 4 mg/l F and an SMCL value of 2 mg/l F (see Table 3.2). In some regions, especially where mean annual temperatures are high, national standards with lower fluoride limits of 0.7–0.8 mg/l F are stipulated. Seawater has a low fluoride concentration of around 1.5 mg/l F, which is further reduced during the desalination process. This means that fluoride is only present in trace amounts in the product waters of the first and second RO passes. If the potable water produced by the SWRO plant is mixed with water from existing sources that have correspondingly higher fluoride content and if the fluoride target value specified by the national drinking water guidelines or the drinking water supply company is thus achieved, fluoride dosing in the SWRO post-treatment stage is not necessary. It may also be advantageous to mix the practically fluoride-free SWRO potable water with existing drinking water that has a fluoride content above the specified limit of the national guidelines, if this results in the fluoride target in the blended water being met so that treatment to reduce the fluoride content of existing water sources can be dispensed with. If fluoride dosing is required in the conditioning step of the SWRO post-treatment stage, the dosing equipment installed there is normally sized for dosing rates in the range of 0.6–1.1 mg/l F depending on the national limits for fluoride. But then dosing in the SWRO plant must be dimensioned so that when the SWRO product water is mixed with existing water sources, the sum of the fluoride supplied with the SWRO part stream and the existing fluoride concentration does not exceed the potable water guideline value for the permissible maximum fluoride concentration in the blended water. In order to set a predetermined fluoride concentration in the SWRO product water, the following fluorine-containing compounds are used as dosing chemicals: • Sodium fluoride, NaF • Sodium fluorosilicate or disodium hexafluorosilicate, Na2SiF6 • Hexafluorosilicic acid, H2SiF6 Sodium fluoride dissociates in water to form sodium and fluoride ions, while the two fluorosilicate compounds react with water to form fluoride and SiO2 (see the chemical equations below).

6

Guidelines for Drinking Waters Quality-4th Edition, 2011, World Health Organisation (WHO). 2018 Edition of the Drinking Water Standards and Health Advisories Tables-United States Environmental Protection Agency.

7

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NaF⇄Naþ þ F Na2 SiF6 þ 2H2 O⇄2Naþ þ 4Hþ þ 6F þ SiO2 H2 SiF6 þ 2H2 O⇄6Hþ þ 6F þ SiO2 Sodium fluoride and sodium fluorosilicate are commercially available as solid compounds and are prepared and dosed as solutions, while hexafluorosilicic acid is commercially available as a 20–30% solution and is also used in this form for dosing. The necessary dosing rate RFc,100% of the 100% respective fluoride compound is calculated with its specific stoichiometric factor fFc,stoich from the required increase of fluoride concentration ΔcF target,SWRO pw in the SWRO product water or, after blending it with existing water sources, with that value of ΔcF target,SWRO pw needed to obtain the specified fluoride concentration (Eq. 3.49). The effective hourly demand of the dosed chemical in commercial concentration DFc,eff is calculated using Eq. (3.49a) and the dosing pump delivery of the fluorine dosing equipment FD,Fc,sol using Eq. (3.49b). RFc,100% ¼ f Fc,stoich  ΔcFtarget,SWRO pw

ð3:49Þ fFc, stoich 2.210 1.650 1.264

Fluoride compound NaF Na2SiF6 H2SiF6

DFc,eff ¼

F Pr,SWRO  RFc,100% 1000  f pur,Fc

F D,Fc,sol ¼

DFc,eff  100 %Fc,sol  ρFc,sol

ð3:49aÞ ð3:49bÞ

RFc, 100% ¼ dosing rate of fluorine compound 100% [mg/l] fFc, stoich ¼ stoichiometric factor of fluorine compound [–] ΔcFtarget,SWRO pw ¼ fluorine target concentration increase [mg/l] DFc, eff ¼ effective hourly demand of fluorine compound of x % [kg/h] fpur, Fc ¼ purity factor of fluorine compound [–] FD, Fc, sol ¼ dosing flow of solution of fluorine compound [l/h] %Fc, sol ¼ percentage of fluorine compound in solution [%] ρFc, sol ¼ density of solution of fluorine compound [kg/l] The chemicals are dosed in proportion to the flow rate of the SWRO product water.

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329

Commercially available hexafluorosilicic acid solutions are strongly acidic with a pH of 1.2 and are highly corrosive to both metallic and ceramic materials. Suitable plastics such as polyethylene (PE) or polyvinyl chloride (PVC) or else rubberized steel should therefore be used as materials for tanks, pipelines, and pumps. Small amounts of gaseous hydrogen fluoride are released from the solutions during storage, and due to its harmful and corrosive effect, storage tanks and day tanks must be properly vented either using alkaline absorption vessels or by discharge to the atmosphere outside of the chemical storage facility. To avoid overdosing of fluoride, periodic monitoring of the fluoride concentration in the conditioned water is necessary. This can be done by online measurement with fluoride-sensitive electrodes together with regular taking of water samples for laboratory analysis to check the fluoride concentration. For dosing, storage, material selection, and monitoring the fluoride concentration during fluoridization, see also [32].

3.4.4

Disinfection

The RO systems and upstream pretreatment stages of an SWRO plant present multiple barriers which to a very large extent intercept the microbiological substances present in seawater, so only minuscule quantities get as far as the SWRO product water. With the process combinations of pretreatment and desalination stages commonly used in membrane seawater desalination, maximum inactivation rates of more than 3 log–4 log, or 99.9–99.99%, can be achieved for microorganisms such as protozoa (Cryptosporidium oocysts and Giardia cysts), bacteria, and viruses. For calculating the percentage inactivation rate I% from log inactivation Ilog, see Sects. 2.2.1.2 and 2.2.1.2.1, Eqs. (2.17d)–(2.17f). However, attainment and maintenance of such inactivation rates depend on the quality of the membrane system and specifically its integrity. Defects and leaks in the membrane elements, especially as they age, can significantly reduce the inactivation rates, especially for viruses, so that the membrane process alone is not sufficient to disinfect the product water. In addition, recontamination of the RO product water is possible during its post-treatment by introduction of microorganisms with the reaction products and chemicals used in this stage as well as through contact with the atmosphere from air exchange during filling and emptying during intermediate storage and buffer storage of the water before it is fed into the water supply network. The drinking water produced in the SWRO plant must therefore be disinfected in the conditioning step of post-treatment to hygienize it and prevent microbial contamination during its storage (primary disinfection) as well as before it is fed into the water distribution network (secondary disinfection). The degree of inactivation of microbiological constituents of the water by a disinfectant depends on its existing concentration C and the contact time t with the component to be inactivated, the CT value. The lower this CT value is, the greater the inactivation potential of the disinfectant. The CT concept for disinfection together with the associated calculation equations is described in detail under

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3 Post-Treatment

Sects. 2.2.1.2 and 2.2.1.2.1. In order to maintain hygienization of the water in the storage tanks for the duration of its residence time, the disinfectant used for primary disinfection must be capable of remaining active at least to a certain extent throughout the residence time of the water in the plant’s tank installation. The tank volumetric capacity for drinking water storage may range from the hourly to the daily production capacity of the SWRO plant, which means a storage period of 1–24 h. This also applies to the disinfectant selected for secondary disinfection, which itself must have an appropriate residual concentration for remaining active at the point where drinking water is drawn from the water distribution system. For these two disinfection steps, only disinfectants are suitable which, in addition to the desired inactivation effect, also have a good persistence of its residual active concentration over a lengthy period.

3.4.4.1 Disinfectant Properties and Selection For primary and secondary disinfection of SWRO product water, a selection is to be made from the disinfection processes used in drinking water treatment such as: • Chlorination—dosing of chlorine in gaseous form or as a hypochlorite solution • Chloramination—production and dosing of monochloramine from chlorine and ammonium • Chlorine dioxide—generation and dosing of chlorine dioxide • Ozonization—dosing of ozone • Ultraviolet (UV)—irradiation with UV light of the mode of disinfection that is most suitable for the respective disinfection step for hygienizing the water under the operating conditions specific to the SWRO process. Table 3.16 compares the chemical properties and operating conditions of the disinfectants used in the above disinfection processes that must be taken into account when selecting the processes to be used for the SWRO product water in the first and second disinfection steps. The disinfectants react with organic and inorganic components of the water to be disinfected to form various disinfection by-products (DBPs), as identified in Table 3.16. Since most of these DBPs are classified as harmful to human health, the reference values for these products quoted in international and national drinking water guidelines are therefore correspondingly low (Table 3.16). However, the separation membranes of a possible membrane step in the pretreatment stage and those in the RO seawater desalination stage intercept a high proportion of the organic constituents of the seawater and thus also the precursors, such as natural organic matter (NOM) or organic amines and protein substances from municipal and industrial wastewater discharges, from which organic DBPs are formed, primarily during disinfection with chlorine [33]. As a result, when disinfecting SWRO product water with chlorine, the risk of the formation of organic halogenated disinfection by-products is low. This also applies to the formation of oxidized organic substances

Maximum contaminant level MCL Parameter [mg/l] Type of by-product generated during disinfection Trihalomethans total 0.05–0.25 TTHM Haloacetic acids HAA 0.06–0.08 Oxidized organics – Bromate BrO3 0.01–0.02 Chlorite ClO2 0.7–1.0 Chlorate ClO3 0.7–1.0 Ammonia as NH3 0.1–0.5 Dichlor-/trichloramine – Nitrite as NO2 0.5–3.0 Nitrosamines NDMA 9–100 [ng/l] Cyanogen halides 0.07–0.08 ○

○ ○ ○

○

0.5–2.0

○

○ ○ ○ ○

1.0–3.0

● ●

○

0.3–1.0

Type of disinfectant Chlorine Chlorine OCl2, Monochloramine dioxide ClO2 HOCl NH2Cl Maximum residual health level [mg/l] 0.4–1.0 as 1.2–5 as 3–4 as NH2Cl ClO2 Cl2 Dosing rate range [mg/l]

Table 3.16 By-product generation and characteristics of disinfectants

–

< 0.1– 0.5

(continued)

–

10 as O3

○ ○ ○

Ultraviolet radiation UV

Ozone O3

3.4 Post-Treatment Configuration and Treatment Systems 331

Moderately High Moderately

High Good High ● ● ●

1.0–3.0

0.5–2.0 High Good Low ● ●

0.3–1.0

Type of disinfectant Chlorine Chlorine OCl2, Monochloramine dioxide ClO2 HOCl NH2Cl Maximum residual health level [mg/l] 0.4–1.0 as 1.2–5 as 3–4 as NH2Cl ClO2 Cl2 Dosing rate range [mg/l]

○Depending on composition of SWRO product water, RO/post-treatment and disinfection operating conditions

Maximum contaminant level MCL Parameter [mg/l] Properties of disinfectant Inactivation of viruses Persistence of residual pH dependency Primary disinfectant Secondary disinfectant

Table 3.16 (continued)

–

< 0.1– 0.5

Moderately None None ○

–

10 as O3

High None Low ○

Ultraviolet radiation UV

Ozone O3

332 3 Post-Treatment

3.4 Post-Treatment Configuration and Treatment Systems

333

as DBPs when chlorine dioxide and ozone are used as well as of nitrosamines and cyanogenic halides when disinfecting with chloramine. Bromate BrO3 is an inorganic DBP which is formed during the reaction of bromide Br with strongly oxidizing disinfectants such as ozone (see simplifying equation below). Br þ 3O3 ! BrO 3 þ 3O2 It has a drinking water guideline value of 0.01–0.02 mg BrO3/l so that ozone can only be used as a primary disinfectant for SWRO product water if the bromide content is reduced in the seawater desalination and the post-desalination stages to such an extent that the very low bromate limits of the national or international drinking water guidelines applicable to the desalination plant are not reached or exceeded. Bromate is also formed during the electrolysis of seawater to produce hypochlorite solution, a process that is used in part for chlorine disinfection to pretreat the seawater feed to an SWRO plant (see Sects. 2.2.1.3 and 2.2.1.3.3). The bromate formed there is effectively retained in the seawater desalination and postdesalination stages, so that the residual contents in the product water are below the specified maximum guideline values for BrO3. However, the hypochlorite solution produced by seawater electrolysis may under no circumstances be used for disinfection in the SWRO post-treatment stage, as then the membrane barrier for bromate retention is bypassed and its permissible limit value in the SWRO product water may then be exceeded. Other inorganic disinfection by-products are chlorite ClO2 and chlorate ClO3. Both compounds are formed when chlorine dioxide is used as a disinfectant (see Sects. 2.2.1.1 and 2.2.1.1.2) and have drinking water limit values of 0.7–1.0 mg/l depending on the applicable international or national guidelines. Up to 50–70% of the chlorine dioxide can be converted into chlorite. Chlorine dioxide is often used to prevent the formation of halogenated DBPs, as is the case with chlorine disinfection. Since such DBPs are hardly ever formed during hygienization of SWRO product water as described above due to its low organic content, there is no need to use chlorine dioxide under this aspect. Commercial hypochlorite solutions may also contain chlorate. Monochloramine is formed during the reaction of ammonium with chlorine in a molar ratio of 1:1, corresponding to a weight ratio of the two components chlorine and ammonium of 4.2–1 (see Sects. 2.2.1.1 and 2.2.1.1.3). By-products such as dichloramine NHCl2 and trichloramine NCl3 are formed when the molar proportion of chlorine is greater than that of ammonium, i.e. the Cl2/NH3 weight ratio is greater than 4.2. Neither of these compounds is a maximum concentration specified in the drinking water guidelines. However, if these two by-products are present following chlorination, this can lead to aesthetic impairments with regard to odour and taste. If the molar ammonium content is greater than 1, i.e. the ratio of Cl2/NH3 by weight is less than 4.2, nitrite NO2 can be formed by nitrification of the excess ammonium then present, according to the equation

334

3 Post-Treatment  þ 2NHþ 4 þ 3O2 ! 2NO2 þ 2H2 O þ 4H  2NO 2 þ O2 ! 2NO3

Nitrification arises due to bacterial oxidation of ammonium. In the first reaction stage, nitrite is formed, this then being further oxidized to nitrate in a subsequent bacterial reaction. As the above equation shows, nitrification liberates H+ ions which lead to a noticeable reduction in the pH of drinking water disinfected with chloramine. Nitrite is classified as hazardous to health, and its maximum concentration in drinking water is limited to 0.5–3.0 mg/l as NO2 in the national and international drinking water guidelines. The formation of these by-products can be prevented or at least greatly reduced by monitoring and control of monochloramine production, that is, by largely maintaining the molar ratio of the ammonium and chlorine components. If nitrification occurs, the bacteria that cause this reaction can be inactivated by increasing the weight ratio of the chlorine component accordingly or by switching to pure chlorine disinfection for a limited time, although this may result in increased formation of the other by-products described above and of DBPs. During the formation of organic disinfection by-products (DBPs), a portion of the added disinfectant is consumed, depending on the content of organic substances in the water to be disinfected. Due to the high retention of organic substances by the SWRO plant’s reverse osmosis stages, SWRO product water has much lower values of total organic carbon TOC and dissolved organic carbon DOC than drinking water obtained from surface water without membrane desalination. The decay of added disinfectant due to its reaction with water constituents is correspondingly lower. Consequently, the dosing rate of disinfectants for SWRO product water is more in the lower range of the dosing bandwidths, as shown in Table 3.16. Protozoa and bacteria are largely rejected by seawater RO membranes. The extent of disinfection measures for SWRO product water is therefore mainly determined by the inactivation of viruses and the prevention of recontamination during drinking water storage and distribution. The dimensioning of disinfection equipment for the primary disinfection of SWRO product water is therefore usually based on the CT values of the disinfectants for the inactivation of viruses. Since viruses require a higher inactivation potential of the disinfectants than most bacteria, this design also ensures sufficient inactivation to counter bacterial recontamination. The graphs of Fig. 3.21a–c show the dependence of the CT values for the disinfectants chlorine, chloramine, and chlorine dioxide for virus inactivation on the degree of inactivation (log inactivation) and the temperature. These curves show how temperature significantly impacts the disinfectants’ effectiveness. With increasing temperature, the CT value decreases, which means that the inactivation potential of the compounds increases. In order for the log inactivation at different temperatures to remain constant, a higher dosage rate of the disinfectant is therefore needed for operating conditions with low temperatures.

3.4 Post-Treatment Configuration and Treatment Systems

335

Ratio Cl2, HOCl and OCl– to active chlorine 1.0 Hypochlorous acid HOCl

0.9 0.8

Temperature 20 - 40°C

0.7 0.6 0.5

40°C → 20°C

40°C → 20°C

0.4 0.3

Hypochlorite OCl–

Chlorine Cl2

0.2 0.1 0.0 0

1

2

3

4

5

6

7

8

9

10

11

pH

Fig. 3.20 pH dependence of chlorine, hypochlorous acid, and hypochlorite portion on total active chlorine content

When dimensioning the dosing equipment of a disinfection system, the range of seawater temperature at the location must therefore be taken into account. If chlorine is used for disinfection, the pH also has a strong influence on its effectiveness. The chlorine compound with the strongest disinfecting action is hypochlorous acid HOCl, and the effectiveness of chlorine as a disinfectant depends on the proportion of the total chlorine concentration of this compound that is present at a particular pH. With increasing pH, this proportion decreases, and the concentration of the less effective hypochlorite OCl increases, as shown in Fig. 3.20 (see Sect. 2.2.1.1, Eqs. 2.2 and 2.2a), and thus the inactivation potential of chlorine decreases accordingly. This influence of pH on the effectiveness of chlorine disinfection is not in detail considered for the pH range of 6–9 in the curves of Fig. 3.21a. However, the influence of pH in this range can be estimated on the assumption that solely HOCl has an inactivating effect and that the disinfection efficiency of chlorine increases or decreases accordingly at a certain pH value as shown in Fig. 3.20. The graphs in Fig. 3.21a–c show how the effectiveness of the three disinfectants in inactivating viruses differs. Chlorine is the most effective, followed by chlorine dioxide and then monochloramine, which is much less effective than the other two substances. Various options for combining the disinfectants described above for primary and secondary disinfection of SWRO product water are listed in Table 3.17.

336

3 Post-Treatment CT [mg*min*l-1] 7.0 6.0

Disinfecting agent: Chlorine pH = 6.0 - 9.0

5.0 4.0 Log inactivation 3.0

4

2.0

3 2

1.0 0.0 10

15

20

25

Temperature [°C] CT [mg*min*l–1] 1600 1400 Disinfecting agent: Chloramine

1200 1000

Log inactivation 800 4 600 3 400 2 200 0 10

15

20

25

Temperature [°C]

Fig. 3.21 Disinfection: inactivation of viruses by (a) chlorine CT values versus temperature, (b) monochloramine CT values versus temperature, (c) chlorine dioxide CT values versus temperature (data source: [34])

3.4 Post-Treatment Configuration and Treatment Systems

337

CT [mg*min*l–1] 30.0

25.0

Disinfecting agent: Chlorine dioxide pH = 6.0 - 9.0

20.0

15.0

Log inactivation 4

10.0 3 5.0 2 0.0 10

15

20

25

Temperature [°C]

Fig. 3.21 (continued)

Table 3.17 Disinfection process options for primary and secondary disinfection of SWRO product water Option no. 1 2 3 4 6 7 8 9

Process options for primary and secondary disinfection Primary disinfection Secondary disinfection Chlorine Cl2, OCl Chlorine Cl2, OCl Monochloramine NH2Cl Chlorine dioxide ClO2 Chlorine dioxide ClO2 Chlorine dioxide ClO2 Monochloramine NH2Cl Ozone O3 Chlorine Cl2, OCl Monochloramine NH2Cl Chlorine dioxide ClO2

Due to the low TOC and DOC contents of SWRO product water, hardly any organic disinfection by-products are formed when it is disinfected with chlorine, so that at this water quality this disinfectant is well suited for primary disinfection and does not need to be replaced by-products with lower DBP formation, such as chlorine dioxide in options 4–6 or ozone in options 7–9. However, if the drinking water produced in the SWRO is blended in the water distribution network with drinking water of a different quality and composition, the choice of disinfectant for secondary disinfection must be based on this blended water composition and the

338

3 Post-Treatment

method of secondary disinfection applied in the network. This may result in the selection of option 2, chlorine/monochloramine, or option 3, chlorine/chlorine dioxide, instead of option 1 chlorine/chlorine to minimize DBP formation in the distribution network. Monochloramine does not provide sufficient inactivation of pathogenic germs during primary disinfection and during the above stated period of drinking water storage before its release into the water distribution system. But because hardly any organic DBPs are formed when it is used and due to the very high persistence of its residual content as well as the prolonged contact time in the distribution system, it is suitable for secondary disinfection. Thus monochloramine is used as a secondary disinfectant, especially in very extensive water distribution systems with relatively high water temperatures. The use of chlorine in the primary and secondary disinfection stage (option 1) and its combination with monochloramine as a secondary disinfectant (option 2) are the most common disinfection processes at seawater desalination plants for product water hygienization. Hypochlorite NaOCl can be purchased from a commercial source as well as generated on site by electrolysis from a NaCl solution that has been prepared from solid sodium chloride. For this NaCl electrolysis, however, sodium chloride with a correspondingly low bromide content must be used (conforming to European standards EN 16370 and EN 16401 or the NSF/ANSI 60 standard), in order to prevent the hypochlorite solution obtained in this way from having a bromate content due to the electrolytic oxidation of the bromide ion Br to bromate BrO3 which, when the disinfectant solution obtained is dosed into the SWRO product water, can lead to a certain bromate limit value being exceeded. Chlorine dioxide can be used for both primary and secondary disinfection because it exhibits both a favourable inactivation potential and a high stability of its residual content, while hardly any organic disinfection by-products are generated through its use. However, it is then necessary to monitor how much chlorite and chlorate are formed as well as compliance with the drinking water limits for these two compounds. Although ozone is highly effective as a disinfectant after a very short residence time, however, its content rapidly drops which means that it can be used as a primary disinfectant in accordance with options 7–9 only if the drinking water is stored for a very short time. This applies equally for its use for secondary disinfection. Its potential use is also limited by the fact that, for its application, the bromide content of the SWRO product water to be disinfected must be reduced by the RO desalination stages to such an extent that the bromate limit laid down in national or international drinking water guidelines will not be exceeded. Since ultraviolet irradiation does not produce a residual content of active disinfectant that remains stable over a lengthy period and therefore cannot prevent re-infection of the already disinfected drinking water, it should not be used for primary or secondary disinfection. It is though suitable as a safety disinfection measure when withdrawing drinking water from the water distribution network.

3.4 Post-Treatment Configuration and Treatment Systems

339

3.4.4.2 Disinfection Process Design 3.4.4.2.1 Selection of Disinfectant for Primary and Secondary Disinfection and Calculation of Disinfection Dosing and Residual Rate From the CT value CTX  log inact, germ at which a specific microbial log inactivation can be achieved by a disinfectant at a specific temperature (see graphs in Fig. 3.21a– c), it is possible to determine the contact time required for a given residual content cdis, res of the disinfectant or, if the contact time τct, nes is known, the residual content of the disinfectant required for a specified degree of inactivation (Eqs. 3.50 and 3.50a) [34]. CTX log

inact,germ

τct,nes ¼

¼ cdis,res  τct,nes

CTX log inact,germ cdis,res

ð3:50Þ ð3:50aÞ

CTX  log inact, germ ¼ CT value for the germ necessary to achieve X-log inactivation at temperature t [mg*min*l1] τct, nes ¼ necessary contact time to achieve X-log inactivation at disinfectant residual [min] cdis, res ¼ concentration of disinfectant residual [mg/l] The time τct, si during which contact between the microbes to be inactivated and the disinfectant takes place in a volume segment is calculated from the theoretical detention time τdt, si in this segment and a correction factor fbaff, si, referred to as the baffling factor, which characterizes the flow conditions in this volume (Eq. 3.50b). τct,si ¼ τdt,si  f baff,si

ð3:50bÞ

τct, si ¼ contact time at segment i [min] τdt, si ¼ theoretical detention time at segment i [min] fbaff, si ¼ baffling factor at segment i [–] The theoretical detention time τdt, si in the segment is determined from its smallest contact volume Vc, min , si, for example, the minimum level in a storage tank and its maximum flow FPr, max (Eq. 3.50c). τdt,si ¼

V c, min ,si  60 F Pr, max

ð3:50cÞ

Vc, min , si ¼ contact volume of segment i (min. volume for tanks and basins) [m3] FPr, max ¼ product water flow max. through contact volume [m3/h]

340

3 Post-Treatment

Table 3.18 Flow distribution and baffling conditions in pipes and basins—determination of baffling factor (source: [34, 35]) Baffling conditions Unbaffled (mixed flow) Poor

Baffling factor fbaff 0.1 0.3

Average Superior

0.5 0.7

Perfect (plug flow)

1.0

Flow distribution description No baffles, agitated basin, very low length-to-width ratio, high inlet and outlet flow velocities Single or multiple unbaffled inlets and outlets, no intra-basin baffles Baffled inlet or outlet with some intra-basin baffles Perforated inlet baffles, serpentine or perforated intra-basin baffles, outlet weir or perforated launders, filters Static mixers or diffusor system at dosing point and pipeline flow or basins with very high length-to-width ratio, perforated inlet, outlet, and intra-basin baffles in basins

The baffling factor fbaff, si is calculated as a coefficient from the time τ10, si during which 10% of the water passes through the contact volume, i.e. the detention time in which 90% of the water is retained in the contact volume (Eq. 3.50d). f baff,si ¼

τ10,si τdt,si

ð3:50dÞ

fbaff, si ¼ baffling factor at segment i [–] τ10, si ¼ time for 10% of the feed flow to pass through the volume of segment i [min] The value of the baffling factor can range from 0.1 to 1.0, and it is determined by the extent to which the flow pattern of the contact volume deviates from ideal plug flow conditions. fbaff, si has a value of 1 if the contact volume has plug flow conditions, like if the volume consists of a lengthy pipe through which the disinfectant is evenly distributed after the disinfectant has been added by, for example, a static mixer. The more the flow in the contact volume is short-circuited, the lower is the value of the baffling factor [35]. The flow conditions in the contact volume, for instance, of drinking water storage tanks or clearwells, can be evened out by installing baffles and by optimizing its inflow and outflow conditions. In existing plants, τ10, si is determined by tracer tests and the baffling factor is then calculated from this using the theoretical detention time τdt, si of the contact volume according to Eq. (3.50d) [35]. When designing a plant, the specific baffling factors of each contact volume for a particular combination of, for example, several segments with different flow characteristics can be approximated using the criteria listed in Table 3.18. If the contact volume is fixed by the design or, in an existing plant, by the different volumes of piping or storage tanks, the total volume should be divided into segments, since the volumes of piping and tanks differ by their baffling factors and thus also for calculating their respective contact times τct, si with Eq. (3.50b).

3.4 Post-Treatment Configuration and Treatment Systems

341

For a certain residual content of the disinfectant cdis, res, si and a contact time τct, si in segment i, its CT value CTcalc, si is calculated by Eq. (3.51). CTcalc,si ¼ cdis,res,si  τct,si

ð3:51Þ

CTcalc, si ¼ calculated CT value for segment i [mg*min*l1] cdis, res, si ¼ concentration of disinfectant residual at segment i and contact time τct, si The quotient of the calculated CT value CTcalc, si and the CT value CTX  log inact, required for a specific microbial log inactivation in a contact volume as shown by the graphs in Fig. 3.21a–c is termed the inactivation ratio rinact, si (Eq. 3.51a) and indicates the degree of inactivation that becomes established as determined by the contact time τct, si and the residual content of the disinfectant cdis, res, si in a segment i of the contact volume. germ

r inact,si ¼

CTcalc,si CTX log

inact,germ

ð3:51aÞ

rinact, si ¼ inactivation ratio at segment i [–] If the value of the inactivation ratio rinact, si is equal to or greater than 1, the log inactivation Ilog, germ fixed by the selection of the CT value CTX  log inact, germ can be achieved. The value of the log inactivation Ilog, calc, si resulting in a segment i due to its inactivation ratio is calculated by multiplying the inactivation ratio rinact, si by the target value of the log inactivation Ilog, germ, i.e. with numerical values for Ilog, germ of 2–4 (Eq. 3.51b). I log ,calc,si ¼ r inact,si  I log ,germ

ð3:51bÞ

Ilog, calc, si ¼ log inactivation calculated for segment i [–] Ilog, germ ¼ log inactivation target for germ ¼ 2–4 [.] The inactivation ratio of the total contact volume rinact, total is the sum of the individual inactivation ratios rinact, si of the number of segments of the volume (Eq. 3.51c), and accordingly the total log inactivation Ilog, calc, total of the contact volume is calculated from the total value of the inactivation ratio rinact, total and the target value for log inactivation of the microbe to be inactivated Ilog, germ (Eq. 3.51d). r inact,total ¼

i¼n X i¼1

r inact,si

ð3:51cÞ

342

3 Post-Treatment

I log ,calc,total ¼ r inact,total  I log ,germ

ð3:51dÞ

rinact, total ¼ inactivation ratio of all segments i ¼ 1 to i ¼ n [–] Ilog, calc, total¼ log inactivation calculated total of all segments i ¼ 1 to i ¼ n [–] To calculate the respective microbial inactivations in several successive segments of a contact volume, the residual content of the disinfectant cdis, res, si in the segment i to be calculated must be known. This is calculated from the concentration of the feed to the segment cdis, feed, si, i.e. the residual disinfectant content from the preceding segment, and the reduction of the concentration of the disinfectant in the segment to be calculated. This reduction in concentration depends on the feed concentration, the TOC content in the segment volume concerned, the temperature, flow conditions, detention time, and substances with which the disinfectant comes into contact within the segment. It is characterized by the decay factor, which therefore has different values depending on the type of segment and must be determined empirically (Eq. 3.51e). For the product water of SWRO plants, values for fdecay of around 0.1–0.2 ¼ 10–20% have been measured for chlorine as disinfectant, depending on the operating conditions. In the period just after dosing the disinfectant, the decay factor is at its highest value, and then it decreases to a level that changes only slightly, even after a lengthy contact time. f decay,si ¼

cdis,feed,si  cdis,res,si c ¼ 1  dis,res,si cdis,feed,si cdis,feed,si

ð3:51eÞ

fdecay, si ¼ disinfectant residue decay factor at section i [–] cdis, feed, si ¼ feed concentration of disinfectant to segment i [mg/l] The active content of disinfectant cdis, res, s2 in segment 2 of the contact volume then results from the residual content cdis, res, s1 in segment 1 according to Eq. (3.51f).   cdis,res,s2 ¼ cdis,res,s1  1  f decay,s2

ð3:51fÞ

cdis, res, s1 ¼ concentration of disinfectant residual at segment 1 and contact time τct, 1 [mg/l] fdecay, s2 ¼ disinfectant residue decay factor at section 2 [–] cdis, res, s2 ¼ concentration of disinfectant residual at segment 2 and contact time τct, 2 [mg/l] If the residual disinfectant content cdis, res, si + 1 in the final segment is known, its concentration in the preceding segment cdis, res, si is calculated by Eq. (3.51g).

3.4 Post-Treatment Configuration and Treatment Systems

cdis,res,si ¼ 

cdis,res,siþ1  1  f decay,siþ1

343

ð3:51gÞ

Accordingly, for a contact volume consisting of only one segment, the dosing concentration RDisc, 100% of the disinfectant required for its disinfection is calculated using Eq. (3.52), and for a volume with two segments, of which the residual concentration of the disinfectant in the second segment cdis, res, s2 and the decay factors fdecay, s1 and fdecay, s2 for the two segments or the total decay factor fdecay, total for the contact volume are known, then the dosing concentration RDisc, 100% for disinfecting the total volume is calculated with Eq. (3.52a). RDisc,100% ¼  RDisc,100% ¼ 

cdis,res,s1  1  f decay,s1

cdis,res,s2 cdis,res,s2   ¼ 1  f decay,total 1  f decay,s1  1  f decay,s2

ð3:52Þ ð3:52aÞ

RDisc, 100% ¼ dosing rate of disinfectant compound 100% [ mg/l] fdecay, s1 ¼ disinfectant residue decay factor at section 1 [–] fdecay, s2 ¼ disinfectant residue decay factor at section 2 [–] fdecay, total ¼ disinfectant residue decay factor of contact volume total [–] When designing a disinfection system, it must be dimensioned to cope with the most unfavourable disinfection conditions. For the calculation procedures described above to determine the dosing concentration of the disinfectant RDisc, 100%, its residual content cdis, res, si, and the associated inactivation ratio rinact, total, a CT value CTX  log inact, germ must be selected from the graphs in Fig. 3.21a–c at the lowest temperatures of the SWRO product water and of the drinking water in the water distribution system. The required residual content of disinfectant cdis, res, si + 1 in the final segment of the contact volume and thus also the dosing concentration RDisc, 100% to the contact volume are then defined by the degree of log inactivation Ilog, calc, total for viruses as specified for the first disinfection step in the SWRO product water and for the secondary disinfection of the drinking water in the water distribution system and the disinfectant residue decay factor fdecay, si in the segments i of the contact volume. At the same time, it must be ensured that the permissible values for the residual content of disinfectants in the drinking water as laid down in the drinking water guidelines applicable to the SWRO plant at the location will not be exceeded (Table 3.16). For the primary disinfection step, operating values for the excess of disinfectant are obtained when using chlorine and chlorine dioxide ranging, respectively, from 0.3 to 0.5 mg/l Cl2 and from 0.4 to 0.8 mg/l ClO2. For the secondary disinfection step, the excess values for chlorine at the drinking water extraction points are in the range of 0.1–0.5 mg/l Cl2, with the use of monochloramine as secondary disinfectant 0.5–2.0 mg/l NH2Cl, and for chlorine dioxide 0.2–0.8 mg/l ClO2, depending on the

344

3 Post-Treatment

location of the extraction point and the contact time in the water distribution system. A further critical factor for secondary disinfection is whether and to what extent the drinking water from the SWRO plant is blended in the distribution system with water from other sources, with this blended water then determining the selection and dimensioning of the secondary disinfection step. The results of a design calculation for primary and secondary disinfection of SWRO product water with chlorine as primary disinfectant and monochloramine for secondary disinfection are presented in Annex 3.A3. 3.4.4.2.2 Calculation of Effective Hourly Demand and Dosing Flow of Primary and Secondary Disinfectants When using chlorine as disinfectant, its effective hourly demand DCl2 ,eff can be calculated from the chlorine dosing rate RCl2 ,100% with Eq. (3.53). The design of a chlorine vacuum dosing unit, which is the type predominantly used for dosing chlorine gas, and the configuration for supply and storage of liquid chlorine are described under Sects. 2.2.1.3 and 2.2.1.3.1. For dimensioning a chlorine gas dosing unit, the equations required for calculating the dosing flow FD of the chlorine water generated at the dosing point (Eq. 2.26), the motive water flow Fmw for the vacuum system (Eq. 2.26a), and the power take-up of the motive water pump E D,Fmw (Eq. 2.27) are given in Sect. 2.2.1.4.2. DCl2 ,eff ¼

F Pr,SWRO  RCl2 ,100% 1000  f pur,Cl2

ð3:53Þ

DCl2 ,eff ¼ effective hourly demand of chlorine [kg/h] RCl2 ,100% ¼ dosing rate of chlorine 100% [mg/l] f pur,Cl2 ¼ purity factor of chlorine [–] If NaOCl solution is used for chlorine disinfection, the effective hourly demand for hypochlorite solution is calculated as shown by Eq. (3.53a). Since the chlorine content of the hypochlorite solution decreases the longer it is stored, the resulting reduction in the available chlorine content of the solution must be taken into account when specifying the free chlorine content. DNaOCl,sol,eff ¼

F Pr,SWRO  RCl2 ,100% ¼ F DNaOCl,sol cCl2,NaOClsol

ð3:53aÞ

DNaOCl, sol, eff ¼ effective hourly demand of sodium hypochlorite solution [l/h] cCl2,NaOCl,sol ¼ chlorine content of NaOCl solution [g Cl2/l] FDNaOCl, sol ¼ dosing flow of NaOCl solution [l/h] The dependence of the chlorine content of the solution on its temperature and storage time as well as the initial chlorine concentration of the solution are as shown

3.4 Post-Treatment Configuration and Treatment Systems

345

in Fig. 2.6 in Sects. 2.2.1.3 and 2.2.1.3.2. The available initial chlorine content of the NaOCl solution and its other physico-chemical data are summarized in Table 2.10 of this chapter. Also, a process description and a process flow scheme of a hypochlorite dosing system can be found. If the hypochlorite solution is not to be dosed at the delivery concentration but with a lower concentration, the required dilution water flow FDilw, i, design at dosing point i can be calculated from the dosing flow of the undiluted solution FDNaOCl, sol, its density ρNaOCl,del, the density of the dilution water ρDilw, and the ratio of the chlorine concentration cCl2,NaOCl of the undiluted and the dosing concentration cCl2,dos of the diluted solution with Eq. (2.29), and the resulting dosing flow rate at the dosing point can be calculated using Eq. (2.29b) in Sects. 2.2. 1.4.2 and 2.2.1.4.3. If chlorine dioxide is used as disinfectant, its hourly demand is determined according to Eq. (3.54). DClO2 ,100% ¼

FPr,SWRO  RClO2 ,100% 1000

ð3:54Þ

DClO2 ,100% ¼ hourly demand of ClO2 100% [kg/h] RClO2 ,100% ¼ dosing rate of chlorine dioxide 100% [mg/l] For industrial-scale drinking water treatment to generate ClO2, most commonly sodium chlorite NaClO2 is reacted with either hydrochloric acid HCl or chlorine Cl2 as shown in the equations below. 5NaClO2 þ 4HCl ! 4 ClO2 þ 5NaCl þ 2 H2 O 2NaClO2 þ Cl2 ! 2 ClO2 þ 2NaCl For the chemistry of chlorine dioxide disinfection, see also Sects. 2.2.1.1 and 2.2. 1.1.2. The effective hourly requirement of the respective reaction component NaClO2, HCl or Cl2 is derived from the chlorine dioxide dosing rate RClO2 ,100%, the flow to be disinfected FPr, SWRO, the purity factors of the reaction components fpur, and the efficiency ηClO2 process of the process in converting the components to ClO2, together with a stoichiometric factor fstoich specific to the combination of the reactants for the component in each case as shown in the following table, in accordance with Eq. (3.54a) for sodium chlorite, Eq. (3.54b) for chlorine, and Eqs. (3.54c) and (3.54d) for hydrochloric acid. The efficiency ηClO2 process of chlorine dioxide generation can be up to 80% for the NaClO2/HCl reaction and 90–95% for the NaClO2/Cl2 reaction, depending on the chosen reaction process. Main compound NaClO2

f stoich,NaClO2 1.676 1.341

Reaction compound HCl Cl2

fstoich, HCl 0.541 –

f stoich,Cl2 – 0.526

346

3 Post-Treatment

DNaClO2 ,eff ¼ DCl2 eff ¼

F Pr,SWRO  f stoich,NaClO2  RClO2 ,100% 1000  f pur,NaClO2  ηClO2 process

ð3:54aÞ

F Pr,SWRO  f stoich,Cl2  RClO2 ,100% 1000  f pur,Cl2  ηClO2 process

ð3:54bÞ

DHCl,100% ¼

F Pr,SWRO  f stoich,HCl  RClO2 ,100% 1000  ηClO2 process

F DHCl,sol X% ¼

DHCl,eff  100 %HCl,sol  ρHCl,sol

ð3:54cÞ ð3:54dÞ

DNaClO2 ,eff ¼ effective hourly demand of NaClO2 [kg/h] f stoich,NaClO2 ¼ stoichiometric factor NaClO2/ClO2 [–] f pur,NaClO2 ¼ purity factor of NaClO2 [–] ηClO2 process ¼ efficiency of ClO2 generation process [–] DCl2 eff ¼ effective hourly demand of Cl2 [kg/h] f stoich,Cl2 ¼ stoichiometric factor Cl2/ClO2 [–] DHCl, 100% ¼ effective hourly demand of HCl 100% [kg/h] fstoich, HCl ¼ stoichiometric factor HCl/ClO2 [–] FDHCl, sol X% ¼ dosing flow of hydrochloric acid of x% [l/h] %HCl, sol ¼ Percentage of hydrochloric acid [%] ρHCl, sol ¼ density of hydrochloric acid of x% [kg/l] If monochloramine is used for secondary disinfection of the drinking water generated by the SWRO plant, its hourly demand DNH2 Cl,100% is calculated from its dosing rate RNH2 Cl,100% and the flow of drinking water to be disinfected FDw in accordance with Eq. (3.55). DNH2 Cl,100% ¼

F Dw  RNH2 Cl,100% 1000

ð3:55Þ

FDw ¼ drinking water supply flow to water distribution system [m3/h] DNH2 Cl,100% ¼ hourly demand of NH2Cl 100% [kg/h] RNH2 Cl,100% ¼ dosing rate of NH2 Cl 100% [mg/l] Monochloramine can be obtained from ammonia NH3 and ammonium compounds, such as ammonium hydroxide NH4OH or ammonium sulphate (NH4)2SO4, by reaction with chlorine Cl2 or hypochlorite OCl, as shown by the following chemical reaction equations and the corresponding table. The table shows the stoichiometric factors f stoich,NH3 ,c for the ammonium components and the

3.4 Post-Treatment Configuration and Treatment Systems

347

stoichiometric factor f stoich,Cl2 ,c relevant for the combination with the respective chlorine component. NH3 þ Cl2 ! NH2 Cl þ HCl NH4 OH þ Cl2 ! NH2 Cl þ H2 O þ HCl ðNH4 Þ2 SO4 þ 2Cl2 ! 2NH2 Cl þ H2 SO4 þ 2HCl NH3 þ NaOCl ! NH2 Cl þ NaOH NH4 OH þ NaOCl ! NH2 Cl þ H2 O þ NaOH ðNH4 Þ2 SO4 þ 2NaOCl ! 2NH2 Cl þ Na2 SO4 þ 2H2 O NH3 compound NH3

f stoich,NH3 ,c 0.331

NH4OH

0.681

(NH4)2SO4

1.284

Cl2 compound Cl2 NaOCl Cl2 NaOCl Cl2 NaOCl

f stoich,Cl2 ,c 1.378 1.446 1.378 1.446 1.378 1.446

The factors f stoich,Cl2 ,c and f stoich,NH3 ,c for the ammonium and chlorine compounds correspond to the stoichiometric molar ratios of these two components in their conversion to monochloramine. If the reaction is to take place with an increased proportion of either the ammonium or chlorine component, the stoichiometric factor for the ammonium component f stoich,NH3 ,c must be increased for an ammonium surplus, and the factor for the chlorine component f stoich,Cl2 ,c must be increased accordingly for a chlorine surplus. From the drinking water flow FDw to be added to the water distribution network, the monochloramine dosing rate for secondary disinfection, the stoichiometric factor of the ammonium component f stoich,NH3 ,c , and the factor f pur,NH3 ,c for its purity, the effective hourly demand DNH3 ,c,eff of the ammonium compound can be calculated using Eq. (3.55a). The corresponding dosing flow F DNH3 ,c,sol for the ammonium component solution is then calculated from DNH3 ,c,eff , the concentration of the ammonium component %NH3 ,c,sol in the solution, and its density ρNH3 ,c,sol (Eq. 3.55b). In the same way, the corresponding effective hourly demand of the chlorine component DCl2 ,c,eff and its dosing flow F DCl2 ,c,sol can be calculated if, for example, it is reacted with the ammonium component as a hypochlorite solution (Eqs. 3.55c and 3.55d).

348

3 Post-Treatment

DNH3 ,c,eff ¼

F Dw  f stoich,NH3 ,c  RNH2 Cl,100% 1000  f pur,NH3 ,c

ð3:55aÞ

DNH3 ,c,eff  100 %NH3 ,c,sol  ρNH3 ,c,sol

ð3:55bÞ

F DNH3 ,c,sol ¼ DCl2 ,c,eff ¼

F Dw  f stoich,Cl2 ,c  RNH2 Cl,100% 1000  f pur,Cl2 ,c

ð3:55cÞ

DCl2 ,c,eff  100 %Cl2 ,c,sol  ρCl2 ,c,sol

ð3:55dÞ

F DCl2 ,c,sol ¼

DNH3 ,c,eff ¼ effective hourly demand of NH3 compound [kg/h] f stoich,NH3 ,c ¼ stoichiometric factor NH3 compound/NH2Cl [–] f pur,NH3 ,c ¼ purity factor of NH3 compound [–] F DNH3 ,c,sol ¼ dosing flow of NH3 compound solution [l/h] %NH3 ,c,sol ¼ percentage of NH3 compound in solution [%] DCl2 ,c,eff ¼ effective hourly demand of Cl2 compound [kg/h] f stoich,Cl2 ,c ¼ stoichiometric factor Cl2 compound/NH2Cl [–] f pur,Cl2 ,c ¼ purity factor of Cl2 compound [–] F DCl2 ,c,sol ¼ dosing flow of Cl2 compound solution [l/h] %Cl2 ,c,sol ¼ percentage of Cl2 compound in solution [%] ρCl2 ,c,sol ¼ density of solution of Cl2 compound [kg/l] The dosage of disinfectants for primary and secondary disinfection is proportional to the flow of SWRO product water to be disinfected for intermediate storage FPr, SWRO and the flow of drinking water supplied by the SWRO FDw to the water distribution system. The drinking water flow FDw to be supplied by the desalination plant can fluctuate considerably over the course of the day, with values that may be significantly below or significantly above the SWRO’s net design capacity over certain operating time intervals. This fluctuation range must be taken into account when designing the dosing system for secondary disinfection, i.e. it must be possible to adjust the available dosing flow to the minimum and maximum values of FDw. Monitoring of the residual content of disinfectant after the two disinfection steps or the dosing rate required for this is done by online measurement of this in combination with regular sampling and laboratory analysis of the water samples for checking and calibrating the online measuring systems. In addition, pH and temperature are measured online, since the effectiveness of the disinfectants as described above but also the measuring characteristics of these online detectors are influenced by both parameters. The configuration of the online measuring systems depends on the type of disinfectants used in the two disinfection steps. If chlorine is used, the contents of total chlorine and free chlorine can be determined either amperometrically or colorimetrically using the DPD (N,N-diethyl-p-phenylenediamine) method. The

3.5 Power Demand of Post-Treatment Systems and Drinking Water Supply

349

excess of chlorine dioxide can also be monitored with special electrodes whose mode of operation is based on the amperometric principle. When using ClO2, however, the chlorite concentration has to be additionally monitored either by laboratory analysis or by means of special online systems. If monochloramine is used as a secondary disinfectant, the dosage rate and residual content of NH2Cl as well as any excess ammonium can be determined colorimetrically according to the phenate-indophenol method. In this case, how the pH value varies in the drinking water distribution system must also be documented, as it will decrease if nitrification occurs. Measuring instruments that work on the principle of UV absorption spectrophotometry, i.e. which measure UV absorption at wavelengths specific to the respective compounds, are also employed as online measuring systems, particularly for monitoring chloramination.

3.5

Power Demand of Post-Treatment Systems and Drinking Water Supply

3.5.1

Post-Treatment Power Demand

The power demand PD, posttr of an SWRO plant’s post-treatment stage is made up of the power demands of its individual systems, i.e. post-desalination PD, postdes, remineralization/alkalinization PD, remin, conditioning PD, cond, and the total power demand PD, disinf for primary and secondary disinfection (Eq. 3.56). PD,posttr ¼ PD,postdes þ PD,remin þ PD,cond þ PD,disinf

ð3:56Þ

PD, posttr ¼ power demand post-treatment [kW] PD, postdes ¼ power demand post-desalination (second pass RO) [kW] PD, remin ¼ power demand remineralization/alkalinization [kW] PD, cond ¼ power demand conditioning [kW] PD, disinf ¼ power demand primary and secondary disinfection [kW]

3.5.1.1 Post-Desalination Power Demand The major part of the power demand for post-desalination PD, postdes is accounted for by the feed pumps, which generate the pressure to feed the RO unit. It is calculated from the feed flow to the membrane system F F,RO2 , the required pressure head pf,RO2 , and the efficiency of the pumps ηPf and their drives ηM according to Eq. (3.57). PD,postdes ¼

F F,RO2  pf,RO2 36  ηPf  ηM

ð3:57Þ

350

3 Post-Treatment

FF, RO2 ¼ feed flow to post-desalination (RO2) [m3/h] pf,RO2 ¼ feed pressure RO second pass [bar] ηPf ¼ efficiency factor of feed pump [–] ηM ¼ efficiency factor of motor of feed pump [–] The feed flow F F,RO2 depends on the recovery rate Y RO2 at which the membrane system is operated (Eq. 3.57a) and on the proportion of product water from the seawater desalination pass RO1 that is directed through the second RO pass according to the capacity factor f C,RO2 for post-desalination (Eq. 3.57b) (see Sect. 3.4.1, Eq. 3.15, Fig. 3.4). F F,RO2 ¼

F Pr,RO2 Y RO2

F Pr,RO2 ¼ f C,RO2  F Pr,M,RO

ð3:57aÞ ð3:57bÞ

FPr, RO2 ¼ product flow of RO2 [m3/h] YRO2 ¼ recovery coefficient of RO2 [–] f C,RO2 ¼ capacity factor post-desalination (RO second pass) [–] FPr, M, RO ¼ RO systems mixed product flow to remineralization [m3/h] Equation (3.57c), which shows the dependence of the feed flow FF, RO2 to the second RO pass on the above operating conditions derives from Eqs. (3.57a) and (3.57b). If the internal water demand of the post-treatment steps following postdesalination is neglected, Eq. (3.57c) shows the direct dependence of the feed flow to post-desalination FF, RO2 on the total product flow FPr, SWRO of the SWRO plant, the capacity factor f C,RO2 , and the recovery rate YRO2 at which the second RO pass is operated. F F,RO2 ¼

f C,RO2  F Pr,M,RO f  F Pr,SWRO ffi C,RO2 Y RO2 Y RO2

ð3:57cÞ

In accordance with Eqs. (3.57a) and (3.57c), the power demand for desalination is then calculated as shown in Eq. (3.57d). PD,postdes ¼

f C,RO2  F Pr,M,RO  pf,RO2 f  F Pr,SWRO  pf,RO2 ffi C,RO2 Y RO2  36  ηPf  ηM Y RO2  36  ηPf  ηM

ð3:57dÞ

In addition to the dependencies of the power demand PD, postdes for postdesalination as shown in the above algorithms, further operating conditions influence this parameter. The feed pressure pf,RO2 required for post-desalination increases with the ageing of the reverse osmosis membranes. At the same time, it is also dependent on feed temperature, i.e. it decreases with increasing temperature and vice versa. The

3.5 Power Demand of Post-Treatment Systems and Drinking Water Supply

351

capacity factor f C,RO2 is also influenced by the membrane age of both the RO1 and the RO2 membranes. With increasing age, the membranes’ salt rejection capability declines (see Sect. 3.4.1, Fig. 3.5), which means that more product water from the seawater RO pass RO1 has to be passed through the post-desalination pass RO2. The temperature, too, influences the product quality of the two RO passes, which means that with increasing temperature salt rejection declines, while with decreasing temperature it increases with a corresponding influence on the capacity factor f C,RO2 of the post-desalination pass. The recovery rate Y RO2 of the RO2 post-desalination process itself likewise influences the capacity factor. As the recovery rate increases, the membranes’ salt rejection capability is reduced, and a higher capacity factor f C,RO2 is required to maintain the quality of the product water. When designing the post-desalination pass and calculating its maximum power demand PD, postdes, max, the worst-case conditions must therefore be selected, these being a specified maximum membrane age, the highest feed pressure to RO2, and the lowest salt rejection rates of RO1 and RO2, in line with the operating conditions.

3.5.1.2 Remineralization/Alkalinization Power Demand The power demand for remineralization is made up of the amount needed for the production or provision and dosing of carbon dioxide plus the amount needed to make the other reaction components of remineralization available, i.e. hydrated lime, limestone, or dolomite, via the respective process technology, and to react them with the CO2. 3.5.1.2.1 Power Demand for CO2 Supply If carbon dioxide is generated in situ at the post-treatment stage, the required power demand consists of the electrical power of the production unit for pumping cooling water, chemical solutions and the generated CO2 as well as for its liquefaction and evaporation, and the thermal power from the natural gas or oil required for combustion (see Sect. 3.4.2.1, Fig. 3.12). If commercially available liquid CO2 is used, e is consumed for its evaporation and possibly for additional cooling of the CO2 storage facility. In both applications, additional energy is required to generate a meterable CO2 solution in a part stream of the RO product water. By introducing the gaseous CO2 into this part stream, a CO2 dosing solution is generated, and this is then mixed with the main product water stream after its pressure has been increased as needed. If in a hybrid configuration consisting of a thermal seawater desalination process paired with an SWRO process, CO2 from the vent gases of the thermal plant are used for remineralization, for which purpose, after pre-cleaning in an adsorber system, they are similarly placed in contact with a part stream of the RO product water. This option also requires energy due to the need to increase the pressure of the CO2 dosing part stream.

352

3 Post-Treatment

3.5.1.2.2 Power Demand for the Lime, Limestone, and Dolomite Process Part In the hydrated lime/CO2 alkalinization process, energy is needed to produce milk of lime, transfer it to the lime saturator and operate it, and then dose the lime water into the RO product water. Additional energy is required to transport the lime sludge from the lime saturator to the SWRO wastewater treatment plant and dewater it there (see Sect. 3.4.2.2.1, Fig. 3.13). For filter processes with limestone and dolomite (see Sect. 3.4.2.2.2, Fig. 3.16), the total power demand is made up of the shares needed: • To keep the filters at their operating pressure • To replenish the used filter material by refilling the filters • To backwash the filters and transport the backwash wastewater to the SWRO wastewater treatment plant • To operate the CO2 degasifiers • For dosing the NaOH • For conveyance of the treated product water to the intermediate storage tanks If the filtration process also incorporates recovery and recycling of the filter backwash and rinse water, additional energy is required to operate these sedimentation and filtration facilities and, in this case, to transport the sludge thus generated to the SWRO wastewater treatment plant for dewatering. With the limestone/H2SO4 process, no energy is needed for CO2 supply and dosing. Rather, energy is required for dosing sulphuric acid and the power demand when filling the limestone filters and for filter backwashing is greater than with the limestone/CO2 and dolomite/CO2 processes due to the higher limestone consumption of this type of alkalinization process.

3.5.1.3 Conditioning and Disinfection Power Demand 3.5.1.3.1 Power Demand of Conditioning The power demand PD, cond for the conditioning step of the post-treatment stage depends on the dosing equipment there in use (see Sect. 3.4.3). It is calculated as the sum of the power demands PD, cond, i of the dosing systems of the installed components as shown by Eq. (3.58). PD,cond ¼

i¼n X

PD,cond,i

ð3:58Þ

i¼1

PD, cond, i ¼ power demand of conditioning dosing compound i [kW] For a dosing component i, it comprises its dosing flow FD, dcomp, i, dos and the associated operating pressure pdp, i at its dosing point (Eqs. 3.58a and 3.58b).

3.5 Power Demand of Post-Treatment Systems and Drinking Water Supply

PD,dcomp:i ¼

F D,dcomp,i,dos  pdp,i 36  103  ηpdp,i  ηMdp,i

F D,dcomp,i,dos ¼

Ddcomp,i,eff  100 %dcomp,i,sol  ρdcomp,i,sol

353

ð3:58aÞ ð3:58bÞ

FD, dcomp, i, dos ¼ dosing flow of compound i solution [l/h] pdp, i ¼ pressure at dosing point of dosing compound i [bar] ηpdp,i ¼ efficiency of dosing pump of compound i [–] ηMdp,i ¼ efficiency of motor of dosing pump of compound i [–] %dcomp, i, sol ¼ percentage of compound i in solution [%] ρdcomp, i, sol ¼ density of solution of compound i [kg/l] The dosing flow FD, dcomp, i, dos is based either on a solution of the dosing product at its delivery concentration or a preparation concentration %dcomp, i, sol or, after dilution, on a lower dosing concentration %dcomp, i, dos with a defined diluting water flow FD, dcomp, i, dos, dil. The dosing flow after adding diluting water is then calculated with Eq. (3.58c).  

ρdcomp,i,sol %dcomp,i,sol  1 F D,dcomp,i,dos,dil ¼ F D,dcomp,i,dos  1 þ ρDilw %dcomp,i,dos

ð3:58cÞ

FD, dcomp, i, dos, dil ¼ dosing flow of compound i solution after dilution [l/h] ρDilw ¼ density of dilution water [kg/l] %dcomp, i, dos ¼ dosing concentration of compound i [%] 3.5.1.3.2 Power Demand of Disinfection The total power demand PD, disinf of the disinfection step consists of the power demands of the primary PD, disinf1 and of the secondary disinfection PD, disinf2 (Eq. 3.59). PD,disinf ¼ PD,disinf1 þ PD,disinf2

ð3:59Þ

PD, disinf1 ¼ power demand of primary disinfection [kW] PD, disinf2 ¼ power demand of secondary disinfection [kW] PD, disinf1 and PD, disinf2 are calculated analogously as shown by Eq. (3.58a) with the dosing flow FD, dcomp, i, dos and the pressure pdp, i at the dosing point for the disinfectant concerned. The effective hourly consumption of the disinfectant Ddcomp, i, eff that has to be known for determining the dosing flow FD, dcomp, i, dos can be determined for chlorine and hypochlorite solution as well as, when using chlorine

354

3 Post-Treatment

dioxide and monochloramine, for their reaction components as described in Sect. 3.4.4.2.2 (Eqs. 3.53–3.55d). Should the rate of take-up FDw by the water distribution system of the water supplied from the SWRO plant greatly fluctuate, it must be possible to control the dosing flow FD, dcomp, i, dos of the dosing system for secondary disinfection that sufficient disinfectant will always be injected for hygienization at the minimum and the maximum water flows.

3.5.2

Design and Power Demand of Product Water Supply

The product water supply pumping system of an SWRO plant is normally equipped with several centrifugal pumps with drives fitted with infinitely variable speed control systems. By varying the number of pumps in operation and controlling their delivery by adjusting their speed, the supply system is able to respond rapidly to large swings in the water supply flow to the water distribution system due to fluctuations in consumption while minimizing energy consumption. The design of the supply pumps must allow for worst-case scenarios in which the delivery flow either exceeds or falls short of the SWRO’s design product flow by a certain margin. For details on the operation and efficiency of speed-controlled centrifugal pumps, see Sect. 5.5.2.2 in [1]. The equations given there for calculating the power demand of centrifugal pumps as a function of speed, the influence of speed control on pump efficiency, details of the type of infinitely variable speed control, and calculation of the necessary NPSH value not only are applicable to the high-pressure RO pumps described there but also apply in general to the use of speed-controlled centrifugal pumps for fresh water delivery and thus also to the pumps in the water supply system of an SWRO plant. Supply pumping stations, especially in larger desalination plants, are often equipped with hydro-pneumatic pressure vessels to serve as surge tanks on the pressure side of the supply pumps, which: • Buffer the water supply flow when the SWRO plant delivers water at a very low rate • Even out the operating pressures across the pumps • Cushion pressure surges arising at start-up and shutdown of the pumps or during pump trips due to power failure • Damp out water hammer arising within the water distribution network Such a surge protection system may also prolong the service life of the entire installation by reducing the number of switching cycles of the supply pumps. The surge tanks are equipped with air compression equipment that generates and maintains the required proportion of compressed air volume within them. The design of the air compression system follows an analysis of the hydraulic system in which possible variations in flow and pressure are simulated with software tools.

3.5 Power Demand of Post-Treatment Systems and Drinking Water Supply

355

The product water supply pumping station also houses the service pumps that supply the SWRO plant’s own requirements with desalinated water and water from the post-treatment plant. The power demand PD, Dws for supplying the water consumers with the desalinated and treated water generated in the SWRO plant is calculated from the water supply flow FDw required by the consumers, the operating pressure pDwp needed to convey the water into the water distribution system, and the efficiency of the pumping installation, i.e. the efficiency of its pumps ηP and their drives ηM and ηVSD (Eq. 3.60). The operating pressure required for the feed pumps pDwp is calculated from the pressure at which the water distribution system is operated, the pressure loss ΔHsupplyl of the pipeline from the SWRO to the water distribution system, and the geodetic height difference ΔHgeo between the supply pumping station of the SWRO plant and the highest geodetic point in the hydraulic profile of the SWRO pumping station and the water distribution system (Eq. 3.60a). F Dw  pDwp 36  ηP  ηM  ηVSD   ΔH supplyl þ ΔH geo  ρDw  g ¼ pDistrs þ 102 PD,Dws ¼

pDwp

ð3:60Þ ð3:60aÞ

PD, Dws ¼ power demand water supply pumping [kW] FDw ¼ drinking water supply flow to water distribution system [m3/h] pDwp ¼ operating pressure of drinking water supply pumps [bar] ηP ¼ efficiency factor of drinking water supply pumps [–] ηM ¼ efficiency factor of motor of drinking water pumps [–] ηVSD ¼ efficiency factor of motor of variable speed drive [–] pDistrs ¼ operating pressure in water distribution system [bar] ΔHsupplyl ¼ head loss in supply pipe to water distribution system [m] ΔHgeo ¼ height difference between level of supply pumps and water distribution system according to hydraulic profile [m] ρDw ¼ density of SWRO product water [kg/l] g ¼ standard gravity acceleration ¼ 9.80665 [m/s2] The power take-up of the feed pumps is most when they are operated at their maximum delivery FDw, max and highest pressure pDwp, max, and it is least when the proportion of the SWRO plant’s design flow FDw, min piped to the water distribution system is at a minimum. The effective power demand of the water supply pump station during a certain operating time is between these two limits. The average power demand PD, Dws, ∅ of the pumping station over a prolonged operating period τop is therefore calculated as shown by Eq. (3.60b) from the sum of the effective power demand values PD,Dws,eff,Δτop over specific periods of operation Δτop, eff under the operating conditions during these intervals.

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3 Post-Treatment

P PD,Dws,∅ ¼

PD,Dws,eff,Δτop  Δτop,eff τop

ð3:60bÞ

PD, Dws, ∅ ¼ average power demand water supply pumping [kW] PD,Dws,eff,Δτop ¼ effective power demand drinking water supply pumping during Δτop, eff [kW] Δτop, eff ¼ operation time interval with effective operating conditions [h] τop ¼ prolonged operation period of water supply pumping system [h]

Annexes 3.A1 Post-Treatment: Remineralization/Alkalinization—Lime/CO2 Process Design

Annexes

357

RO Product flow rate Temperature max. of process water TDS target in SWRO product Alkalinity target in SWRO product Alkalinity in RO product Target of Alkalinity increase

4,166.7 25.0 150 1.5 0.0090 1.4910

m3/h °C mg/l mmol/l mmol/l mmol/l

CO2 concentration in RO product water

1.2 0.0273

mg/l mmol/l

CO2 concentration in SWRO product water

0.790 0.0045 0.98 0.95 0.85

mg/l mmol/l [-] [-] [-]

Carbondioxide purity factor Calcium hydroxide purity factor Calcium hydroxide reactivity factor Solubility of Ca(OH)2 Solids concentration of lime milk Solids concentration of saturator feed Number of saturators

1.59 50 2.0 2

g Ca(OH)2/l g /l g /l No.

Surface loading rate saturator Sedimentation/reaction zone ratio Solids content of saturator sludge

1.5 0.1 6

m3/m2*h [-] %

CO2 stoichiometric specific demand

65.8

mg/l

CO2 effective demand

67.1

mg/l

CO2 effective hourly demand

279.8

kg/h

Ca(OH)2 stoichiometric specific demand

55.3

mg/l

Ca(OH)2 stoichiometric hourly demand

230.5

kg/h

Ca(OH)2 effective demand

71.3

mg/l

Process water hourly Ca(OH)2 demand

0.4

kg/h

Ca(OH)2 effective hourly demand

285.8

kg/h

Lime water flow rate max

145.0

m3/h

Lime milk flow rate max

5.7

m3/h

Prozesswater flow rate max

144.9

m3/h

Saturator system feed rate

145.2

m3/h

Saturator units feed rate Saturator inner area Saturator diameter

72.6 53.2 8

m3/h m2 m

Specific surface load of saturator Solids amount formed in process water Solids amount produced in saturators Sludge amount produced in saturators

2.2 0.5 55.9 931.3

kg Ca(OH)2 /m2 kg/h kg/h kg/h

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3 Post-Treatment

3.A2 Post-Treatment: Remineralization/Alkalinization— Limestone/CO2 Process Design m3/h

RO Product flow rate Temperature max. of process water TDS target in SWRO product Alkalinity target in SWRO product

4,166.7 25.0 150 1.5

°C mg/l mmol/l

Alkalinity in RO product Target of Alkalinity increase total

0.0090

mmol/l

1.49

mmol/l

Alkalinity increase from NaOH dosing Alkalinity increase with limestone Alkalinity increase in filter part Capacity factor filter part

0.12

mmol/l

1.37

mmol/l

2.74

mmol/l

0.50

[-]

2,083.4

m3/h

2,083.4 By-pass capacity CO2, limestone and NaOH demand 0.98 Carbondioxide purity factor 0.90 Carbondioxide adsorption rate factor 125.7 Stoichiometric CO2 demand

m3/h

Filtration plant treatment capacity

Effective CO2 demand Limestone purity factor Limestone backwash loss factor Stoichiometric CaCO3 demand

218.5 0.98 0.10 286.2

[-] [-] kg 100%/h kg X %/h [-] kg 100% /h

321.2 kg X%/h Effective CaCO3 demand with BW losses 0.45 [-] NaOH purity factor 5.2 mg 100%/l NaOH dosing amount calculated 21.7 kg 100% /h NaOH demand calculated 48.1 kg X%/h Effective NaOH demand Empty bed contact time (EBCT) calculation 2.0 mm Limestone particle effective diameter Material density limestone

2,700

kg/m3

Bulk density limestone Porosity filter bed

1,400

kg/m3

0.48

[-]

Yamauchi coefficient

8.229E-03 mm/sec

Reactivity coefficient reaction

2.133E-03 sec-1

CO2 concentration filter feed

92.5

mg/l

CO2 equilibrium concentration filtrate

9.1

mg/l

CO2 concentration filtrate EBCT calculated

32.2

mg/l

10.0

min

Conversion rate of filter bed

72.3

%

Annexes

359

Filter unit design Number of filters in operation Height of filter bed full EBCT selected

8.00 3.00 10

No. m min

Feed flow per filter unit Filtration velocity

260.4

m3/h

18.0

m/h

Filtration surface area per filter Filter inner diameter (vertical filter)

14.5

m2

4.3

m

Limestone volume of filter bed full Limestone mass of filter bed full

43.4

m3

60,764

kg

Filter refilling and backwash Filter bed volume change factor

0.10

[-]

Mass loss during operation cycle Operation cycle duration Mass for refilling per unit Refilling and backwash time

6,076

kg

166.5 6,684

h kg

- Filter shut - down & drain down - Limestone filling - Air / water backwash - Water backwash

5 20 5 10

min. min. min. min.

- Start - up / Infiltration

10

min.

50 0.047

min No/h

60 12

m/h m/h

- Water backwash

20

m/h

- Infiltration

18

m/h

868

Nm3/h

- Air / water backwash

174

m3/h

- Water backwash

289

m3/h

- Infiltration flow

260

m3/h

- Backwash at every refilling

2.95

m3/h

- Infiltration at every refilling

2.04

m3/h

4.99

m3/h

Average wastewater discharge

2.95

m3/h

Average solids discharge

29.2

kg/h

Total Average number of refilling & backwash Backwash velocities - Air / water backwash - Air - Water

Backwash pump and blower flow - Air blower - Backwash water pump

Internal water demand

Total

360

3 Post-Treatment

3.A3 Post-Treatment: Disinfection Process Design—Primary and Secondary Disinfection 4,167 SWRO product flow rate max. 20 Water temperature min. Primary disinfection Chlorine Type of disinfectant 3.0 CT value for virus inactivation 4 at log-inactivation 0.4 Disinfectant residue last segment Disinfection segments 1st segment Pipe Type of segment 1.5 Velocity 50 Length 1.0 Baffling factor 0.20 Decay factor of segment 0.56 Hydraulic detention time 0.56 Contact time Disinfectant residue of segment Calculated CT of segment Inactivation ratio of segment log-inactivation of segment

0.44 0.25 0.082 0.33

m3/h °C mg*min/l mg/l

m/s m min min mg/l mg*min/l -

2nd segment Type of segment Volume Baffling factor Decay factor of segment Hydraulic detention time

Storage tank 5,000 0.3 0.10 72.0 1.2 21.6 0.40 8.6 2.88 11.5

m3 min h min mg/l mg*min/l -

0.56 0.6

mg/l mg/l

Contact time Disinfectant residue of segment Calculated CT of segment Inactivation ratio of segment log-inactivation of segment Contact volume total 2.96 Inactivation ratio total 11.8 log-inactivation total 0.28 Decay factor total Dosing rate disinfectant • calculated • selected

-

References

361

Type of disinfectant chemical Purity factor Effective hourly demand

Chlorine 0.98 2.55

kg/h

Secondary disinfection 15 Water temperature min. Chloramine Type of disinfectant 712 CT value for virus inactivation 3 at log-inactivation Disinfectant residue distribution 1 system Pipe Type of contact volume 0.1 Baffling factor 0.3 Decay factor distribution system 712 Necessary contact time 11.9

min h

Dosing rate disinfectant • calculated • selected

1.43 1.50 6.25

mg/l mg/l

7.09

kg/h

25

% kg/l l/h

Hourly demand NH2Cl Type of chemicals → NH2Cl • Chlorine • Effective hourly demand • Ammonium hydroxide solution • Percentage of NH4OH • Density of solution • Effective hourly demand

0.907 18.8

°C mg*min/l mg/l -

kg/h 100%

= Input parameter¼ Input parameter

References 1. Ludwig, H., Reverse Osmosis Seawater Desalination, Volume 1, Springer, 2022. 2. Ayers R.S., Westcot D.W., Water quality for agriculture - FAO Irrigation and drainage paper 29 Rev. 1, Rome: Food and Agriculture Organization of the United Nations FAO, 1985. 3. Plummer N.L., Busenberg E., “The solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90
C, and an evaluation of the aqueous model for the system CaCO3CO2-H2O,” Geochimics et Cosmochimica Acta, vol. 46, pp. 1011–1040, 1982. 4. Harned H.S., Owen B.B., The Physical Chemistry of Electrolytic Solutions, 3rd edition, New York: Reinhold Publishing Corp., 1958.

362

3 Post-Treatment

5. Joint Task Group on Calcium Carbonate Saturation, “Suggested Methods for Calculating and Interpreting Calcium Carbonate Saturation Indexes,” Journal AWWA (American Water Works Association), vol. 82, no. 7, pp. 71–77, 1990. 6. de Moel P.J., van der Helm A.W.C., van Rijn M., van Dijk J.C., van der Meer W.G.J., “Assessment of calculation methods for calcium carbonate saturation in drinking for DIN 38404-10 compliance,” Drinking Water Engineering and Science, vol. 6, no. 2, pp. 115–124, 2013. 7. Ryznar J.W., “A New Index for Determining Amount of Calcium Carbonate Scale Formed by a Water,” Journal American Water Works Association, vol. 36, no. 4, pp. 472–486, 1944. 8. Puckorius P.R., Brooke J.M., “A New Practical Index for Calcium Carbonate Scale Prediction in Cooling Tower Systems,” CORROSION, vol. 47, no. 4, pp. 280–284, 1991. 9. Larson J.E., Skold R.V., “Laboratory Studies Relating Mineral Quality of Water to Corrosion of Steel and Cast Iron,” CORROSION, vol. 14, no. 6, pp. 285–288, 1958. 10. Gorenflo A., Brusilovsky M., Faigon M., Liberman B., “High pH operation in seawater reverse osmosis permeate: First results from the world’s largest SWRO plant in Ashkelon,” Desalination, vol. 203, pp. 82–90, 2007. 11. Molina V.G., Taub M., Yohay L., Busch M., “Long term membrane process and performance in Ashkelon Seawater Reverse Osmosis Desalination plant,” Desalination and Water Treatmernt, vol. 31, pp. 115–120, 2011. 12. I.D.E Technologies Ltd, Liberman B., Liberman I., “US Patent 7,097,769 B2 - METHOD OF BORON REMOVAL IN PRESENCE OF MAGNESIUM IONS,” 2006. 13. I.D.E Technologies Ltd, Liberman B., Liberman I., “European Patent EP 1 363 856 B1 METHOD OF BORON REMOVAL IN PRESENCE OF MAGNESIUM IONS,” 2006. 14. MEDRC Series of R & D Reports - MEDRC Project: 12-CoE-007- Ouda A.S., “Investigate the Boron Content Reduction from Seawater Reverse Osmosis Permeate by Ion Exchange method (Case Study: SWRO Desalination Plant of Gaza Power Station),” The Middle East Desalination Research Center, Muscat, Sultanate of Oman, 2014. 15. Nadav N., “Boron removal from seawater reverse osmosis permeate utilizing selective ion exchange resin,” Desalination, vol. 124, pp. 131–135, 1999. 16. Ludwig H., Hetschel M., “Treatment of Distillates and Permeates from Seawater Desalination Plants,” Desalination, vol. 58, pp. 135–154, 1986. 17. Glade H., Meyer J.,Will S., “The release of CO2 in MSF and ME distillers and its use for the recarbonation if the distillate: a comparison,” Desalination, vol. 182, pp. 99–110, 2005. 18. Hasson D., Bendrihem O., “ Modeling demineralization of desalinated water by limestone dissolution,” Desalination, vol. 190, pp. 189–200, 2006. 19. Baldauf G., Henkel M., “Möglichkeiten und Grenzen der Marmorentsäuerung vor dem Hintergrund der neuen Trinkwasserverordnung,” GWF Wasser - Abwasser, vol. 132, no. 3, pp. 132–140, 1991. 20. Yamauchi Y., Tanaka K., Hattori K., Kondo M., Ukawa N., “REMINERALIZATION OF DESALINATION WATER BY LIMESTONE DISSOLUTION FILTER,” Desalination, vol. 66, pp. 365–383, 1987. 21. Shemer H., Hasson D., Semiat R., Priel M., Nadav N., Shulman A., Gelman E., “Remineralization of desalinated water by limestone dissolution with carbon dioxide,” Desalination and Water Treatment , vol. 51, pp. 877–881, 2013. 22. Ludwig H., “Post-treatment and Potabilisation - A Quality Determining Factor in the Generation of Drinking Water by Means of Sea Water Desalination,” in International Desalination Assoziation - Congress on Desalination and Water Reuse , Madrid, Spain, 1997. 23. Lehmann O., Birnhack L., Lahav O., “Design aspects of calcite-dissolution reactors applied for post treatment of desalinated water,” Desalination, vol. 314, pp. 1–9, 2013. 24. Wiegleb K., “Planungskriterien zur Enteisenung und Entsäuerung durch Filtration über halbgebrannte Dolomite,” gwf Wasser - Abwasser, vol. 142, no. 6, pp. 417–422, 2001. 25. Cotruvo J., Bartram J., eds., “Calcium and Magnesium in Drinking-water : Public health significance,” World Health Organization WHO, Geneva, 2009.

References

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26. “Safe Drinking-water from Desalination - WHO/HSE/WSH/11.03,” World Health Organisation WHO, Geneva, 2011. 27. Hasson D., Semiat R., Shemer H., Priel M., Nadav N., “Simple process for hardening desalinated water with Mg2+ ions,” Desalination and Water Treatment, vol. 51, pp. 924–929, 2013. 28. Birnhack L., Lahav O., “A new post-treatment process for attaining Ca2+, Mg2+, SO42- and alkalinity criteria in desalinated water,” Water Research, vol. 41, pp. 3989–3997, 2007. 29. Birnhack L., Oren S., Lehmann O., Lahav O., “Development of an additional step to current CO2-based CaCO3 dissolution post-treatment processes for cost-effective Mg2+ supply to desalinated water,” Chemical Engineering Journal, vol. 160, pp. 48–56, 2010. 30. Birnhack L., Penn R., Oren S., Lehmann O., Lahav O., “Pilot scale evaluation of a novel posttreatment process for desalinated water,” Desalination and Water Treatment, vol. 13, pp. 128–136, 2010. 31. Fawell J., Bailey K., Chilton J., Dahi E., Fewtrell L., Magara Y., “Fluoride in Drinking-water,” World Health Organization WHO / IWA Publishing, 2006. 32. American Water Works Association AWWA, Water Fluoridation - Principles & Practices Manual of Water Supply Practices - M4, Denver, 2016. 33. Kim D., Amy G.L., Karanfil T., “Disinfection by-product formation during seawater desalination: A review,” Water Research, vol. 81, pp. 343–355, 2015. 34. U.S. Environmental Protection Agency, Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems using Surface Water Sources, Washington, USA: U.S. Environmental Protection Agency, 1991. 35. U.S. Environmental Protection Agency, „LT1ESWTR Disinfection Profiling and Benchmarking - Technical Guidance Manual,“ U.S. Environmental Protection Agency - Office of Water, Washington USA, 2003.

4

Seawater Extraction and Supply and Concentrate Discharge

The seawater required to produce a specified output of desalinated product water by a membrane desalination plant is drawn from the sea by an intake and supply system and piped to the plant. The concentrate produced during the desalination process in the SWRO plant as well as the wastewater that is also generated is returned to the sea via an outfall system. In order to avoid as far as possible any influence on the extracted seawater by the elevated salinity of the plant discharge and other substances contained in it, the plant’s seawater extraction point and the outfall discharge point must be located at a sufficient distance from each other, taking into account the conditions regarding tides and currents in the extraction and discharge locations.

4.1

Seawater Extraction and Supply

A system for supplying a seawater desalination plant with the seawater feed flow required for its operation consists essentially of the components for extracting and transporting the seawater and, if necessary, equipment for coarse screening of the water to remove particulate impurities. The flow rate for which the respective components have to be dimensioned depends on the type and configuration of the extraction system itself together with the configuration of the pretreatment and reverse osmosis systems of the SWRO plant as well as their design and operating conditions. The total withdrawal flow of the intake system FW,SW,T is made up of the feed flow FF,SWRO to the SWRO plant and the seawater flow FF,Int,Scr required for the intake system consumers (Eq. 4.1). F W,SW,T ¼ F F,SWRO þ F F,Int,Scr

# The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 H. Ludwig, Reverse Osmosis Seawater Desalination Volume 2, https://doi.org/10.1007/978-3-030-81927-9_4

ð4:1Þ

365

366

4

Seawater Extraction and Supply and Concentrate Discharge

FW, SW, T ¼ total seawater withdrawal flow [m3/h] FF, SWRO ¼ feed flow to SWRO plant [m3/h] FF, Int, Scr ¼ feed flow for consumers in intake and screening systems [m3/h] The seawater intake consumers are: • Equipment for dosing disinfectants such as chlorine or hypochlorite and the motive water or dilution water flow required for this. • Electrochlorination equipment if this is used to generate hypochlorite solution from seawater and the seawater flow required for the electrolysis itself plus the filter system for treating the seawater feed to prepare it for electrolysis. • Spray water for the fine screening equipment to dislodge the caked on filtered solids. In accordance with Eq. 4.1a, the feed flow FF,SWRO to the SWRO plant is calculated as the quotient of the SWRO plant’s product flow FPro,SWRO and its total product recovery factor YT,SWRO plus the seawater feed flow FF,rip required for infiltration of the filter units of the pretreatment filtration systems, comprising gravel or stratified-bed filters or applying membrane filtration. F F,SWRO ¼

F Pro,SWRO þ F F,rip Y T,SWRO

ð4:1aÞ

FPro, SWRO ¼ product flow from the SWRO plant, including RO internal consumption [m3/h] YT, SWRO ¼ product recovery factor total of SWRO plant [] FF, rip ¼ infiltration (ripening) feed flow of pretreatment filtration systems [m3/h] The SWRO plant’s total product recovery factor YT,SWRO is calculated by multiplying the product recovery factor of its reverse osmosis systems Y ROsystem by the total product recovery factor of the pretreatment stage YP, Pr ,T. YP, Pr ,T in turn is obtained by multiplying the product recovery factors YP, Pr, i of the individual treatment processes that make up the pretreatment stage (Eq. 4.1b) (see Sect. 2.3. 2.3, Eqs. 2.67e to 2.67g; Sect. 2.3.3.3, Eqs. 2.76d to 2.76e; Sect. 2.3.4.3, Eqs. 2.87 to 2.87a, Eqs. 2.94 to 2.94d; Sect. 2.3.5.3, Eqs. 2.127 to 2.127a). Y T,SWRO ¼ Y ROsystem  Y P,Pr,T ¼ Y ROsystem 

i¼n Y

Y P,Pr,i

i¼1

Y ROsystem ¼ product recovery factor of RO systems of the SWRO plant [] YP, Pr , T ¼ product recovery factor of the pretreatment stage [] YP, Pr , i ¼ product recovery factor of individual pretreatment processes []

ð4:1bÞ

4.1 Seawater Extraction and Supply

367

If the SWRO plant’s reverse osmosis systems are operated only with the seawater desalination stage, i.e. in the single-pass mode, then for the product recovery factor Y ROsystem of the reverse osmosis systems Y ROsystem ¼ Y RO1 applies, and when operated with a post-desalination stage in the second pass mode Y ROsystem ¼ Y RO1,RO2 or Y ROsystem ¼ Y RO1,RO2,R applies. The value of the product recovery factor YRO1, RO2 of the reverse osmosis tract depends on the product recovery YRO1 of the RO1 seawater desalination unit and the product recovery factor YRO2 of the RO2 post-desalination unit as well as the capacity factor fC, RO2, i.e. the fraction of the product water of RO1 which is passed through the RO2 desalination stage (Eq. 4.1c). Y RO1,RO2 ¼

Y RO1 1  f C,RO2 þ

f C,RO2 Y RO2

ð4:1cÞ

YRO1 ¼ product recovery factor of RO1 [] YRO2 ¼ product recovery factor of RO2 [] fC, RO2 ¼ capacity factor of RO2 (RO second pass) [] Further, the product recovery factor YRO1, RO2 of the SWRO plant’s reverse osmosis systems is also determined by whether the concentrate from the RO2 post-desalination process is recirculated into the feed pipe of the RO1 seawater desalination stage. For this operating mode, which is common for a seawater desalination system with permeate gradation, the value of YRO1, RO2, R is calculated as shown in Eq. 4.1d (see [1] Sects. 4.2.1.4.1 and [1] 4.2.1.4.2). Y RO1,RO2,R ¼

1  f C,RO2 þ

f C,RO2 Y RO2

Y RO1  ð1  Y RO1 þ Y RO1  Y RO2 Þ

ð4:1dÞ

YRO1, RO2, R ¼ product recovery factor of RO systems with RO2 concentrate recycle [] Equation 4.1e is derived from Eqs. 4.1a and 4.1b. This is used to calculate the total seawater extraction flow FW,SW,T from the above parameters. F W,SW,T ¼

F Pr,SWRO þ F F,rip þ F F,Int,Scr iQ ¼n Y ROsystem  Y P,Pr,i

ð4:1eÞ

i¼1

For the design of an intake system, the operating parameters described above must be assigned values corresponding to a maximum permissible worst-case scenario for the withdrawal system itself as well as for the downstream SWRO plant and its operation under these conditions.

368

4

Seawater Extraction and Supply and Concentrate Discharge

Fig. 4.1 Overview of seawater intake system types

For the SWRO plant’s total product recovery factor YT,SWRO, this means: • The operation of the pretreatment stage should its own water demand increase and thus the product recovery factor YP, Pr ,i of its process components decrease, that is, if there is a rise in the concentration of solids from the seawater and from the addition of flocculants, in which case the filtration units of the pretreatment stage would have to be backflushed more frequently, so that the seawater inlet flow FF, rip for infiltration also increases. • Operation of the RO units at the lowest permissible value of the product recovery factor YRO1,RO2 of RO1 and RO2 and the highest capacity coefficient fC,RO2 for RO2. As the quality of the seawater feed drops, i.e. the concentration of solids increases, there is a rise in the amount of disinfectant that has to be dosed into the intake flow and thus of seawater consumption to meet the needs of the associated systems, namely, the dosing equipment or seawater electrolysis plant. The need for spray water for the fine screening equipment also increases due to the greater amount of solids to be filtered, so the total seawater requirement FF,Int,Scr of the intake and its components likewise rises. Seawater extraction as a subcomponent of an intake system can be implemented in a variety of ways, and an overview of these options is given in Fig. 4.1. The main distinction is made between surface water (direct) intake and its extraction after it has flowed through soil structures in the seabed or shore area, termed indirect (soil filtration) intake. Direct intake is further broken down into systems in which seawater is taken from a channel or lagoon at the sea surface—open intake channel/lagoon type—or from

4.1 Seawater Extraction and Supply

369

greater depths via submerged pipes or tunnels. Depending on the design of the screening systems for removing coarser solids from the extracted water within the intake, the intake designs are divided into active systems, comprising coarse and fine screens with a moving screen surface, and passive systems, in which screening units are installed at the extraction point in the seawater to screen and intercept solids. With indirect extraction, a basic distinction is made between options in which the seawater is extracted after passing through sea- and beach-side ground formations by means of various designs of wells arranged in the shore area, termed onshore intakes, and those types in which the seawater is preferably extracted in the inundated shore area after passing through the sea bed—offshore intakes. Because of the different options for indirect extraction with regard to the design of the beach wells and of the well galleries in the beach and sea zones as well as the technologies for offshore extraction, there are a large number of variants for this type of seawater extraction, and these are described in more detail below, together with the options for direct seawater extraction. The seawater intake is a key component of a reverse osmosis seawater desalination plant. The task of an intake system is to ensure the supply of seawater to an SWRO plant depending on its operating conditions while minimizing the ecological impact on the marine fauna and flora. This objective is to be achieved at reasonable cost for the construction of the facilities as well as the cost of their operation over the lifetime of the desalination plant. Further, the choice of the intake system design— open surface water intake, submerged pipe intake, or indirect intake—significantly influences the SWRO plant’s pretreatment process and its mode of operation. In order to identify the optimal solution for a site by comparing the technical and economic characteristics of the components of the extensive range of possible intake systems shown in Fig. 4.1, comprehensive information on the onshore and offshore conditions at the desalination plant’s site is needed, so surveys and investigations must be carried out to obtain this. Site-related information needed for the onshore zone is as follows: • Its topography. • Its geology and hydrogeology with pumping tests to determine hydraulic conductivity and the prospective well yield for evaluating indirect intake options. • Interactions between the seawater aquifer in coastal waters and fresh water aquifers more inland. • Urban, industrial, and agricultural activities in the vicinity of the plant site and in the coastal region. • Environmental requirements with regard to the protection of flora and fauna in the onshore part of the site. Information as follows has to be known for the offshore area: • Seabed bathymetry. • The coastal seabed geology with investigations of its hydraulic conductivity and pumping tests to evaluate offshore intake systems.

370

4

Seawater Extraction and Supply and Concentrate Discharge

• Marine fauna and flora at the projected seawater extraction location and environmental regulations for their protection, such as requirements for minimizing impingement and entrainment of marine organisms in seawater extraction systems, e.g. in accordance with the USEPA Clean Waters Act Rule 316(b). Impingement is when organisms that are larger than the bar spacing of a coarse screen are retained on the screen’s surface due to the seawater flow through it. Entrainment is when organisms pass through the intake’s raking and screening equipment and are killed by the mechanical action of the system’s piping, pumps and filters [2]. • Oceanographic conditions such as the timing and range of tides and currents as well as wave heights. • Meteorological conditions regarding temperature, rain, and wind. • Maritime activities in and around the planned extraction area. • Range of seawater quality including the frequency of algal and jellyfish blooms. • Urban and industrial wastewater discharges near the site. • River estuaries near the site and their impact on seawater quality.

4.1.1

Surface Water (Direct) Intakes

4.1.1.1 Open Intake Channel or Lagoon Type In this type of intake, seawater is drawn from intake structures that are arranged in the form of a lagoon near the shore or as a channel near the shoreline down to medium water depth. The seawater abstracted in this way is first passed through coarse screens, also referred to as trashracks, in the raking and screening section of the intake, to remove coarse solids and then through a fine screening section. The coarse screens are configured as vertical bar screens with a bar spacing from 20 to 100 mm and even up to 200 mm. So as to keep the impingement of marine organisms as low as possible right when the solids are pre-separated, the flow velocity in the seawater extraction channel and thus also the approach speed to the coarse screens should not exceed 0.15 m/s, and the bar spacing of the screens should be selected rather towards the lower part of the range. The coarse screens are equipped with a mechanical rake with which the intercepted solids are scraped from the bars of the screen. The cleaning process is either time-controlled or triggered when the build-up of debris on the inlet side of the trashrack attains a specified thickness. After coarse screening, the content of smaller-diameter solids is removed by fine screening using travelling band screens or rotating drum screens (see a schematic of this intake system in Fig. 4.2). The fine screens have mesh aperture of 1–10 mm. Here too, if impingement and entrainment are to be minimized, a smaller mesh aperture of 1 mm to a maximum of 3 mm should be selected. The solid material that is caked onto the screen elements of the travelling band screen or the rotating drum screen is loosened by a pressurized water spray, washed off, and conveyed to a collection system. From there, the marine residues are either returned to the sea or transported away for disposal on land. The cleaning process for

4.1 Seawater Extraction and Supply

371

Coarse screen

Travelling band screen

SWRO seawater supply pumps

Fig. 4.2 Schematic of an open onshore active screening intake process

the fine screens is either time-controlled or initiated at a specified value of the difference in water level between the inlet and outlet of the screens due to the backed up water on the inlet side caused by the debris loading of the screen elements. The fine screens can be driven at different speed settings, thus varying their solids retention capacity to suit current operational requirements. An increased solids interception capacity in the rake and screening section of a seawater intake is particularly necessary in the event of a jellyfish bloom, so that blockage of the intake’s solids pre-separation section by the increased solids accumulation then occurring can be avoided. Solids can be intercepted by the moving fine screening systems in various ways [3]. With travelling band screens, a distinction is made between the through-flow configuration (see Fig. 4.3), the out-to-in or dual-flow configuration, and the in-toout or centre-flow configuration. With rotating drum screens, a distinction is made between dual-flow and centre-flow patterns. The units are then set up in the intake structure to match the direction of flow. To reduce the extent of marine life impingement in the fine screening section, the screening units can be equipped with special fish buckets for safe capture and return of fish to the sea. These are termed Ristroph screens. The raking and fine screening section of an intake is normally divided into more than one train, usually two or three. By means of stop logs, the raking and screening units of individual trains can be separated from each other to allow them to be maintained, while other trains continue to operate (see Fig. 4.2). The design feed flow FF,Int,train,d of each train is calculated from its net feed FF,Int, train,net plus a reserve factor fC,reserve so that, during maintenance of one train, i.e. when the number of trains in operation is reduced, the remaining trains can be operated at higher feed flows, and thus the entire intake facility can be operated at nearly or completely its total design flow FW,SW,T (Eq. 4.2).

372

4

Seawater Extraction and Supply and Concentrate Discharge

Fig. 4.3 Travelling band screen cut-away view

  F F,Int,train,d ¼ F F,Int,train,net  1 þ f C,reserve

ð4:2Þ

FF, Int, train, d ¼ design feed per intake train [m3/h] FF, Int, train, net ¼ net feed per intake train [m3/h] fC, reserve ¼ factor of reserve capacity per train [] The net feed FF,Int,train,net results from the total feed FW,SW,T to the intake as calculated by Eq. 4.1e and the number of trains NInt,tr (Eqs. 4.2a and 4.2b).

4.1 Seawater Extraction and Supply

373

F W,SW,T N Int,tr

ð4:2aÞ

 F W,SW,T   1 þ f C,reserve N Int,tr

ð4:2bÞ

F F,Int,train,net ¼ F F,Int,train,d ¼

NInt, tr ¼ number of intake trains [no.] The intake’s total installed feed FF,Int,installed is calculated from the design feed FF, per train and their number NInt,tr (Eq. 4.2c).

Int,train,d

F F,Int,installed ¼ F F,Int,train,d  N Int,tr

ð4:2cÞ

FF, Int, installed ¼ installed feed flow of intake [m3/h] During maintenance of the intake installations, it is operated with a reduced number of trains (Eq. 4.2d). However, if the intake is always to be operated at 100% of its treatment flow during maintenance, a sufficient reserve must be factored into the design flow FF,Int,train,d of each train, depending on the number of trains NInt, tr and the number that are to remain in operation NInt,tr,maint. The factor for the additional reserve flow fC,reserve required for this is calculated from the number of trains in the intake NInt,tr and the number of trains in operation during maintenance NInt,tr,maint using Eq. 4.2e. N Int,tr,maint ¼ N Int,tr  N Int,tr,out f C,reserve ¼

N Int,tr 1 N Int,tr,maint

ð4:2dÞ ð4:2eÞ

NInt, tr, maint ¼ number of intake trains in operation during maintenance [no.] NInt,tr,out ¼ number of intake trains in outage during maintenance [no.] For desalination plants that require very high seawater flow rates for their operation, this intake option in particular can provide the needed extraction capacities as the dimensions of their structures can be very large. Open intake channel or lagoon intakes are therefore mainly used for the thermal seawater desalination processes of multistage flash evaporation (MSF) and multi-effect distillation (MED), which require considerably higher seawater extraction flows compared to seawater membrane desalination. This intake alternative is also preferred for thermal power plants that are operated with once-through cooling using seawater. However, due to extraction taking place in shallow water and near the water surface, seawater from this type of intake contains all surface contaminants, and its quality is also greatly influenced by coastal municipal and industrial wastewater

374

4

Seawater Extraction and Supply and Concentrate Discharge

Fig. 4.4 Open onshore intake at an SWRO plant with mechanical coarse screening and travelling bands for fine screening (source: author)

discharges as well as maritime activities and shipping traffic near the coast. This intake process is well suited to the MSF and MED thermal processes as their requirements regarding seawater quality are much less stringent than for SWRO systems. But if it is applied for reverse osmosis seawater desalination, the outlay for process technology and the operating expenditure in the pretreatment stage are correspondingly higher to provide the requisite feed quality for the membrane systems. For this reason, this type of seawater extraction for SWRO plants has so far only been used to a limited extent and only at locations where the use of a submerged process from greater water depths is not possible due to local bathymetric and oceanographic conditions. Figure 4.4 shows an open intake system for an SWRO plant with mechanical coarse screening and fine screening with travelling band screens. In hybrid plants too, in which thermal desalination processes are paired with reverse osmosis seawater desalination, the seawater for the SWRO plant can be drawn off via a common open intake channel arrangement. This likewise applies if the membrane desalination process is integrated with a power plant in a so-called co-location configuration and the SWRO plant uses the existing seawater withdrawal and outfall system as well as other power plant facilities (see also [1] Sect. 4.2.3.5.2).

4.1.1.2 Submerged Pipe/Tunnel Active Screening Type In the case of an intake design with direct but submerged seawater extraction, the water is supplied via intake heads which are installed on the seabed at greater water depths. These intake heads are connected to the screening components of the intake structure by pipelines or pipes in tunnels through which the seawater is directed to the fine screen. Coarse screening is already done in part during seawater extraction, for which purpose the inlet cross-sections of the extraction heads are fitted with rake bars at a spacing of 80–300 mm [4, 5]. Figure 4.5 shows a schematic of this intake design which is equipped with a rotary drum filter for fine screening.

4.1 Seawater Extraction and Supply

375 Spray water for depris removal Marine debris

Stop log

M ̴

Stop log

M ̴

Rotary drum screen

to SWRO plant

SWRO seawater supply pumps

Seawater

Intake heads Intake Structure

Fig. 4.5 Schematic of open onshore submerged pipe intake with active screening Fig. 4.6 Cut-away view of rotating drum screen

A cut-away view of a rotary drum screening unit is shown in Fig. 4.6, while Fig. 4.7 shows a photo of a dual-train fine screening installation with rotary drum screens for an SWRO plant. This type of fine screen, too, is equipped with screening fabric with a mesh size of 1–10 mm.

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Fig. 4.7 Rotating drum screen installations at an SWRO plant (source: author)

Instead of this fine screening unit design, travelling band screens as described under Sect. 4.1.1.1 can also be used for such intakes. However, rotary drum filters are mostly used for intake systems with a high extraction rate, as these can handle up to twice the flow rate of travelling band screens. With this type of intake, seawater can be extracted at greater water depths, so that the amount of particulate contamination in seawater and also the negative influence of municipal and industrial discharges as well as other factors contributing to the reduction in water quality, as is the case in particular with extraction from the surface and from shallow waters, can be reduced. But with increasing water depth, the temperature of the seawater drops (see [1] Sects. 3.1 and 3.1.2, Fig. 3.6) which influences the permeability of the SWRO separation and desalination membranes and the operating pressure that would then be required. The intake heads are installed at a depth ΔhSsf of 5–25 m below the water surface to match the marine topography of the coastal zone of the desalination plant location and at up to 1500 m from the shoreline depending on how water depth changes with distance from this. The height of the intake head openings above the seabed, ΔhSb, has to be chosen in consideration of the nature of the seabed, that is, whether it is sandy or rocky (see Fig. 4.8). Depending on sea current conditions and seabed structure, a distance of between 2 m and 6 m between the intake heads and the seabed is recommended. For sandy soils and with a heavy swell, a value towards the higher end of this range is advisable. The intake heads are fitted with “velocity caps” to close off their tops so that seawater enters its cylindrical section laterally (see Fig. 4.8). The purpose of this design in conjunction with a correspondingly low approach velocity vF,Ih to the intake area is to minimize the impingement of marine organisms on the rake bars of the intake head. Dimensioning of a cylindrical intake head and determination of its extraction flow can be based on the total seawater extraction flow FW,SW,T of the SWRO plant with Eq. 4.1e and the approach velocity to the intake feed zone vF,Ih together with its height hAIh and the number of intake head units NIh to be provided for the desalination plant by means of Eqs. 4.3, 4.3a and 4.3b.

4.1 Seawater Extraction and Supply

377

Surface dIh,i Velocity cap

∆hSsf

Feed area

hAIh

vF,Ih

dIhbs,i ∆hSb Intake structure

Intake riser pipe

dIhrp,i

Sea bottom

Fig. 4.8 Surface water submerged pipe intake: intake head

AIh ¼

F W,SW,T 3:6  103  N Ih  vF,Ih

AIh ¼ π  d Ih,i  hAIh dIh,i ¼

F W,SW,T 3:6  103  N Ih  vF,Ih  π  hAIh

ð4:3Þ ð4:3aÞ ð4:3bÞ

AIh ¼ free and clean intake head feed area [m2] dIh, i ¼ inner diameter of intake head feed area [m] hAIh ¼ height of intake head feed area [m] NIh ¼ number of intake heads [no.] vF, Ih ¼ approach (face) velocity to intake head feed area [m/s] The approach velocity vF, Ih to be selected for the design of the intake head depends on the maximum through-screen velocity vThrough, Ih at which the head’s

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intake zone is to be operated. This value vThrough, Ih, too, is determined by the need to minimize marine life impingement and should normally not exceed 0.15 m/s. Studies have shown that at this speed, most fish are not caught on the screen surface [2]. The flow velocity through the intake zone vThrough, Ih increases in relation to the proportion of the area of the rake bars installed there on the free surface of the zone and additionally by the build-up of marine organisms that are trapped on the head materials. The influence of these two factors is taken into account by the correction factor f Aeff (Eq. 4.3c). This in turn is made up of a factor f Asb for the reduction of the surface area due to the screen bars and a factor f Amg fixed by the anticipated increase in trapped marine life (Eq. 4.3d). The correction factor for the reduction of the free surface by the screen bars can be determined using Eq. 4.3e from the ratio of the width of the screen bars and the selected bar spacing in the intake zone. vThrough,Ih ¼

vF,Ih 1  f Aeff

f Aeff ¼ f Asb þ f Amg f Asb 

wb sb þ wb

ð4:3cÞ ð4:3dÞ ð4:3eÞ

vThrough, Ih ¼ through-screen velocity at intake head feed area [m/s] f Aeff ¼ correction factor for feed area due to reduction by screen bars and marine growth [] f Asb ¼ correction factor for reduction of area by screen bars [] wb ¼ width of screen bars [mm] sb ¼ spacing of screen bars [mm] f Amg ¼ correction factor for reduction of area due to marine growth [] For a given design value of the approach velocity vF, Ih, the through-screen velocity vThrough, Ih is calculated, which stabilizes at this value according to Eq. 4.3c. But if the permissible flow velocity vThrough, Ih through the intake zone is specified, the corresponding approach velocity vF, Ih is obtained from Eq. 4.3f.   vF,Ih ¼ vThrough,Ih  1  f Aeff

ð4:3fÞ

After dimensioning the intake head, its intake flow capacity FC, Ih is determined from the total seawater extraction flow FW, SW, T of the SWRO plant and the number of intake heads NIh that are to be installed (Eq. 4.3g). F C,Ih ¼

F W,SW,T N Ih

ð4:3gÞ

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379

FC, Ih ¼ flow capacity of intake head [m3/h] The factor f Amg allowing for the build-up of marine life depends on the conditions at the seawater extraction location and the frequency with which the intake heads of the SWRO plant are cleaned manually by divers. The range of values for this factor f Amg is normally 0.3–0.5. To enable manual cleaning inside the intake head, a manhole of sufficient size must be provided on top of it, i.e. on the velocity cap, to provide access. Also the bottom of the head shall be sized so that there is sufficient space for divers to carry out cleaning. So as to minimize the build-up of marine life at the heads and within the pipeline or tunnel connecting the seawater intake points with the fine screening equipment, disinfectants such as chlorine or chlorine dioxide are dosed at both the heads and within the pipes. The build-up of marine life on the rake bars of the intake head can be reduced by making these of corrosion-resistant copper-containing materials such as CuNi steels (CuNi 90–10, CuNi 70–30). Although these measures normally reduce such build-up, they cannot completely prevent it. Thus, in the pipelines, too, physical cleaning must be carried out at certain intervals, such as by pigging, which involves passing a cleaning device through the pipeline or tunnel to remove the deposits from their walls. The internal diameter dIhbs, i of the bottom structure of the intake head is calculated using Eq. 4.3h from its extraction flow FC, Ih and at a flow velocity vIh, bs through this component within which there is sufficient space for it to be cleaned manually. d Ihbs,i

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4  F C,Ih ¼ 3:6  103  vIh,bs  π

ð4:3hÞ

vIh, bs ¼ flow velocity in bottom structure [m/s] dIhbs, i ¼ internal diameter of bottom structure of intake head [m] The internal diameter dIhrp, i of the pipe connected to the intake head, that is, the intake riser pipe with the conveyance pipeline of the seawater intake to the fine screening systems, is also calculated from the intake head’s extraction flow FC, Ih using Eq. 4.3i, while for dimensioning the common conveyance pipeline for the total seawater intake, the SWRO plant’s extraction flow FW, SW, T (see Eq. 4.1c) is taken as the basis in accordance with Eq. 4.3j. For both calculations, a flow velocity of 2.0–2.5 m/s can be selected, which is usual for pipeline dimensioning.

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dIhrp,i

dcp,i

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4  F C,Ih ¼ 3:6  103  vIh,rp  π

ð4:3iÞ

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4  F W,SW,T ¼ 3:6  103  vcp  π

ð4:3jÞ

vIh, rp ¼ flow velocity in riser pipe [m/s] dIhrp, i ¼ internal diameter of riser pipe [m] vcp ¼ flow velocity in conveyance pipe [m/s] dcp, i ¼ internal diameter of conveyance pipe [m] The design and dimensioning of the intake heads for an SWRO plant with a net product flow of 100,000 m3/d are illustrated by a calculation example in Table 4.1. The pretreatment stage of the SWRO plant consists of floc filtration with filter backwash with filtered seawater. There is no recirculation of the filtrate during infiltration of the filters. On this basis, the factor YP,Pr,T is calculated for the pretreatment recovery rate. The above-described intake design with direct extraction of seawater through submerged intake heads and with onshore dynamic fine screening by means of travelling band screens or rotating drum screens is the type of direct surface water extraction most frequently used for SWRO plants over their entire capacity range. Table 4.1 Intake extraction head design for an SWRO plant with 100,000 m3/d product capacity Parameter SWRO plant seawater withdrawal Product flow, gross Product recovery, SWRO Recovery rate pretreatment Infiltration flow through one filter Recovery rate, SWRO total Total seawater withdrawal Intake head design Number of intake heads Through-screen velocity Height of feed area Velocity at intake head bottom structure Velocity in intake riser pipe Velocity in conveyance pipe Screen bar spacing Width of bars Correction factor for marine growth Correction factor for screen bars

Symbol

Unit

Value

FPr,SWRO YRO1,RO2 YP,Pr,T FF,mat YT,SWRO FW,SW,T

m3/h – – m3/h – m3/h

4168 0.409 0.955 783 0.391 11,454

NIh vthrough,Ih hAlh vIh,bs vIh,rp vcp sb wb fAmg fAsb

No. m/s m m/s m/s m/s mm mm – –

2 0.15 2.0 0.8 2.0 2.0 100 25 0.30 0.20 (continued)

4.1 Seawater Extraction and Supply

381

Table 4.1 (continued) Parameter Correction factor, feed area total Approach velocity to feed area Internal diameter of feed area • Calculated • Selected Withdrawal capacity of intake head Internal diameter of bottom structure • Calculated • Selected Internal diameter of intake riser pipe • Calculated • Selected Internal diameter of conveyance pipe • Calculated • Selected

Symbol fAeff vF,Ih dIh,i

FC,Ih dIh,bs,i

Unit m/s

Value 0.50 0.075

m m m3/h

3.38 3.50 5727

m m

1.59 2.00

m m

1.01 1.00

m m

1.42 1.50

dIh,rp,i

dcp,i

4.1.1.3 Submerged Pipe Passive Screening Type An intake system with submerged passive screening consists of intake heads that are connected to the SWRO feedwater pumping station via conveyance piping and a compressed air system that provides compressed air to flush the intake heads (see intake system schematic Fig. 4.9). The solids suspended in the seawater are separated by screening over the entire range from coarse to fine material at the intake heads. The debris layer deposited on the inlet surface of the intake heads is removed by the current present there together with intermittent air flushing of the heads. For this to be effective, at the intake zone of the extraction heads, there has to be a sufficiently strong and continuous current, i.e. a sweeping velocity, to clean the heads and transport the debris dislodged by the air purge away from them [6]. The screen aperture size of the intake heads is in the range of 1–3 mm, and they are designed for a through-screen velocity of 0.10–0.15 m/s. For the approach velocity to the heads’ intake zones as calculated according to Eq. 4.3f, at the extraction location, the sweeping velocity parallel to the intake head surfaces would have to be at least 0.30 m/s to prevent their excessive, long-term fouling. For open intakes, dimensioning of the heads with regard to screen aperture size and through-screen velocity as described above achieves the best reduction of impingement and entrainment with this design of intake system. With the above-mentioned dimensioning of the heads with regard to screen aperture size and through-screen velocity, for this type of intake systems, the best possible reduction of impingement and entrainment can be achieved. The intake heads are designed as wedge-wire screens either in drum form or in a horizontal T configuration or else as polyhedron-shaped units with perforated screen surfaces. A wedge-wire screen in a T-configuration is depicted in Fig. 4.10a with the wedge wires of the screen element shown in Fig. 4.10b [6].

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Fig. 4.9 Schematic of open onshore passive screen intake process

Fig. 4.10 (a) Cylindrical passive screen unit (courtesy: Aqseptence Group-Johnson Screens®). (b) Wedge-wire screen profile

4.1 Seawater Extraction and Supply

383

Fig. 4.11 (a) Polyhedrally shaped passive screen (courtesy: Taprogge). (b) Polyhedral system screen element (courtesy: Taprogge). (c) Extraction head of a polyhedrally shaped passive screen intake (courtesy: Taprogge

Figures 4.11a and b show a polyhedral unit used as an intake head and one of the perforated screen elements as it is inserted into its metal frame, and in Fig. 4.22c an intake head of such system is illustrated. The compressed air station usually installed in the SWRO feedwater pumping station for air blast flushing of the intake heads consists of a pressure vessel and air compressors that generate compressed air at a pressure of 8–10 bar. Each intake head is connected to the compressed air system by an airline. Backflushing with air is triggered intermittently for each head by a time control at an interval depending on the seawater’s solid matter content. The intake heads can be located up to 300–400 m away from the compressed air station. If the extraction lines were to be any longer, the compressed air pressure would have to be increased or else air blast purging

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Seawater Extraction and Supply and Concentrate Discharge

would be less effective, so in this case the compressed air station is placed close to the extraction heads on a barge or a platform set up on the seabed. Despite the seawater flow across the heads’ intake zones and the air burst backflush, marine growth in both the heads and the intake pipelines cannot be prevented. For this reason, with this type of intake, disinfectants such as chlorine are dosed intermittently at the intake heads as well as into the pipelines. The use of CuNi materials—CuNi 90–10, CuNi 70–30—for the intake heads can also reduce the build-up of biological matter on the heads. Pigging of the conveyance piping must also be considered if a high level of biological activity is to be expected at the extraction location. The option of manual cleaning of the intake heads by divers both on their outsides and internally must also be considered when designing the system. Consequently, intake heads with a sufficiently large diameter shall be provided with a manhole for access. The flow FC,Ih,ww through a cylindrical intake head with a wedge-wire screen may be estimated from the flow velocity through the screen vThrough,Ih together with the diameter dIh,ww and length lIh,ww of the head’s intake zone multiplied by a correction factor f Aeff to take account of the obstruction of the free area of the through-flow zone by the wedge wires and by marine growth that cannot be removed by the sweeping water flow and air backflushing.   F C,Ih,ww  3:6  103  vThrough,Ih  π  dIh,ww  lIh,ww  1  f Aeff

ð4:4Þ

FC, Ih, ww ¼ flow through intake wedge-wire screen unit [m3/h] vThrough, Ih ¼ through-screen velocity at intake head feed area [m/s] dIh, ww ¼ diameter of wedge-wire screen unit [m] lIh, ww ¼ length of feed surface area of wedge-wire screen [m] f Aeff ¼ correction factor for feed area due to its reduction by wedge wires and marine growth [] The correction factor f Aeff is made up of the factor for the reduction of the free intake zone by the wedge wires f Aww and the factor for its reduction by biological growth f Amg (Eq. 4.4a). f Amg depends on the water quality and biological activity at the extraction location and can have a value of 0.1–0.5. f Aeff ¼ f Aww þ f Amg

ð4:4aÞ

f Aww ¼ correction factor for reduction of area by wedge wires [] f Amg ¼ correction factor for reduction of area by marine growth [] For the screen configuration shown in Fig. 4.10b, the correction factor f Aww can be determined from the wire width ww and the screen slot width sw using Eq. 4.4b.

4.1 Seawater Extraction and Supply

385

f Aww 

ww sw þ w w

ð4:4bÞ

ww ¼ wire width [mm] sw ¼ slot width [mm] The number of intake heads NIh, ww needed to provide the total intake flow FW, specified for the SWRO plant is calculated according to Eq. 4.4c from the intake flow of a single head as calculated by Eq. 4.4. SW, T

N Ih,ww ¼

F W,SW,T F C,Ih,ww

ð4:4cÞ

NIh, ww ¼ number of wedge-wire screen units [no.] Cylindrical intake heads should be installed at a minimum spacing of one head diameter. Up to now, this type of intake has not been widely adopted and is mainly used in industrial and municipal SWRO plants with small to medium seawater extraction capacities. If only direct withdrawal of seawater is possible at the site of a desalination plant and very low impingement and entrainment rates are required, this type of intake can also be considered for large-scale plants if the seawater quality and hydraulic conditions, especially flow conditions, at the location are suitable for the process.

4.1.2

Indirect (Soil Filtration) Intakes

In the case of intake structures with indirect subsurface withdrawal, seawater is drawn from seawater aquifers in the immediate onshore or nearshore area after it has percolated through soil formations. The soil must have sufficiently high water permeability so that a free or unconfined aquifer connected to the sea can develop and the amount of water taken from it is constantly replenished by influx from the sea. The permeability of soil is characterized by its hydraulic conductivity kf, a proportionality factor of the Darcy equation for laminar flow through a porous medium. Darcy’s law states that the laminar flow rate FW through a cross-sectional area A of a material is directly proportional to the hydraulic gradient i, this being the quotient of the pressure head differential Δh and the flow length l (Eqs. 4.5 to 4.5b). FW ¼

k f  A  Δh l

ð4:4Þ

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4

Seawater Extraction and Supply and Concentrate Discharge

Δh ¼i l

ð4:4aÞ

FW ¼ k f  A  i

ð4:4bÞ

FW ¼ flow of water [m3/s] A ¼ through flow area of material [m2] l ¼ length of flow through material [m] Δh ¼ pressure head differential [m] kf¼ hydraulic conductivity [m/s] i ¼ hydraulic gradient [] The hydraulic conductivity kf is then obtained by transforming Eq. 4.4 into Eq. 4.4c. kf ¼

FW  l F v ¼ W ¼ W A  Δh A  i i

ð4:4cÞ

Determination of the hydraulic conductivity kf of soil formations is just one of the investigations for establishing the geological and hydrogeological conditions of a site at which the installation of systems for underground extraction of seawater is planned. This is done by pumping tests during which the potential well extraction flow and the quality of the water pumped are assessed by means of test wells, while how much the seawater level is lowered in the aquifer at certain distances from the test wells is measured at additional test wells (see Sect. 4.1.2.1). The permeability of rocks and porous media is also characterized by the permeability coefficient K which is calculated with Eq. 4.4e. The hydraulic conductivity kf results from K which, as shown in Eq. 4.4f, is a function of the density ρW and dynamic viscosity μ of the water. K¼

μ  F W  l μ  vW  l ¼ A  Δp Δp kf ¼ K 

g  ρW μ

ð4:4eÞ ð4:4fÞ

K ¼ permeability coefficient [m2] vW ¼ velocity of water flow [m/s] Δp ¼ pressure differential [Pa] μ ¼ dynamic viscosity [Pa*s] ρW ¼ density of water [kg/m3] g ¼ standard gravitational acceleration ¼ 9.80665 [m/s2] The density of seawater ρSW and its dynamic viscosity μSW depend on temperature and salinity (see [1] Sects. 3.2.2, 3.2.2.1, and [1] 3.2.2.2). Derived from Eq. 4.4f

4.1 Seawater Extraction and Supply

387

is Eq. 4.4g, which describes the dependence of kf on the seawater’s density ρSWts and dynamic viscosity μSWts

1,2

1,2

as temperature and salinity vary. Using this relation-

ship, the influence of temperature and salinity on the hydraulic conductivity can be determined via the parameters density and viscosity. k

f ts2

¼k

f ts1



ρSW ts  μSW ts 2 1 ρSW ts1  μSW t2

ð4:4gÞ

kfts1 ¼ hydraulic conductivity at temperature and salinity 1 [m/s] kfts2 ¼ hydraulic conductivity at temperature and salinity 2 [m/s] ρSWts ¼ density of seawater at temperature and salinity 1 [kg/m3] 1 ρSWts2 ¼ density of seawater at temperature and salinity 2 [kg/m3] μSWts ¼ dynamic viscosity at temperature and salinity 1 [Pa*s] 1 μSWts2 ¼ dynamic viscosity at temperature and salinity 2 [Pa*s] The permeability of porous materials can be classified according to their hydraulic conductivity kf. The permeability of bulk materials and rock is thus assigned to a specific hydraulic conductivity range (see Table 4.2). For indirect seawater extraction through wells, the hydraulic conductivity of the subsoil at the planned extraction site should be in the range from “very high” to the upper range of “medium”, i.e. within the value range of kf from 102 to 104. As the table shows, for rocks in the range of very high permeability, this is mainly the case for karst limestone, for high permeability additionally for permeable basalt and for medium permeability to some extent also for limestone/dolomite and sandstone. For loose material fills, very high permeability is achieved with gravel, whereas high to medium permeability is achieved with sand of appropriate coarse, medium, and fine grain sizes. Planning of indirect extraction systems requires knowledge not only of the onshore hydrogeological conditions but also of potential hydraulic connections of the seawater aquifer with possible confined onshore fresh water aquifers near the Table 4.2 Classification of permeability and range of hydraulic conductivity of various clean materials Classification of permeability Very high High Medium

Hydraulic conductivity range kf [m/s] > 102 102–104 104–106

Low

106–108

Very low

< 108

Type of cleans material Gravel, karst limestone Karst limestone, permeable basalt, coarse sand, gravel Karst limestone, limestone/dolomite, permeable basalt, sandstone, silt, coarse sand, medium sand, fine sand, silt Sandstone, limestone/dolomite, permeable basalt, coarse sand, medium sand, fine sand, silt Clay, basalt, limestone/dolomite, sandstone, silt

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coast. Infiltration of fresh water into a seawater aquifer can reduce the salinity of the extracted seawater or may cause this to fluctuate widely. By the same token, though, it is also possible for seawater to infiltrate the fresh water aquifer so that its salinity may increase to that of brackish water. Due to the filtration effect of the soil formations that seawater passes through during extraction, its quality in terms of the content of particulate impurities and colloids, i.e. its silt density index SDI, may improve significantly. Bank filtration can also greatly reduce the content of organic matter in the form of natural organic matter NOM and transparent exopolymer particles TEP as well as of biological matter such as bacteria and algae. However, how and to what degree the quality of seawater is influenced depend on the chemical composition and physical properties as well as the thickness of the soil strata through which the seawater passes before its extraction. But also the type of extraction system influences the extent of seawater quality change prior to its extraction [7]. Biological growth such as algal bloom and jellyfish bloom has hardly any negative influence on the membrane desalination plant’s operation with indirect seawater extraction. Impingement and entrainment, which have a significant influence on the design of open intake systems under both ecological and economic aspects, are also not determining factors with indirect extraction. The seawater extracted in this way can therefore usually be assigned to quality class A for the design of an SWRO pretreatment stage (see Sect. 2.2.2.1.2, Table 2.12), for which, in Group A b, sand filtration in combination with cartridge filters ahead of the plant’s reverse osmosis system is sufficient. However, indirect extraction can also introduce impurities such as iron and manganese into seawater in concentrations of several mg/l, which are normally not present at all in seawater or only in trace amounts. These metallic ions are either absorbed as they flow through the layers of soil or they enter the seawater as a result of infiltration from a fresh water aquifer which is contaminated with them. In this way, other pollutants can likewise get into the seawater aquifer if the groundwater is contaminated by substances from industrial or municipal sources. Hydrogen sulphide H2S and other sulphur compounds may also be present as further contaminants because, in most anaerobic conditions in the soil strata, these constituents are generated by bacteria from sulphate compounds. They are then also absorbed by the seawater as it passes through these strata. Seawater contaminated in this way can no longer be cleaned just with the basic pretreatment processes described above. Hydrogen sulphide can be removed physically by air stripping the water in a degasifying tower. To remove iron and manganese, the water must be aerated or dosed with chlorine or chlorine dioxide for oxidation, and then it has to be passed through dual-media filters and/or treated with greensand filters. These filters are operated at a significantly lower filtration rate than is the case for sand filtration using pretreatment option A b. If iron and manganese are present at higher concentrations, it may be necessary to add a sedimentation or flotation step to the filtration stage. Should such additional pretreatment measures become necessary for the indirect extraction of seawater, their CAPEX and OPEX may become equal to or even exceed those of the pretreatment configurations used for open intakes [8–10].

4.1 Seawater Extraction and Supply

389

4.1.2.1 Vertical Intakes (Beach Wells) The process schematic of a vertical beach well for seawater extraction is shown in Fig. 4.12. The seawater flows through the soil formations of the onshore area to the intake wells from where it is directed by submerged well pumps to the SWRO plant. The vertical wells are installed as near as possible to the offshore zone to minimize the length of the soil material to be traversed by the seawater during extraction but also to prevent possible influencing of the seawater aquifer on onshore fresh water resources. In order to determine the parameters for the design of such a well system, a number of pilot wells are installed at the intended extraction site during the planning phase of the facility. These wells and additional test boreholes are used to determine the geological and hydrogeological parameters within the beach area that are to be expected for the extraction wells. These design parameters include the hydraulic conductivity kf and transmissivity T of the soil formations in the extraction zone and the resulting values for: • The inflow FAqui,in to be expected from the seawater aquifer to the shore. • The flow rate FWell,c of a well to be expected. • The radius R of a well’s zone of influence. The following equations are used to roughly estimate the dimensions and the order of magnitude of the potential flow rate of a vertical well [11]. For an unconfined aquifer, its available inflow FAqui,in to a vertical well is calculated using the Dupuit-Thiem equation in accordance with Eq. 4.5 and as shown in Fig. 4.13.

Fig. 4.12 Schematic of a vertical beach well intake process

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Seawater Extraction and Supply and Concentrate Discharge

Fig. 4.13 Vertical beach well intakes: water levels and radiuses of wells’ zones of influence

F Aqui,in

  3:6  103  π  k f  H 2  h2w ¼ ln rRw

ð4:5Þ

FAqui, in ¼ inflow from aquifer to well [m3/h] H ¼ water level at distance R (at undisturbed water table head) [m] hW ¼ water level at well during pumping [m] R ¼ radius of well’s zone of influence [m] rW ¼ radius of well borehole [m] By transforming Eq. 4.5 into Eq. 4.5a, the value of the hydraulic conductivity kf can be determined from the withdrawal rate Fw,well as measured at the test wells and the test boreholes together with the aquifer water levels h1 and h2 at distances r1 and r2 from the well (see Fig. 4.13). kf ¼

F w,well  ln rr21   π  h22  h21

Fw, well ¼ withdrawal rate from well during pumping test [m3/s] r1 ¼ distance 1 from well [m] r2 ¼ distance 2 from well [m] h1 ¼ water level at distance r1 [m] h2 ¼ water level at distance r2 [m]

ð4:5aÞ

4.1 Seawater Extraction and Supply

391

With Eq. 4.5b, the transmissivity T is obtained from the hydraulic conductivity kf and the aquifer’s thickness from H, if the well pipe extends to the bottom of the aquifer. T ¼ kf  H

ð4:5bÞ

T ¼ transmissivity [m2/s] H ¼ water level at distance R (at undisturbed water table head) ¼ thickness of aquifer [m] The radius of the well’s zone of influence, i.e. the distance R from the well pipe at which the undisturbed water level is again H after lowering of the aquifer’s water level by seawater extraction from the well, can be estimated with Eq. 4.5c. R ¼ 3, 000  s ∙

pffiffiffiffiffi pffiffiffiffiffi k f ¼ 3, 000  ðH  hw Þ ∙ k f

ð4:5cÞ

s ¼ drawdown of undisturbed water level during pumping [m] hW ¼ water level at well during pumping [m] In a vertical well system, the wells should be so spaced out that the radius R of the zone of influence is taken into account when determining the spacing between them so as to minimize their mutual interference and thus each well’s reduction in extraction capacity. If R is known, Eq. 4.5 can be used to determine the available inflow to the well from the aquifer FAqui, in and Eq. 4.5d to determine the well’s potential extraction capacity FWell, c. F Well,c ¼ 7:2  103  r w  π  ðH  sÞ 

pffiffiffiffiffi kf f emp

ð4:5dÞ

FWell, c ¼ well capacity [m3/h] rW ¼ radius of well borehole [m] femp ¼ empirical factor ¼ 15 For a seawater aquifer, it should be noted that the height H of its water level and the depth of the aquifer changes with tidal fluctuations of the seawater level. This means that the inflow to the well from the aquifer FAqui, in, the radius R of the well’s zone of influence, and the well’s extraction capacity FWell, c change accordingly. Consequently, the design of the vertical well system has to be based on the aquifer’s water level at low tide. The number of wells required to provide the extraction capacity FW, SW, T needed for an SWRO plant is derived from the value of FW, SW, T and the well’s capacity

392

4

Seawater Extraction and Supply and Concentrate Discharge

FWell, c calculated with Eq. 4.5d together with a factor fcl,well to account for the well’s reduction in capacity (Eq. 4.5e). This factor depends on the quality of the extracted seawater, the age of the well installation, the frequency of maintenance, and the likely maintenance outlay for each well. This factor fcl,well should not exceed a range of 0.2–0.4. Adequate space reserves for installing additional wells to compensate for the loss of extraction capacity with time of the initial wells should be available at the extraction location. N wells ¼

F W,SW,T   F Well,c  1  f cl,well

ð4:5eÞ

Nwells ¼ number of wells [no.] fcl,well ¼ capacity loss factor of wells ¼ 0.2–0.4 [] The number Nwells of vertical wells required for a defined seawater extraction flow FW,SW,T is primarily dependent on the permeability of the soil strata through which the seawater passes during extraction. However, a well’s capacity FWell,c is also governed by its dimensioning, i.e. its depth and diameter, and the degree of drawdown s. The dimensioning of a vertical well system with the above design algorithms for a seawater withdrawal rate of 11,454 m3/h, corresponding to an SWRO product flow rate of 100,000 m3/h, is shown in Table 4.3. The calculation is based on a hydraulic conductivity, kf, of 2*104 [m/s], or 17.3 [m/d], which represents a high Table 4.3 Vertical well design Parameter Total seawater withdrawal Hydraulic conductivity

Symbol FW,SW,T kf

Undisturbed water level aquifer Drawdown Radius of well borehole Capacity loss factor Transmissivity

H s rW fcl,well T

Water level of well during pumping Radius of well’s zone of influence Inflow from aquifer Well capacity Number of wells • Without capacity loss • With capacity loss • Selected

R FAqui,in FWell,c

Unit m3/h m/s m/d m m m – m2/s m2/d m m m3/h m3/h

Value 11,454.0 2.0E-04 17.3 70 8 0.25 0.2 1.4E–02 1.210 62 339.4 331.1 330.6

NWells

no.

34.7 43.3 44

4.1 Seawater Extraction and Supply

393

permeability. Considering a capacity loss of the wells of 20%, i.e. fcl,well is 0.2, 44 wells each with a capacity of 331 m3/h are required for this withdrawal rate. The radius of the zone of influence R under these design conditions is about 340 m. Mutual influencing of the wells is not considered in the calculation, which means that it applies to the dimensioning of each individual well. However, if the spacing of the wells is less than R, which will usually be the case in practice, there will be mutual interference between them, which will lead to a reduction in the withdrawal rate of each one. If the soil structure has a very high permeability with a hydraulic conductivity, kf, of 1.0  103 [m/s] or 86 [m/d], then for a seawater withdrawal flow as given in Table 4.3 and with the same well dimensions but a reduced drawdown, s, of 4 m, its withdrawal capacity increases to 787 m3/h, so that then only 20 wells would be required. However, as the calculation for the vertical well system shows, even for SWRO plants with a medium product capacity, a large number of wells would be needed, and these would have to be spaced at adequate distances from each other to keep any reduction in the flow of each one within limits. Correspondingly large shore areas would therefore be required to implement such an indirect seawater extraction system. Moreover, additional space would be needed for expanding the well system if the quality of the extracted seawater and ageing of the wells would cause a reduction of their withdrawal capacity. Up until now, vertical wells have been most commonly used for indirect seawater extraction. However, due to the considerable shore space they take up, these are used predominantly for SWRO plants with low and medium product capacity. The largest shore well system of this kind in operation to date is for the Sur SWRO Plant in Oman. This has an extraction capacity of 9000 m3/h, or 216,000 m3/d, and consists of 33 wells, each 80–100 m deep with a diameter of 0.40 m. The seawater thus extracted is pretreated by pressurized sand filtration before being piped to the SWRO plant [12].

4.1.2.2 Horizontal Intakes With horizontal well systems, the seawater is drawn from the seawater aquifer through well pipes that are arranged horizontally in the ground. They are perforated in the pipe sections where the seawater is withdrawn so they act as a sieve. The well pipes are either arranged radially around a collector well or caisson or are directed towards the sea if water withdrawal from the land is to be avoided due to the risk of fresh water intrusion. 4.1.2.2.1 Radial Collector Wells (Ranney-Type Wells) A schematic of a horizontal well system with radially arranged well pipes is shown in Fig. 4.14. With comparable permeability of the soil in the shore zone, a significantly higher well extraction capacity can be achieved with this well type compared to a vertical well. If the soil structure has a higher permeability on the sea side than in the onshore zone, the output of a radial well can be further increased by positioning the well

394

4

Seawater Extraction and Supply and Concentrate Discharge

Fig. 4.14 Horizontal beach well intake

pipes more towards this soil area. In addition, a hydraulic connection to a landside fresh water aquifer can be prevented by reducing water withdrawal from the land side, i.e. by arranging the lateral pipes accordingly. However, the expenditure for development and construction of this type of well is significantly higher than for vertical wells. This horizontal (radial) shore well type is widely used for onshore filtration of fresh water, but has so far only been used to a limited extent for indirect seawater extraction. Such an intake with three horizontal, radial wells is operated at the SWRO plant of Pemex Salina Cruz Refinery in Mexico with a total extraction capacity of 1875 m3/h or 45,000 m3/d [9].

4.1.2.2.2 Horizontal Directionally Drilled Wells When constructing horizontal wells by means of the horizontal directional drilling (HDD) technique, boreholes are drilled at a slight angle from the horizontal from the shore, or close to the shore, through the subsoil to below the seabed. The perforated or porous pipes or hoses are then installed in the boreholes, through which the seawater is extracted and fed to the collection well for transfer to the SWRO plant. With this construction, too, test wells and their lengthy trial operation are needed to determine the flow of each well string and of the entire installation, depending on the subsoil conditions in the intended withdrawal area, together with the prospective quality of the extracted seawater. Because the horizontal boreholes are oriented towards the sea, a hydraulic connection to onshore fresh water aquifers is avoided. At the same time, if the boreholes start at an appropriate distance from the shore, the negative influence of the marine atmosphere on the supply equipment to be installed in the well system and also intrusive visual aspects of the intake systems in the beach area can be reduced. Environmental influences during construction and operation of the extraction facilities in the shore zone, in the inland area close to the shore, and in the seawater zone where the extraction takes place can be kept lower with this

4.1 Seawater Extraction and Supply

395 Pump house

M

to SWRO plant

Shore

Seawater

Collection well

Porous filtration pipe

Fig. 4.15 HDD well intake

horizontal drilling technology than is the case with vertical wells, horizontal well systems of the Ranney type, or shore and sea gallery systems. An example of an application of this technology is the NEODREN design [13, 14]. With this technology, upon completion of the HDD boreholes, extraction lines comprised of HDPE hoses of 200–450 mm diameter are installed in them. Along their extraction sections, they have a porous structure of sintered HDPE with a pore size of 120 μm. The hose length can be up to 700 m. The extraction hoses are inserted into the boreholes from the sea side, installed over their length, and secured in the boreholes with cement at the non-porous sections of them (Fig. 4.15). The withdrawal zone is located in coastal seawater, and the extraction lines are emplaced 5–10 m below the seabed. This technology can also be used to advantage in both medium and shallow seawater, that is, where the sea level is subject to wide tidal fluctuations. . An extraction system consists of a number of extraction lines in a fan configuration that come together at a collector well or caisson (see Fig. 4.16). From there the extracted seawater is directed to the SWRO plant with supply pumps. Greater removal rates can be achieved in a modular arrangement for which several such arrays are combined. The fan configuration of the extraction lines is so designed that in the infiltration zone, the distance between them is sufficient to minimize their mutual interaction. The NEODREN process is currently being used at SWRO plants in Spain. The largest installed capacity with this process design is the intake of the SWRO plant of San Pedro del Pinatar I with an extraction rate of 7200 m3/h or 172,800 m3/d [15]. This installation consists of 20 extraction lines, each with a length of around 500 m per unit and a hose diameter of 350 mm. The total installed length of the hoses in this plant is 9191 m, and the length of their porous filter sections is altogether 4545 m. This results in a specific flow of an extraction line per unit of length within

396

4

Seawater Extraction and Supply and Concentrate Discharge

Fig. 4.16 Schematic showing a top view of horizontally drilled wells

its filter zone of 0.44 l/s,m or about 360 m3/h,m. The SWRO’s pretreatment process for the extracted seawater is based on sand filtration. Horizontal directed drilling is also used for the installation of horizontal slant wells. For these, too, boreholes are drilled from the shore or close to the shore, in a straight line into the seabed. Depending on the local terrain profile and morphology of the seabed as well as the depth from which the seawater is to be taken at the seabed, the slant angle is defined for the drilling operation. A schematic of this technology is shown in Fig. 4.17. The well pipes, which in this process are of corrosion-resistant stainless steel, are inserted into the drilled wells from the shore side. They also include a filtration and extraction section in which the pipes are permeable and are configured as a screen. This permeable section is located within the borehole in an engineered filter pack built up of material of mineral origin. The seawater can be drawn from the well pipe as an inflow under gravity into the collection well, or a submersible pump can be installed at the starting point of the well pipe to extract the seawater from it. The slant wells are constructed of telescoping pipes. The well pipe section containing the submersible pump, namely, the pump chamber, has a diameter of 460 mm, while the diameter of its extraction section is 350 mm. The well pipes can be up to 300 m long. To attain seawater withdrawal rates that go beyond the capacity of one of the well

4.1 Seawater Extraction and Supply

397

Fig. 4.17 Slant well intake

pipes, they are configured in fan-shaped arrays consisting of two to four pipes (see Fig. 4.17). Details of the design and construction of horizontal well installations and results from the operation of test wells in terms of flow capacity and water quality can be found in [16] and [17]. For seawater extraction, this process is currently still at the pilot plant stage. A number of test wells have been operated in the USA on the Californian coast. Under the local geological conditions in the seabed and shore area there, extraction rates of 417–667 m3/h or 10,000–16,000 m3/d per well pipe were measured. The number of withdrawal lines required for the total extraction flow FW,SW,T of the intake system of an SWRO plant is calculated with Eq. 4.6 from FW,SW,T, the initial flow Fwl,ini of an withdrawal line according to the experience of the system manufacturer, or as determined from test wells depending on the geological conditions at the extraction location and including a factor fcl,wl for the loss of flow capacity with increasing operating time of the well system. N wl ¼

F W,SW,T   F wl,ini  1  f cl,wl

ð4:6Þ

Nwl ¼ number of withdrawal lines [no.] Fwl, ini ¼ initial flow capacity of withdrawal line [m3/h] fcl, wl ¼ capacity loss factor of withdrawal lines ¼ 0.3–0.5 [] The flow capacity loss of such a horizontal direct extraction system that has to be taken into account in its design is largely determined by its technology, in terms of both how the water is extracted from the aquifer and the possibility of reducing or compensating for any capacity loss through system maintenance. Greatly influencing the loss of capacity, however, are also the water quality during extraction

398

4

Seawater Extraction and Supply and Concentrate Discharge

with regard to scaling and fouling constituents as well as the geological structures at the extraction location. For an SWRO plant with a product capacity of 100,000 m3/d and a seawater extraction rate of 11,454 m3/h, assuming a capacity loss of 30%, i.e. fcl,wl is 0.3, in line with the system manufacturers’ specifications for the possible flow capacity per withdrawal line of their systems, the necessary number of withdrawal lines to provide the seawater extraction rate as stated above calculates • For the NEODREN system with a line flow of 360 m3/h, unit to about 45 withdrawal lines. • For the slant well system with a line flow of 542 m3/h, unit to about 30 withdrawal lines.

4.1.2.3 Beach Galleries and Seabed Filters If the existing natural soil formations in the available shore area do not have sufficient permeability to provide the seawater extraction flow required for the SWRO plant via vertical or horizontal shore wells, these soil layers can be removed in the area of the shore intended for the installation of the extraction system and replaced by materials with a higher hydraulic conductivity. These so-called beach galleries are made up of layers of more permeable materials such as gravel and sand with a suitably graded grain size. The seawater is extracted from this engineered soil structure using a horizontal well configuration in which the horizontal pipes of the well’s extraction system are embedded in a layer of coarse gravel. Above this are fine-grained sand layers that filter the seawater. The seawater taken from the aquifer thus formed is fed to the collection well, from which it is pumped to the SWRO plant by the supply pumps (see the extraction system schematic in Fig. 4.18). For higher extraction flows that cannot be attained with just one beach gallery, additional gallery installations are combined in a modular configuration to create a pooled extraction system for the SWRO plant (see Fig. 4.19). If the natural soil formations are replaced by an engineered soil structure not on the shore but on the seabed in the coastal surf zone, this type of gallery is termed a

Fig. 4.18 Beach gallery intake

4.1 Seawater Extraction and Supply

399

Fig. 4.19 Schematic showing a top view of a beach gallery intake

Fig. 4.20 Seabed filter gallery intake

seabed filter gallery (Fig. 4.20). This gallery acts like a slow sand filter whose surface is swept free of deposited solids by the surf’s wave motion. For this reason, the seabed surface over the gallery has to remain covered with seawater even at low tide, and the wave swell must be adequate, so the seawater filter gallery has to be installed at a sufficient distance from the shore zone. The structure of the subsoil layering of the seabed filter gallery is similar to that described above for the beach gallery, with coarse gravel, in which the extraction pipes of the horizontal filter system are embedded, and, on top of this, layers of graded grain sizes of medium and fine gravel. Here, too, the extracted seawater is brought together in collection wells from where it is supplied to the SWRO plant [10, 18].

400

4

Seawater Extraction and Supply and Concentrate Discharge

The required filtration area As,sbf of the seabed filter gallery is calculated with Eq. 4.7 from the total seawater extraction flow FW,SW,T and the filtration velocity or surface load vsbf. The range of values of vsbf from 0.1 to 0.3 [m3/m2,h] for the surface load used for dimensioning the gallery is the same with which slow sand filters, too, are operated, when they are used for surface water filtration in fresh water treatment. As,sbf ¼

F W,SW,T vsbf

ð4:7Þ

As, sbf ¼ area of seabed filter gallery [m2] vsbf ¼ filtration velocity/surface load of seabed filter gallery ¼ 0.1–0.3 [m3/m2,h] The Fukuoka Uminonakamichi Nata SWRO plant in Japan has been operating with such an intake system since 2005. This desalination plant has a product capacity of 50,000 m3/d, and the extraction flow rate of the intake is 103,000 m3/d or 4292 m3/h. The gallery has an intake area of some 20,100 m2 (314 m  64 m), which works out to a surface load of approx. 0.21 m3/m2,h, with a maximum extraction flow of 0.25 m3/m2,h. The intake area is located 640 m from the shore, and the maximum depth of the filter surface below sea level is 11.5 m. The emplaced gravel and sand layer is 3.85 m thick, while the filter layer from the collection pipe to the filter surface is 2.95 m thick. The collector pipe is embedded in gravel with a grain size of 30–40 mm with filter layers of sand with grain sizes of 2.5–13 mm above it and then a layer matching the natural grain size distribution of the seabed. The thickness of these layers is 1.5 m. The seawater at Fukuoka has a silt density index, SDI, of around 5–10, and this is reduced to 2–3 after it has passed through the filter gallery. It is subsequently treated by ultrafiltration in the SWRO plant’s pretreatment stage.

4.1.3

Assessment and Selection of Intake Withdrawal Systems

The selection of a seawater intake system from the range of available options described above that is best suited to the specific conditions prevailing at the site of an SWRO plant depends on a number of factors, which in turn are determined both by the extraction flow of the desalination plant and the specific characteristics of the potential intake systems but also to a large extent by the local topographical, geological and hydrogeological conditions at the withdrawal location. In addition, there are ecological aspects with regard to flora and fauna conservation during construction and operation of the intake systems, both onshore in the shore zone and offshore. Various selection criteria that have to be considered under planning, technical, operational, and ecological aspects are listed in Table 4.4, and for each intake option, the respective criterion has been assigned a general comparative rating.

Land use Dependence on • Natural soil geology • Coastal & seabed structure • Sea tide & sea level • Surf & currents • Seawater quality Fresh water intrusion possibility

5 6 6.1 6.2

6.3 6.4 6.5 7

4

3

L Y to L Y N

N Y

N Y

L Y Y N

M

H

L

M to H

M to H

L to M

M

H

L

H to VH

M to VH

• Risks of quality changes SWRO pretreatment measures Performance degradation Lifetime expectancy

1.3

2

L

• Quality

1.2

H to VH

L Y to M Y N

N Y

L

H

L

M to H

M to H

M to H

M to H

L N N to L Y

Y L

H to VH

L to M

M to H

L to M

L to M

M to H

L N N to L Y

Y L

M to H M to H L to M L to M M to H L to M H

L N N to L N

Y L

H

L to M

M to H

L to M

L to M

M to H

M to H

L to M

H to VH

Criterion Extracted water • Rate

No. 1 1.1

Subsurface indirect withdrawal Horizontal wells Vertical Directional wells Radial HDD

Intake withdrawal options Surface direct withdrawal Submerged type Channel Active Passive type screening screening

Table 4.4 Assessment and comparison of direct and indirect intake withdrawal systems

L N N to L N

Y L

H

L to M

M to H

L to M

L to M

M to H

M to H

Slant wells

L N N to L Y

L L

H

M

L to M

L

L

M to H

M to H

Galleries Beach galleries

(continued)

Y Y to H N to L N

N Y

H to VH

M

L to M

L

L

M to H

M to H

Seabed galleries

4.1 Seawater Extraction and Supply 401

• During operation • Impingement rate • Entrainment rate Testing and planning outlays Construction complexity Construction risks Operation • Flexibility • Reliability Maintenance • Complexity • Re-establishing performance • Frequency of maintenance

8.2 8.3 8.4 9

L to M –

L – L to M

M H

M H

L

L to M

L

M

M M to H H M

L to M

L to M –

L to M M to H

L to M

L to M

M L to M M to H M

Abbreviations: N no, Y yes, L low, M moderate, H high, VH very high

13.3

11 12 12.1 12.2 13 13.1 13.2

M

H H VH M

M

M L to M

L M to H

M

L

L N N H

M L to M M

L M

M

M

M to H L N N H

M

M to H L to M

L M

M

M

L N N H

L

L to M

M to H

H

M to H

Subsurface indirect withdrawal Horizontal wells Vertical Directional wells Radial HDD

Intake withdrawal options Surface direct withdrawal Submerged type Channel Active Passive type screening screening

M

M to H L to M

L M

M

M

L N N H

L

Slant wells

M

M L to M

L M

M

H

L N N H

M to H

Galleries Beach galleries

M

M to H L to M

L M

M

H

L N N H

H

Seabed galleries

4

10

Criterion Environmental impacts • During construction

No. 8 8.1

Table 4.4 (continued)

402 Seawater Extraction and Supply and Concentrate Discharge

4.1 Seawater Extraction and Supply

403

The rating given to each criterion in Table 4.4 regarding the achievable extraction flow (No. 1.1), the performance degradation during prolonged operation (No. 3), and also the quality of the extracted water (No. 1.2) is, in the case of indirect extraction systems, greatly dependent on local soil conditions and how these influence the composition of the extracted water. The frequency of maintenance (No. 13.3), its efficiency in re-establishing reduced extraction performance (No. 13.2), and thus also the life expectancy of a well system (No. 4) are also very much dependent on these factors. Accordingly, for these criteria, assessment ranges are given for which the maximum and minimum ratings are for what is to be expected under respectively the most favourable and the most unfavourable site conditions. In the case of direct surface withdrawal, it is in particular the quality of seawater and its variability due to biological growth as well as contamination by industrial or municipal discharges and shipping traffic that lead to fluctuations in the quality of the extracted seawater (No. 1.2). These differ depending on the location and have to be considered in the design of the pretreatment stage so that this will be able to cope with the expected conditions. This is reflected by quoting a range for the assessment ratings for these criteria. The CAPEX for constructing the respective seawater extraction facility is also a major selection criterion. When comparing and selecting a technology, in addition to the costs of the withdrawal installation also to be taken into account are the costs of the pretreatment stage (No. 2) to ensure that the water quality specified for feeding into the RO plant is achieved. During the selection process, these costs likewise have to be allocated to the particular withdrawal system. The costs to be considered when selecting the extraction process besides the product capacity of the SWRO depend to a large extent on the site conditions and are derived for a specific technology from the site-specific expenditure for testing and planning (No. 9), the CAPEX for construction and installation (Nos. 10, 11), and the cost for operation (No. 12) and maintenance (No. 13) of the respective extraction process. To be added to this are the costs that result from the pretreatment stage. Further, the costs for the differing space requirements of the process options (No. 5) must be considered. The costs (CAPEX and OPEX) of the various extraction systems can therefore not be assessed in a general comparative manner, and a cost criterion is therefore not included in Table 4.4. The reference situation for the various seawater extraction options under the prevailing site conditions is also a decisive selection criterion. This influences the evaluation of the criteria construction risk (No. 11) and operational reliability (No. 12.2). As the technologies evolve, but also in the light of experience gained from the operation of existing plants, the reference situation for each seawater extraction option changes, and corresponding research must be undertaken at each stage of the selection process. A further criterion for the selection and definition of the type of intake and its extraction systems is the involvement of public and municipal stakeholder groups in the decision-making process and that their suggestions and concerns regarding socio-economic and regional planning aspects are taken into account. One

404

4

Seawater Extraction and Supply and Concentrate Discharge

possibility for making transparent the selection process also for the stakeholders is to apply “multi-criteria analysis (MCA)” as described in detail under [1] Sect. 4.2.3.5.2 and to involve members of the groups in defining the evaluation criteria and their scores. Preliminary screening of seawater withdrawal systems can be done by applying a range of exclusion criteria. Some screening criteria are listed below: • For indirect extraction: – Check whether for these extraction systems, with a known or assumed average permeability of the onshore and offshore soil formations, the area in the shore zone or in the nearshore sea is sufficient and available to provide the seawater withdrawal flow specified for the site. In this way either a technology can be preselected or indirect extraction can be ruled out altogether. – Check whether the installation of wells or galleries in the shore zone is not possible for ecological or socio-ecological reasons. This would exclude vertical and horizontal well systems and also well galleries and would favour HDD solutions. – Check whether intrusion could result in salination of inland fresh water aquifers. If this is the case, horizontal wells with the extraction pipes and HDD systems directed towards the sea as well as seabed filter galleries would be favoured. – Check whether, due to conservation of marine fauna and flora, a seabed filter gallery would not be possible. • For direct extraction: – Check whether environmental protection regulations mandate a significant reduction in the impingement rate, which would then require active screening systems to be specially equipped with facilities for capturing and returning fishes to the sea without harming them. This necessitates the application of passive screening systems. – Check whether environmental protection regulations mandate a very substantial reduction in both impingement and entrainment that would not be attainable with active nor with passive screening equipment. If this is the case, indirect seawater extraction systems have to be installed. – Check whether it would not be possible to situate the intake structures for ecological or socio-ecological reasons or because of nature conservation or historical sites in the near- or far-shore area. Should this be the case, the installations from the seawater intake point to the active screening equipment and the SWRO supply pumps would have to be constructed underneath these protected areas either by tunnelling or by means of HDD. This criterion rules out direct extraction via a channel or lagoon but also excludes the use of passive screening. On completion of such a preliminary screening exercise, the remaining options are then compared in detail under the criteria as set out in Table 4.4, including the costs of the various options, while also considering the references for the extraction technologies corresponding with the site conditions.

4.1 Seawater Extraction and Supply

4.1.4

405

SWRO: Seawater Supply Pumping Facilities and Intake Power Demand

4.1.4.1 Seawater Supply Pumping For SWRO plants towards the high capacity end of the range, primarily centrifugal pumps are used for supplying the seawater, these being installed in the intake structures in either a wet-pit or a dry-pit configuration. Vertical submersible centrifugal pumps are usually used in the form of turbine pumps as shown in Fig. 4.21 for wet-pit arrangements as well as in the mixed flow Fig. 4.21 Intake seawater supply pumps: vertical turbo pump for wet-pit configuration (courtesy: Sulzer) (Vertical turbine pump model SJT/SJM, typically used as seawater intake pump in desalination applications)

406

4

Seawater Extraction and Supply and Concentrate Discharge

Fig. 4.22 Intake seawater supply pumps: dry-pit configuration with vertical inline pumps (source: author)

configuration in the seawater collection wells of indirect extraction systems or, for direct extraction, in the seawater basins following the screening facilities of this type of intake systems. For dry-pit arrangements, horizontal centrifugal pumps or vertical inline pumps as shown in Fig. 4.22 find application. With inline pumps, their drives are mounted vertically above the pump section, so this design takes up less space than conventional horizontal centrifugal pumps. The pumps are installed in seawater pumping stations designed as a dry pump pit with bottom connection of their suction lines to the seawater collection wells or the seawater basin, with seawater inflow from these. Depending on the delivery head ΔH PF,SWRO for which the pumps of the seawater pumping station are to be dimensioned, the centrifugal pumps are single-stage or multistage. The delivery head for which the seawater supply pumps are to be designed is calculated with Eqs. 4.8 and 4.8a from the geodetic height difference ΔHgeo between the water level height above the suction point of the pumps ΔHW,SWs and the water level height HWl,F,pretret in the SWRO plant’s pretreatment stage, the pressure losses in the pipeline and the fittings on the suction side of the pumps ∑ΔHSp, and the sum

4.1 Seawater Extraction and Supply

407

of the pressure losses ∑ΔHFp in the SWRO plant’s supply line. If, in addition to overcoming the geodetic height, pressure must also be provided for the processing equipment in the pretreatment stage, such as membrane or pressure filtration, or if there is a closed pressure system from pretreatment to the high-pressure pumps, the delivery head of the supply pumps must be so calculated that it is sufficient to provide the increased pressure ∑ΔHpretr then required in accordance with Eq. 4.8b for operating the filter systems in the pretreatment stage and for cartridge filtration as well as to provide the inlet pressure for the high-pressure RO pumps. ΔH PF,SWRO ¼ ΔH geo þ

X

ΔH Fp þ

X

ΔH pretr

  X ΔH geo ¼ H Wl,F,pretret  ΔH W,SWs  ΔH Sp X

ΔH pretr

P 105  ppretr ¼ ρSW  g

ð4:8Þ ð4:8aÞ ð4:8bÞ

ΔH PF,SWRO ¼ seawater supply pump head [m] ΔHgeo ¼ geodetic delivery head [m] ∑ΔHFp ¼ sum of pressure head losses in feed pipe (pipe wall friction, valves, etc.) [m] ∑ΔHpretr ¼ additional head for pretreatment systems or a closed pressure system up to the RO units [m] HWl, F, pretret ¼ water level height at feed to pretreatment [m] ΔHW, SWs ¼ water level height at suction point of SW supply pump [m] ∑ΔHSp ¼ sum of pressure head losses in suction pipe (pipe wall friction, valves, etc.) [m] ∑ppretr ¼ sum of pressure increases in pretreatment and RO feed [bar] ρSW ¼ density of seawater [kg/m3] g ¼ standard gravitational acceleration ¼ 9.80665 [m/s2] The water level above the suction point of the supply pumps ΔHW,SWs fluctuates under tidal influences at the plant’s location. The delivery head of the pumps ΔH PF,SWRO therefore has to be calculated to cater for the lowest seawater level ΔHW,SWs,min at low tide and the resulting highest value for the geodetic height ΔHgeo. When selecting the supply pumps, this must also be considered with regard to their NPSHr3% value. For cavitation-free operation of a centrifugal pump, the relationships of Eqs. 4.9 to 4.9c apply (see also [1] Sect. 5.5.2.2.2). The calculated NPSH value NPSHa of the pump system under certain operating conditions should be greater by a factor f NPSHsafety than the NPSH value NPSHr,3% of the feed pumps under the same conditions.

408

4

Seawater Extraction and Supply and Concentrate Discharge

NPSHa > NPSHr,3%

ð4:9Þ

NPSHa  NPSHr,3%  f NPSHsafety

ð4:9aÞ

NPSHr,3% 

NPSHa f NPSHsafety

ð4:9bÞ

NPSHa¼ net positive suction head available [m] NPSHr, 3% ¼ net positive suction head requested at 3% head loss [m] f NPSHsafety ¼ NPSHr3% safety factor: 1.2–1.5 The design NPSH for the pump station NPSHa must be determined for the worstcase operating conditions of the SWRO seawater supply pumps, namely, for the minimum water level at low tide on the suction side ΔHW,SWs,min and the maximum vapour pressure of the seawater pv,SW,max at the highest seawater temperature occurring at the location. The NPSHa value at these conditions is then calculated with Eq. 4.9c for atmospheric inlet operation of the pumps from the water level at their intakes ΔHW,SWs,min, the atmospheric pressure pa, the maximum seawater vapour pressure pv,SW.max (see [1] Chap. 3, Sects. 3.2 and 3.2.1.4, and [1] Annex 3.A2, Table 3.20), and the pressure losses ∑ΔHSp in the suction line. NPSHa ¼ 105 

X pa  pv,SW, max ΔH Sp þ ΔH W,SWs, min  ρF  g

ð4:9cÞ

pa ¼ atmospheric pressure [bar] pv,SW,max ¼ vapour pressure of seawater at max. Operating temperature [bar] ΔHW, SWs, min ¼ water level at suction point of SW supply pump at low tide [m] In accordance with the NPSHa value calculated for these operating conditions, according to Eq. 4.9b the pumps selected for seawater supply to the SWRO plant should have an NPSHr, 3% value that will ensure low-vibration and cavitation-free operation and which is at least a factor of f NPSHsafety less than the value of the NPSHa. The delivery FP, supply, SWRO of the individual supply pumps is derived from the total seawater withdrawal flow FW, SW, T for the SWRO plant and the number of pumps nPu, op installed to operate the intake (Eq. 4.9d). The total number of pumps installed nPu, total is made up of the pumps required to operate the intake nPu, op and the number nPu, sb of standby units. F P,supply,SWRO ¼

F W,SW,T nPu,op

nPu,total ¼ nPu,op þ nPu,sb

ð4:9dÞ

4.1 Seawater Extraction and Supply

409

FP, supply, SWRO ¼ SWRO supply pump capacity [m3/h] nPu, op ¼ number of pumps for operation [no.] nPu, total ¼ total number of pumps installed [no.] nPu, sb ¼ number of standby pumps [no.] The number of supply pumps of the intake installation is largely determined by the bandwidth over which the SWRO system has to deliver its product output flow FPr,SWRO. The specified seawater extraction flow FW,SW,T for the desalination plant and thus also the required delivery range of the seawater pumping station of the intake is designed accordingly. The respective delivery flows of the supply pumps shall be so selected that, if the number of pumps in operation is appropriately staggered, the full range of extraction flows can be provided from the minimum up to the maximum required flow. If it is required to operate the SWRO plant with wide product water flow swings, it is advisable to equip the seawater supply pumps with infinitely variable speed controls to avoid throttling of the pump delivery and thus increased energy consumption. Speed control is especially necessary if the SWRO plant is designed with a pressurized system from the intake to the high-pressure RO pumps. In this case, the design of the intake pumps must also allow their delivery flow and head to be adjusted to match the varying inlet flow to the pretreatment stage and cartridge filtration units and the fluctuation in their differential pressure, as well as to any necessary changes in the feed pressure to the high-pressure RO pumps. In the case of indirect seawater withdrawal, the extraction flow bandwidth also plays a role in determining the total number of well installations needed, the number of units in operation, the number of standby wells, and the resulting numbers of well pumps and feed pumps.

4.1.4.2 Power Demand of Intake Systems The power demand PD,int of an intake system with direct extraction is the sum of the power demands of the SWRO supply pumps PD,P,supply,SWRO,i plus the power demand of the intake screening systems PD,Ws,i according to Eqs. 4.10 and 4.10a. For active screening equipment, their power demand PD,Ws,i is determined by the share for driving the moving screening elements or the rotating drum and that for the pressurized water spray for dislodging solids from the screens’ surfaces. A passive screening system consumes power to operate the compressor for generating the compressed air required to flush the screen elements of its extraction heads. PD,int ¼

i¼n Pu,op X

PD,P,supply,SWRO,i þ

X

PD,Ws,i

ð4:10Þ

i

PD,P,supply,SWRO,i ¼

F P,supply,SWRO,i  ΔH PF,SWRO,i  ρSW  g 3:6  106  ηP,i  ηM,i  ηVSD,i

ð4:10aÞ

410

4

Seawater Extraction and Supply and Concentrate Discharge

PD, int ¼ power demand intake system [kW] ∑PD, P, supply, SWRO, i ¼ total power demand of SWRO supply pump units i [kW] ∑PD, Ws, i ¼ total power demand of SW withdrawal and screening system units i [kW] ΔH PF,SWRO,i ¼ head of seawater supply pump unit i [m] ρSW ¼ density of seawater [kg/m3] ηP,i ¼ efficiency factor of pump unit i [] ηM,i ¼ efficiency factor of motor of pump unit i [] ηVSD,i ¼ efficiency factor of variable speed drive of pump unit i For indirect seawater extraction, the power demand of the intake PD,int results from that needed to operate the pumps of the wells or the well installations and additional supply pumps, should these be needed. The power take-up of the individual pumps at their respective operating conditions is calculated with Eq. 4.10a.

4.2

Concentrate and Wastewater Discharge

The concentrate resulting from the desalination process in the SWRO plant is returned to the sea via the SWRO outfall together with the plant’s wastewater.

4.2.1

Discharge Flow of SWRO

The discharge flow FD,SWRO for which the outfall system is to be dimensioned is made up of the concentrate discharge flow of the SWRO plant FC,SWRO, the wastewater discharge flow FWT,SWRO and, in the case of partial load of the desalination plant, the seawater flow FD,byp which may be fed to the outfall in a bypass to the SWRO (see Eq. 4.11). F D,SWRO ¼ F C,SWRO þ F WT,SWRO þ F D,byp

ð4:11Þ

FD, SWRO ¼ discharge (outfall) flow of SWRO [m3/h] FC, SWRO ¼ concentrate discharge flow of SWRO [m3/h] FWT, SWRO ¼ wastewater discharge flow of SWRO [m3/h] FD, byp ¼ bypass seawater flow to outfall at SWRO part-load operation [m3/h] The wastewater capacity FWT, SWRO to be discharged consists of the portion of wastewater from the filtration systems of the pretreatment FW, Pr, the wastewater from the reverse osmosis systems FW, RO1 and FW, RO2 and the wastewater from the post-treatment FW, Posttr (Eq. 4.11a).

4.2 Concentrate and Wastewater Discharge

411

F WT,SWRO ¼ F W,Pr þ F W,RO1 þ F W,RO2 þ F W,Posttr

ð4:11aÞ

FW, Pr ¼ wastewater discharge flow of pretreatment [m3/h] FW, RO1 ¼ wastewater discharge flow of RO1 [m3/h] FW, RO2 ¼ wastewater discharge flow of RO2 [m3/h] FW, Posttr ¼ wastewater discharge flow of post-treatment [m3/h] Depending on the configuration of the RO stages and how they are operated, the concentrate discharge flow FC, SWRO will be different. If only one RO stage is operated with the plant in single-pass mode, the concentrate flow FC, SWRO ¼ FC, RO1 is calculated from the plant’s product water flow FPro, SWRO and the product recovery factor YRO1 of the membrane stage as given by Eq. 4.11b.  F C,SWRO ¼ F C,RO1 ¼ F Pro,SWRO 

1

Y RO1

 1

ð4:11bÞ

FC, RO1 ¼ concentrate discharge flow of RO1 [m3/h] FPro, SWRO ¼ product flow from the SWRO plant including its internal consumption [m3/h] If the desalination plant is operated as a two-pass system, i.e. with a secondary desalination stage but without returning its concentrate to the inlet of the first pass, the concentrate flow FC, SWRO to be discharged via the outfall depends on the product recovery factor of the first pass YRO1, that of the second pass YRO2, the total product recovery factor of the RO tract YRO1, RO2, and the capacity factor fC, RO2 of the second pass as shown by Eq. 4.11c. The total product recovery factor YRO1, RO2 for this mode of operation is determined using Eq. 4.1c (see also [1] Sects. 4.2.1.4.1 and [1] 4.2.1.4.2). F C,SWRO ¼ F C,RO1 þ F C,RO2  

1  Y RO1 1 ¼ F Pro,SWRO  þ f C,RO2  1 Y RO2 Y RO1,RO2 FC, RO2 ¼ concentrate discharge flow of RO2 [m3/h] YRO1, RO2 ¼ product recovery factor of RO tract of SWRO [] YRO1 ¼ product recovery factor of RO1 [] YRO2 ¼ product recovery factor of RO2 [] fC, RO2 ¼ capacity factor of RO2 (RO second pass) []

ð4:11cÞ

412

4

Seawater Extraction and Supply and Concentrate Discharge

For a two-pass system with concentrate recycling to the inlet of the first pass, the concentrate discharge flow FC, SWRO, R is as shown by Eq. 4.11d. For this operating mode, the product recovery factor YRO1, RO2, R of the RO tract is calculated with Eq. 4.1d. F C,SWRO,R ¼ F C,RO1,R ¼ F Pro,SWRO  ð1  Y RO1 Þ  

1 1  þ f C,RO2  1 Y RO1,RO2,R Y RO2

ð4:11dÞ

FC, SWRO, R ¼ concentrate discharge flow of SWRO with RO2 concentrate recirculation [m3/h] FC, RO1, R ¼ concentrate discharge flow of RO1 with RO2 concentrate recirculation [m3/h] YRO1, RO2, R ¼ product recovery factor of RO systems of SWRO with RO2 concentrate recirculation [] The lowest concentrate flow is generated in the single-pass system operating mode, the highest concentrate flow in a two-pass system without concentrate recirculation, while with concentrate recirculation the concentrate flow of a two-pass system lies between these two operating modes. However, the concentrate flow that has to be discharged from two-pass systems increases with increasing values of the capacity factor fC, RO2 of the second pass. This means that when designing the outfall, the influence of membrane age, temperature, and fouling on this parameter and its resulting increase up to when the post-desalination stage is operated with the SWRO plant’s total product flow ( fC, RO2 ¼ 1) must be taken into account. The total wastewater of an SWRO plant consists of that generated in the pretreatment stage during filter backwash FW, Pr; the wastewater from the RO stages FW, RO1 and FW, RO2 for rinsing, cleaning, and preservation of the membranes; and the wastewater from the product post-treatment stage FW, Posttr, i.e. backwash water from the limestone filters (Eq. 4.11a). The largest wastewater flow to be discharged is that from pretreatment FW, Pr, which also occurs periodically and at the shortest time intervals (see Sect. 2.3.4.3.1). The wastewater from the RO stages FW, RO1 and FW, RO2 arises only at greater intervals (see [1] Sect. 5.5.2.5), while wastewater from product water post-treatment FW, Posttr only occurs if this is equipped with limestone filters and there is no recovery of the backwash water within the filtration system (Sect. 3.4.2.2.2). All wastewater flows from the SWRO plant occur in batches, and by providing appropriate buffer capacities, the wastewater flow to be discharged via the outfall can be evened out. The outfall has then to be designed as a worst-case scenario on the basis of parallel discharge of the pretreatment wastewater FW, Pr plus that of an RO pass FW, RO1 or FW, RO2, plus possibly the post-treatment wastewater flow FW, Posttr.

4.2 Concentrate and Wastewater Discharge

413

In the absence of judicious wastewater buffering, the maximum wastewater flow generated in batches in the pretreatment stage should be selected as the value for the wastewater flow of the pretreatment stage FW, Pr (see Eqs. 2.91d and 2.108). Wastewater discharge peak flows that would result from simultaneous discharges of the various partial wastewater flows must be flattened by their appropriate timing. If there is sufficient buffering, the wastewater flow from the pretreatment stage can be estimated using Eq. 4.11e. F W,Pr 

F Pro,SWRO  ð1  Y P,Pr,T Þ Y T,SWRO

ð4:11eÞ

YT, SWRO ¼ overall product recovery factor of SWRO plant [] YP, Pr , T ¼ product recovery factor of the pretreatment stage []

4.2.2

Salt Content of SWRO Discharge

The salinity cC, RO of the concentrate to be discharged via the outfall is determined by the salinity of the seawater and the configuration of the RO stages of an SWRO plant together with its mode of operation and depends on the product recovery factors YRO1, YRO2, YRO1, RO2, or YRO1, RO2, R. For operation as a single-pass reverse osmosis stage, the concentrate salinity cC, RO1 is calculated from Eqs. 4.12 to 4.12b. cF,SW  ð1  Y RO1  SPRO1 Þ 1  Y RO1

ð4:12Þ

cF,SW  ð1  Y RO1  ð1  RRO1 ÞÞ 1  Y RO1

ð4:12aÞ

cC,RO1 ¼ cC,RO1 ¼

cC,RO1 ¼ cF,SW 

1 Y RO1  cPro,RO1  1  Y RO1 1  Y RO1

ð4:12bÞ

cC, RO1 ¼ concentration (TDS) of concentrate of RO1 [mg/l] cF, SW ¼ concentration (TDS) of seawater [mg/l] SPRO1 ¼ salt passage factor of RO1 [] RRO1 ¼ salt rejection factor of RO1 [] cPro, RO1 ¼ concentration (TDS) of product water of RO1 [mg/l] By neglecting the low salt concentration cPro, RO1 of the reverse osmosis product water of RO1, Eq. 4.12c is obtained.

414

4

Seawater Extraction and Supply and Concentrate Discharge

cC,RO1 ¼ cF,SW 

1 1  Y RO1

ð4:12cÞ

For a two-pass system without concentrate recirculation, the salinity cC, RO1, RO2 of the concentrate of this RO stage is given by Eq. 4.12d. If the salinity of the SWRO plant’s product water is neglected for the calculation, Eq. 4.12e results. The total product recovery factor YRO1, RO2 is calculated using Eq. 4.1c. cC,SWRO ¼ cC,RO1,RO2 ¼

cF,SW  Y RO1,RO2  cPro,RO1,RO2 1  Y RO1,RO2

cC,SWRO ¼ cC,RO1,RO2 ¼ cF,SW 

cC,

RO1, RO2

1 1  Y RO1,RO2

ð4:12dÞ ð4:12eÞ

¼ TDS of concentrate of RO tract without concentrate recirculation

[mg/l] YRO1, RO2 ¼ product recovery factor of RO tract of SWRO without concentrate recirculation [] cPro, RO1, RO2 ¼ TDS of product water of RO tract without concentrate recirculation [mg/l] If in a two-pass RO system concentrate from the second pass is fed back into the feed to the first pass, the salinity of the concentrate cC, RO1, RO2, R is calculated as shown in Eq. 4.12f. If the salinity of the SWRO product water is not taken into account, Eq. 4.12g is obtained. The value of the product recovery factor YRO1, RO2, R of such a system is calculated using Eq. 4.1d. cC,SWRO,R ¼ cC,RO1,RO2,R ¼

cF,SW  Y RO1,RO2,R  cPro,RO1,RO2,R 1  Y RO1,RO2,R

cC,SWRO,R ¼ cC,RO1,RO2,R ¼ cF,SW 

1 1  Y RO1,RO2,R

ð4:12fÞ ð4:12gÞ

cC, SWRO, R ¼ TDS of concentrate of SWRO with RO2 concentrate recirculation to feed of RO1 [mg/l] cC, RO1, RO2, R ¼ TDS of concentrate of RO tract with RO2 concentrate recirculation to feed of RO1 [mg/l] cPro, RO1, RO2, R ¼ TDS of product water of RO tract with concentrate recirculation [mg/l] YRO1, RO2, R ¼ product recovery factor of RO tract of SWRO with concentrate recirculation [] The salinity cD, SWRO of the effluent flow FD, SWRO to be discharged via the outfall is derived from the salinity cC, SWRO of the concentrate from the RO tract, the salinity

4.2 Concentrate and Wastewater Discharge

415

cWT, SWRO of the plant’s wastewater that is added to the concentrate, and the salinity cSW of the seawater that is fed into the outfall when the SWRO plant is bypassed at part load (Eq. 4.12h). cD,SWRO ¼

F C,SWRO  cC,SWRO þ F WT,SWRO  cWT,SWRO þ F D,byp  cSW F C,SWRO þ F WT,SWRO þ F D,byp

ð4:12hÞ

cD,SWRO ¼ concentration of discharge of SWRO [mg/l] cWT, SWRO ¼ total concentration (TDS) of wastewater discharge of SWRO [mg/l] FD, byp ¼ bypass seawater flow to outfall at SWRO part-load operation [m3/h] cSW ¼ TDS of seawater [mg/l] Depending on whether the filter systems in the pretreatment stage are backwashed with reverse osmosis concentrate or seawater, the salt content of the outfall is reduced or remains unchanged by adding this wastewater. The elevated salinity in the SWRO plant’s outfall discharge means that it has a higher density as well as greater dynamic and kinematic viscosities compared to seawater. The density corresponding to the salinity of the SWRO discharge can be calculated using [1] Eqs. 3.39 and 3.40 in [1] Sects. 3.2.2 and 3.2.2.2.1 or taken from [1] Fig. 3.15 or the table in [1] Annex 3.A2, Table 3.23. The viscosities corresponding to the salinity of the outfall are obtained from [1] Sect. 3.2.2.2, Eqs. 3.41 and 3.42, Fig. 3.16, Annex 3.A2, Table 3.24 and [1] Sect. 3.2.2.3, Eq. 3.43, Fig. 3.17, Annex 3.A2, Table 3.25.

4.2.3

Discharge Diffuser Systems and Their Design

When discharging the concentrate and wastewater of an SWRO plant into the sea, a spatially limited zone is usually defined around the discharge point in accordance with the environmental legislation applying at the plant’s location. Within this zone, the outfall of the SWRO has to be mixed with the seawater, and a maximum value for the increase in seawater salinity is specified at the zone’s boundary. For such legal or regulatory mixing zones, a radius for the mixing zone of 50–300 m around the point of discharge of the outfall is defined by the water authorities on the basis of the wastewater regulations applying at the SWRO’s location but also depending on bathymetric conditions, such as the water depth at the point of discharge. The permissible increase in salinity at the mixing zone boundary Δcregul is stipulated either as an increment in the range of 1–4 parts per thousand (ppt) or as a percentage Δcregul, % of the seawater salinity of up to 5% [19, 20]. The dilution of the salinity SR of the SWRO plant’s discharge required for the specified allowable increment of salinity increase Δcregul or Δcregul, % up to the mixing zone boundary is calculated with Eqs. 4.13 to 4.13c. The salinity which then results at this boundary is calculated according to Eq. 4.13d.

416

4

Seawater Extraction and Supply and Concentrate Discharge

cD,SWRO  cSW Δcregul

ð4:13Þ

ðcD,SWRO  cSW Þ  100 Δcregul,%  cSW

ð4:13aÞ

SR ¼ SR ¼

Δcregul,% ¼

Δcregul  100 cSW

ð4:13bÞ

cD,SWRO  cSW cemz  cSW

ð4:13cÞ

cemz ¼ cSW þ Δcregul

ð4:13dÞ

SR ¼

SR ¼ required dilution rate [] Δcregul ¼ max. Concentration increase at the regulatory mixing zone boundary [mg/l, ppt] Δcregul, % ¼ percentage of concentration increase at the regulatory mixing zone boundary [%] cemz ¼ concentration at the regulatory mixing zone boundary [mg/l, ppt] Thus, for an SWRO plant’s discharge with a salinity of 61 ppt into seawater with a salinity of 35 ppt at a maximum allowable increment of the increase in salinity Δcregul of 1 ppt, with Δcregul, % corresponding to 2.9%, a dilution rate SR of 26 is required to comply with these values for the allowable increase in salinity at the mixing zone boundary. Due to its elevated salinity, the SWRO discharge has a correspondingly higher density compared to that of seawater. For example, at a temperature of 20  C, a discharge with a salinity of 61 ppt compared to seawater with a salinity of 35 ppt has a density that is about 17 kg/m3 higher. As a result, upon its discharge into the sea SWRO effluent sinks to the seabed. The extent to which this sinking occurs is characterized by the buoyant acceleration g00 as shown by Eq. 4.14. For the outfall discharge of seawater reverse osmosis plants, g00 always shows strongly negative values, i.e. it has a significant negative buoyancy. g00 ¼ g 

ρD,SWRO  ρSW ρD,SWRO

ð4:14Þ

g00 ¼ buoyant acceleration [m/s2] < 0 negatively, > 0 positively buoyant g ¼ standard gravitational acceleration ¼ 9.80665 [m/s2] ρD, SWRO ¼ density of SWRO outfall discharge at temperature t [kg/m3] ρSW ¼ density of seawater at temperature t [kg/m3] If an effluent is discharged in an uncontrolled mode into the sea, its mixing with seawater is dependent on the bathymetric structure of the seabed as well as on

4.2 Concentrate and Wastewater Discharge

417

Fig. 4.23 SWRO outfall: multiport diffuser configurations

currents and wave motion at the point of discharge, so a controlled mixing, as it is necessary, when a mixing zone of a limited size is defined cannot be achieved in this way. Intensive and controlled mixing of SWRO effluent with the seawater is achieved by means of diffusers, for which purpose the exit point of the liquid is designed as a nozzle. The denser SWRO effluent is then discharged as a liquid jet into the surrounding seawater with which it mixes due to the resulting turbulence (see Fig. 4.24). Depending on the flow to be discharged via the SWRO outfall, the diffusers are configured with a number of ports, i.e. designed as multiport devices, to split up the SWRO discharge. Figure 4.23 shows various designs of such multiport diffusers to cater for discharge flows at the higher end of the range as side and top views. The ports can be attached on one side (A) or both sides (B) of the SWRO outfall pipe or on risers (C), which allows a certain distance from the seabed to be maintained. A special type of multiport is the rosette diffuser, where a number of ports are attached to a riser and the jets are directed equally split up in different directions (D). The port nozzles are directed upwards at a certain angle to the sea floor, so that the outflowing high-density liquid jet is first directed upwards towards the sea surface and then sinks again as a result of its negative buoyancy. The point where the jet drops to the seabed is called the impact point (Fig. 4.24). The region between the exit of the liquid jets from the diffusers and the point of impact is referred to first as the

418

4

Seawater Extraction and Supply and Concentrate Discharge

Fig. 4.24 SWRO outfall: single-port configuration and jet dimensions

near-field zone and then the intermediate-field zone, while the region beyond the point of impact is called the far-field zone. How and to what extent the liquid jet emerging from the diffuser’s nozzle is formed depends on the jet’s exit speed ve,diff and the diameter ddiff of the port’s nozzle. Both parameters are characterized by the densimetric Froude number Fr0 according to Eq. 4.15. ve,diff Fr 0 ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  g0   ddiff 0

ð4:15Þ

Fr0 ¼ densimetric Froude number [] ve, diff ¼ velocity at diffusor nozzle exit of port [m/s] ddiff ¼ diameter of diffusor nozzle exit of port [m] If the Froude number Fr0 or the flow per port of the diffuser Fport is specified and the diameter of the nozzle ddiff of the port is known, the exit velocity ve, diff is calculated as shown in Eq. 4.15a. Fport is derived from the SWRO plant’s outfall flow and the number of ports of the diffuser system with Eq. 4.15b. ve,diff ¼ Fr0  F port ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   g0   ddiff ¼ 4  F port 0 π  d2diff

F D,SWRO π ¼  d2  ve,diff 3:6  103  nports 4 diff

ð4:15aÞ ð4:15bÞ

4.2 Concentrate and Wastewater Discharge

419

Fport ¼ discharge flow of port [m3/s] nports ¼ number of ports of the diffusor system [no.] FD, SWRO ¼ discharge (outfall) flow of SWRO [m3/h] The diameter ddiff of the port nozzle is given by the parameters Fr0, g00 , ve, diff, or Fport as shown by Eq. 4.15c.  ddiff ¼

ve,diff Fr0

2

0

125  F port ¼@  1 A Fr0  g0 2 4 π

   g0  0

ð4:15cÞ

0

To achieve thorough mixing of the discharge jet with the seawater, its Reynolds number Rejet (see Eq. 4.15d) is to be in the turbulent range of >4000. Re jet ¼

ve,diff  d diff νD,SWRO

ð4:15dÞ

Rejet ¼ Reynolds number of jet [] νD, SWRO ¼ kinematic viscosity of SWRO discharge at specified temperature and salinity [m2/s] The angle θjet (see Fig. 4.24) at which the port nozzles are aligned to the horizontal plus the slope angle of the seabed θbottom also influence how the exit jet is formed and thus the degree of mixing and the extent of dilution which can be achieved in the near- and intermediate-field zones by the diffuser system. In order to determine the geometry of the discharge jet at different Froude numbers, nozzle diameters and nozzle angles, and the resulting degree of dilution at various points of the jet, extensive laboratory tests were carried out under defined Froude numbers and flow conditions in the discharge area of diffuser ports for diffusers in single-port, multiport, and rosette configurations [21–26]. The outcome of these investigations are equations with empirical coefficients as compiled in Table 4.5 for a diffuser with a single port and under stagnant flow Table 4.5 Outfall diffuser—design C-factor values for dimensions and possible dilution rate of single-jet diffusers under stagnant conditions

No. 4.5.1

Parameter Dilution at Impact point

4.5.2

Near field

4.5.3

Location of impact point

4.5.4

Near-field length

4.5.5

Height of jet rise

Equation Si Fr 0 Sn Fr 0

 C1  C2

xi d diff Fr0 xn d diff Fr0 yt d diff Fr0

Nozzle angle 45 30 C-factor 1.20 1.60

60

75

1.65

1.50

1.85

2.50

2.60

2.10

 C3

3.50

3.60

2.75

1.90

 C4

10.30

11.00

9.50

8.40

 C5

1.18

1.80

2.25

2.64

420

4

Seawater Extraction and Supply and Concentrate Discharge

Table 4.6 Outfall diffuser—recommended value ranges for design parameters Parameter Froude number Nozzle exit velocity Nozzle exit diameter Reynolds number of jet Discharge angle Port height Max. height of jet rise

Symbol Fr0 ve, diff ddiff Rejet θjet hport hport + yt

Unit – m/s m – 

m m

Design value range  10, recommended 20–25 4–6 0.1–1, recommended 0.25 > 4000 30–60, for deep water 60 0.5–1  0.75*hwd to 0.9*hwd

conditions in the discharge area [25], with which the dilution Si at the point of impact xi of the jet (Eq. 4.5.1) and the dilution Sn at the end of the near-field zone (Eq. 4.5.2) can be determined. Further, with Eqs. 4.5.3, 4.5.4 and 4.5.5, respectively, the position of the impact point xi, the length of the near-field zone xn, and the maximum height of the jet rise yt can be determined. This simplifying model shows that the achievable dilution is greatest at a nozzle angle of 60 degrees [24], and this finding can also be transferred to more complex diffusion systems. For the design of outfall diffuser systems in practice, if the depth of water is sufficient, this is the angle that is most often used. In shallower water, though, the inclination of the nozzles has to be reduced [22, 25]. With multiport and rosette diffusers, the discharge jets interact with each other, and the extent of this depends on the configuration of the ports in the diffuser systems [22, 23, 26]. In the context of a project of the Middle East Desalination Research Center, MEDRC, on the environmentally oriented planning and management of the discharge of brine from desalination plants [19], Bleninger et al. developed an Excel calculation tool, the Desalination plant discharge calculator, with which a largescale multiport diffuser system for stationary, non-flowing ambient conditions can be designed [19, 27]. The report on this project as well as the calculation tool can be downloaded from the Internet address1 given below. The ranges of design values for the various design parameters for dimensioning diffuser systems according to [25, 27] are shown in Table 4.6. Depending on the angle of inclination of the ports of the diffuser system and the resulting height that the discharge jet attains, in order to avoid contact of the jet with the surface of the sea, the point of discharge of the SWRO outfall is located at a distance of from several hundred metres up to more than one kilometre from the coast, depending on the slope of the seabed. The conceptual design of a multiport diffuser system for the reference SWRO plant with 110,000 m3/d product flow under stagnant flow conditions, based on the calculation tool referred to above, can be found as a table in Annex 4.A1.

1

http://www.ifh.uni-karlsruhe.de/science/envflu/Research/brinedis/default.htm

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421

With a diffuser system dimensioned for a specific SWRO flow and thus also a corresponding outfall flow, a defined degree of dilution within the specified dimensions of the regulatory mixing zone can only be achieved with the discharge velocity ve,diff and Froude number Fr0 corresponding to the design value of the discharge flow. When the SWRO is under part load, these operating parameters are reduced, and consequently the turbulence in the diffusers’ discharge area is also lessened. To maintain the degree of dilution in the mixing zone during part-load operation of the SWRO plant, there are three possibilities: • Some of the ports of the diffuser system are shut off by means of remotecontrolled valves installed in them. • An appropriate share of the seawater flow from the plant’s intake is piped to the outfall via the SWRO plant’s bypass. • The ports are equipped with so-called duck bill valves, whose cross-section is reduced as flow decreases.

4.2.3.1 Software Systems for Modelling of Near-Field and Far-Field Zone Conditions 4.2.3.1.1 Near- and Intermediate-Field Zones In the near-field zone, the attainable degree of dilution depends mainly on turbulence, which is a function of the physical conditions fixed by the design of the diffuser systems such as their Froude number, diameter of the port nozzles, and jet exit angle. In reality, however, an SWRO plant’s outfall does not discharge into the sea under steady-state conditions for which the current and density conditions at the point of discharge would be stagnant but into a dynamic environment in which local currents, tidal currents, and wave motion, as well as density changes occurring during dilution of the outlet jets, also influence the degree of dilution. In addition, the bathymetric characteristics of the seabed affect the mixing of the outfall discharge with the seawater. Whereas in the near-field zone it is primarily the physical conditions during operation of the diffuser systems that determine the degree of mixing, in the intermediate-field zone in particular the specific ambient conditions, such as ambient and tidal currents, density stratification, boundary conditions, etc., in the wider area around the discharge point impact the mixing action and thus the degree of dilution. The influence of non-stagnant flow conditions at the discharge point on the initial design of a diffuser system as derived with the desalination plant discharge calculator for stagnant conditions can be ascertained in a further step with the aid of more complex calculation software with which these non-stagnant conditions, and also other ambient conditions can be mathematically modelled. These calculation results can then be used for detailed design of the system on the basis of the operating conditions of the diffusers that are required for a specified degree of dilution of the salinity in the near-field zone. Such software suites, which for the discharge of SWRO outfall into the sea also include corresponding algorithms for calculating non-stagnant flow conditions in the

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near- and intermediate-field zones, are the commercial computer programs CORMIX, the Cornell Mixing Zone Expert System2 [19] and VISJET.3 CORMIX software is used to model the mixing and dilution processes in the near- and intermediate-field zones. It comprises a number of calculation modules and modules for tabular and graphical output of the calculation results. Submerged diffuser systems are best calculated with the DHYDRO calculation module for the discharge of the SWRO outfall with a higher density than seawater, i.e. for the discharge of brine with negative buoyancy. With this module, it is possible to select whether a calculation should be prepared for single-port or multiport diffusors. However, calculation of a discharge with a higher density compared to seawater is also possible with the modules CORMIX 1 for single-port diffusers and CORMIX 2 for diffuser systems with several ports. The number of ports must be entered and which outlet diameter and outlet angle they should have. For calculating multiport units, the number and configuration of ports within the diffuser systems can be specified accordingly. In the three calculation modules, it is also possible to represent a simplified model of the influence of bathymetry. With all calculation modules, the dimensions of a regulatory mixing zone and the permissible percentage increase in salinity at its boundary can also be specified. Another calculation module in CORMIX is CORJET, which is the Cornell Buoyant Jet Integral Model. Within this module, too, a distinction is made between single-port and multiport diffusors in the calculation. In addition, when entering the ambient conditions, this calculation model allows the input of different salinity levels and densities, temperatures, and flow velocities at various flow angles for up to ten water depths. This routine finds application within the calculation modules CORMIX 1, CORMIX 2, and DHYDRO for calculating the near-field zone. It can also be used separately with calculation files of the three calculation modules mentioned above or independently of them with separate input data to model the close-up zone of a diffuser system [28]. The CORMIX software suite contains as a further calculation module the CorHyd program, with which the hydraulic conditions within the entire outfall system from the outfall basin via the discharge pipe and including the diffuser system with its corresponding number of ports, their configuration, and even for the use of duck bill valves at the ports can be calculated. With this program, it is possible both to determine for a certain design of an outfall system the flow rate, the flow velocity, and the pressure as well as the pressure loss in individual sections of the system and thus to determine either its possible discharge flow or, for a given discharge flow, the required geodetic height or required filling level in the outfall basin to achieve this. Depending on the hydraulic profile of an SWRO plant’s outfall system, this calculation module can therefore also be used to determine whether the outfall can be operated under gravity or whether pumping stations have to be installed.

2 3

http://www.cormix.info/index.php http://www.aoe-water.hku.hk/visjet/visjet.htm

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VISJET software is also a calculation program for mathematical modelling in the near- and intermediate-field zones and includes the JETLAG module for calculating the processes in the near-field zone. The type and extent of data input for this modelling correspond to the comparable calculation modules of the CORMIX software suite with the exception that in VISJET, bathymetric influences are not considered. The calculation program is equipped in its program interface with a number of visualization routines, with which the formation and orientation of the discharge jets, calculated according to the dimensioning and configuration of the diffuser systems and the specified velocity and direction of the flow, are graphically displayed already during the calculation run. By selecting the respective points or sections of the graphic representations shown on the screen, the corresponding calculated values can be retrieved from the graphic in real time and displayed in associated tables. 4.2.3.1.2 Far-Field Zone From the discharge jets of the diffuser system generated in the active mixing area of the near-field zone, a plume develops in the intermediate-field zone with concentrations of salt and other substances, such as particulate matter, that differ from the surrounding seawater conditions. This outfall plume extends beyond the intermediate-field zone into the far-field zone, and knowledge of its extent and the direction of its propagation within the dynamic ambience of the intermediate and far field is essential for the positioning of the SWRO plant’s intake and outfall relative to each other in order to avoid a drop in the quality of the seawater extracted at the intake by the outfall discharge, i.e. to prevent recirculation or short-circuiting of elevated salinity and other constituents into the SWRO plant’s inlet. The range of the near- and intermediate-field zones extends from up to several tens of metres for the near-field zone to several hundred metres for both zones. The further the plume moves away from the vicinity of the point of discharge of the outfall, the more its transport and extent as well as its further dilution will be influenced by the natural environmental conditions of the sea such as • • • • • •

The prevailing flow conditions and their direction Wave motion and turbulence caused by wind stress Tidal influences Temperature and temperature fluctuations Recirculation Bathymetric and topographic conditions

and the resulting combination of mixing and diffusion processes. It is therefore not possible to use the calculation software applied for modelling the transport and mixing processes in the near- and intermediate-field zones for mathematical modelling of the various dynamic processes occurring in the far-field zone. More complex hydrodynamic 3D software packages find application for these which, unlike the calculation software used for the near- and intermediate-field zones as a basis for modelling, require to a much greater extent the input of data on the

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conditions prevailing in the offshore area under investigation and how these vary. In order to obtain these data, existing oceanographic surveys must be drawn on, appropriate measurements must be taken during field studies, and available data must be supplemented with field measurements. Such calculation software is included in the commercial Delft 3D suite for dynamic simulation among others of flows, wave motions, and density and concentration profiles in the far-field zone.4 With these applications, in particular the module Delft 3D-FLOW, it is possible to model the further propagation of the outfall plume and the development of its concentration profile in the far-field zone on the basis of the data generated by calculation software for the outfall plume forming in near-field zone, such as CORMIX. Just as the discharge conditions in the near-field zone influence the behaviour of the outfall plume in the far-field zone, the conditions there can also have repercussions for the mixing intensity and the direction of the discharge jets of the diffuser systems in the near- and intermediate-field zones. In order to model these mutual influences, the two types of calculation software, for instance, the CORMIX near-field/intermediate-field software and the Delft 3D-FLOW far-field software, are coupled so that there is a mutual data exchange between the modules [19, 29]. With what is referred to as dynamic coupling between the calculation software suites, both changes in the concentrations of the SWRO discharge and the operating mode of the diffusers can be simulated in their effects on the behaviour of the outfall plume in the far-field zone, as well as how the far-field zone, such as changes there in the flow conditions and intensity of the wave motion, impact the development over time of the plume in the near- and intermediate-field zones.5 This makes it easier to model the conditions in the general surroundings of the outfall discharge point, also with regard to the effects of how the intake and outfall are configured relative to each other. The processes involved in the discharge of the SWRO outfall and its mixing in the existing seawater environment as well as the propagation of the resulting outfall plume in the vicinity of the discharge point are complex, are influenced by a large number of design and environmental parameters, and are therefore difficult to model. For this reason, the calculation software suites described above contain in part assumptions and simplifications of an empirical nature, in order to nevertheless allow an estimate of certain influences. In the case of the near-field/intermediatefield calculation software, this applies in particular to the bathymetric situation, which, depending on the software, is either not taken into account at all or only highly simplified. This can lead to simulation outcomes that differ between calculation models and depend on the conditions under which the empirical assumptions were derived at the time of their development. To what extent the results of mathematical modelling in the near-field/intermediate-field correspond to the actual situation in these zones depends greatly on the nature and complexity of the prevailing ambient conditions [30].

4 5

https://www.deltares.nl/en/software/delft3d-4-suite/ http://www.cormix.info/cormix-delft3d.php

Annexes

425

When designing large-scale multiport and rosette diffuser systems, to supplement numerical modelling, trials on a pilot scale are conducted for physical modelling of such systems. For their further optimization, the formation and mutual influencing of the discharge jets as well as the development of the outfall plume and the resulting degree of mixing and dilution in the near-field zone are investigated, depending on the configuration of the ports in the diffuser system regarding their mutual spacing and alignment as well as the ports’ angle of inclination from the horizontal. The results of this together with the outcome of numerical modelling then form the basis for the design of the diffuser system. In the far field, the quality of the modelling is largely determined by the amount of available oceanographic data and how they are influenced by weather conditions, as well as the available bathymetric and topographic information. Although the numerical models can only predict the actual concentration ratios in the near-field/intermediate-field and far-field zones area with certain limitations, they are an important aid for SWRO outfall design, since it is possible to apply these, especially with dynamic coupling of the near-field and far-field calculation software, to represent the impacts on the general surroundings that would result from a change of particular operating or environmental parameters in terms of orders of magnitude or at least relative to the initial situation.

Annexes 4.A1 SWRO Outfall Discharge: Diffuser System Conceptual Design

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Seawater characteristics 20.00 x Temperature 35.00 x Salinity 1,024.53 x Density 1.05E-06 x Kin. viscosity SWRO product water and intake flow rate 4,140 x Product flowrate 43 x Recovery rate SWRO 2.68 x Intake flowrate SWRO concentrate characteristics 1.53 x Plant effluent flowrate 25.00 x Temperature 61.40 x Salinity 1,042.99 x Density Admixture to concentrate (wastewater etc.) 468 x Flowrate 0.13 25.00 x Temperature 35.00 x Salinity 1,023.03 x Density SWRO discharge characteristics 5,976 x Flowrate 1.66 25.00 x Effluent temperature 59.30 x Effluent salinity 1,041.39 x Effluent density -0.16151 x Buoyant acceleration 9.75E-07 x Kinematic viscosity Diffuser system design characteristics: 0.25 x Port diameter 8 x Number of openings 60 x Discharge angle 0.5 x Port height 400 x Diffuser offshore location 18 x Off shore slope x Water depth at location 13 min 0.21 x Flowrate at port 4.22 x Port discharge velocity 21.03 x Dens. Froude Number 1.08E+06 x Reynolds Number Jet and dilution properties 10 x Height of jet rise max 13 x Impact point location 36 x Dilution at impact point

= Input parameter

°C ppt kg/m3 m2/s m3/h % m³/s m3/s °C ppt kg/m3 m3/h m3/s °C ppt kg/m3 m3/h m3/s °C ppt kg/m3 m/s2 m2/s m ° m m ° m m3/s m/s m m -

References

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References 1. Ludwig, H., Reverse Osmosis Seawater Desalination Volume 1, Springer, 2022 2. Hogan T. W., “Impingement and Entrainment at SWRO Desalination Facility Intakes,” in Missimer M.T., Burton J., Maliva R.G. - Intakes and Outfalls for Seawater Reverse-Osmosis Desalination Facilities - Innovations and Environmental Impacts, Springer, 2015, pp. 57 - 78. 3. Rogers N.R., “A Review of Onshore Screening Technologies for Fine Screening of Large Intakes,” in International Desalination Association IDA - World Congress on Desalination and Water Reuse, Dubai, 2009. 4. Baudish P., “Design Considerations for Tunnelled Seawater Intakes,” in Missimer M.T., Burton J., Maliva R.G. - Intakes and Outfalls for Seawater Reverse-Osmosis Desalination Facilities Innovations and Environmental Impacts, Springer, 2015, pp. 19 - 37. 5. Craig K., “Sydney and Gold Coast Desalination Plant Intake Design, Construction and Operating Experience,” in Missimer M.T., Burton J., Maliva R.G. - Intakes and Outfalls for Seawater Reverse-Osmosis Desalination Facilities - Innovations and Environmental Impacts,, Springer, 2015, pp. 39 - 56. 6. Missimer T.M., Hogan T.W., Pankratz T., “Passive Screen Intakes: Design, Construction, Operation and Environmental Impacts,” in Missimer M.T., Burton J., Maliva R.G. - Intakes and Outfalls for Seawater Reverse-Osmosis Desalination Facilities - Innovations and Environmental Impacts, Springer, 2015, pp. 79 - 104. 7. Rachman R.M., Li S., Missimer T., “SWRO feed water quality improvement using subsurface intakes in Oman, Spain, Turks and Caicos Islands, and Saudi Arabia,” Desalination, vol. 351, pp. 88 - 100, 2014. 8. Missimer T.M., Watson I., Maliva R.G., Ghaffour N., Dehwah A.H.A., Woolschlager J., Hegy M., “Impacts of natural pore-water and offshore aquifer chemistry on the operation and economics of some subsurface intake types for SWRO plants,” Desalination and Water Treatment, vol. 132, pp. 1 - 9, 2018. 9. Maliva R.G., Missimer T.M., “Well Intake Systems for SWRO Systems: Design and Limitations,” in Missimer M.T., Burton J., Maliva R.G. - Intakes and Outfalls for Seawater Reverse-Osmosis Desalination Facilities - Innovations and Environmental Impacts,, Springer, 2015, pp. 147 - 162. 10. Missimer T.M., Ghaffour N., Dehwah A.H.A., Rachman R., Maliva R.G., Amy G., “Subsurface intakes for seawater reverse osmosis facilities: Capacity limitation, water quality improvement, and economics,” Desalination, vol. 322, pp. 37 - 51, 2013. 11. Hölting B., Coldeway W.G., Hydrogeologie - Einführung in die Allgemeine und Angewandte Hydrogeologie, Springer, 2013. 12. David B., Pinot J-P., Morrillon M., “Beach Wells for Large-Scale Reverse Osmosis Plants: The Sur Case Study,” in IDA World Congress on Desalination and Water Reuse, Dubai, 2009. 13. Peters T., Pinto D., Pinto E., “Improved seawater intake and pre-treatment system based on Neodren technology,” Desalination, vol. 203, pp. 134 - 140, 2007. 14. Peters T., Pinto D., “Seawater intake and pre-treatment / brine discharge - environmental issues,” Desalination, vol. 221, pp. 576 - 584, 2008. 15. Farinas M., Lopez A.L., “New and innovative sea water intake system for the desalination plant at San Pedro del Pinatar,” Desalination, vol. 203, pp. 199 - 217, 2007. 16. Williams D.E., “Yield and Sustainability of Large Scale Slant Well Feedwater Supplies for Ocean Water Desalination Plants,” in IDA World Congress on Desalination and Water Reuse, San Diego USA, 2015. 17. Williams D.E., „Slant Well Intake Systems: Design and Construction,“ in Intakes and Outfalls for Seawater Reverse-Osmosis Desalination Facilities - Innovations and Environmental Impacts, Springer, 2015, pp. 275-320. 18. Maliva G., Missimer T.M., “Self-cleaning Beach Intake Galleries: Design and Global Applications,” in Intakes and Outfalls for Seawater Reverse-Osmosis Desalination Facilities - Innovations and Environmental Impacts, Springer, 2015, pp. 195-213.

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19. MEDRC Series of R&D Reports - MEDRC Project: 07-AS-003 – Bleninger T., Jirka G.H., “Environmental planning, prediction and management of brine discharges from desalination plants,” The Middle East Desalination Research Center, Muscat, Sultanate of Oman, 2010. 20. Roberts P.J.W, “Near Field Flow Dynamics of Concentrate Discharges and Diffuser Design,” in Intakes and Outfalls for Seawater Reverse-Osmosis Desalination Facilities - Innovations and Environmental Impacts, Springer, 2015, pp. 369-396. 21. Roberts P.J.W., Ferrier A., Daviero G., “Mixing in Inclined Dense Jets,” Journal of Hydraulic Engineering, vol. 123, no. 8, pp. 693-699, 1997. 22. Desalination and Water Purification Research and Development Program Report No. 167Roberts P.J.W., Abessi O., „Optimization of Desalination Diffusers Using Three-Dimensional Laser-Induced Fluorescence,“ U.S Department of Interior - Bureau of Reclamation, Denver, Colorado, 2014. 23. Abessi O., Roberts P.J.W., “Multiport Diffusers for Dense Discharges,” Journal of Hydraulic Engineering, vol. 140, no. 8, 2014. 24. Abessi O., Roberts P.J.W, “Effect of Nozzle Orientation on Dense Jets in Stagnant Environments,” Journal of Hydraulic Engineering, vol. 141, no. 8, 2015. 25. Abessi O., Roberts P.J.W., “Dense Jet Discharges in Shallow Water,” Journal of Hydraulic Engineering, vol. 142, no. 1, 2015. 26. Abessi O., Roberts P.J.W., „Rosette Diffusers for Dense Effluents in Flowing Currents,“ Journal of Hydraulic Engineering, Bd. 144, Nr. 1, 2018. 27. Bleninger T., Niepelt A., Jirka G., „Desalination plant discharge calculator,“ Desalination and Water Treatment, Bd. 13, pp. 156 - 173, 2010. 28. Bleninger T., Jirka G.H., “Modelling and environmentally sound management of brine discharges from desalination plants,” Desalination, vol. 221, pp. 585 - 597, 2008. 29. Bleninger T., Morelissen R., „Tiered Modeling Approach for Desalination Effluent Discharges,“ in Intakes and Outfalls for Seawater Reverse-Osmosis Desalination Facilities Innovations and Environmental Impacts, Springer, 2015, pp. 397 - 449. 30. Boerlage S.F.E., Gordon F.G., “Assessing Diffuser Performance and Discharge Footprint for the Gold Coast Desalination Plant,” in World Congress of International Desalination Assoziation IDA, Perth, Western Australia, 2011.

5

SWRO Effluents and Residues: Composition, Environmental Impacts, Discharge and Disposal Regulations, and Treatments Measures

The construction and operation of an SWRO plant generates emissions that could have impact on the maritime and terrestrial environment at the plant’s location in many different ways. A comprehensive overview of the diverse environmental impacts of SWRO facilities, as they may arise during the implementation of such a project and its subsequent operation, is presented in [17] Sects. 4.2.3.5 and 4.2.3.5.1 in Volume 1, and Fig. 4.11. Therefore it is important for reducing the extent of adverse environmental consequences or even largely avoid such impacts both during the construction phase and the subsequent operation of the plants to implement in these phases appropriate organizational and technical measures as well as to design the plant’s processing systems by suitably considering environmental aspects. The full scope of such measures is described in more detail in [17] Sects. 4.2.3.5 and 4.2.3.5.1 (in Volume 1) from an overall planning perspective for the construction phase as well as during operation of an SWRO plant. In [17] Sect. 4.2.3.5.3 in Volume 1, a description is given of how these environmental protection measures are to be integrated and presented as part of an environmental impact assessment (EIA) to be prepared when applying for construction and operating permits for the plant from the regulatory authorities. In this chapter, the liquid and solid reaction and waste products resulting from operation of the SWRO plant are described and explained in greater detail with regard to the following: • Their quantities and composition • The potential environmental impacts of their constituents • The guidelines for mitigating these impacts as mandated by environmental legislation • The technical measures necessary within the SWRO processing stages, as laid down by these legislative provisions as well as additional issues specific to the location of the facility like socioeconomic facts and ecological and nature conservation aspects

# The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 H. Ludwig, Reverse Osmosis Seawater Desalination Volume 2, https://doi.org/10.1007/978-3-030-81927-9_5

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5 SWRO Effluents and Residues: Composition, Environmental Impacts, Discharge. . .

5.1

Overview of SWRO Plant Effluents and Solid Residues

5.1.1

SWRO Processing Stages and Wastes Produced During Their Operation

Depending on its design and configuration, operation of an SWRO plant generates waste products in varying quantities and compositions that arise as concentrates, wastewater and sludges as well as solids. In Table 5.1, the respective liquid and solid waste products are allocated to the various processing stages of an SWRO plant as they are generated during their operation. The concentrates of the plant’s RO desalination stages that are generated continuously account for the major share of the liquid effluents (No. 3.1 and No. 3.2 of Table 5.1). In terms of quantity, these are followed by the wastewater from backwashing the filter units of the pretreatment stage, this being generated intermittently depending on how long they are operated during the intervals between backwashing (No. 2.2 and No. 2.3). The duration of filter operation times depends on the degree to which the seawater to be treated is loaded with suspended solids and whether the filters in the SWRO pretreatment stage are preceded by a solids pre-separation step in the form of sedimentation or flotation. If this is the case, most of the suspended solids contained in seawater and the solid fractions resulting from coagulant dosing upstream of the pretreatment steps are retained in the sludge they produce (No. 2.1) while in the downstream filters only the residual solids fraction is intercepted. This extends their operating time between backwashing cycles and reduces the amount of wastewater from these pretreatment steps. If backwashable cartridge filters are installed upstream of the seawater RO stages, these also intermittently generate backwash water (No. 2.4). Wastewater is also generated during rinsing and chemical cleaning of the membranes in the SWRO pretreatment stage if it includes a membrane filtration system (No. 2.3) as well as in the RO desalination stages of the plant (No. 3.1 and No. 3.2). This wastewater from membrane treatment also arises intermittently but at longer intervals which, depending on the quality of the seawater to be desalinated and the efficiency of the SWRO pretreatment stage, may be from once or twice up to four times per year. The membranes can be flushed more frequently at weekly or monthly intervals with filtered or pre-cleaned seawater or else product water. Wastewater is also generated when the membrane elements of the RO stages are preserved with a chemical solution to inhibit biological growth during prolonged shutdowns of the SWRO plant. The membranes are chemically cleaned before preservation and the preservation solution must be flushed out when the plant is returned to operation. The membranes of a membrane filtration system in the pretreatment stage are treated and preserved in a similar way. When supplied, new membrane elements are also preserved with such a solution, and this has to be removed from the modules when membranes are replaced and when an SWRO plant is returned to service, again generating wastewater. If the posttreatment stage of an SWRO plant is equipped with limestone filters for alkalinization of the product water, these are backwashed each time limestone is

5.1 Overview of SWRO Plant Effluents and Solid Residues

431

Table 5.1 SWRO plant stages: effluents and solid residues for discharge and disposal No. 1 1.1

SWRO systems Intake • Active screening systems

1.2

• Seawater electrolysis Pretreatment • Sedimentation or flotation

2 2.1

2.2

2.3

2.4 3 3.1

3.2

4 4.1 4.2

• Granular media filtration

Wastes to be disposed of Effluents Spray water after solids separation

Backwash water with suspended solids from seawater feed solids and coagulant

Backwash water with suspended solids from seawater feed solids and coagulant (plus chlorine) • Membrane • Backwash water with suspended filtration solids from seawater feed solids and coagulant • Rinsing water • Cleaning wastewater • Membrane preservation wastewater • Cartridge Backwash water with suspended solids if filtration backwashable cartridges are installed Reverse osmosis system • RO first pass • Concentrate • Rinsing water • Cleaning wastewater • Membrane preservation wastewater • RO second • Concentrate (if concentrate is not pass recirculated to first pass) • Rinsing water • Cleaning wastewater • Membrane preservation wastewater Posttreatment • Lime saturation Alkaline rinse water of lime milk and system lime water systems with suspended solids • Limestone Backwash water with suspended filter system limestone solids if backwash water is not recycled

Solids Debris with solids removed from seawater Replacement of filter sand and gravel Sludge from seawater feed and coagulant solids Replacement of filter materials and gravel Replacement of membranes

Replacement of cartridges Replacement of membranes

Replacement of membranes

Alkaline lime sludge Limestone sludge if filter backwash water is recycled

replenished. If the filter plant is not equipped for recycling the backwash water, this arises intermittently as wastewater. Otherwise, the fine grain portion of the limestone contained in the backwash water arises as sludge in the recycling system. If hydrated lime for lime water saturation is used for alkalizing the SWRO product water, wastewater is only generated from flushing the plant’s pipework, which is done at set intervals to prevent buildup of hydrated lime or calcium carbonate in the pipes and valves.

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5 SWRO Effluents and Residues: Composition, Environmental Impacts, Discharge. . .

If the chlorine needed to chlorinate the extracted seawater is produced by seawater electrolysis, the solids content in the feed to the electrolysis system must be reduced. This can be done by using seawater for electrolysis after its treatment in the SWRO pretreatment stage or by purifying seawater from the plant’s intake in a dedicated filtration plant for the electrolysis equipment. In the latter case, wastewater additionally arises from backwashing these filters (No. 1.2). Solid waste as sludges is generated: • During operation of the systems for preliminary solids separation, comprising sedimentation and flotation: these contain a major part of the solids present in seawater as suspensions plus the solid products from dosing of coagulants such as iron compounds upstream of these treatment units (No. 2.1) • In the SWRO posttreatment stage: – In the form of alkaline lime sludge, if posttreatment involves lime water saturation with hydrated lime (No. 4.1) – As neutral sludge consisting mainly of limestone particles, if limestone filters with recycling of the filter backwash water are installed in this stage (No. 4.2) • In active screening systems for intercepting coarse solids at the seawater intake: the resulting sludge contains mainly naturally occurring coarse organic substances of biological origin but also other seawater impurities (No. 1.1) Other solid residues arising from operation of an SWRO plant are as follows: • Spent membrane elements from the RO stages (No. 3.1 and No. 3.2) as well as from membrane filtration (No. 2.3) if this is installed in the pretreatment stage, which have to be replaced by new membranes after a certain period of operation • Filter units from a cartridge filtration installed upstream of the RO stages, especially if this is equipped with non-backwashable filters (No. 2.4) • Filter gravel, hydroanthracite, coarse gravel, and other wastes that arise from the replacement of granular filter media in the filtration systems of the SWRO pretreatment stage

5.2

SWRO Effluents

5.2.1

Composition of Main Discharges

The wastewater and sludge generated in the various SWRO processing stages that are to be disposed of and the substances they contain are listed in Table 5.2. Also shown is how the concentrations of these compounds are influenced or determined by the design and operating conditions of the SWRO plant and its components. Data and algorithms from other sections of this book that are needed for calculating the concentrations of the constituents, but also more detailed information on the different substances contained in the effluents and sludges, are given in the “Remarks” column.

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Table 5.2 Composition of SWRO discharges Type of discharge Constituents Pretreatment systems • Sedimentation/flotation Sludge SW salinity, suspended solids of SW feed Solids formed by coagulant dosing

Flocculant • Granular media filtration Backwash SW or RO concentrate water salinity; suspended solids of SW feed

Solids formed by coagulant dosing Flocculant • Membrane filtration Backwash SW or RO concentrate and CEP salinity; suspended wastewater solids of SW feed Solids formed by coagulant dosing

Chlorine Acid or caustic soda CIP wastewater

Membrane preservation wastewater

Chlorine, mineral acids or caustic soda, citric acid or oxalic acid, surfactants Sodium bisulphite or organic biocides

Concentration

Depending on SW suspended solids concentration, SW feed dosing rate of coagulant (5–20 mg Fe3+/l), and sludge thickening efficiency of sedimentation/flotation Depending on SW feed dosing rate Dependent on SW suspended solids concentration, SW feed dosing rate of coagulant (1–10 mg Fe3+/l), filtration velocity, BW velocity, and length of filtration cycle Dependent on SW feed dosing rate Depending on SW suspended solids concentration, SW feed dosing rate of coagulant (0.5–5 mg Fe3+/l), filtration velocity, backwash velocity, and length of filtration cycle 5–50 and up to 100– 500 mg Cl2/l Dependent on type of fouling Dependent on type and extent of fouling

Dependent on type of membranes and membrane manufacturer

Remarks

Section 2.3.1.1.3, Table 2.14, and Eqs. (2.40–2.40e)

Excess that is not absorbed into the sludge SW or concentrate salinity of BW water depends on whether backwashing is done with SW or concentrate Section 2.3.1.1.3, Table 2.14, and Eqs. (2.40–2.40e) Excess that is not adsorbed by solids

Section 2.3.1.1.3, Table 2.14, and Eqs. (2.40–2.40e)

During chemical enhanced backwash CEB; see Sect. 2.3.5. 3.2, Table 2.26 Section 2.3.5.3.2, Table 2.27

For new membranes and membrane preservation during shutdown, see Sect. 2.3.5.3.2 (continued)

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Table 5.2 (continued) Type of discharge Constituents Reverse osmosis systems Concentrate High salinity

Antiscalant

Chlorine

CIP wastewater

Mineral acids or caustic soda, citric acid, phosphates, surfactants, chlorine for CA membranes Sodium bisulphite or organic biocides

Concentration

Remarks

Dependent on SW salinity; RO recovery rates Dependent on SWRO feed dosing rate and RO recovery rates

Salinity calculation with Eqs. (4.12–4.12g)

Residual Cl2 concentration after chlorine decay in membrane elements Dependent on type and extent of fouling

Antiscalant concentration in concentrate calculated with Eq. (5.1) If CTA membranes installed

For more detailed information, see [17] Sect. 5.5.2.5.1, Table 5.30 in volume 1

Membrane See [17] Sect. 5.5.2.5.6, For new membranes and preservation Table 5.35 in Volume 1 membrane preservation wastewater during shutdown Posttreatment • Limestone filtration With no BW water recovery: See Backwash water Neutral pH range with limestone particles Sect. 3.4.2.2.2 (process design) Sludge With BW water recovery • lime water saturation Wastewater from Alkaline water with lime and See Sect. 3.4.2.2.1 (process design) pipework flushing calcium carbonate particles Sludge from Alkaline sludge with lime and saturators calcium carbonate particles

5.2.1.1 Reverse Osmosis Concentrate The salinity cC,SWRO or cC,SWRO,R of the concentrate generated by the SWRO plant depends on the salinity of the seawater and the recovery rate of the RO stages YRO1 or YRO1,RO2, that is its configuration as a one-pass or two-pass system, the capacity of the second pass and the extent to which its concentrate is recirculated to the feed of the first RO stage. Depending on the design of the RO stages, to calculate the salinity of the concentrate, the relevant equation has to be selected from the systems of equations in Sect. 4.2.2, Eqs. (4.12 to 4.12g). For seawater salinities of between 35,000 and 50,000 mg/l and recovery rates YRO of seawater desalination membrane systems of 35% to 50%, the salinity of concentrates from SWRO plants is normally in the range of 60 to 70 g/l but can be as high as 80 g/l and above for special RO systems designs.

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435

To stabilize scaling compounds, antiscalants are dosed into the feed lines to the RO systems (see also [17] Sect. 5.3.4.2.1 in Volume 1). As desalination proceeds, their original dosing concentration is likewise raised to a higher concentration level within the RO concentrate. The antiscalants are almost completely retained by the membranes and their concentration cC,RO,antisc in the concentrate is therefore calculated from the feed concentration cF,RO,antisc to the RO stage and its recovery rate YRO with Eq. (5.1). cC,RO,antisc ¼ cF,RO,antisc 

1 1  Y RO

ð5:1Þ

cF,RO,antisc ¼ antiscalant dosing concentration in RO feed [mg/l] cC, RO, antisc ¼ antiscalant concentration in RO concentrate [mg/l] At a dosing concentration cF,RO,antisc of the antiscalant in the RO feed of 1 mg/l and a recovery rate YRO of 0.45, i.e. 45%, the antiscalant concentration cC,RO,antisc in this concentrate is 1.8 mg/l. In the second pass of an SWRO plant with a recovery rate YRO of 0.90, i.e. 90%, the concentration of the antiscalant increases to 10 mg/l in the concentrate for the same feed concentration of 1 mg/l. If the SWRO plant is equipped with polyamide membranes, disinfectants such as chlorine or chlorine dioxide, which are dosed in its seawater extraction and pretreatment stages, are removed in the feed line to the RO stages by reaction with sulphite compounds such as sodium bisulphite NaHSO3 or sodium metabisulphite Na2S2O5, so these biocides are no longer present in the concentrate generated by the RO stages. If cellulose acetate membranes are installed in the main desalination tract of the SWRO plant, due to their better resistance to oxidation, biological growth on these is suppressed by intermittently applying to them chlorine at a dosage rate of 0.3 to 1.0 mg/l, while the chlorine concentration should not drop below a minimum of 0.2 mg/l in the concentrate (see also [17] Sect. 5.5.2.4.2 in Volume 1). Thus when this type of membrane is installed in the SWRO seawater desalination stage, the RO concentrate will still contain chlorine.

5.2.1.2 Backwash Water of Pretreatment Filtration Systems 5.2.1.2.1 Physical Backwash (PB) The backwash water resulting from physical backwashing of filtration systems in the pretreatment stage contains the suspended solids from the feed to the filters that are captured by these during the filtration cycles between backwashes. This solids content is made up of the solids present in seawater and the additional amount of solids, which is generated by coagulant dosing in the pretreatment stage. The amount of solid product, ferric hydroxide Fe(OH)3, which is formed from coagulants such as ferric chloride FeCl3 or ferric sulphate Fe2(SO4)3, is calculated as described in Sect. 2.3.1.1.3 (Eqs. 2.40 to 2.40e). Regarding the coagulant dosing rates for different

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methods of filtration in the pretreatment stage, see Table 2.14 in this section. The solids content of the backwash water cbw,ss is calculated with Eqs. (5.2 and 5.2a) from the amount of solids Δar,ss intercepted hourly in the filters as well as the average wastewater flow FWW,bw,∅ from backwashing the filters. Δar,ss ¼

F F,fs  cF,ss,T 1000

ð5:2Þ

cbw,ss ¼

Δar,ss  1000 F WW,bw,∅

ð5:2aÞ

Δar, ss ¼ amount of suspended solids removed by filtration [kg/h] cbw, ss ¼ suspended solids concentration in backwash water [mg/l][g/m3] F F,fs ¼ FGmf, F, fs ¼ FM, Filt, S ¼ feed/filtrate flow through filters [m3/h] cF, ss, T ¼ total suspended solids concentration in filter feed [mg/l][g/m3] FWW, bw, ∅ ¼ FGmf, WW, bw, total∅ ¼ FM, WW, bw, total, ∅¼ average backwash wastewater flow [m3/h] To calculate FGmf,WW,bw,total∅ for filters with granular media, see Eqs. (5.4a and 5.4b), and of FM,WW,bw,total,∅ for membrane filtration, see Eqs. (5.5 and 5.5e). With direct flocculation filtration of seawater, i.e. without pre-separation of solids by sedimentation or flotation upstream of the filters and with a seawater content of suspended solids in the range of 2–4 mg/l and dosing rates of ferric chloride FeCl3 of 5–10 mg/l, the solids content in the wastewater of the pretreatment filtration installations is about 100–300 mg/l. Higher levels of suspended solids in the wastewater of up to 400–600 mg/l are possible depending not only on the quality of the seawater, i.e. its content and type of suspended solids and the necessary dosing rate of coagulant, but also on the type and mode of operation of the filtration systems of the pretreatment stage. If seawater is used for physical backwashing, the backwash water has the same salinity as the seawater. The concentrate from the SWRO plant’s RO systems is often used for backwashing filters with granular media. In this case the backwash water has the same salinity as that of the RO concentrate.

5.2.1.2.2 Chemical Enhanced Backwash (CEB) In membrane filter installations, in addition to physical backwashing, at set intervals a backwashing process takes place for which biocide in the form of chlorine is added to the backwash water, sometimes at an elevated pH of up to 10–13 or else the filtration membranes are treated at low pH values of up to 1–2 simply by adding hydrochloric acid or sulphuric acid during backwashing (see also Sect. 2.3.5.3.2, Table 2.26). The frequency and manner in which CEB treatment is done depends on the extent of membrane fouling and how it occurs. For CEB treatment, alkaline and acidic rinses may have to be initiated directly one after the other.

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The alkaline CEB rinse is done with a chlorine content of 5–50 mg/l Cl2 but also with Cl2 dosing concentrations of up to 100–500 mg/l. The residual chlorine content in the generated wastewater is then calculated from the dosing concentration minus the chlorine decay during membrane treatment. If the CEB treatment is carried out with seawater as backwash medium, the wastewater has the same salinity as the seawater. In order to avoid the formation of scaling during alkaline rinsing, product water of the first RO pass RO1 of the SWRO plant may also be used, which results in a correspondingly lower wastewater salinity.

5.2.1.3 Cleaning-in-Place (CIP) Fouling and scaling of membranes that cannot be removed by the physical treatment processes of rinsing or backflushing or by chemical enhanced backwashing (CEB) in membrane filtration systems is reduced or removed by more intensive rinsing with chemical solutions selected depending on the type of membrane fouling. This is termed cleaning-in-place (CIP). 5.2.1.3.1 Reverse Osmosis Membranes A detailed description of the cleaning-in-place (CIP) process for an SWRO plant, both for polyamide membranes and cellulose acetate membranes, is given in [17] Sect. 5.5.2.5 in Volume 1. Formulations for cleaning solutions, as recommended by membrane manufacturers for different types of RO membranes and the respective cleaning parameters with regard to pH and temperature, are listed in [17] Sect. 5.5.2.5, Table 5.30 in Volume 1. As can be seen from this table, a variety of chemicals are used for the CIP of polyamide membranes both in strongly acidic solutions at a pH range of 1–2 and in strongly alkaline solutions at a pH range of 10–12. With this type of membrane, strong acids such as hydrochloric acid or phosphoric acid are used to remove deposits composed of alkaline earth carbonates. For coatings made up of a mix of substances in which alkaline earth sulphates and metal hydroxides are also present, organic complexing agents such as citric acid and sodium EDTA, reducing agents such as sodium dithionite, Na2S2O8, and inorganic phosphates such as sodium tripolyphosphate are used in combination with complexing agents in solution concentrations of around 1%–2% of the compound concerned. So that these coatings can be more easily detached from the membrane surfaces, organic surfactants in concentrations of 1000 to >10,000 mg/l, the antiscalants have no or only a very limited

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ecotoxicological effect on marine organisms under these conditions [5]. However, phosphonates in particular are very stable and, with a biodegradability of at most 20% to 40% over 30 days in seawater, and persist as compounds over a long time [6]. Intensified efforts by environmental authorities at national and international levels for enforcing regulations for the protection of the marine ecology have resulted in more stringent restrictions on the discharge of synthetic organic components with low biodegradability and higher ecotoxicity into the sea. These limitations on discharging certain substances can be specific for a particular desalination plant site as well as a result of international agreements covering extensive marine regions that thus apply to a number of plant locations. For example, within the framework of the Oslo-Paris (OSPAR) Convention,1 which was agreed upon by the riparian states of the North-East Atlantic for the protection of the marine environment, criteria as follows are laid down for the discharge of organic compounds in this marine region. It is not permissible to discharge organic compounds into the sea if: • Their biodegradability is less than 20% in 28 days • Two out of the following three conditions apply: – Their biodegradability is less than 60% in 28 days – A bioaccumulation of log (POW)