Coagulation and Flocculation in Water and Wastewater Treatment Second Edition John Bratby LONDON • SEATTLE Published by IWA Publishing, Alliance House, 12 Caxton Street, London SW1H 0QS, UK Telephone: +44 (0) 20 7654 5500; Fax:+44 (0) 20 7654 5555; Email: [email protected] Web: www.iwapublishing.com First edition 1980 Second edition 2006 C 2006 John Bratby Copyedited and typeset by TechBooks, India Printed by TJ International, Padstow, Cornwall Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright, Designs and Patents Act (1998), no part of this publication may be reproduced, stored or transmitted in any form or by any means, without the prior permission in writing of the publisher, or, in the case of photographic reproduction, in accordance with the terms of licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licenses issued by the appropriate reproduction rights organization outside the UK. 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British Library Cataloguing in Publication Data A CIP catalogue record for this book is available from the British Library Library of Congress Cataloguing-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 1843391066 ISBN13: 9781843391067 Contents Preface xi 1 Introduction 1.1 General 1.2 Stability and Destabilization 1.3 Definitions 1.4 Performance Criteria 1.5 Summary 1.6 References 1 1 3 5 6 7 8 2 Colloids and interfaces 2.1 Introduction 2.2 Origin of Surface Charge 2.3 Effect of Surface Charge 2.4 Adsorption 2.5 Inner Part of Electrical Double Layer 2.6 Diffuse Part of Electrical Double Layer 2.6.1 Assumptions 2.6.2 Distribution of Potential with Distance from the Charged Surface 2.6.3 Thickness of Double Layer 2.6.4 Effect of Ionic Strength on Double Layer 2.6.5 Effect of Nature of Counter Ions 2.7 Stern’s Model of Complete Double Layer 2.8 Colloid Stability in Terms of the Double Layer 2.8.1 Energy of Interaction Between Particles v 9 9 13 14 14 17 18 18 20 22 24 25 25 26 26 vi Contents 2.8.2 Theoretical Optimal Concentration of Electrolyte Required for Destabilization 2.8.3 Schulze–Hardy Rule 2.9 Electrokinetic Measurements 2.9.1 Introduction 2.10 References 28 29 29 29 30 3 Coagulants 3.1 Introduction 3.2 Metal Coagulants 3.2.1 Commonly Used Metal Coagulants 3.2.2 Chemistry of Metal Coagulants 3.3 Polymers 3.3.1 General 3.3.2 Activated Silica 3.3.3 Natural Polyelectrolytes 3.3.4 Synthetic Polymers 3.4 References 31 31 32 32 42 50 50 51 52 56 69 4 Treatment with metal coagulants 4.1 Introduction 4.2 Destabilization of Hydrophobic Colloids 4.2.1 Extent of Hydrolysis and Adsorption 4.2.2 Effect of Coagulant Dosage 4.2.3 Effect of Colloid Concentration 4.2.4 Effect of pH 4.3 Destabilization of Hydrophilic Colloids 4.4 Removal of Natural Organic Matter 4.4.1 Organic Color 4.4.2 Enhanced Coagulation 4.5 Pathogen Removal 4.5.1 Removal of Giardia and Cryptosporidium 4.5.2 Virus Removal 4.6 Effect of Anions 4.6.1 General 4.6.2 Effect of Sulfate 4.6.3 Effect of Phosphate 4.7 Chemical Phosphorus Removal in Wastewater Treatment 4.7.1 General 4.7.2 Mechanisms of Chemical Phosphorus Removal 4.7.3 Applications of Chemical Phosphorus Removal 4.8 Wastewater Treatment by Coagulation 4.9 Arsenic Removal 4.10 Staged Coagulation and Sequencing 4.11 Effects of Preozonation 4.12 Effects of Temperature 72 72 75 75 75 77 80 86 87 87 98 113 113 114 116 116 117 118 120 120 124 138 152 159 161 169 171 Contents vii 4.13 Residual Aluminum 4.14 References 173 176 5 Treatment with polymers 5.1 Introduction 5.2 Mechanisms of Destabilization 5.2.1 General 5.2.2 The Bridging Mechanism 5.2.3 The Electrostatic Patch Mechanism 5.3 Polyelectrolytes as Primary Coagulants 5.3.1 General 5.3.2 Turbidity Removal Using Polyelectrolytes 5.3.3 Organics Removal Using Polyelectrolytes 5.3.4 Removal of Microorganisms Using Polyelectrolytes 5.4 Polyelectrolytes as Flocculant Aids 5.4.1 Polymers as Filter Aids 5.5 Polymers as Sludge Conditioners 5.6 References 186 186 187 187 188 192 194 194 196 200 204 206 210 211 214 6 Rapid mixing 6.1 Introduction 6.2 Requirements for Rapid Mixing Devices 6.2.1 General 6.2.2 Comparison of Back-Mix and Plug-Flow Reactors 6.2.3 Velocity Gradient Requirements 6.2.4 Rapid Mixer Retention Time 6.2.5 Tapered Rapid Mix Velocity Gradient 6.2.6 Coagulant Feed Concentration 6.2.7 Sequence of Chemical Addition 6.3 Design of Rapid Mixing Devices 6.3.1 General 6.3.2 Back-mix Reactors 6.3.3 In-line Mixers without Controlled Velocity Gradient 6.3.4 In-line Mixers With Controlled Velocity Gradient 6.4 References 219 219 220 220 222 225 226 227 227 228 228 228 229 229 235 238 7 Flocculation 7.1 Introduction 7.2 Perikinetic Flocculation 7.3 Orthokinetic Flocculation 7.3.1 Theoretical Development 7.3.2 Working Equation 7.3.3 Flocculation Reactors in Series 7.3.4 Adequacy of G and GT as Design Parameters 7.3.5 Experimental Determination of Flocculation Parameters 240 240 241 243 243 249 250 250 252 viii 8 Contents 7.4 Design of Flocculation Basins 7.4.1 General 7.4.2 Types of Flocculation Chambers and Devices 7.4.3 Short-Circuiting in Flocculation Reactors 7.4.4 Compartmentalization 7.4.5 Combined Flocculation–Sedimentation Basins 7.4.6 Transfer of Flocculated Water 7.5 References 253 253 254 267 268 270 274 276 Testing and control of coagulation and flocculation 8.1 Introduction 8.2 Optimizing Primary Coagulant Type, Dosage and pH 8.2.1 General 8.2.2 Apparatus 8.2.3 Chemical Solutions 8.2.4 Criteria Describing Process Performance 8.2.5 Jar Test Procedure 8.2.6 Analysis of Results 8.3 Using the Jar Test to Evaluate Settling 8.4 Evaluating Flocculant Aids 8.4.1 General 8.4.2 Initial Choice of Flocculant Aid 8.4.3 Preparation of Polyelectrolyte Solutions 8.4.4 Experimental Procedure 8.5 Evaluating Sludge Conditioners 8.5.1 General 8.5.2 Experimental Procedures 8.6 Optimizing Flocculation Parameters 8.6.1 General 8.6.2 Apparatus 8.6.3 Experimental Procedure 8.6.4 Analysis of Data 8.7 Control Systems for Coagulation 8.7.1 Introduction 8.7.2 Electrokinetic Measurements 8.7.3 Monitoring Floc Formation 8.7.4 Data-Driven Control Systems 8.8 References 279 279 280 280 281 282 285 289 291 293 296 296 296 297 298 299 299 299 304 304 306 308 312 315 315 315 323 327 332 Appendix: Processing and Disposal of Coagulant Sludges 338 A.1 Introduction A.2 Production of Water Plant Residuals A.2.1 Estimating Sludge Quantities A.2.2 Alternative Coagulants and Dosage Reduction A.2.3 Sludge Characteristics A.2.4 Sludge Conditioning 338 341 341 344 345 347 Contents ix A.3 A.4 A.5 A.6 Filter Backwash Sludge Lagoons Sludge Drying Beds Mechanical Thickening and Dewatering A.6.1 Sludge Thickening A.6.2 Sludge Dewatering A.7 Coagulant Recovery A.8 Sludge Disposal A.8.1 Introduction A.8.2 Disposal to Municipal Sewers A.8.3 Land Application of Water Plant Sludge A.9 References 353 357 360 365 365 373 380 388 388 390 393 394 Index 401 Preface I believe that a colloid chemist, if asked today to explain the coagulation of a lyophobic hydrosol by electrolytes, will make a rather unhappy face . . . if a professor is obliged to discuss this unsatisfactory condition of the theory of coagulation for thirty or more years, in every term of the academic year, then it may easily happen that he becomes more and more impatient. Either he becomes resigned or he commences to curse. The latter course is in general more fruitful.1 There have been a number of developments since the publication of the first edition of this book in 1980. Not the least of these is the advent of new classes of coagulants, and the increasing adoption of membranes in water and wastewater treatment. However, despite these developments, most of the comments in the Preface to the first edition are still relevant today. Coagulation and flocculation still play a dominant role in many water and wastewater treatment schemes, including those incorporating membrane treatment. Because of the complex interdependence of numerous factors inherent in the coagulation and flocculation processes, a thorough understanding of the phenomena involved is essential. The above quotation is from a paper by Professor Wolfgang Ostwald in 1938. Notwithstanding the frustrations expressed, his work along with others made a positive contribution to a deeper understanding of the phenomena involved— although, at times, it appears that some of the important principles are not fully appreciated, or are forgotten. There are several excellent books on water treatment that include chapters and sections on coagulation and flocculation. However, current literature generally does not present an in-depth account of coagulation and flocculation in a single 1 W. Ostwald in Electrolyte coagulation and coefficient of electrolyte activity, Fifteenth Colloid Symposium, Cambridge, MA, June 9–11 (1938) xi xii Preface publication. Moreover, a given publication may emphasize the theoretical aspects with little indication of the practical significance of the phenomena, or may largely ignore the theoretical development and present essentially practical material only. In most cases, the uninitiated reader is left without an overall picture of the processes as a whole and, without further extensive literature searching, succeeds in achieving only a superficial understanding. The intention of this book is to help remedy these inadequacies. An attempt has been made to present the subject matter logically and sequentially from theoretical principles to practical applications. Successive chapters deal with, in turn, properties of materials present in waters and wastewaters; characteristics and types of coagulants commonly in use; mechanisms and practical implications of destabilization of waterborne material using metal coagulants and polyelectrolytes; considerations related to coagulant addition at the rapid mixing stage; theoretical and practical considerations of flocculation; and details of experimental procedures for assessing primary coagulants, flocculant aids, sludge conditioners, and flocculation parameters. Numerous examples are included where appropriate. Treatment and disposal of sludges resulting from coagulation-flocculation related operations is dealt with in the Appendix. This important topic has been separated from the main text to avoid disturbing the continuum of the presentation. The book in its present form evolved from material offered to undergraduate and postgraduate university students. Although it has been progressively modified and increased in scope to cater for the requirements of practicing chemists and chemical and civil engineers involved with water and wastewater treatment, the book still retains a didactic nature. Therefore, it is hoped that the book will serve three functions: (1) A readable and useful presentation for the water scientist and engineer. (2) A convenient reference handbook in the form of numerous examples and appended information. (3) Course material for undergraduate and postgraduate students.