Coagulation and Flocculation in
Water and Wastewater Treatment
Second Edition
John Bratby
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
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A CIP catalogue record for this book is available from the British Library
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A catalog record for this book is available from the Library of Congress ISBN: 1843391066
ISBN13: 9781843391067
Contents
Preface xi
1 Introduction 1
1.1 General 1
1.2 Stability and Destabilization 3
1.3 Definitions 5
1.4 Performance Criteria 6
1.5 Summary 7
1.6 References 8
2 Colloids and interfaces 9
2.1 Introduction 9
2.2 Origin of Surface Charge 13 2.3 Effect of Surface Charge 14
2.4 Adsorption 14
2.5 Inner Part of Electrical Double Layer 17 2.6 Diffuse Part of Electrical Double Layer 18 2.6.1 Assumptions 18 2.6.2 Distribution of Potential with Distance from the
Charged Surface 20 2.6.3 Thickness of Double Layer 22 2.6.4 Effect of Ionic Strength on Double Layer 24 2.6.5 Effect of Nature of Counter Ions 25 2.7 Stern’s Model of Complete Double Layer 25 2.8 Colloid Stability in Terms of the Double Layer 26 2.8.1 Energy of Interaction Between Particles 26
2.8.2 Theoretical Optimal Concentration of Electrolyte
Required for Destabilization 28 2.8.3 Schulze–Hardy Rule 29 2.9 Electrokinetic Measurements 29 2.9.1 Introduction 29 2.10 References 30 3 Coagulants 31 3.1 Introduction 31 3.2 Metal Coagulants 32 3.2.1 Commonly Used Metal Coagulants 32 3.2.2 Chemistry of Metal Coagulants 42
3.3 Polymers 50 3.3.1 General 50 3.3.2 Activated Silica 51 3.3.3 Natural Polyelectrolytes 52 3.3.4 Synthetic Polymers 56 3.4 References 69
4 Treatment with metal coagulants 72
4.1 Introduction 72
4.2 Destabilization of Hydrophobic Colloids 75 4.2.1 Extent of Hydrolysis and Adsorption 75 4.2.2 Effect of Coagulant Dosage 75 4.2.3 Effect of Colloid Concentration 77 4.2.4 Effect of pH 80 4.3 Destabilization of Hydrophilic Colloids 86 4.4 Removal of Natural Organic Matter 87 4.4.1 Organic Color 87 4.4.2 Enhanced Coagulation 98 4.5 Pathogen Removal 113 4.5.1 Removal of Giardia and Cryptosporidium 113 4.5.2 Virus Removal 114 4.6 Effect of Anions 116
4.6.1 General 116
4.6.2 Effect of Sulfate 117 4.6.3 Effect of Phosphate 118 4.7 Chemical Phosphorus Removal in Wastewater Treatment 120
4.7.1 General 120
4.7.2 Mechanisms of Chemical Phosphorus Removal 124 4.7.3 Applications of Chemical Phosphorus Removal 138 4.8 Wastewater Treatment by Coagulation 152 4.9 Arsenic Removal 159 4.10 Staged Coagulation and Sequencing 161 4.11 Effects of Preozonation 169 4.12 Effects of Temperature 171
Contents vii 4.13 Residual Aluminum 173
4.14 References 176
5 Treatment with polymers 186 5.1 Introduction 186 5.2 Mechanisms of Destabilization 187
5.2.1 General 187
5.2.2 The Bridging Mechanism 188 5.2.3 The Electrostatic Patch Mechanism 192 5.3 Polyelectrolytes as Primary Coagulants 194
5.3.1 General 194
5.3.2 Turbidity Removal Using Polyelectrolytes 196 5.3.3 Organics Removal Using Polyelectrolytes 200 5.3.4 Removal of Microorganisms Using Polyelectrolytes 204 5.4 Polyelectrolytes as Flocculant Aids 206 5.4.1 Polymers as Filter Aids 210 5.5 Polymers as Sludge Conditioners 211
5.6 References 214
6 Rapid mixing 219
6.1 Introduction 219 6.2 Requirements for Rapid Mixing Devices 220
6.2.1 General 220
6.2.2 Comparison of Back-Mix and Plug-Flow Reactors 222 6.2.3 Velocity Gradient Requirements 225 6.2.4 Rapid Mixer Retention Time 226 6.2.5 Tapered Rapid Mix Velocity Gradient 227 6.2.6 Coagulant Feed Concentration 227 6.2.7 Sequence of Chemical Addition 228 6.3 Design of Rapid Mixing Devices 228
6.3.1 General 228
6.3.2 Back-mix Reactors 229 6.3.3 In-line Mixers without Controlled Velocity Gradient 229 6.3.4 In-line Mixers With Controlled Velocity Gradient 235
6.4 References 238 7 Flocculation 240 7.1 Introduction 240 7.2 Perikinetic Flocculation 241 7.3 Orthokinetic Flocculation 243 7.3.1 Theoretical Development 243 7.3.2 Working Equation 249 7.3.3 Flocculation Reactors in Series 250 7.3.4 Adequacy of G and GT as Design Parameters 250 7.3.5 Experimental Determination of Flocculation
7.4 Design of Flocculation Basins 253
7.4.1 General 253
7.4.2 Types of Flocculation Chambers and Devices 254 7.4.3 Short-Circuiting in Flocculation Reactors 267 7.4.4 Compartmentalization 268 7.4.5 Combined Flocculation–Sedimentation Basins 270 7.4.6 Transfer of Flocculated Water 274
7.5 References 276
8 Testing and control of coagulation and flocculation 279 8.1 Introduction 279 8.2 Optimizing Primary Coagulant Type, Dosage and pH 280
8.2.1 General 280
8.2.2 Apparatus 281 8.2.3 Chemical Solutions 282 8.2.4 Criteria Describing Process Performance 285 8.2.5 Jar Test Procedure 289 8.2.6 Analysis of Results 291 8.3 Using the Jar Test to Evaluate Settling 293 8.4 Evaluating Flocculant Aids 296
8.4.1 General 296
8.4.2 Initial Choice of Flocculant Aid 296 8.4.3 Preparation of Polyelectrolyte Solutions 297 8.4.4 Experimental Procedure 298 8.5 Evaluating Sludge Conditioners 299
8.5.1 General 299
8.5.2 Experimental Procedures 299 8.6 Optimizing Flocculation Parameters 304
8.6.1 General 304
8.6.2 Apparatus 306 8.6.3 Experimental Procedure 308 8.6.4 Analysis of Data 312 8.7 Control Systems for Coagulation 315 8.7.1 Introduction 315 8.7.2 Electrokinetic Measurements 315 8.7.3 Monitoring Floc Formation 323 8.7.4 Data-Driven Control Systems 327
8.8 References 332
Appendix: Processing and Disposal of Coagulant Sludges 338 A.1 Introduction 338 A.2 Production of Water Plant Residuals 341 A.2.1 Estimating Sludge Quantities 341 A.2.2 Alternative Coagulants and Dosage Reduction 344 A.2.3 Sludge Characteristics 345 A.2.4 Sludge Conditioning 347
Contents ix A.3 Filter Backwash 353 A.4 Sludge Lagoons 357 A.5 Sludge Drying Beds 360 A.6 Mechanical Thickening and Dewatering 365 A.6.1 Sludge Thickening 365 A.6.2 Sludge Dewatering 373 A.7 Coagulant Recovery 380 A.8 Sludge Disposal 388 A.8.1 Introduction 388 A.8.2 Disposal to Municipal Sewers 390 A.8.3 Land Application of Water Plant Sludge 393
A.9 References 394
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 co-agulation 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
1W. Ostwald in Electrolyte coagulation and coefficient of electrolyte activity, Fifteenth Colloid
Symposium, Cambridge, MA, June 9–11 (1938)
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 pro-cesses 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 sequen-tially from theoretical principles to practical applications. Successive chapters deal with, in turn, properties of materials present in waters and wastewaters; char-acteristics 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
mix-ing 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 re-lated operations is dealt with in the Appendix. This important topic has been separated from the main text to avoid disturbing the continuum of the presenta-tion.
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.