Coagulation and Flocculation in Water and Wastewater Treatment Second Edition John Bratby

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: publications@iwap.co.uk
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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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.