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COAGULATION and FLOCCULATION with an Emphasis on Water and Wastewater Treatment COAGULATION and FLOCCULATION With an emphasis on water and wastewater treatment by JOHN BRATBY "Coagulation and Flocculation" Coagulation and flocculation play a dominant role in many water and wastewater schemes. Because of a complex interdependence of numerous factors inherent in the coagulation and flocculation processes, a thorough understanding of the phenomena involved is essential. To date, pertinent current literature does not present an overall account of coagulation and flocculation in a single publication; as a consequence, the reader lacks an overall picture of the processes as a whole and, without further extensive literature searching, succeeds in achieving only a superficial understanding. This new book has been written to fill the conspicuous gap in the current literature. The subject matter is presented logically and sequentially from theoretical principles to practical applications such that the reader gains an overail in-depth understanding of ttie processes as a whole. The Author The form of presentation of the book is a reflection of the author's background. Dr. Bratby, author of numerous technical papers and articles, held the position of Senior Research Officer of the Water Research Commission of South Africa at the University of Cape Town and, at the present time is Contract Technical Consultant with CAESB, the Brazilian government organisation responsible for environmental protection, water and sewage treatment etc. in the Federal District of Brasilia. In its present form the book evolved from material offered to undergraduate and postgraduate university students. Although it has been progressively modified and increased in scope so as to cater for the requirements of practising chemists, chemical and civil engineers involved with water and wastewater treatment, the book retains a didactic nature. Therefore, the book is designed to serve three purposes: (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 and (3) Course material for under- and postgraduate students. COAGULATION JOHN BRATBY AND pLOCCULATION WITH AN EMPHASIS ON WATER AND WASTEWATER TREATMENT UPLANDS PRESS LTD Publishers of "Filtration & Separation " magazine CROYDON CR9 1LB, ENGLAND 1980 First published in 1980 by Uplands Press Ltd. Croydon CR9 1LB Uplands Press Ltd. All rights reserved. No part of the publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means without the prior permission of Uplands Press Ltd. An Uplands Press Publication TO JENNY AaTEpa? doaSpETS daTTJp i\l6%. EV8E Y£VO'MT1V oupovoS, QS TTOXAOTS 6\niaoiv tK oi p PLATO (Greek Anthology. Bk.vii, epig.669) Preface Coagulation and flocculation play a dominant role in many water and wastewater treatment, schemes. Because of a complex interdependence of numerous factors inherent in the coagulation and flocculation processes, a thorough un- derstanding of the phenomena involved is essential. In general, pertinent current literature does not present an overall account of coagulation and flocculation in a single publication. Moreover, a given publication may emphasise theoretical aspects with little indication of the practical significance of the phenomena, or, largely ignores the theoretical development and presents essentially practical material only. In most cases, the uninitiated reader lacks an overall picture of the processes as a whole and, without further extensive literature searching succeeds in achieving only a superficial understanding. This book has the intention of remedying the above in- adequacies. An attempt has been made to present the subject matter logically and sequentially from theoretical princi- ples 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 co- agulants and polyelectrolyt.es respectively; 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 floc- culation parameters. Numerous examples are included as appropriate. Disposal of sludges resulting from coagulation-floc- culation related operations is dealt with in the Appendix. This important topic has been separated from the main text so as to avoid disturbing the continuum of the presentation. The Appendix also includes a "ready reference" information source of coagulation-flocculation applications. The objective in compiling information in this fashion is to provide an a priori guide to coagulant types , dosages and conditions of application; provisions for flocculation; related solid- liquid separation facilities and so on, for numerous applic- ations . The book in its present form evolved from material of- fered to undergraduate and postgraduate university students. Although it has been progressively modified and increased in scope so as to cater for the requirements of practicing chemists, 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 under- and postgraduate students. I wish to express my appreciation of the following: Professor G.V.R. Marais, University of Cape Town, whose exam- ple as a research worker has always been an inspiration; The Water Research Commission of South Africa under the Chairman- ship of Dr. G. J. Stander who, through their intelligent sponsorship, contributed to a healthy research atmosphere which I enjoyed at the University of Cape Town; Mr. Ian Reed, consultant engineer, Ninham Shand and Partners Inc., for many fruitful discussions and especially his help with parts of Chapter 6; Universidade de Brasilia, in particular Professor Marco Antonio A. Souza of the Department of Civil Engineering, for authorization to use the University library facilities; Mr. Derek B. Purchas, consultant chemical engineer and Ill Mr. John Pickford, senior lecturer at the University of Tech- nology, both of U.K., for advice concerning publication; The South African Water Information Centre, The Water Research Centre, U.K. and Environmental Protection Agency, U.S.A. in particular Mr. M. R. Steyn, Mr. P. J. Russell and Ms. Carole Cumiford respectively as invaluable sources of information; and Sra. Eleusis Gurgel Machado who produced an excellent typescript from an impossible draft. JOHN BRATBY IV Contents Preface i 1 Introduction 1 CONTENTS 1 1.0 STABILITY AND DESTABIL[ZATION 1 1.1 DEFINITIONS 5 1.2 PERFORMANCE CRITERIA 7 1.2.1 Turbidity 9 1.2.2 Colour 11 1.3 SUMMARY 11 1.4 REFERENCES 13 2 Colloids and Interfaces 14 CONTENTS 14 2.0 INTRODUCTION IS 2.1 ORIGIN OF SURFACE CHARGE 19 2.2 EFFECT OF SURFACE CHARGE 21 2.3 ADSORPTION 2 2 2.4 INNER PART OF ELECTRICAL DOUBLE LAYER 2S 2.5 DIFFUSE PART OF ELECTRICAL DOUBLE LAYER 28 5.S.I Assumptions 28 2.5.2 Distribution of Potential With Distance From Charged Surface 29 2.5.3 Thickness of Double Layer 31 2.5.4 Effect of Ionic Strength on Double Layer 34 2.5.5 Effect of Nature of Counter Ions 35 2.6 STERN'S MODEL OF COMPLETE DOUBLE LAYER 35 2.7 COLLOID STABILITY IN TERMS OF THB DOUBLH LAYER 37 2.7.1 Energy of Interaction Between Particles 37 2.7.2 Theoretical Optimal Concentration of Electrolyte for Destabilization 39 2.7.3 Schulze-Hardy Rule 40 2.8 ELECTROKINETIC MEASUREMENTS 41 2.8.1 Zeta Potential 41 2.8.2 Electrophoresis Measurements 44 2.8.3 Streaming Current Measurements 47 2.8.4 Colloid Titration 48 2.9 RKFERliNCHS 50 3 Coagulants 54 CONTENTS 54 3.0 INTRODUCTION 55 3.1 METAL COAGULANTS 55 3.1.1 Commonly Used Metal Coagulants 55 3.1.1.1 Aluminium Sulphate 56 3.1.1.2 Aluminium Chloride 58 3.1.1.3 Poly aluminium Chloride 58 3.1.1.4 Sodium Aluminate 59 3.1.1.6 Ferric Sulphate 59 3.1.1.6 Ferrous Sulphate 60 3.1.1.7 Chlorinated Copperas 60 3.1.1.8 Ferric Chloride 61 3.1.1.9 Dissolution of Dry Coagulant Products 62 3.1.2 Chemistry of Metal Coagulants 6 3 3.1.2.1 Hydration and Stepwise Substitution Reactions 63 3.1.2.2 Stability of Metal-Ion Hydrolysis Species 66 3.1.2.3 Average Coordination Number 69 3.1.2.4 Species Distribution During Destabilization 71 3.2 POLYELECTROLYTES 76 3.2.1 General 76 3.2.2 Activated Silica 77 VI 3.2.3 Natural Polyelectrolytes 77 3.2.3.1 Guar Gume 78 3.2.3.2 Starches 78 3.2.3.3 Tannins 78 3.2.3.4 Sodium Alginate 78 3.2.4 Synthetic Polyelectrolytes 79 3.2.4.1 Structure of Synthetic Polyelectrolytes 80 3.3.4.2 Charge Density and Molecular Weight 83 3.2.4.3 Dissolution of Synthetic Polyelectrolytes 84 3.2.4.4 Toxicity of Synthetic Polyelectrolytes 88 3.3 REFERENCES 89 4 Destabilization with Metal Coagulants 9Z CONTENTS 9 2 4.0 INTRODUCTION 9 3 4.1 DESTABILIZATION OF HYDROPHOBIC COLLOIDS 96 4.1.1 Extent of Hydrolysis and Adsorption 96 4.1.2 Effect of Coagulant Dosage 98 4.1.3 Effect of Colloid Concentration 100 4.1.4 Effect of pH 104 4.1.4.1 General 104 4.1.4.2 pH 1,0 105 4.1.4.3 pH 2,0 105 4.1.4.4 pH 3,0 to 5,0 106 4.1.4.5 pH B,0 to 9,0 107 4.1.4.6 Further Considerations 107 4.2 DESTABILIZATION OF HYDROPHILIC COLLOIDS 109 4.2.1 General
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