The Scientific Basis of Flocculation PDF
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THE SCIENTIFIC BASIS OF FLOCCULATION NATO ADVANCED STUDY INSTITUTES SERIES Proceedings of the Advanced Study Institute Programme, which aims at the dissemination of advanced knowledge and the formation of contacts among scientists from different countries. The series is published by an international board of publishers in conjunction with NATO Scientific Affairs Division A Life Sciences Plenum Publishing Corporation B Physics London and New York C Mathematical and D. Reidel Publishing Company Physical Sciences Dordrecht and Boston D Behavioural and Sijthoff & Noordhoff International Publishers B.V. Social Sciences E Applied Science Alphen aan den Rijn, The Netherlands and Winchester, Mass., USA Series E: Applied Science - No. 27 THE SCIENTIFIC BASIS OF FLOCCULATION edited by KENNETH J. IVES Professor of civil engineering Department of Civil and Municipal Engineering University College London, u. K. SIJTHOFF & NOORDHOFF 1978 Alphen aan den Rijn - The Netherlands Proceedings of the NATO Advanced Study Institute on The Scientific Basis of Flocculation Christ's College, Cambridge, U. K. July 3-15, 1977 ISBN-13: 978-94-009-9940-4 e-ISBN-13: 978-94-009-9938-1 001: 10.1007/978-94-009-9938-1 © Copyright 1 978 Sijthoff & Noordhoff International Publishers B.v., Alphen aan den Rijn, The Netherlands. Softcover reprint of the hardcover 15t edition 1978 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photo copying, recording, or otherwise, without the prior permission of the copyright owner. v TABLE OF CONTENTS K.J. Ives Introduction 1 J. Lyklema Surface Chemistry of Colloids in Connection with Stability 3 K.J. Ives Rate Theories 37 L.A. Spielman Hydrodynamic Aspects of Flocculation J. Gregory Flocculation by Inorganic Salts J. Gregory Effects of Polymers on Colloid Stability 101 R.J. Akers Experimental Methods (1) Concentrated Suspensions 131 K.J. Ives Experimental Methods (2) Dilute Suspensions 165 H. Sontheimer Flocculation ln Water Treatment 193 C.R. O'Melia Coagulation in Wastewater Treatment 219 VI K. Roberts Flocculation and Dewatering of Sludges J.A. Kitchener Flocculation in Mineral Processing D.C. Freshwater Integral Design and Flocculation Systems Index INTRODUCTION K.J. Ives Professor of Public Health Engineering University College London The aggregation of small particles in liquids, to form flocs which are large enough to settle, or to be filtered, is a common operation in industrial processes, and water and wastewater treatment. This aggregation, given the general title flliocculation in this book, may be brought about by the addition of chemicals to reduce the stability of the original suspension, by neutralising electrical forces of repulsion, by the addition of chemicals (polymers) to link particles by bridging action, by the addition of chemicals which form particles to increase collision proba bilities, and by the input of energy leading to hydrodynamically induced collisions. The particles undergoing flocculation may range from colloidal in the nanometer size range, through micro scopic (micron) size, up to visible particles in the millimeter size range; that is a total size range of six orders of magnitude. Consequently the colloid chemist and the hydrodynamicist are both concerned with the interactions that take place, and to them the engineer must turn, to obtain the fundamental information ne cessary for the process design and its associated hardware. The particles involved may be highly defined in characteris tics (such as the silver iodide sols and polystyrene latices beloved of colloid scientists), or less well-defined as in mine ral processing or water treatment, or very ill-defined as in wastewater treatment. The particles may also be inorganic, or ganic or biological to further complicated understanding of the process. 2 In an attempt to rationalise the information available on floc culation in liquids, the lecture series of the NATO Advanced Study Institute held in Cambridge, England in July 1977 set out to provide the scientific basis of flocculation in a logical se quence. Consequently the fundamental surface chemistry of colloids is followed by kinetic theories, and the basic hydrodynamic as pects. Practical chemicals, principally inorganic salts and poly mers, are described with their effects on colloid particles, and then laboratory experimental methods for the study of floccula tion of both concentrated and dilute suspensions are considered. Then follows descriptions of the application of flocculation: to water treatment, for wastewater treatment, for sludge de watering and in mineral processing including selective floccu lation. Finally the integral design of flocculation systems is described in terms of linked processes designed for optimal, reliable operation. As the individual lectures, which form the chapters of this book, were prepared independently by expert authors, there is no uniformity of style or symbols throughout. Indeed the dis ciplines of colloid chemistry on the one hand and hydrodynamics on the other have their own traditional nomenclatures, and any attempt to harmonise them would probably produce a hybrid that would suit neither. However, where necessary, each chapter has its own notation defined. Although reference is made to well-known process units and engineering hardware, there is no attempt to proceed to the practical design stage involving the choice of constructional materials, sizing of units and specifications for pumps, machinery etc. This omission is consistent with the aim of this book to provide the scientific basis of the subject. Also a certain level of knowledge of science is assumed on the part of the reader, so that fundamental laws and concepts of chemistry, physics, fluid mechanics and mathematics are taken for granted. Where the reader feels deficient in any of these topics, he should consult the reference lists provided with each chapter. This book can be used as a companion volume to "The Scientific Basis of Filtration" published in the same series in 1975. 3 SURFACE CHEMISTRY OF COLLOIDS IN CONNECTION HITH STABILITY J. Lyklema Lab. for Physical and Colloid Chemistry, Agricultural University, De Dreijen 6, Wageningen, The Netherlands ABSTRACT. A review is given of the fundamental features of the inter facial electrochemistry of colloid particles, emphasizing its role in colloid stability. The discussion includes a description of double layers in the presence of simple electrolytes and with polymers. 1. SOHE GENERAL REMARKS ON COLLOID STABILITY Colloid stability is a widely used term that usually covers the broad field of the interaction of dispersed particles, their aggre gation or resistance to aggregation. The term is also used beyond the field of colloids proper if the character of the interactions has a colloid character. For instance, the stability of soils and suspensions can, to a certain extent, be interpreted by the same laws that apply to colloidal solutions. Some authors consider dis solved polymers also as colloids. The aggregation of colloids is known as coagulation or floccu lation. In the past, the two terms were used interchangeably, but nowadays there is a trend to distinguish between aggregation due to simple ions (coagulation) and aggregation due to polymers (floccu lation). This distinction is adhered to in this paper, althou~h it is understood that the title of the entire course is meant to in clude both. Aggregation by inorganic hydrolyzed species is taken to belong to the first group. Except under idealized laboratory conditions, colloid stabili ty is a very complex matter. Several forces can be operative be tween the colloids; some attractive, others repUlsive. These forces may react in different ways upon variations in the conditions (pH, T, salt concentration etc.). In order to understand what forces are operative in a given case, it is mandatory to characterize the col loidal particles, and especially the surfaces of these particles. 4 Even that is not yet enough, because one has to consider also the rate at which particle surface properties can alter relatively to the rate at which two particles approach each other. In other words, both static and dynamic properties deserve attention. Hence, if flocculation has a scientific basis, its root has to be found in the particle surfaces, the description of which constitutes the theme of this article. The most fre~uently occurring forces between colloids are: van der Waals forces, electrostatic forces and forces due to adsorbed macromolecules. Van der Waals forces are ubi~uitous; electric for ces almost so. Adsorbed polymers - or adsorbed lower molecular weight materials - obviously operate only if they are present, but in natural waters such is fre~uently the case. In addition, specif ic forces may act in special cases. For instance, magnetic colloid particles may attract each other magnetically or chemical bonds may be formed between two colloids. Some of these forces have a long range. Van der Waals forces are long-range, in this case implying that they can operate over several tens of nm. This applies also for electrostatic repulsion, with the utmost important addition that the range is reduced if electrolyte is added. On this principle the fundamental picture of colloid stability is based: sols stabilized by electric repulsion between the particles can be destabilized by electrolytes. Chemical bonds are short-range, hence they can come into operation only if there are no other forces keeping approaching particles apart. The treatment of colloid stability is different for dilute and concentrated sols. In dilute sols, aggregation is typically a bi molecular process. In concentrated systems, all particles interact simultaneously with several neighbours (as in a li~uid), and the treatment is rather given in terms of statistical thermodynamics. However, the state of such systems depends ultimately on pair in teraction energies (often erroneously called "potentials"). In the following sections, the most important interfacial pro perties of colloid particles will be presented and their contribu tion to particle interaction studied. The emphasis is strongly on electrostatic phenomena. 2. INTERFACIAL ELECTROCHEMISTRY 2.1 Origin of surface charge Charges on dispersed particles are invariably due to an une~ual distribution of ions over the particle and the surrounding solution. The particle charge and the charge in the solution are e~ual in mag nitude but opposite in sign. The particle charge and the counter charge together form an electrical double layer. The reason why an uneven charge distribution between particle and solution arises can be ~uite different, depending on the nature of the particle. Some familiar processes are:
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