Developments in Water Treatment 2 (Development Series) 0853349037, 9780853349037, 9780203974926

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DEVELOPMENTS IN WATER TREATMENT—2

THE DEVELOPMENTS SERIES Developments in many fields of science and technology occur at such a pace that frequently there is a long delay before information about them becomes available and usually it is inconveniently scattered among several journals. Developments Series books overcome these disadvantages by bringing together within one cover papers dealing with the latest trends and developments in a specific field of study and publishing them within six months of their being written. Many subjects are covered by the series including food science and technology, polymer science, civil and public health engineering, pressure vessels, composite materials, concrete, building science, petroleum technology, geology, etc. Information on other titles in the series will gladly be sent on application to the publisher.

DEVELOPMENTS IN WATER TREATMENT—2 Edited by

W.M.LEWIS M.Chem.A., C.Chem., F.R.I.C., F.I.W.E.S. WHO Consultant EURO, Environmental Health—Drinking Water Quality, Copenhagen, Denmark Managing Director, Coventry Chemical Consultancy Ltd, Coventry, UK

APPLIED SCIENCE PUBLISHERS LTD LONDON

APPLIED SCIENCE PUBLISHERS LTD RIPPLE ROAD, BARKING, ESSEX, ENGLAND This edition published in the Taylor & Francis e-Library, 2005. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to http://www.ebookstore.tandf.co.uk/.” British Library Cataloguing in Publication Data Developments in water treatment.—(Developments series). 2 1. Water—Purification I. Lewis, W.M. II. Series 628.1′6 TD430 ISBN 0-203-97492-1 Master e-book ISBN

ISBN 0-85334-903-7 (Print Edition) WITH 24 TABLES AND 54 ILLUSTRATIONS © APPLIED SCIENCE PUBLISHERS LTD 1980 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, photocopying, recording, or otherwise, without the prior written permission of the publishers, Applied Science Publishers Ltd, Ripple Road, Barking, Essex, England

PREFACE ‘Surveillance of Drinking Water Quality’ published by WHO in 1976 in its introduction stated that ‘Public health protection of drinking-water supplies should assure that each component of the system—source, treatment, storage and distribution—functions without risk of failure’. Drinking water is perhaps, together with the air we breathe, a unique commodity in that the general population is normally permitted no freedom of choice, so the assurance that the water available for drinking is of the highest quality is of paramount importance. Since the universal introduction of disinfecting agents in water supplies in developed countries, risks to health from microbially contaminated drinking water have been dramatically reduced. Today the problem confronting personnel responsible for ensuring the public’s water supply is perhaps of a much more subtle character brought about, in part at least, by the rapid progress in analytical chemistry and the environmental awareness of the general public resulting in the demand for the creation of standards of quality for drinking water. Chemicals present in raw water supply range from simple ions extracted from soil and minerals in the watershed to (in some instances) unidentified waste products from the chemical industry, the length of the list being limited only by the capabilities of the analytical chemists and their instruments. Some medical researchers proclaim that the presence, or absence, of a certain substance in drinking water is directly associated with the differences in death rates from specific diseases or the incidence of morbidity. The problem is compounded due to the fact that the toxic effects of many identified chemicals are insufficiently understood. Thus the responsibility devolving upon the shoulders of personnel responsible for ‘Treatment’ is to provide a process (or combination of processes) which will ensure, as far as is practicable, that the water supply is not only aesthetically acceptable, but also of the best chemical standard and in a condition which will not damage the integrity of the distribution system which could result in subsequent and additional contamination. With rivers of the calibre of the Danube, Trent and Rhine, to mention but three, the resulting treatment, to provide drinking water whose quality is beyond suspicion, needs to be very sophisticated. The ‘treatment process’ is not a single identifiable parameter but is dependent upon the nature and quality of the raw material and may for example involve only simple filtration or filtration plus disinfection. On the other hand if the quality of the supply water is from a lowland river, such as the three previously mentioned, then it follows that a combination of individual processes, commencing with coagulation for the removal of suspended matter, etc., and employing perhaps the majority of the techniques described, will be essential to provide the required quality of drinking water. Within this series will be found the various important facets of treatment each written by an author, expert in the particular field, who has introduced his topic with a brief

historical background before providing the reader with the most up-to-date information available on the subject. It is a salutary thought that in 1975 (latest information available) some 78% of the world’s rural population and even 22% of the urban population were without an adequate water supply. Of the urban population of the world having access to a piped water supply (77%), 57% only had house connections and 54% of the population served by public piped supply received it only on an intermittent flow basis. Conscious of the urgent need to rectify these shortcomings, the UN Water Conference—Mar del Plata, March 1977—urged the adoption of ‘The International Drinking-Water Supply and Sanitation Decade, 1981–1990’. The aim of the latter is to encourage and assist all countries of the world to adopt programmes with realistic standards for both quality and quantity and to provide water to all people by 1990, if possible. It is unfortunate, but nevertheless true that at present, and for how long into the future we know not, many countries—not only the developing ones—are experiencing financial constraints of varying magnitude. To achieve the above objectives will therefore strain the ingenuity and professional expertise of all concerned with the task of supplying the community with drinking water. It is consequently singularly appropriate that these first two volumes on ‘Developments in Water Treatment’ should be available at this time, for within their pages will be found that information on ‘Treatment’, appropriate to the needs, to enable people to overcome the financial constraints laid upon them. W.M.LEWIS

CONTENTS Preface

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List of Contributors

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1. Filtration T.H.Y.TEBBUTT 2. Removal of Organic Compounds C.S.SHORT 3. Removal of Nitrogen Compounds R.B.GAUNTLETT 4. Desalination M.J.BURLEY and J.D.MELBOURNE 5. Disinfection A.T.PALIN 6. Sludge Treatment and Disposal M.A.HILSON 7. Water Quality Monitoring P.J.MORLEY and J.COPE Index

1 22 53 79 122 142 162

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LIST OF CONTRIBUTORS M.J.BURLEY Consultant, Sir M.Mac Donald & Partners, Demeter House, Station Road, Cambridge, CB1 2RS, UK. J.COPE Scientific Officer, Headquarters Staff, Severn-Trent Water Authority, Tame House, Newhall Street, Birmingham, B3 1SE, UK. R.B.GAUNTLETT Treatment Division, Water Research Centre, Medmenham, Marlow, Bucks, SL7 2HD, UK. M.A.HILSON Principal Scientist, Water Treatment and Supply, North West Water Authority, Dawson House, Great Sankey, Warrington, WA5 3LW, UK. J.D.MELBOURNE Managing Director, Melcon Water International Ltd, 165 Reading Road, Henley-onThames, Oxon., RG9 1DP, UK. P.J.MORLEY Principal Scientist, Avon Division, Severn-Trent Water Authority, Avon House, De Montford Way, Cannon Park, Coventry, CV4 7EJ, UK. A.T.PALIN Consulting Chemist, 7 Montagu Court, Montagu Avenue, Newcastle upon Tyne, NE3 4JL, UK. C.S.SHORT Yorkshire Water Authority, Olympia House, Gelderd Road, Leeds, LS12 6DD, UK. T.H.Y.TEBBUTT Senior Lecturer, Department of Civil Engineering, The University of Birmingham, P.O. Box 363, Birmingham, B15 2TT, UK.

Chapter 1 FILTRATION T.H.Y.TEBBUTT, B.Sc., S.M., Ph.D., M.I.C.E., M.I.W.E.S. Senior Lecturer, Department of Civil Engineering, The University of Birmingham, Birmingham, UK

SUMMARY Developments in water filtration have enabled a more efficient use of the process and have provided a better understanding of the mechanisms involved. Good filtrate quality can be achieved at higher loadings than would have been considered possible some years ago and the advent of dual and multi media beds has brought economic benefits. A consequence of more effective use of bed capacity is that the process is more likely to be influenced by the nature of the suspended solids in the feed. A consequence of obtaining deeper penetration of suspended matter into a filter bed is the need to ensure adequate cleaning arrangements. The complexity of the filtration process is such that mathematical models cannot as yet provide a universal solution for any situation but their existence is of value in helping to develop more efficient filtration plants.

1.1. INTRODUCTION Some form of filtration is almost certain to be incorporated in the treatment used to produce potable water from most surface water sources. Indeed, the principle of filtering rain water through a layer of porous material to remove suspended matter has been used for centuries although the first recorded purpose-built sand filters for a public water supply were not installed until 1804 in Paisley. The early installations were of the slow filter type and their considerable purification ability in no small way contributed to the great improvement in public health which occurred during the second half of last century. By 1870 the rapid pressure filter concept made its appearance, to be shortly followed by the rapid gravity filter whose basic design has not changed greatly to the present day. However, since the introduction of filtration as a water treatment process a considerable amount of knowledge has been accumulated about the behaviour and performance of the process. The continuing pressures to require more efficient treatment, in operational and economic terms, and the demands for higher quality filtrates have meant that a variety of developments to and modifications of the conventional filtration process have been introduced. Coupled with these developments has been an intensive study of the actual mechanisms involved in filtration with the aim of providing a better understanding of the process to aid in future design. Filtration through porous media is however a process of great complexity and it is not yet possible to produce a completely satisfactory theoretical model.

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1.2. APPLICATIONS OF FILTRATION Except in rare cases all surface waters will contain, at least at times, sufficient suspended matter to given turbidities in excess of 5 units which is the highest desirable level stipulated in the World Health Organisation International Drinking Water Standards. EEC standards set out a guide level of 5 FTU with a maximum admissible concentration of 10 FTU. In the USA the National Interim Drinking Water Standards lay down a mandatory turbidity limit of 1 unit for most surface-water-derived supplies. The American Water Works Association has adopted a turbidity goal of 0.1 units and it has been suggested that this may in the future be reduced to 0.05 units. The rationale behind the adoption of such low turbidity levels is not that the turbidity itself is likely to be harmful but that its presence may inhibit disinfection processes. With such limits for turbidity being required, even high quality impounded waters would need turbidity removal for much of the time and all lowland river derived supplies would necessitate comprehensive treatment for turbidity removal. With high turbidity raw waters, chemical coagulation and sedimentation are commonly employed to remove the bulk of the turbidity with filtration providing the final polishing stage to bring turbidity down to around 1 unit. With coloured upland sources, turbidity in the raw water may not be a serious problem but chemical coagulation is frequently used to remove the colour by what is in effect a precipitation process so that here again filtration is necessary following sedimentation. With low turbidity raw waters direct filtration using either gravity or pressure rapid filters aided by chemical coagulation is widely used and in parts of the UK and Europe slow sand filters are used either as a single stage of treatment or as a second stage following rapid filtration used as a roughing or preliminary stage of treatment. Whilst all these types of filtration should be able to produce filtrates with turbidity less than 1 unit the reliable production of turbidities less than 0·1 or 0·05 units is another matter. It is important to appreciate that most treatment processes, including filtration, follow a law of diminishing returns in that the unit cost of successive increments of purification increases, sometimes almost exponentially. Before setting stringent turbidity levels it should therefore be considered whether such levels are achievable by normal means of treatment and whether the cost of such treatment is justifiable in respect of the marginal improvement in water quality which they would produce. It is of interest to note that developments in wastewater purification have resulted in the adoption of filtration as a tertiary treatment stage in situations where the effluent is discharged to a receiving water with little dilution or which provides a raw water source some distance downstream of the outfall. In such circumstances sand filtration, usually in rapid gravity units, removes a considerable amount of the suspended solids which escape from final settling tanks together with the organic matter associated with these suspended solids. Thus a normal secondary effluent of 30 mg/litre suspended solids and 20 mg/litre biochemical oxygen demand can be polished by filtration to give a final effluent of around 10 mg/litre suspended solids and biochemical oxygen demand. The adoption of tertiary filters has encouraged a considerable amount of research into design and operational factors which has enhanced knowledge of the filtration process.

Filtration

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1.3. CONVENTIONAL FILTRATION Before discussing recent developments in the filtration of water it is useful to briefly outline what may be considered as conventional usage which had not changed greatly until the early 1970s and which is summarised in Table 1. 1.3.1. Slow Filters As indicated earlier, the first form of sand filter was the slow type, so called because of its relatively low hydraulic loading of 1–4m3/m2d. Because of this low hydraulic loading there is only superficial penetration of suspended matter into the bed and filter runs of several weeks or even months are

TABLE 1 CHARACTERISTICS OF CONVENTIONAL FILTERS Characteristic

Slow filter Rapid filter

Filtration rate

1–4 m3/m2d

Size of bed

About 2000 m2 1m unstratified 0·5–1·0 mm 2·0–2·5

100–150 m3/m2d About 100 m2 0·5–0·8 m stratified 0·5–1·5 mm 1·2–1·5

Up to 1 m 20–90 days 0·2–0·6% of output

Up to 2·5 m 1–5 days 1–6 % of output

Depth of bed Sand size Uniformity coefficient Head loss Length of run Cleaning water consumption

possible before excessive head loss terminates operation. The usual depth of water above the sand surface is 1·0–1·50 m so that when the head loss approaches this figure filtration must be stopped to prevent the possibility of sub-atmospheric pressures being created within the bed with consequent deterioration in performance. The bed can be restored to operation by scraping off the top few cm of sand which can be washed and later used to replenish the bed. The long interval possible between cleaning procedures enables biological activity to become established in the bed particularly in the surface layers where a biological slime, the schmutzdecke, contributes significantly to the removal of fine suspended matter. In addition the biological growth utilises organic constituents in the water as a food source oxidising them to inorganic end-products and often preventing or reducing possible taste and odour problems in the finished water. Nitrification of ammonia is also usually a feature of slow sand filtration and this can make the ensuing disinfection stage more reliable. With small levels of ammonia, which would be all that would be likely to be present in a potable water source, the small addition of nitrate to the

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finished water is unlikely to be objectionable although of course levels of nitrate nitrogen of more than a few mg/litre are to be viewed with suspicion. Because of their large size slow filters are labour intensive as regards cleaning operations and although a number of plants have been equipped with mechanical scraping devices, the cleaning of slow filters is a costly operation. For this reason it is imperative that they are only used for relatively low turbidity (10–20 units) raw waters. Because of this limitation, slow filters are not normally suitable for sources such as lowland rivers with high turbidities which usually require chemical coagulation since the additional carry-over of floc from settling tanks would be sufficient to rapidly clog the filter surface. The increased dependence on lowland sources has resulted in a decline in the use of slow sand filters in many parts of the world. However in the UK much of London’s water is treated by slow filters which follow primary rapid filters. In the Netherlands slow filters have found considerable application as the final treatment stage when purifying heavily polluted river waters such as those from the Rhine. 1.3.2. Rapid Filters The introduction of rapid sand filters no doubt arose partly from the seemingly inevitable trend to increase the capacity of treatment units and the need to handle high turbidity raw waters. The land area required for slow filters became a factor when constructing treatment plants for large communities so that considerable pressures were exerted on designers to develop more compact units. The result was the rapid filter with a hydraulic loading of around 100 m3/m2d. At this rate of flow, penetration of solids into the bed is considerable and clogging to a maximum allowable head loss takes only a day or two depending upon feed quality. Alternatively, penetration of turbidity may be complete resulting in breakthrough of suspended matter in the filtrate which will terminate the run even though the head loss limit may not have been reached. The deep penetration of solids and frequent need for cleaning would of course rule out the type of cleaning operation used for slow filters. Fortunately, however, the incorporation of backwashing systems was able to provide a practical and economical means of cleaning rapid filters. When using the normal bed of graded media, backwashing results in a stratified bed which is not desirable from the point of view of optimum utilisation of bed capacity but which has been generally accepted as a characteristic of rapid filtration. The bulk of filtration capacity worldwide is in the form of rapid gravity filters preceded, where appropriate, by chemical coagulation and sedimentation units. In the UK, rapid pressure filters with direct injection of coagulants find favour in hilly areas where their hydraulic characteristics prove useful when siting treatment plants but in other parts of the world pressure filters tend to be mainly used for industrial water supplies. For low turbidity raw waters rapid filtration alone, possibly with the addition to the raw water of a small dose of coagulant, can provide a filtrate of satisfactory quality. In most cases, however, the bulk of the turbidity is removed by preliminary coagulation and sedimentation processes which prolong filter runs. As mentioned in the previous section, rapid gravity filters are also used as the primary stage in a double filtration process. In the case of pressure filters separate coagulation and sedimentation tanks are not feasible so that the coagulant is dosed directly into the raw water and flocculation takes place largely within the filter bed. This does of course mean that a much larger

Filtration

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amount of suspended matter is trapped by the filters and thus filter runs are much shorter than if the same water were treated by conventional gravity filters preceded by coagulation and sedimentation units. The omission of these units does however give a cost reduction to offset the shorter filter runs and more frequent washing required by pressure filters. An important feature of rapid gravity filters is the arrangement made for backwashing and in particular the design of the underdrain and filter floor. It is important that the active filter medium is supported in such a way that it is contained within the filter shell whilst preventing any short circuiting of flow during filtration or backwashing. The commonest system in the UK is to employ a concrete floor with plastic collection and distribution nozzles covered by a layer of graded gravel but there are a considerable number of proprietary filter bottoms. Some of these are of complex and costly design and it is by no means certain that any advantages they may have are cost effective. There are also variations in the form of filter control and backwash procedures adopted by different manufacturers and designers. A variation of the pressure filter, used quite widely in the UK for swimming pool water treatment, employs diatomaceous earth or cellulose powder deposited on rigid formers as the filtering medium. These units can give excellent turbidity removal and are found suitable for small potable supplies in some parts of the world, notably in the USA. In operation it may be beneficial to dose the raw water with a small concentration of the filtering medium. At the end of a run the medium together with the trapped suspended matter is discharged to waste and the filter element recoated with powder. The units are relatively small in size and can provide an economic form of treatment for low turbidity supplies.

1.4. THE FILTRATION PROCESS At first sight the manner in which a bed of porous medium removes suspended particles from a fluid passing through the bed might appear to be easily explained as some form of straining action. Clearly, if the suspended particles are larger than the voids in the bed they will be removed in the same way as a sieve prevents the passage of particles larger than the mesh size. Such a removal mechanism would mean that no penetration of solids into the bed could occur and that there would therefore be no need for a deep bed of medium. Examination of the solids removal performance of a granular bed shows, however, that particles very much smaller than the voids in the bed are effectively removed. Thus in a sand bed with grain size of 0·5–1·0 mm the voids are likely to be of the order of 100–200 µm. In such a bed, removal of fine colloidal particles such as silt and bacteria with sizes of about 1 µm are readily demonstrated. It is evident, therefore, that straining cannot be the only mechanism operating in a bed of granular medium and indeed in most situations straining plays a relatively insignificant part in the overall removal of turbidity. Removal of suspended particles is controlled by a number of transport and attachment mechanisms. The transport mechanisms move particles into the vicinity of grains of medium in the bed when the attachment mechanisms can operate to trap the particle.

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1.4.1. Transport Mechanisms In most filters laminar flow conditions prevail and thus the velocity in a void within the bed varies from zero at the grain surface to a maximum at the centre of the void. Transport mechanisms thus have to be able to move suspended particles out of the flow streamlines into lower velocities near the bed grains. The important transport mechanisms in water filtration are: 1. Interception: streamlines may pass close enough to bed grains so that suspended particles actually come into contact with the grain surface. 2. Diffusion: colloidal particles are influenced by random Brownian movements which may bring them into the vicinity of the grain surface. This mechanism is only important for very small particles (