Eawag_07549

Draft IRCWD Report No. 10/92

Surface Water Treatmentby Roughing Filterswith special emphasis on

horizontal—flow roughing filtration

Martin Wegelin

International Reference Centre for Waste Disposal (IRCWD)Ueberlandstr. 133, CH — 8600 Duebenclorf, Switzerland

Surface Water Treatment by Roughing Filterswith special emphasis on horizontal-flow roughing filtration

, byMartin Wegelin

in collaboration withGerardo Galvis and Jan Teun Visscher

IRCWD, June l992

Text Revisor: Sylvie PeterScript Processing: Brigitte HauserIllustrations: Heidi Bolliger

Dominik SchweizerPaul Schlup

Copyright © by the International Reference Centre for Waste Disposal (IRCWD), CH-8600 Duebendorf, Switzerland

Permission is granted for reproduction of this material, in whole or part, for education,scientific or development related purposes except those involving commercial sale,provided that

- full citation of the source is given- written request is submitted to IRCWD.

Foto on Cover Page: Roughing and Slow Sand Filter Plant Restrepo,Colombia (CINARA, University del Valle, Cali) '

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Surface Water Treatment by Roughing Filterswith special emphasis on horizontal-flow roughing filtration

Part1

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Part2

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byMartin Wegelin

in collaboration withGerardo Galvis and Jan Teun Visscher

Table of Contents

PrefaceIntroduction

General Aspects of Roughing Filter Application _

Water Treatment ConceptSolid Matter SeparationSedimentationRoughing FiltrationRoughing Filter DesignSelection Criteria for Roughing Filter ApplicationBacteriological Water QualitylmprovementChlorinationSlow Sand FiltrationLayout of a Water Supply SchemeGeneral ConsiderationsHydraulic Profile‘Treatment StepsWater DistributionApplication of Roughing FiltersHistoric UseRecent Development of Horizontal-flow Roughing Filters

Design, Construction and Operation Manualon Horizontal-flow Roughing Filters (HRF) (in preparation)

Raw Water CharacterizationCatchment AreaWater QualityHRF DesignMain Features of a HRFDesign AspectsDesign GuidelinesPilot Plant TestsFlow and Headloss ControlFilter Drainage System

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HRF ConstructionFilter Box LevelingFilter Box StructureFilter MediumIn and Outlet StructuresDrainage System

HRF Operation and MaintenanceCaretaker TrainingCommisioning of the FiltersFlow PatternFlow Control AWater Quality ControlFilter CleaningFilter Maintenance

HRF Costs10.1 Construction Cost Structure10.2 HRF and SSF Specific Cost Comparison10.3 Cost Comparison between HRF and Flocculation/Sedimentation10.4 General Costs of a Water Supply Scheme '

11. Conclusions and a Final Remark

ReferencesGlossary

Annexes

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Institutions involved in Roughing Filtration DevelopmentSimple Methods for Water Quality AnalysisSimple Methods for Discharge MeasurementsHRF Filtration TheoryPilot Plant Design ExamplesHRF Design ExamplesHRF Drawings tOutline for Caretaker TrainingMonitoring of Filter OperationHRF Field Experiences ~Salient Data and Features of Slow Sand FiltersConversion Table

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Preface

This publication presents water treatment processes particularly applicable to rural watersupplies of developing countries. This book concentrates on the pretreatment ofturbid surface water and is divided in two parts.

Part 1 contains a general introduction in the subject of rural water treatment.It describes the water treatment concept, summarizes the different water treatmentprocesses used for solid matter separation, gives a brief account of bacteriologicalwater quality improvement and of the general layout of water supply schemes and,finally, discusses different aspects of roughing filter application

Part 2 is a design, construction and operation manual on horizontal-flowroughing filters. It gives detailed information on the layout of this filter type,discusses operational and economic aspects and, together with the annexes, itdiscloses valuable information on the practical experience with this filtrationtechnology. Part 2 is in preparation and not included in this document.

Hence, Part l focuses on general aspects of rural water treatment and allows the interestedreader to get a keyhole glance at the different challenges presented by the water treatmenttechnologies. Part 2 then fully opens the door to one pretreatment process - to horizontal-flowroughing filtration - and describes in detail the implementation of this technology.

This publication might be useful as general text book to teachers looking for anintroduction to rural water treatment technologies, to decision makers interested in obtain-ing a short overview of the different water treatment options, to engineers having to selectand design appropriate treatment installations, and to treatment plant operators in need ofrunning adequately their facilities.

This book is mainly a technical publication, which by nature is often very rational, reservedand dry. However, efforts were made to present the text and illustrations in a lively, easilyunderstandable and attractive form. The "hardware information" is complementedwith "software stories", scattered in the text as inserts. I hope you will not onlyenjoy this book but relax and also meditate on the adventures I experienced during mychallenging work as rural water treatment promoter. ‘

Duebendorf, 20- June 1992Martin Wegelin

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Introduction

Life on earth would not be possible without water. Man, animals and plants require a sufficientand safe water supply. Water availability is dependent on the local climatic andhydrogeological conditions. The whims of the weather often cause extreme conditions;droughts from water scarcity, and floods from water surplus might endanger life. A sufficientwater quantity is essential for good public health conditions. A daily water volume of 15 - 20litres per person is often considered as absolute minimum to maintain an acceptable healthsituation. However, since the available water quantity is very much influenced by nature itself,it can only be partly controlled by man.

Water quality, however, is and can very much be influenced by man. Human activitiesmight change the microbiological, chemical and physical characteristics of the original waterquality, which is determined by the water type as well as by the superficial and hydrogeologicalconditions of the catchment area. Appropriate source selection, prevention of water pollutionand, as a last step, water treatment, are the instruments with which the water quality can becontrolled and changed. In general, drinking water should not contain any disease-causingmicroorganisms or substances affecting health.

Springs and groundwater are generally characterised by a good microbiological quality.The water drawn from such sources undergoes natural purification processes when percolatingand flowing through the pore system of the underground. lt is normally safe for consumptionprovided the respective installations such as spring catchment facilities and deep or shallowwells are adequately located, constructed and maintained. Rainwater will also yield safedrinking water if adequate catchment and storage facilities are available.

With regard to water quality, the use of surface Water presents the option of lastpreference. Surface water is unprotected and permanently exposed to possible sources oicontamination. Water drawn from polluted rivers, irrigation canals, ponds, and lakes mightcarry disease—causing organisms such as harmful bacteria, viruses, protozoa, eggs and wormsknown as pathogens. Therefore, apart from being collected, stored and distributed, surfacewater also has to be treated. Water treatment is usually by far the most difficultoperational task and should, hence, be avoided whenever possible and priority be given toalternative water sources of better quality.

The primary target of any water treatment is the removal of disease-causingorganisms. The removal of toxic substances or the reduction of high mineral or organic

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concentrations generally requires advanced treatment processes which are not only costly butoften beyond the scope of rural application. In such situations, elimination of the pollution atthe source or catchment of another water source is recommended.Rural water treatment must aim at applying the minimum required treatment level. Operationaland maintenance aspects are key factors requiring careful consideration during the design andconstruction of water treatment installations. Rural water treatment plants have to be operatedand maintained at village level and sustained by local means.

In brief, the best water quality source such as springs, groundwater orrainwater must be used first, and water treatment avoided whenever possible.If surface Water has to be utilized as raw water source, water treatment has toconcentrate, above all, on microbiological water quality improvement.Sustainable operation and maintenance by the local community is an importantfactor which should be fully integrated during the design and construction ofrural water treatment plants. t

from water scarcity and water transmitted diseases to safe water

In former times, the popuiation ofMafi Kumase, a §hanian 1/iiiage in the‘I/o[ta Region, greatiy suffered fromwater scarcity. ‘The peopie drew thewater from sma[[ crevices formed in thehi[[y rocks surrounding the z/i[[age.Rainwater fed these hoies which,however, ran dry after on[y a shortperiod. This distressing conditionseemed to find an end at theinsta[[ati0n of a dam constructed hythe Ministry of filgricuiture. Thereservoir stores the rains surfacerunoff and now provides sufficient

£45 water throughout the year. 9-fowever,the chi[dren and women fetching thesurface water from the sha[[owshoreiine were faced with a newprohiem. CBi[harzia and guinea worm,two water-hased diseases, settied in thereservoir and impaired the hea[th of the

water drawers. In order to overcome this dep[orah[e condition, the 1/i[[agers organisedthemseives and constructeda puhiic water suppiy scheme consisting ofa water treatmentpiant anda distrihution system. Since the inauguration of the seh‘-he[p project, hiiharziaandguinea worm incidences have drasticady dropped and cotdd afrnost er/en he eradicated

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Part 1 General Aspects of Roughing Filter Application

l. Water Treatment Concept

Water treatment is usually a complex undertaking, often bound to fail if the objectives remainunclear, the raw water properties not seriously examined and the treatment processes notadequately selected and applied. Failures can be avoided by a clear treatment conceptcomprising a reasonable appreciation of the raw water characteristics and seasonal variations ofthe water quality, and combining, in a logical sequence, the most appropriate treatmentprocesses.

Looking into a bucket filled withwater drawn from a turbid river Sand Matter content

in Surface Water

visible visibleby eye by microscope

floating » ,_a3$=’ 91 ‘ 4"matter ""4 "

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one might find floating matter such asdebris of wood, leaves and grass, fineand coarse sand settled at the bottomof the bucket, and probably also somefine matter in the form of silt and clayparticles suspended in the water.

guafififirnded _ _ microorganisms,. - - suspended solids,

. - - ' and algae

wecoarse ~,; , 1-1settlabie matter - ,-H’; O. 3:”

However, what cannot be detected arethe harmful microorganisms, thecarriers of so many infectious and

.. ,,_. 2'-‘1-‘:‘.- ._.- 0"“tropical diseases transmitted byconsumption or contact of polluted

iFlCWD 1/92water. The size of such organisms

such as protozoa, bacteria and viruses lies within the range of a few micrometers (a thousandthof a millimetre) or even less. The removal or inactivation of these pathogenic organismsremains, however, the first priority target of any water treatment. A difficult task consideringtheir small size and possibly low concentration in a large volume of water to be treated. Slowsand filtration and chlorination are thus the two most widely used treatment processes,which are sensitive and efficient enough to improve particularly the microbiologicalwater quality.

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The efficiency of chlorination and slow sand filtration is strongly influenced by the turbidity ofthe water to be treated. Turbidity mainly reflects the amount of fine suspended solids present inthe water. A considerable amount of microorganisms - tired of swimming around in the water -attach themselves like "boat-people" to the surface of these solids. The microorganisms enjoythe protection of the solids, which might consume the chlorine meant to kill them or, in slowsand filters, the pathogens will maliciously observe how the fine particles block the sandsurface. Hence, an efficient use of chlorine and slow sand filters is only possiblewith low turbidity water virtually free of solid matter.

Multiple BarrierWater Treatment Concept

pretreatment stages main treatment stage

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coarse \ ,matter V _

separation gradual removal of fine matterand microorganisms ‘mew 2/92

Water has to undergo a step-by-step treatment, especially when containing differentlysized impurities. Sound water treatment schemes will start with coarse solids separation whichis usually easy to achieve. Finer particles will be separated in a next pretreatment step andfinally, water treatment will end with the removal oridestruction of small solids andmicroorganisms. Nevertheless, even the different pretreatment steps will contribute to thereduction of the pathogenic microorganisms. The "boat-people" or pathogens attached to thesurface ofsuspended solids will be stranded when separated from the solids. ln addition, someof those microorganisms floating in the water might get into contact with the surface of thetreatment installations and stick to them. Solid matter and microorganisms aretherefore confronted with a multiple barrier system built up by the differenttreatment units.Since treatment efficiency of each barrier increases in flow direction, the

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flow through each subsequent treatment unit becomes increasingly difficult for the impurities topass through the barriers.

Surface Water Treatmentin 2 stages is recommended

turbid _ _surface pretreated drmkmgwater water water

18‘ stage 2nd stage removal of _removal of

solids microorganisms ’

lilil iii(turbidity reduction) (pathogen)

IRCWDG/92

Conclusively,_surface water treatment generally requires at least two treatmentsteps. The first step - also called pretreatment - concentrates mainly on theremoval of the solid matter. Screens, grit chambers, sedimentation tanks,gravel and coarse sand filters are usually applied as pretreatment units. Thesecond step, commonly considered as the main treatment part, is especiallyused to remove or destroy the remaining microorganisms and the last traces ofsolid matter. For this second step, slow sand filtration and chlorination are themost commonly applied treatment processes.

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The microhioiogicaf henefits ofgraveifdters

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The community of Cocharcas is [ocated in a agricuiturai area of the high Peruvian jierra.The viiiagers, approrg. 1000 inhahitants, constructed their own water suppiy scheme in1977. Its typicai design comprises an ahstraction from an irrigation canaf, asedimentation tank, 2 siow sandfi[ters, a reservoir, anda distrihution system with sing[etap househoid connections. Tiy 1985', the water suppiy scheme had seriousfy deteriorated.The system was working oniy 2 - 3 hours per day due to constant interferences hy otherusers of the irrigation canai. Mo f[ow controi system was instafied neither at the intakenor at the treatment pfant. The caretaker was oniy concerned with fi[[ing up the reservoiras quichiy as possihie. Consequentfy, the siow sandfiiters were hear/i@ overioaded and noteffective in their treatment. ‘Water with a gross faecai po[[ution was suppfied toCocharcas. The system had to he rehahihtated. Fin agreement regarding water use with theneighhouring communities was achieved and a sma[[ weir, insta[[ed at the intake site,reveaied the sma[[ need of water for the vi[[age suppiy compared to that requiredforagricidture. In order to adow adequate operation of the s[ow sandfifters, 2 graveifiitersas additional raw water pretreatment were constructed with community participation.The treatment piant was now running at constantfiow and the efforts were rewarded hythe fo[[owing water quaiity improvement:

faecaf Cohforms mean % reduction % reduction(counts/100 mi) vaiue per trrnt stage cumuiative

raw water 20,000 - -sedimentation tank 14,500 27 27graveifiiter 1,200 92 94siow sandfilter 20 98 99,9

The Teruvian ergperience reveafls that gravei prefiiters enhance sIow sandfiiter operationand increase the overa[[ piant performance. The mzdtipie harrier system proves to lie anappropriate conceptfor rura[ water supp[y. (Refs. 1, 2)

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2. Solid Matter Separation

Let us now look at the first treatment step, i.e. at solid Matter Contentthe separation of solid matter. We might be Separationconfronted with a large variety of solids as

. . . lloatln 1 43;-’=' Ql 4'“type of surface water, whether natural punfication matterg " ‘ '“‘= °

observed in our bucket filled with turbid riverwater. However, this variety depends much on the

processes can separate part of the solids and_ _ _ suspended _

possibly generate undesired new ones by organic solids - _ - _ ' _growth. Natural purification should already be ' ' 'extensively integrated into the treatment concept by flne <

. coarsean adequate selection of the surface water source settlable matterI10 0 a a

on 00 pa no on0 .-‘3O 0'.

and intake location. 'IRCWD 4/92

Sedimentation and filtration processes are mainly applied in water treatmentto separate the solids found in the raw water. These shall be discussed in details in the nextsection.

Yet, let us first focus on the particularities of different categories of surface water andtheir impact on _the type and concentration of the solid matter found in the water:

Cl The properties of the drained catchment area and the characteristics of the surfacewater influence type and concentration of the solid matter in the raw water. Flowvelocity and rate of erosion determine the amount of settleable solids carried by thewater. With respect to the type of solid matter, flowing and stagnant surfacewater differ much from each other. While the turbulent flow of a water coursemay carry coarse settleable solids, which might settle in gently flowing or impoundedsurface water, the suspended solids might partly sediment in ponds and lakes wherealgal growth is likely to occur.

Cl Flowing surface water often changes drastically its quantity and quality. Theannual rainfall distribution influences the seasonal fluctuation of the surface watermainly with regard to turbidity and solids concentration. Flowing surface water willusually carry settleable solid matter at varying concentrations during different periodsof time. Small upland rivers are generally low in turbidity during the dry season butcan exhibit high turbidity peaks of short duration during heavy rainfalls. Larger

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lowland rivers may be of moderate turbidity throughout the year with increasedturbidity levels lasting for several months.

Cl Stagnant surface water changes only gradually the amount and the type ofsolid matter during the course of a year. In fact, the large volume of stored water inlakes, reservoirs and ponds preconditions the water quality. Coarse inorganic particlessettle to the bottom of the receiving water body, light organic solid debris tend to floaton the water surface and, dissolved organic matter will be transformed byphotosynthetic processes to algae and plankton. Hence, each standing watersource acts as a first pretreatment step as the incoming and stored water isexposed to natural purification processes. As a result, impounded water is generallycharacterized by smaller water quality fluctuations. This higher stability of the rawwater quality facilitates the operation of a treatment plant.

U Flowing surface water carries solids of different sizes, varying fromcoarse sand and silt to fine clay. Due to the distinct flow conditions, the solids areunevenly distributed over the cross-section in a river bend. Coarse solids drift towardsthe bend's outside whereas the fine solids are washed towards the inner side of a riverbend and develop there a silting zone. A sound location of the intake structurealready reduces part of the fine particles more difficult to remove. The intake shouldtherefore be placed at the outer or erosion side of a river bend in order to redtice theabstraction of fine matter and to avoid the silting of intake works.

El Surface water can also carry coarse floating matter which might block oreven damage part of the water supply installations. The undesirable material is thusretained right at the beginning of the water treatment facilities either by screens or by ascum-board. Fixed screens are most commonly used; the installation oftenconsists of a coarse screen followed by a finer screen to avoid blockage and excessiveheadlosses.

In summary, preference should be given to stagnant water, in case surfacewater has to be used as a raw water source for a Water supply scheme. Thenatural purification processes and the smaller water quality variations reducethe required degree of treatment and its control during operation. Flowingsurface water often exhibits rapid water quality changes which render watertreatment more difficult.

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2.1 Sedimentation

There is no doubt, that small pebbles or sand particles will settle in standing water. Thisprocess, called sedimentation, depends on the physical properties of the solid matter and water.The settling velocity is influenced by the mass density, size and shape of the particle as well asby the viscosity and hydraulic conditions of the water. Grit chambers and sedimentationtanks remove quite efficiently relatively heavy and coarse solids such as sand andsilt particles. Inorganic matter larger than approximately 20 um can usually be removed byplain sedimentation without the use of chemicals.

The shape of sedimentation tanks is either rectangular, square, or circular. The tanks areoperated continuously or intermittently. The flow direction in continuously operated tanks iseither horizontal or vertical. The flow pattern in circular tanks is complex and the conditions ofvertically operated tanks unstable. Therefore, rectangular tanks operated on ahorizontal flow and continuous basis are recommended for rural water supplyschemes. Sedimentation tanks can be further classified according to the separated solids.

Grit and sand are removed by small grit chambers. Theiprovision of such smallsettling tanks is recommended for water treatment schemes drawing water from fast-flowin gupland rivers. The grit chamber may form part of the intake structure or can be combined withthe flow distributor box at the side of the treatment plant.

Layout and Design of Grit Chamber(grit chamber combined with iiow distributor box)

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List of SymbolsL (m) lengthW (m) widthH average depth

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Design GuidelinesQ

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Sedimentation tanks separate finer solids such as silt, clay and part of thesuspended solids. The raw and turbid water enters on one side of the tank and is evenlydistributed over the tank's cross-section, the solids settle under laminar flow conditions to thetank bottom and the clarified water is abstracted uniformly over the full width at the oppositeside of the tank. In order to be separated, each solid particle has to overcome asettling distance which is equal to the tank depth, e.g. in the order of 1 to 3meter. The accumulated sludge is periodically removed from the tank bottom. The solidsremoval efficiency of a sedimentation tank depends mainly on the hydraulic surface load, tankdepth, and detention time. The design values for a sedimentation tank should be chosenaccording to the settling characteristics of the solids. These can be determined in asedimentation test using a transparent test tube, for additional information please consultChapter 7.5. The time recorded in the test to attain a certain clarification level has to bemultiplied by a factor of three to allow for unfavourable flow conditions in the full-sized tank.Low surface loads should be applied with raw water of poor settling properties and in smallplants with variable operating conditions.

Layout and Design of Sedimentation TanksPlan List of Symbols

pi-_-5_-__->1 (m) length) width

Q depth-P - q - ) flow rate

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Even properly designed and operated sedimentation tanks will separate only part ofthe suspended solids. With the help of coagulants such as alum and iron salts, suspensionscan be destabilised. The small particles lose their repulsive forces, cluster together and coalesceto larger flocs of improved settling characteristics. Coagulants are extensively used inconventional water treatment technology. However, the flocculation/sedimentationprocess is already an advanced treatment technique requiring highly qualified

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personnel and well-equipped facilities; both hardly available in rural areas of developingcountries. Chemicals often have to be imported from abroad and paid for in foreign currency.Since transport problems are pertinent to many developing countries, the adequate and reliablesupply of chemicals to remote treatment plants is yet another difficulty. Once at the treatmentplant, the chemicals have to be dosed correctly to the water. The dosage must be adapted to thevariation of the raw water quality and therefore requires a for a professional water qtialitymonitoring. The corrosive action of the chemicals attacks the accurate and thus sensitiveinstallations of the dosing equipment. Finally, chemical water treatment requires skilledpersonnel capable of monitoring the water quality, adjusting the doses, as well as maintainingand repairing the dosing equipment. In fact, rural water supplies often facetremendous problems with chemical water treatment. A reliable and successfulapplication of chemical flocculation is therefore rather unrealistic for many small water supplyschemes. The chemical flocculation and sedimentation process applied in conventional watertreatment schemes for the separation of suspended solids and colloidal matter will thereforegenerally fail in rural water supply schemes and can therefore not be recommended.

In conclusion, grit chambers and sedimentation tanks remove quite efficientlycoarse and easily settleable solid matter. They are used as prelirninar_vtreatment step especially to treat raw water drawn from running water courseswith high solid matter concentrations. The use of chemicals to entrancesedimentation by flocculation is in rural water supply schemes difficult and,under such conditions,ltherefore hardly reliable and successful.

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Locked chemiccti dosing room

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‘We thoroughfy enjoyed the Chinese hospitahtg of Mr gictng, water officer responsihfe fora district town in Zhejiang Province. ‘The mandarins and the green tea in the china cupswere deiicious and the atmosphere in the reception room of the administrative huifdingfriendfg. fraud of his water suppiy system and pfeased hg our visit, Mr Qiang informedus with the hefp ofa translator that the treatment piant had Been constructed 2 yearsago at totaf costs of 1,2 mio 9’en. ‘With a design capacity of 1,500 m3/d, it suppfies15,000 people. Chforine and tdum su[phate are apparentfy added to the ahstracied riverwater pumped to the top of a smtdi hi[[ where 2 'non~va[ve' fiiters and a reservoir areiocated. Curious to see the treatment instaifations, we ciimhed on the ciose sma[[ hi[[accompaniedfrom £1 de[egation of engineers, administrators and operators. The [onecrossed through a tea pfantation from where we had ct nice view on one of the numerousiightig huift paviiions so common in this province. Fl sma[[ portaigives access to thetreatment pfant which is surrounded 6y a massive white waif. ‘Zhe 2 'non-vo[ve' fifiersturned out to he rapid sandfifters equipped with a sgphon for fifter hachwashing. ‘Weciimhed on the fiiters and were given decoded e;(p[anations hg our Chinese friends.However, we cotdd not the chemicai dosing room at the p[ant and were toid that thefacifities are [ocated in a room nezgt to the administrative Budding. flfter the ineviiahfcgroup picture shot at the entrance of the treatment pfant, we returned to the mainhui[ding in the hope to see the chemicaf part of the pfant. Our request created someconfusion and under cover discussions among the engineers and operators. jfinafiy, ourtransiator informedus that the room, which is protected hg decorative wrought iron liarson the windows, is not accessihie due to the unavaiiahiiity of the key.

This was not our first ergperience with an 'unavai[oh[e Ker/'for a chemicai dosing roomofa rurai water suppiy scheme. Marty other Keys around the worid are not avoifohfe orare even '[ost', perhaps cdong with the chemicals and the dosing equipment.

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2.2 Roughing Filtration

Gravel and sand layers significantly improve the water quality of polluted surfacewater which, during infiltration, flows through such layers. Therefore, under favourablehydrogeological conditions, polluted and turbid river water might be drawn as clear and safegroundwater from a shallow well located next to a river. However, the local soil is quite oftenimpervious due to the absence of gravel and sand layers. Nevertheless, why should theexcellent purification capacity of nature be ignored just because the specific hydrogeologicalconditions are not favourable at the site of a new water supply scheme ? Let us then copynature and construct an artificial aquifer by filling a sedimentation tank withgravel. The solids removal efficiency of such a tank will drastically increase on account of thesettling distance which is greatly reduced by the gravel. In other words, the fine solids crossingan ordinary sedimentation tank have to overcome a vertical settling distance of l to 3 metersbefore coming into contact with the tank bottom. Fortunately, the same solids flowing througha filter will touch the surface of a gravel after already a few millimetres. Hence, the settlingdistance is extremely reduced by the filter material and, therefore, filtration isa more effective process for solids removal due to the presence of a small -pore systemand to the large filter surface area available for sedimentation, adsorption, as well as chemical,and biological activities.

Conceptional Layout of 'a Sedimentation Tank and a Roughing FilterSedimentation Tank

horizontal tlow directioninlet I

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coarse particle é distance/ _L. ~;1111,, /Illllllzillllllllllrl/oé

J 1 1 flow velocity oi water<__ _ l‘ -3- ~ _ _} 2 2 settling velocity of particledrain for 3 flow path oi particlesludge removal '

‘Roughing Filterhorizontal flow direction

inlet 'I5"'I;'-"I3ififiiifiiiitt‘ll'EI"11'i3'ii‘-I1’Z'-§"lI'-I3?-I5'lI'5:0“ I |\| I -I,-5|-" 'tam 9': O E.LQ-I -I 2 ' '= -' e 2 —

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settlingdistance ‘rm?st-.=E.=‘:.1’:aE.1‘:94'-'-"-'.-':-'.-I-.-'.--\,-¢,-\,-\-5-I,-1,-I,--H ‘_'. -,'. ~,'. ~ -,'.~_-. ','.r--:-'.r-.-:t<t-:=.='t~: /

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— J4-dfain TOT _ gravel as filter materialfilter cleaning

lFlCWD 7/92

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View in a Roughing Filter loaded with Kaolin (Ref.3)

Classification of Prefiltersintake and Dynamic Filters

M 'L‘I,§§fi,"?§l,'}§’“ i ~ installelfl in thle bed'0 ' O sma C3085 __ . canal

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Roughing Filters

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The design and application of prefilters vary considerably. The different filter types areclassified according to their location within the water supply scheme, to their main purpose ofapplication and to the flow direction. Intake and dynamic filters, which often form part of thewater intake structure, differ from actual roughing filters, which are generally located at thewater treatment plant. Roughing filters are further subdivided into down, up and horizontal.-flow filters. Finally, vertical-flow filters can be classified according to the way the gravel layersare installed. The different gravel fractions of roughing filters in series are installed in separatecompartments, while those of roughing filters in layers are placed on top of each other in theS81T1€ COl'I'lp81'll'DCIlI. -

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Roughing filters usually consist of differently-sized filter material whichsuccessively decrease in size. The bulk of the solids is separated by the coarse filter mediumlocated next to the filter inlet. The subsequent medium and fine filter media further reduce thesuspended solids concentration. The filter medium of a roughing filter is composed ofrelatively coarse (rough) materialranging from approx. 25 to 4 mm in size. Gravel isusually used as filter material. Significant solids removal efficiencies are only achievedunder laminar flow conditions since sedimentation is the predominant process in roughingfiltration. Therefore, roughing filters are operated at small hydraulic loads with afiltration velocity of usually 0.3 - 1.5 m/h. The coarse filter material and the small hydraulicload limit the filter resistance to a few centimetres.

Filter cleaning is carried out manually and hydraulically depending on the pattern ofthe retained solids accumulated in the filter. Intake and dynamic filters separate the solidsusually at the inlet zone of the filter and thus act as surface filters. These filters are thereforemanually cleaned by scouring the top of the filter bed with a shovel or rake. Compared withintake and dynamic filters, roughing filters act as space filters due to the deep penetration of thesolids into the filter medium. The accumulated solid matter is periodically flushed out ofroughing filters by hydraulic filter cleaning. If necessary, these filters have to be cleanedmanually by excavating the filter material from the filter compartment, washing and refilling itinto the the filter boxes as will be presented in the following chapter.

2.2.1 Roughing Filter Design

2.2.1.1 Intake Filters

Remark: Intake and dynamic filters have a similar design but differ in theirapplication. However, they will be presented separately in the following 2 chapters.(Ref. 4)

Intake filters are combined with water abstraction structures and installed next tosmall and narrow river beds. Since their main purpose is a first reduction of solid matter fromthe abstracted surface water, intake filters are often used as first pretreatment unit ina water treatment scheme. A small weir controls the water level of the surface water and directspart of the flow into an adjacent filter compartment. This filter box is filled with differently-sized gravel layers. The top layer consists of relatively fine gravel of less than 10 millimetres indiameter. The lower coarser gravel layers act as filter support and pemiit an even abstraction oithe prefiltered water through perforated drainage pipes. The abstracted raw water, which isevenly distributed over the full width of the filter box by a small weir, then flows gently overthe surface of the gravel bed. Part of this water percolates through the gravel layers. the other

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part is discharged over an outlet weir back to the river. Intake filters are constructed atthe river side, but should not be installed directly in the river bed because the filter materialruns a high risk of being washed out during periods of high river discharge. A separation wall,constructed between river bed and filter box, is recommended instead to protect the filter frombeing washed out.

Intake Filter at the Colegio Colombo-Britanico in Cali, Colombia (Ref. CINARA)G) river, ® weir, © inlet, @ filter, © overflow weir, © gravel washing slab

As an alternative, intake filters can also be installed in the bed of small canals,particularly upland rivers with a steep river bed and a suitable topography might allow fortheaccommodation of a small diverting canal. The filter bed consisting of different gravel layers isinstalled in a limited stretch of the canal. Part of the canal water is filtered through the series offine to coarse gravel layers, while the remaining water is returned to the river. The prefilteredwater is collected by perforated drainage pipes laid at the bottom of the coarse gravel layer, andthe discharge rate regulated by a valve placed at the filter outlet. The flow velocity in thecanal must be controlled by the canal intake structures in order to protect the filter bedfrom being washed out during periods of high river discharge. The flow velocity in the canalshould actually range between 0.10 and 0.30 m/s in order to prevent fine matter from settlingand remaining on top of the gravel bed, and also to avoid fine filter material from being washedout. This layout may also be applied to irrigation canals, provided they are continuouslysupplied and controlled throughout the year.

Finally, "intake" filters may be located directly at the treatment plant site and canbe considered a first treatment step. This particular location is recommended in gravity watersupply schemes with a raw water intake located in a remote area of difficult access.Surveillance and regular filter cleaning will be facilitated by such a layout.

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Filtration rates of intake filters range between 0.3 and 2 m/h. However,substantial solids removal rates can be expected only at low filter velocities. The design of thehydraulic structures should be based on a maximum filter resistance of 20 to 40 centimetres.This f1gure will not be exceeded if regular filter cleaning is observed.

Layout ofIntake and Dynamlc Filters

Fllter Installed next to a small riverPlan Section A - A

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Fllter Installed ln the bed of a canal

Plan F WM Section A - Allltor --

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_ , _ _ ._ V: V; _ . —>_. -lntnko weir WW0 M2

The solid matter mainly accumulates on top of the filter bed. By increasing the flow velocityover the filter surface, a fraction of this accumulated matter might be dragged away by thewater. However, intake filters are normally cleaned manually with a rake or a shovel.The first step in the cleaning process is the closing of the valve on the prefiltered water line.Thereafter, the inlet control valve is opened in order to increase the horizontal flow in the filterbox to approx. 0.20 m/s to 0.40 m/s. Controlled flow velocity changes in canals might bedifficult to achieve and possibly not necessary. The solid matter retained by the filter is firstresuspended by mechanical stirring and then flushed back to the river. Manual cleaning shouldstart at the upstream side of the filter and progress in flow direction in order to avoid silting ofthe cleaned gravel. Filter operation is restarted by draining the prefiltered but possibly turbidwater into the river or to waste for some time before resupplying the treatment plant.

2.2.1.2 Dynamic Filters

Dynamic filters protect the treatment plant units from high solid concentrationshock loads. Highly turbid surface water can quickly clog filters, especially slow sandfilters. Therefore, during periods of extremely high raw water turbidity, the flow may beinterrupted to reduce the work of cumbersome filter cleaning. The separation of solid matter isonly a second priority objective of dynamic filters. -

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The performance of dynamic filters is, as described by its adjective, dynamic. The waterquality between filter inlet and outlet hardly changes during periods of low raw water turbidity.During a raw water turbidity peak, however, the quantitative change is drastic: no water can befound in the filter outlet ! Dynamic filters act like turbidity meters connected to anopen-close valve as they immediately clog with raw water of high turbidity.

In comparison to intake filters, dynamic filters have a similar layout but differ infilter material size and filtration rate. Especially the gravel size of the filter top layer issmaller; i.e. less than 6 millimetres in diameter, while the filtration rate applied to the filter isusually more than 5 m/h. The maximum available headloss is still limited and within the rangeof 20. to 40 cm, in spite of the finer filter material and higher filter velocity. Finally, thehorizontal flow velocity over the filter bed surface should be small or nonexistent; i.e., lessthan 0.05 m/s or nil, in order to prevent the removal of the deposited silt accumulated during aturbidity peak.

Dynamic filters are cleaned after each raw water turbidity peak. The cleaning isalso done manually and the procedure is similar to that of intake filters. During periods of flowinterruptions by dynamic filters, treatment plant operation is maintained at reduced capacity(e.g. by declining filtration rate operation in slow sand filters). Nevertheless, dynamicfilters are to be used with raw water experiencing short turbidity peaks; i.e., inthe order of a few hours to maximum half a day. Dynamic filters are preferably located at thesite of treatment plants in order to reduce the surveillance and cleaning work of the caretaker.

Dynamic Filter at Chorro de Plata in Cali, Colombia (Ref. CINARA)(D irrigation canal, @ canal section with dynamic filter, ® effluent control box

(D slow sand filter units

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Cheap gravei

Tontanero Tuiio, caretaker of ‘Taminangos water suppiy scheme, [ocated in asouthwestern region of Coiomhia, [ead our sma[[ expedition which headed towards theintake structure. flliong the steep path winding through dry grass [and which is hardiyctdtivated nor used, we passed a few run down haziendas providing shade to some cattie.filfthough it was afready [ate in the afternoon, the sun was sti[[ strong. /'4 sp[endid viewover the hifiy highfand of Cauca Q/Department spread peacefuiiy heiow us and rewardedour tedious 2-hour waik. ‘Ihe narrow traii flattened after the panoramic view point andentered into a narrow vaiiey covered with sma[[ trees and hushes, which indicated thepresence of water. ‘We regained our hreath on the fiat stretch and I then enquired whenthe [oca[ peopie had [ast visited the intake. Vlhout two weeks ago’ was the answer ofTtdio, whiie the district engineer pretended to have inspected the site one month ago afterthe intake was reconstructed within the framework of a fieid study project. §ent[espiashing announced the water and, after a finaijump through smaii hashes, we reachedthe hrookiet. ’Ihe rehahiiitation work was we[[ done and the intake fiiter piaced at theriver site as recommended. ‘Water was flowing through the system and everything seemedto he working wed '1/he district engineer hroke a hranch off a tree in order to check thegravei [eve[ in the fiiter. fife siowiy dipped the hranch into the turhid water and [owered itfurther without touching ground. Flt first, everyhody was surprised and thenemharrassed. The infiow through the in[et pipes were quickiy stopped with 2 piasticsheets and the filter hok was drained. Qhe emptied tank reveafed that the gravei had heenremoved; stoien hy some perfidious farmers. ‘We discovered horse dung a the site, a ciearfinger print of the rohhery. The district officer pushed his cap hack onto his neck andstared into the empty fiiter hok. "V1/ou[d it he rationai to refi[[ this remote fiiter hor withgravei P It would certainly require considerahie efforts to transport the materiai to thesite. The piastic sheets were removed, the water siowiy refiiied into the intake structurehefore it flowed again - untreated - trough the [ong suppiy [ine towards ‘Taminagoswater reservoir. '

9\[ot much was said during our descent. I hoped to discover a wheeiharrow recentiy usedfor concrete work and turned upside down against the wa[[ of a diiapidated cottage.9-fowever, this wou[d have not much improved the situation. ’I'u[io, who [ives in theviiiage, is not ahie to properiy take care of this remote intake fiiter.

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2.2.1.3 Vertical-flow Roughing Filters

Roughing filters as major pretreatment process treat highly turbid surfacewater over prolonged periods. They efficiently separate fine solid particles and are usedas major pretreatment step. Roughing filters are, therefore, placed at the treatment plant site andoperated in combination with other pretreatment units such as dynamic filters or sedimentationtanks. Roughing filters precede final treatment processes such as slow sand filtration andchlorination. .

Vertical-flow roughing filters usually consist of 3 filter units arranged inseries. The water to be treated flows in sequence through the 3 filter compartments filled withcoarse, medium and fine filter material. The size of the 3 distinct filter material fractions isgenerally between 20 and 4 mm, and graded for example into fractions of l2 - l8 mm, 8 - l2mm and 4 - 8 mm.

Vertical-flow roughing filters operate either as downflow or upflow filters andhence, are either supplied by inflowing water at the filter top or at the filter bottom. The filtermaterial of vertical-flow roughing filters is completely submerged. A water volume of approx1O to 2O cm depth usually covers the gravel. A layer of coarse stones can be installed in orderto shade the supematant water and thus will reduce or prevent algal growth often experiencedin pretreated water exposed to the sun. Drainage facilities in form of perforated pipes or a falsefilter bottom system are installed on the floor of the filter boxes. Finally, pipes or special inletand outlet compartments are required to convey the water through the subsequent 3 filter units.

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Downflow Roughing Filter in in Series at Atzpitia, Peru (Ref. DelAgua)(D inlet with sedimentation tank, ® first , ® second and @third filter compartment

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Layout and Designoi‘ Vertical-flow Roughing Filters

Roughing Filters in Series A M: open Roughing Filters in Layers~ downllowroughlngillter =N= °'°$°d

_l< L -l F L >1 i- L >4F> .. -.. .

not recommendedH unless sufiiclent

washwater lor iiltercleaning ls available

' ti ii “ti”?drains tor tiller cleaning

~ upilow roughing filter

|. L L .1. L 4 |.___;-_-r

?"Z§> LQ .

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;.- ;.-.-,-.' ;.',_-.‘.-_~.'.-_~._‘.-;-*,_-._‘.-;-2_.-. '.-_;-_1<_,:'.-_,=‘.{:-

-> . Y -> HQ Q

drains lor illter cleaning=»<=ti-

List oi Symbolsd9 (mm) gravel sizeH (m) iiiter depth

Design GuidelinesQ QVF: —A—=LW= 0.3‘ 1 i’T‘l/h

L (m)w (m)A (m2)AH (cm)Q rm“/n)Qd (m3/n)vF (mlh)vd (m/h)

tiller lengthiilierwidthliiter bed areapartial headlossiiow ratedrainage rateiiltration ratedrainage rate

-Q=i_&t_ .vd A _L‘W 40 som/n

AH ~10cm

H=O.60-1.00m

d9=12-18mmdg=8-12mm

— dg=4-BmmIRCWD 10/92

Sedimentation is the main solids removal mechanism in gravel filters. In order to establishlaminar flow conditions which favour sedimentation processes, vertical-flow roughingfilters are usually operated within filtration rates of 0.3 to 1.0 m/h. Vertical-flowroughing filters may be sensitive to hydraulic fluctuations, especially when loaded with largeamounts of solid matter. Settled matter might be resuspended at increased filtration ratescausing solids to breakthrough the filter. Filter operation at constant flow rates is thereforerecommended. Raw water carrying colloidal matter and having a high suspensiorrstability,should be treated at low filtration rates and in filters filled preferably with fine filter material.Filter resistance is usually less than 20 cm per filter unit and hence, not a decisive operationalcriteria for properly designed and operated roughing filters.

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Important in filter operation is a regular cleaning of the filter medium. Filtercleaning is carried out hydraulically and also manually if necessary. Contrary to filter operationunder laminar flow, hydraulic filter cleaning is carried out under turbulent flow conditions. Thewater stored in the filter is drained out of the filter compartment at high drainage velocities.Shock drainage is achieved by a fast opening of the valves or gates connected tothe underdrain system of the filter. Starts and stops of the drainage process induce unstableflow conditions that will loosen and disintegrate the solid deposits accumulated in the filter.Subsequent high drainage rates are applied to flush the resuspended solids out of the filter.Each vertical-flow roughing filter box can be drained separately. This enables individualcleaning of the specific filter compartments. Conventional filter backwashing as applied inrapid sand filtration is not possible since the filter bed of roughing filters cannot be fluidised.

Let us now compare downflow with upflow roughing filters.How do they differfrom each other and which type should preferably be used ? L

Cl The flow direction is obviously the first difference which might interfere orsupport the settling of solids on the filter material. Flow and sedimentation show inthe same direction for downflow filtration and to opposite sides in upflow filters. Thesolid removal efficiency should consequently differ in the two filter types.Theoretically, downflow filters should have a better performance than upflow filtersas the solid particles are more likely to settle on top of the gravel surface in the flowdirection than under countercurrent conditions. However, practical field experienceshow a similar efficiency for both filters. Solids probably settle in dead filterzones where the water flow is reduced to a minimum. The solids enter such zones byforces of inertia induced by the curved streamlines of the porous filter system. Thesedead zones provide quiescent conditions and the necessary time for the solid particle tosettle on the surface of the gravel.

El The accumulation pattern of retained solids is another difference betweendownflow and upflow filters. Since the solids penetrate deep into the filter bed.roughing filters are classified as space filters. However, the bulk of the solids is stilldeposited at the entrance zone of the filter, for downflow filters in the upper part of thefilter and for upflow filters in the filter medium located next to the filter bottom. This,however, has a tremendous impact on hydraulic filter cleaning. lndownflow roughing filters, the major mass of accumulated solids has to be flushedout from the soiled filter top through the lower and cleaner filter part to the filterbottom with a relatively small washwater volume. The opposite is true for upflow

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roughing filters. The bulk of retained solids is accumulated next to the drainagesystem and a relatively large washwater volume, accommodated in the upper filterpart, is available to flush the solids out of the filter. The important aspects offilter cleaning speaks for upflow roughing filters rather than fordownflow filters.

Vertical-flow roughing filters have a relatively small filter depth which, due tostructural constraints, is limited to about 1 m.This results in a filter depth of totally 3 m for 3filter units used in series. This total available filter depth limits the application ofvertical-flow roughing filters, which can generally and efficiently cope with raw waterturbidities of 50 to 150 NTU (Nephelometric Turbidity Unit). The reduction of the filtrationrate or the provision of additional filter boxes would be required to treat raw water containinghigher turbidities. Alternatively, vertical roughing filters in layers, in which all 3 gravelfractions are installed in the same filter box and thus reduce the total filter distance to approx. lm, can be used to pretreat low turbidity water only.

2.2.1.4 Horizontal-flow Roughing Filters

Unlimited filter length anti simple layout are the main advantages ofhorizontal-flow roughing filters. Generally, the shallow structure does not imposestructural problems and, therefore, the filter length is not limited to a few metres. Furthermore,in comparison to the vertical-flow roughing filters, the simple layout does not require additionalhydraulic structures and installations. The raw water runs in horizontal direction from the inletcompartment through a series of differently- graded filter material separated by perforated walls.The size of the filter material is also in the order of 20 to 4 mm. The water level is kept belowthe surface of the filter material by a weir or an effluent pipe placed at the filter outlet to preventalgae growth.

Horizontal-flow roughing filters are also operated at low filtration rates in the order of 0.3 to1.5 m/h. The filtration rate is here defined as hydraulic load (m3/h) per unit cross section area(mg) . The filter length depends on the raw water turbidity and usually lies within 5 to 7 m.Due to the comparatively long filter length, horizontal-flow roughing filters can handleshort turbidity peaks of 500 to 1,000 l\‘TU. Tire filter resistance does not exceed 20cm if normal filter operation and regular cleaning is observed.

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Layout and Designof Horizontal-flow Roughing Filters

i~ L‘ +L2+L“r={>o

' drains for filter cleaning

List of Symbols Design Guidelinesdg (mm) gravel size Q Q _

VF = }WV=T =U.3-1.5m/hH (m) filler depth

_ QL1_2‘3 (m) filter length vd = V~60- 90 m/h

W (m) filter widthA _ Al-l ~ 30 cm

A (ma) illi9i‘ cross-section area

AH (cm) maximal headloss H ~ O.80- 1.20m

Q (ma/h) flow ratedg=12-18mm_ L1 ~2-4m

dg = e-12mm. L2 ~1-am—d9= 4-8mm. L3~1-2m

IRCWD 11/92

Qd (m3/h) drainage rate

vF (m/h) filtration rate

vd (mlh) drainage rate

With progressive filtration time, the solids settle on the top surface of the filter medium andgrow to small heaps of loose aggregates. Parts of the small heaps drift towards the filter bottomwhen they become unstable. This drift regenerates the filter efficiency of the upper filtermaterial and slowly silts the filter from bottom to top. This self~regeneration of the filterenables the accumulation of a considerable amount of retained solids. Therefore, horizontal-flow roughing filters possess a large silt storage capacity. They should also reactless sensitively to filtration rate changes since clusters of resuspended solids will drift towardsthe filter bottom or be retained by the subsequent filter layers. Compared to vertical-flowfilters, horizontal-flow roughing filters are thus less susceptible to solids breakthroughs in theeffluent caused by flow rate changes.

Periodical cleaning is also for horizontal-flow roughing filters essential.Hydraulic cleaning is carried out by a fast drainage of the water stored in the filter. During filterdrainage, the small unstable heaps of accumulated solids collapse and are flushed towards thefilter bottom. The solid matter stored in the filter material is washed off the filter box through

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Water Treatment plant in Mafi Kumase, Ghana(D horizontal-flow roughing filter (2 units), ® slow sand filters (2 units)

the drainage system. Drainage velocities of 60 to 90 m/h are necessary in order toachieve a good hydraulic cleaning efficiency. Depending on the solids concentration in the rawwater, regular hydraulic filter cleaning at intervals of every few weeks is required to avoiddeterioration of filter efficiency and the development of excessive filter resistance. Frequent andefficient filter drainages also postpone the need for manual filter cleaning which, however, hasto be carried out at least after several years of filter operation.

The design, construction, operation and maintenance of horizontal-flow roughing filters will bediscussed in detail in the second part of this manual.

In brief, filtration is compared to sedimentation a more efficient process forthe separation of fine solids. Roughing filters usually consist of differently-sized gravel ranging from approx 20 to 4 mm. They operate at small hydraulicloads of usually 0.3 - 1 m/h. Intake filters are used at the point of waterabstraction as first pretreatment unit whereas dynamic filters protect thesubsequent treatment plant from high solids concentration shock loads.Roughing filters are the major pretreatment step and used to remove thesuspended solids efficiently.Due to their limited filter depth, vertical-flowroughing filters can cope with raw water turbidities up to 50 - 150 NTU.Hydraulic cleaning aspects favour the use of upflow roughing filters. Thefilter length of horizontal-flow roughing filters is not limited and, therefore,turbidity peaks of 500 to 1000 NTU can be treated by this filter type.Roughing filters are cleaned hydraulically by periodic high-rate filterdrainages and, if required, manually after several years of operation.

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2.2.2 Selection Criteria for Roughing Filter Application

Roughing filters are primarily used to separate fine solid matter which is onlypartly or not at all retained by grit chambers or sedimentation tanks. Roughing filters mainly actas physical filters and reduce the solid mass. However, the large filter surface area available forsedimentation, and the relatively small filtration rates also support adsorption as well aschemical and biological processes. Therefore, apart from solid matter separation, roughingfilters also partly improve the bacteriological water quality and, to a minor extent,change some other water quality parameters such as colour or the amount of dissolved organicmatter. T

A great variety of different prefilter types is available. Intake and dynamic filters.downflow and upflow roughing filters and finally horizontal-flow roughing filters makes thechoice of the most appropriate pretreatment method difficult. The selection of an adequatetreatment scheme should be based on the following aspects: V

Cl raw water characteristicsEl type of surface waterCl topography at the intake and treatment plant siteEl operational aspectsEl economic aspects

Cl The raw water characteristics determine to a large extent the type of pretreatmentto be applied. The variation of turbidity and of suspended solids concentration isthereby the most important required information.

Average (Ind maximum levels of turbidity and silspended solids concezzmzrio/1 are oigreat importance in the design of the pretreatment units. In addition, information onthe duration of turbidity and suspended solids concentration peaks are essential. Dosuch peaks last for a few hours, some days or do they occur over a period of a fewmonths 7 Furthermore, the solids should also be characterized according to theirsettleability and size.Do the particles settle easily in the water or are they kept insuspension ?

Information especially with regard to peak values is unfortunately often not available.This forces the sanitary engineer to make his own observations before embarking on

the design of the treatment scheme. Some simple sedimentation tests carried outduring periods of high and normal river discharges enable the study of the settling

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In brief, average and maximum levels of turbidity and suspended solidsconcentrations, the settleability of the solids and the duration of peakconcentrations are the most important raw water characteristics for theselection and design of pretreatment units.

The type of surface water also has a strong impact on the characteristics andamount of solids carried by the water.Small upland rivers, large lowland streams andstagnant surface water generally differ from each other as described in the following.

Small upland rivers draining a catchment area, which is protected by a manifold andrich vegetation might, probably have a clear or coloured water during periods of meandischarge. Coloured water can be observed especially with slowly flowing watercourses in contact with organic matter. This might be the case where rivers flowthrough a dense forest or in swampy areas, where water leaches humic substances outof the decaying material and acquires a yellowish or brownish colour. The colour mayonly be partly reduced by roughing filters. However, it will not affect the operation oislow sand filters which achieve a further colour reduction. The small upland riverswill react to heavy but short rainfalls by an immediate increase in run-off and waterquality change. Turbidity peaks and/or raised levels of colour are usually correlated toriver discharge. These peaks decrease with falling water level soon after rainfalls stop.In such situations, either intake or dynamic filters may be used to reduce the extremepeak values or protect the treatment plant from heavy solid loads discharged by theriver for a few hours.

Large lowland streams have a different regime. Local showers do not greatly affecttheir discharge and water quality. The discharge is more influenced by the annualrainfall distribution and by the wet and the dry seasons. Changes occur gradually overa period of some days or weeks when increased turbidity levels or suspended solidconcentrations are recorded for a few weeks or months. Water quality variationsexpressed as the ratio between average and maximum values are usually smaller thanthose of upland rivers. Therefore, pretreatment is permanently required and the use oiroughing filters, possibly combined with intakes filters, recommended. The choice oiroughing filter depends, among other aspects, on the level and duration of highturbidity or suspended solid concentration. As a general rule, moderate turbidities of

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Treatmentiof Surface Water

turbidity

raw waterturbiditycharacteristic _

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surface last flowing slow flowing stagnant waterwater type upland river lowland river (l'BSB!’V0ll', pond, lake)

treatment l l Lscheme

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iii???iii‘?iiiiiiiéiiiiiéiiiifiiétslow sand fllteri2‘Frieiiitiv‘>}'>$>3ifriiiritiviviiirivitiiivi‘ Mr“: .515 .‘1f\fNI\I‘\{\I_\ \'\I'\€\€\f'i‘ 1 "n{N{'\ _\|I\l‘nI\f'\ _\l‘uI‘\{\{'ii‘v"\"H'.\"v"\'\ \'\|"I'.mam treat-<

ment stage '— —————————————————————————— — -1chlorination |\ l ___________________________ _ _

necessarytreatmenl unit

likely necessary treatment unit

"-1 1| possibly optional treatment unit"" "' IRCWD 12/92

50 to 150 NTU might be treated with vertical-flow roughing filters and higher rawwater turbidities with horizontal-flow filters. However, it could be economical to treatraw water containing higher turbidity levels of short duration also with venical-flowroughing filters provided a more frequent filter cleaning is observed.

Stagnant surface water , f1nally,will probably have the smallest water quality changes.The influent of reservoirs, ponds and lakes undergoes natural treatment processes.

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Settleable matter will settle and microorganisms will die off with increasing retentiontime. Nevertheless, suspended and colloidal matter may still remain suspended anddepending on the degree of eutrophication and the extent of solar radiation, algaemight growth. The use of finely graded roughing filters might be appropriate andnecessary in such situations in order to protect the subsequent slow sand filter unitsfrom excessive loads of fine solid matter and algae.

In short, small upland rivers draining erosion protected catchmentareas are likely to have a small turbidity which, however, mightincrease to peaks of short duration during periods of heavy rainfalls.Such conditions favour the use of dynamic and intake filters. Largelowland streams are in general more turbid and change their qualityonly gradually following the annual climatic pattern. The use ofroughing filters possibly combined with intake filters might representan appropriate option for this situation. The suspended and colloidalmatter as well as algae of a stagnant water source call for theapplication of probably fine graded roughing filters.

The topography has to be taken into consideration especially in the layout of thewater supply scheme. The location of the intake, the topographical conditions at thatsite and operational aspects are essential criteria for the conceptional layout of watersupply schemes. I

The location of surface water intakes to remote places is often required in order toenable the construction of gravity schemes strongly recommended and to be realizedwhenever possible. Access to the remote intake sites is often difficult, usually time-consuming and the regular control and cleaning of the installations not guaranteed andquite often neglected. In such situations, pretreatment at the intake should be reducedto coarse solid removal and effective water treatment should only start at the treatmentplant which is generally located in front of the reservoir and as close to the supply areaas possible.

The local topography might favour the construction of a small canal for the controlleddiversion of surface water and, consequently, the installation of an intake or dynamicfilter. Favourable river bed conditions might also allow the construction of an intakefilter at low costs. . .

Drainage aspects need careful considerations. Intake filters usually operate withsurplus water drained to waste. Hence, their use in water schemes with a pumped rawwater supply calls for a careful economic evaluation. Furthermore, the operation oi

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Q?i]§‘icu[t community participation

Our discussion centred on how to rehahifitate the structure of a part[y finishedhorizontah-flow roughing fihter. One treatment [ine was fi[[ed with grave[, thecompartments of the other [ine were empty and sti[[ waiting to he fi[[ed with graveh Elsma[[ weir in the acfiacent steep creek directed the raw water into the intake pipe whichthen conveyed the water to the roughing fihter. ‘The water entering the fi[ter was ratherc[ear on this heautifu[5eptemher morning. However, a cfoser [ook into the fiiter reveaiedthat the water was flowing over the gravel surface as the fiiter was fu[[y chogged withaccumuhated sfudge. Ohviousfy, this sfudge originates from the harren hi[£s and is erodedduring periods of heavy rain from the catchment area which is covered on[y hy scarcevegetation. It was also during these periods of the year that the peophe of filntanamhe, adistrict centre with ahout 5,000 inhahitants in fliadagascar, addressed their compfaintsahout the turhid water to {Mr Kakotoyao, director of the water supply system. ‘Today,however, Mr Kakotoyao was not [istening to these compiaints hut to the engageddiscussions of the engineers and technicians grouped around Mr Zienjamin, a programmeofficer of a non-governmentai organisation. ‘The fi[ter media has to he removed andcieared off the accumufated siudge. I4 drainage system which a[[ows hydrau[ic fiitercheaning has to he instafhed hefore the c[ean graveh can he reinsta[[ed. ‘The hydrauiic systemhas to he chartged in such a way that one part of the ergisting structure can he operated assedimentation tankfor the separation of the coarse sett[eah[e sofids prior to [etting thewater pass through ‘the graveh fi[ter. fil[[ these modifications wouid improve thetreatment efficiency and prohong the fi[ter run. The director, seated on a [urge f[at stonewas attentivehy foiiowing the technica[ dehate. I t was evident that the work was to hecarried out as a seif-he[p project under community participation with the technicaiassistance of Mr (Benjamin and his group. Zveryhody was [ooking at Mr Kakotoyaowhose face, facing the p[ain hehind the hi[[s, had tumed serious. 9%ohi[isation of thevi[[agers for this rehahi[itation work wouid he one prohiem, the other more seriousdifficulty woufd he the transport to this remote site. On[y the first 9 km of the ’tota[ 15km had truck access. ‘The remoteness of the piant wou[d certainiy greatiy reduce the daiiyprogress of the rehahditation work.Mr Rakotoyao actuaffy achieved to organize materiai and peopfe hut 9\{r CBenjomin andhis crew [earned a hesson: they wi[[ in future more carefu[[y evaiuate differentpossihiiities of treatment phant hocation. .

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roughing filters requires an adequate topography for washwater disposal. Highwastewater discharges must be possible without creating erosion and the installationslarge enough for the runoff to be discharged into the next receiving water course orpreferably into a pond especially constructed for washwater storage.

Summing up, gravity schemes should be constructed wheneverpossible. This often requires remote intakes difficult to maintain.Nevertheless, favourable riverbed conditions should be used for theinstallation of intake and dynamic filters in order to reduce wastewaterdischarge problems at the treatment plant side, an aspect which needscareful attention with the use of roughing filters.

Operational and economic aspects also influence the selection of thepretreatment scheme.Construction costs are related to operational costs and routinework increases the reliability of water treatment.

Construction costs can possibly be reduced by applying the multibarrier concept.Adequate pretreatment units allow the design of subsequent treatment units applyinghigher hydraulic rates or reduced filter lengths. This will lower the construction costsof these units and probably also the total capital costs of the treatment plant. Besidetaking advantage of the natural pretreatment potential by tapping stagnant surfacewater or by locating surface water intakes at optimal sites, the use of grit chambers,sedimentation tanks, intake and dynamic filters should always be considered in orderto permit a roughing filter size reduction. Part of the capital costs might be convertedto operational costs by reducing the treatment unit sizes and increasing the filtercleaning frequency.

Regularfi/ter cleaning is not only important to restore the treatment plant's efficiencybut also to enhance the caretakers responsibility and to keep him on the job. Intake ordynamic filters in need of frequent, respectively of compulsory cleaning, support thisargument. Finally, the washwater demand depends on the type of prefilter andincreases as follows: dynamic filters, intake filters, vertical-flow and horizontal-flowroughing filters. This could also be a decisive criterion for the selection of thepretreatment scheme, especially when water is scarce or has to be pumped.

In short, total costs might often be reduced through a sound treatmentscheme design applying a sequence of different treatment steps.Routine maintenance work is essential for a good treatment plantperformance. t

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Practical pilot plant tests are strongly recommended to evaluate treatment concepts.The design of treatment plants should not merely be based on the theoretical evaluation of thepresented different criteria or on the application of general design guidelines andrecommendations. This is applicable especially to large treatment schemes requiringconsiderable capital investments and where modifications cannot easily be realized at a laterstage. Therefore, field tests with small pilot units are strongly recommended. Such tests shouldrun over a year in order to cover the rainy as well as the dry season. The treatment plant designwill certainly gain greater confidence if it is based on practical experience and on design valuesestablished by field tests.

filfrican kiifer hees supported sustainahiiity

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The rapid sandfifters of Safaga, a district centre in Northern Qhana, were never fi[[edwith fi[ter media and therefore, the popidation of the town exposed to an unsafe watersupply. The o[d treatment phant had to he extended and repiaced hy a treatment schemeappropriate to the [oca[ conditions. Therefore, roughing and show sandfihters were testedin a piiot phant shaded hy a [arge haohah tree. The [ocation was chosen hy the ergternaiexperts sufihering under the §hanian sun. The piiot phant was constructed hy the [oca[water authority and, for the start of the fieid tests, the expatriates were again on thespot. However, also filfrican miter hees seiected the same [arge haohah tree for theirresidence and, they were much attracted hy the white skin of the foreigners. The fieidtests continued - under remote supervision hy the ergpatriates - with [oca[ staff, withpeopie who gained ergperienee and confidence in a treatment scheme they wi[[ have to runin future at fu[[ scaie without foreign assistance. Thus, the /‘Zlfrican ki[[er hees somehowsupported the deve[opment towards [oca[ sustainahiiity.

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3. Bacteriological Water Quality Improvement

Separation ofMicroorganisms

The water in our bucket is now clear butstill unsafe for ‘consumption. The turbidwater drawn from the troubled river has nowchanged its appearance due to the solid matter

discussed in the foregoing chapter. The water haslost the brownish tinge and turned into a clear and ';‘,';;f,}SmSpleasant liquid. However, the water might still notbe as pleasant and safe as it looks. Disease- Mseparation carried out by pretreatment processes 1

causing, pathogenic microorganisms are usuallyIHCWD 18/92not visible to the naked eye of the easy-going con-

sumer who could get an attack of heavy diarrhoea a few hours after drinking the suspect water.Hence, the pretreated water still needs further treatment for the final removal orinactivation of pathogens. Chlorination and slow sand filtration are the two mostcommonly applied treatment processes for bacteriological water quality improvement.

3.1 Chlorination

Chlorination of the water aims at destroying or at least inactivating harmful microorganismssuch as pathogenic bacteria, viruses, and cysts present in the water. Chlorine is a strongoxidizing agent which does not only react with the enzymes vital for the metabolic processesof living cells, but is also involved in other chemical reactions. For instance, chlorine reactswith dissolved organic matter and nitrogen to form chloramines. These fast chemical reactionsdeplete part of the chlorine which then, is not available for water disinfection.

This sounds rather theoretical and let us, for that purpose, make the following practicaldemonstration:

We use our bucket filled with now pretreated river water. After having determined thewater volume and carried out some tricky calculations we add a specific dose ofchlorine solution to the water in the bucket. We stir the water during dosage inorder to achieve a good mix and let the water stand in the bucket for about 20, minutes.While waiting, we again check our calculation by arranging into correct correlationthe water volume, the volume of the chlorine solution and the active chlorine contentindicated on the label of the packing. As expected, we then determine the free

* chlorine content of the water with a test kit and sadly discover that the measured

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free chlorine concennation does not conform with the calculated one. The differencebetween calculated and measured chlorine concentration is called the chlorinedemand of the water, which is essentiallythe part of chlorine used up by the fastchemical reactions, such as the formation of chloro-organic compounds and thedestruction of chloramines and chloro-organics. Hence, the chlorine demanddepends also on the degree of pretreatment. Turbid water carrying organicswill generally have a higher chlorine demand than clear water. It should be notedhowever that all our accurate calculations, readings and determination of the chlorinedemand might be offset by the reduction of the active chlorine content in the stocksolution. Chlorine is unstable and its concentration vanishes during storage andthereby creates an additional problem.

Accurate dosing of chlorine is essential to achieve an efficient disinfection. Only partialdisinfection is achieved with chlorine dosages lower than the chlorine demand of the water.And water with too high chlorine concentrations might be rejected by the consumers sincechlorinated water emits a distinct odour. A strong smell develops when chlorine reacts withammonia to form chloramines. People often reject chlorinated water,_ even whenchlorine is carefully handled and dosed at low concentrations.

Chlorine is available ingaseous, solid, and liquid form.Chlorine gas is extremely toxic,difficult to handle and thereforeusually inappropriate for niral watersupplies. Chlorinated lime, commonlyknown as "bleaching powder",calcium hypochlorite powder, orsodium hypochlorite solution, but alsocalled "lavel water", are used as asource of chlorine. A chlorinesolution is preferably added tothe water and therefore, chlorinatedlime and calcium hypochlorite need tobe dissolved in the waterto a stocksolution usually containing 1 to 3percent active chlorine.

v

Application of Chlorine

Constant - headSolution Feeder

lloating bowl .__ wregulating lnlel lulloxiblo delivery tub

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variable levelol chlorine solutioncontainer

weir

Chemicals Design Guidelines' chlorinated lime or .

"bleaching powder"- 33 - 37 % chlorine content- unstable when exposed

to air, light and moisture- COUOSIVB

- calcium hypochlorite- 60 - 70% chlorine content- more stable powder

than chlorinated lime~ corrosive

- sodium hypochlorite“Javelle water "- 10 - 15 % chlorine content- unstable, corrosive

dosage in form of1 - 3 % solution

dosage dependingon chlorine demandol water

free residual chlorineconcentration

0.5 l aft 30 M'2 conirggitimgr In> 0.2 I in the

distrrii"i%/tion network

IRCWD 14/9'2

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The chlorine solution is added to the water at a constant rate by dosing devices. The relativelysmall doses of chlorine call for accurate dosing equipment which are, however,exposed to and often damaged by the corrosive action of the chlorine. The dose has to beadjusted according to the chlorine demand of the water to be disinfected. In practice, chlorinedosage adjustments are only possible to a limited extent, e.g. on a day to day basis.Consequently, a reliable treatment of the water prior to chlorination is required. Adequatewater disinfection is generally only feasible with water of low turbidity orvirtually free of solid and organic matter.

A reliable supply of chlorine is often difficult. Chlorine must be purchased on aregular basis, since its unstable nature does not allow lengthy storage. The chlorine mustfrequently be imported and therefore requires foreign currency which is often scarce indeveloping countries. Additionally, these countries face other difficulties such ascommunication problems when ordering the chemicals as well as transportation problems forcarrying the chemicals to the treatment plant in time. And finally, chemical water treatmentrequires skilled personnel, often not available in rural areas. All these aspects very much

I u ~question a reliable and efficient application of chlorine and, more generally, ofchemicals in rural water supply schemes in developing countries. This observation isendorsed by numerous cases of malfunctioning or abandoned chemical watertreatment installations.

Conventional disinfection methods are generally not successful in small rural water supplyschemes. Hence, there is a need to develop low-cost, reliable methods of disinfection whichare simple, robust and easy to maintain. New techniques for water disinfection havealready been developed and field tested (Ref.5). One alternative is the use of iodine insteadof chlorine and to bind it onto resins housed in a cartridge. This cartridge is placed into thewater and the microorganisms are killed by oxidative reaction with iodine. Compared withchlorine, the iodine does not react so easily with organic compounds in the water. However,further development especially on the fixation of iodine on an adequate supporting material isrequired before this disinfection method can be used on a greater scale. A second alternative forwater disinfection is the utilization of an electrolytic cell that produces anoxidizing gas by passing an electrical current through a saturated sodium-chloride solution.The Moggod method, as the process is called by the manufacturer, requires normal salt. waterand power and seems to produce a strong oxidizing This method has a considerableapplication potential in developing countries. However, further investigations on the nature ofthe produced gas and on operational aspects when using low-quality salt are necessary beforethis method of disinfection can be recommended for a wider use.

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Ch[orinated water not for drinking purposes

One coafd afmost srne[[ the paint of the recent[y insta[[ed puhfic standpost. Its designdiffers from the many other thousands used around the wor[d. The design of theCameroon standpost app[ies the principfe of water syphoning with fferihfe tuhes. 2ffoatirrg valve Keeps the water at a constant [evef in the c[osed stee[ eyfinder and thusprevents [eahing or hrolien down taps due to heavy puhfic use. 9-fowever, the interest ofour group, composed of representatives of the foreign consuftant and suppfier and [ead 5ya d6.$'k_ ofificer of the nationaf water company and the [oca[ director of the water suppfy,was not onfy restricted to this speciaf standpost design hut to the entire water suppiysystem. ‘Ihe system visited was the first offour which started operation afew weeks ago.‘Zhe construction of an additiona[ twefve schemes were under discussion. /’¢l[[ water suppfyschemes were identicaf in design: a surface water inta/fie, a water treatment consisting ofprechforination, pf-[-controf, aeration, ffoccufation with afum sufphate, tifted pfatesett[ers, rapid sand flftration and fina[ disinfection, as weft as c[ear water pumpingstation suppfying the reservoir and the distrihution scheme. fil modufe system for thetreatment pfant enahfed rapid and efficient construction. 9-fowever, we had some douhtswhether the river water, which drains a dense and unpopufatedforest area, required suchergtensive treatment in this focation. Our visit coincided with the rainy season hut eventhen the ahstracted raw river water was quite cfear. 7\[everthe[ess, the water ran throughthe different treatment stages and chemicals were added

‘Two gir£s passed 6y the standpost site and were asked 6y the desk officer of the watercompany they were enjoying the new water quafity. ‘The answer was unexpected anddepressing. ‘They declllred that their famifies did not use this water for drinking Because ofits artificiaf and strange taste. ' _

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Summarizing, an efficient and reliable disinfection with chlorine requires apretreated water of low turbidity and virtually free of solid and organic matter.The application of chlorine in rural water supply schemes often facesenormous problems and, therefore, is frequently bound to fail as documentedby numerous treatment plants. Furthermore, rural people often rejectchlorinated water. Careful technical, institutional and socioculturalinvestigations are therefore required before introducing chlorination in ruralwater treatment.

3.2 Slow Sand Filtration

Slow sand filtration plays an important role in rural water treatment. Slow sandfilters reduce the number of microorganisms present in the water and hence, they improvethe bacteriological water quality. In addition, fine organic and inorganic matter isseparated and organic compounds dissolved in the water are oxidized. The effluent of a well-designed and operated slow sand filter is virtually free from pathogenic microorganisms and.therefore, Water treated by a well-operated slow sand filter is safe forconsumption.

The slow sand filter technology copies nature. The sand layers of aquifers convertunsafe surface water into a good quality drinking water. Especially the harmful bacteria.viruses, protozoa, eggs, and worms are most effectively removed by physical and biochemical

processes to a level which will no longer endanger human health. This natural purificationprocesses also used by the slow sand filters - a technology discovered the last century.At that time and under the threat of cholera epidemics, European waterworks discovered thebenefits of slow sand filtration. The water treatment technique proved to be efficient againstwater-borne diseases and, in combination with other sanitation improvements, these epidemicswere eradicated in Europe. Numerous water supplies in industrialized countries arestill using slow sand filters. For instance, the Thames Water Authority supplies surfacewater treated by slow sand filters to two thirds of London's population.

The layout of slow sand filters is simple and clear-cut. A slow sand filter consistsof an open box containing-a sand layer of approx. 0.8 to 1.0 meter in depth. The upper part ofthe filter box is filled with water,'which flows by gravity through the sand bed. The filteredwater is then collected by an underdrain system and conveyed through flow control devices tothe clear water tank. The well-graded sand of the filter bed is relatively fine with an effectivesize of 0.15 to 0.30 millimetre.

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Slow sand filter operation is easy and reliable. Slow sand filters are usually operatedat 0.1 to 0.2 m/h filtration rates. Consequently, an area of l m2 sand produces daily approx.2.5 to 5 m3 water. The flow rate is preferably controlled at the filter inlet, and the water levelmaintained at a minimum level above the sand bed by a weir or effluent pipe at the filter outlet.Slow sand filters perform best under continuous operation and at constant flow conditions.Therefore, a 24-hour-day operation is recommended, making maximum use of the availablefilter installations. The initial filter resistance of a clean sand filter ranges between 0.20 and0.30 meter. The headloss gradually increases with progressive filtration time. The sand filterhas to be cleaned when filter resistance increases to about l meter.

Layout and Designof Slow Sand Filter

=i><l= Open:}(= closed

:35?"

List of Symbols Design Guidelinesds

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A

AH

Q (

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VF (

333 vvé

5,23L’

mlma/h)m/h)

sand size

sand depthgravel depth

filter lengthfilter width

filter bed area

headloss

flow rate

filtration rate

VF: £._ -A—.".“Q O102m/hL'W .

AH max

d S Z

hs =

hg =

1.0m (= rnax levelof supernatant water)

0.1 - 0.3 mm (=effectivesize, 10% passing)

0.8 ¢ 0.9 m

0.2 - 0.3 m

IHCWD 15/92

Slow sand filters act mainly as surface filters. The water quality changes at thesurface of the sand bed, in the so-called "Schmutzdecke“ and, to a minor extend, in the first 20to 30 centimetres of the sand bed. A thin layer on the surface of the sand bed, consisting ofretained organic and inorganic matter, and of a great variety of biologically activemicroorganisms, is responsible for the physical, chemical, and biological improvement of the

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water. This thin biological layer must first develop in a new slow sand filter. The ripeningperiod will nomially last from two to four weeks. Cleaned filters will regain their full biologicalactivity within two to four days, provided the cut-down time for filter cleaning is of shortduration, e.g. 6 - 12 hours. '

Filter cleaning must be carried out once the supernatant water has reached its maximumpermissible level; i.e. when the maximum filter resistance of approx. 1 meter is attained. Filtercleaning starts with the drainage of the supernatant water and the dewatering of the top of thesand bed. Subsequently, the biological skin as well as 1 to 2 centimetres of sand are removedfrom the sand bed. Resanding is possibly carried out after removal of the top sand layer and,thereafter, filter operation is immediately restarted in order not to greatly. upset the biologicalfilter activity. The water, filled from the bottom into the filter bed, drives the air out of thepores of the sand and completely saturates the filter bed. Then, normal operation is reassumedby opening the inlet valve and adjusting the filtration rate.

Cleaning of a Slow Sand Filter in Jinxing, China

Well-operated slow sand filters achieve filter runs of 1 to 3 months or more.The term "filter tun" is defined as the time period between two subsequent filter cleanings. Inorder to achieve such long filter runs, slow sand filters have to be supplied with relatively clearwater. Reasonable filter operation can only be expected with raw water turbidities below 20 to30 NTU. Higher turbidities, with consequently higher concentrations of solids, will rapidlyclog the sand surface and interfere with the biological processes. Hence, it is a strong fact thatpretreatment of surface water prior to slow sand filtration is generally necessary.

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Deficiencies cause problems -also With SIOW sand filters. Common Design Faults of Slow Sand Filter

In the past, several slow sand filter

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inadequate operation and poor waterquality Supphad to the Slow Sand @ noflowconfrolinstallalions @lnawr>rQnri=tes-and bed

. . ' Cf ' S1 l0 Hi l i I lfilters are the major reasons for the ® ma equawpfiy em ® we “en WW We ,,,Cw,,,,,,,experienced problems and failures.A lack of flow control installations, inadequate installations of the pipes, a soiled, poorlygraded sand not conforming to the recommended size or the missing water level control system,are the most common design faults encountered. Random filter operation under variable,frequently grossly exceeded filtration rates by not well-trained caretakers are often the reasonsfor insufficient filter efficiency.

Poor raw water quality hinders slow sand filter operation. This is probably themain reason for inadequate slow sand filter performance. Frequently, no pretreatment at all isprovided and that slow sand filters are directly fed with raw water. Slow sand filters are oftenalso combined with inefficient or inappropriate pretreatment processes. Explicitly, slow sandfilters usually face serious operational problems in combination with chemical flocculation andsedimentation used as pretreatment. The local caretaker might not be able to controlflocculation, which is an unstable pretreatment process difficult to operate. Light flocs are oftenwashed on the slow sand filters or the lack of chemicals greatly reduces the solid removalefficiency of the sedimentation tank. Premature, rapid filter clogging and frequent filter cleaningare the consequences. Therefore, an efficient pretreatment of the surface water, for instanceprovided by roughing filters, is necessary to avoid serious operational problems with slow sandfilters. Small slow sand filter units receiving raw water of moderate turbidity can also beprotected by layers of non-woven synthetic filter fabrics. .

Conclusively, slow sand filtration offers the great advantage of being safe andstable, simple and reliable. Filter construction makes extensive use of localmaterial and skills. Filter operation neither requires sophisticated mechanicalparts nor the use of chemicals. Construction, operation and maintenance of thefilters are easy and require only limited skills. However, adequate filteroperation is only possible with raw water of low turbidity virtually free ofsolid matter. Pretreatment of surface water is therefore necessary. Slow sandfiltration is - in combination with adequate pretreatment methods - considereda most appropriate Water treatment technology for developing countries.

43

Defective s[ow sandfi[ter nergt to the cemetery

The photograph is se[f-evident. Standing on the s[ope of a steep va[[ey in the Feruvianhighfands we can see hefow us two s[ow sandfi[ter units fi[[ed with choco[ate hrownishwater, a [arge heap of sand deposited on the soi[ nergt the structure and a hit furtherdown the cemetery of the vi[[age whose popu[ation is suppfied 5y the water of thesedefective fifters. 9\/fist is cfimhing the vaftey and wi[[ soon engufi this gfoomy vision .

In 1985, Cbelflgita surveyed the 18 treatment p[ants in 2 departments of5Peru. ‘Two of thep[ants had rapid sandfi[ters which were not worfing. The study a[so revea[ed that a[[s[ow sand fi[ters and disinfection units had major deficiencies and operating proh[ems.’1he reasons for this faifure can he cfassified into technicaf and institutionaf pro6[emareas. ‘The fundamentaf prohfems with respect to the technicai aspects were thoseassociated with the flow controf and the raw water quatity. The ahsence ofaflow contro[at the raw water intake caused the fifters to he operated at unsta5[e or intermittentconditions. ‘The hzgh[y turhid and contaminated raw water was not adequate[y pretreatedand resu[ted in short fi[ter runs and operationaf pro6[ems. Consequent[y, fitter efficiencyfed considerahiy and, according to the survey, more than ha[f of the pfants had marginafor no effect in reducing turhidity and 6acteria[ contamination. ‘With respect toinstitutional aspects, the caretakers and administrative committees were not adequate[ytrained in the operation and maintenance of the treatment p[ants. The users did notreceive professionaf supervisory support from the responsi5[e nationaf authorities norwere there any incentives to provide a re[ia6[e water supp[y. ‘The descrihed proh[ems weretack[ed 5y a rehahihtation and techno[ogy transfer programme for ruraf water treatment.Effective and appropriate pretreatment processes such as roughing fi[tration wereintroduced and institutional deve[opment and community education supported.

F1 re[ia6[e and sustainahfe operation of treatment p[ants requires the adequateapp[ication of appropriate treatment processes as we[[ as the [oca[ deve[opment oftechnicaf skiff and management capacity. (Ref. 1)

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4. Layout of a Water Supply Scheme

4.1 General Considerations

From the technical point of view, the following three main questions have to be answeredduring the planning phase of a water supply scheme:

Cl which raw Water source should be used for the water supply scheme ?El is treatment necessary, then, what type of treatment scheme is required ?El how much water should be distributed to the consumers, and at what service

level?

Source selection is a very basic decision entailing numerous consequences for the futurewater supply scheme. The different local water sources have to be evaluated with respect totheir availability, accessibility and quality. The present or preferably the future waterdemand must be covered by the selected source, which should be located as close as possible tothe supply area and provide the best possible water quality.

Water treatment, which is usually the most difficult element in any water supply scheme,should, therefore, be avoided whenever possible. The general statement that no treatment isthe best treatment applies especially to rural water supply schemes exhibiting often scarceinfrastructural and institutional support to adequately maintain water treatment facilities. Theuse of better water quality sources is, therefore, always an alternative to be taken into seriousconsidered. If no other alternative exists, rural water treatment must concentrate on the basicimprovement of the water quality by locally sustainable treatment processes.

Water distribution systems depend on the type of water source used, on the topography andon the provided supply service level. Individual water supplies, e.g. rainwater harvesting andshallow groundwater wells equipped with handpumps usually do not need piped supplysystems. Treated surface water, however, is nonnally distributed by a piped system. A suitabletopography often enables the installation of a gravity system which, thereby, greatlystrengthens reliability and supply continuity. Pumped water supply schemes dependon energy supply and spare parts availability and are, thus, very susceptibleto temporarystandstills. Finally, the service level of water supply strongly governs water demand. Waterusage increases drastically with the provided service level, e.g. public standpost - yardconnection - multiple tap house connection. Water supply is always linked withWastewater disposal aspects. The public health situation of a community supplied with

2 traditionalwatersupplg-lg’ 8 & {T Q Q

45

treated water does not necessarily improve, especially when sanitation and wastewater disposalare neglected. Hence, a reliable and safe water supply as well as an adequate disposal of humanwaste are absolutely necessary to obtain a significant improvement of the public healthsituation. .

Surface water has to be collected, treated and stored before reaching the consumer. Theseoperations are attained by the use of different installations which are schematically presented inthe following Table.

RAW WATER SUPPLY I TREATMENT I DISTRIBUTION

gravity scheme ti _River 5 llllt 1Sad ' ___ piped system withintake ‘fink goughing fill - public standpost

Fm” Slow SandFiner Reservoir

semi-Pipedscheme fi/ O Q O‘II pl,

River _-lmake ?::(' Roughing ll cistern with handpump

Finer Slow SandFilterpumpedscheme

llillr -Flaw Water ‘ Eh

Tank RoughingFiller Slow Sand

C Filler Reservoir

‘Ii River intake andPumping Station

IFICWD 17/92

4.2 Hydraulic Profile

The selection of the hydraulic profile is a basic decision when planning a water supply scheme.First choice must be given to gravity supply systems since they guaranteereliable operation at low. running costs. Schemes which integrate the use of handpumpsare given second choice. The installation of mechanically driven pumps should be chosen aslast option and only applied in special cases where a reliable and affordable energy supply isguaranteed as well as the infrastructure for pump maintenance and repair work. Hydraulic ramsmight be an appropriate option where surface water gravity and water volume are sufficient.

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Under special local conditions, collection and pretreatment of the raw water might be combinedin a single installation such as infiltration galleries.Gravity flow should also be foreseen for the water treatment plant installationssince they are usually operated at a free water table. The total headloss through the treatmentplant will be in the order of 2 or 3 m. In general, any water lifting except through handpumpsshould be avoided in order not to depend on the supply of energy and sophisticated spare parts,which is, in most cases, very unreliable. The number of pumping steps must belimited if, due to topographical reasons, water lifting is absolutely necessary.As illustrated in the next Figure, a 1-stage pumping scheme should be chosen to pump the rawwater to an elevated site where the treatment plant and the reservoir are located. Such a l-stagepumping scheme is more advantageous compared to a 2-stage one as it increases reliability ofthe scheme by a factor of 2. In addition, the risk of flooding often cannot be completelyexcluded in lowland areas. Flood protection of a high-lift pumping station is easier than of afull treatment plant. However, a 2-stage pumping system cannot be avoided in a piped supplyon a flat area lacking natural elevation.

2 stage pumping scheme(not recommendable) __

max 7 treatment reserve"

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44 ‘ ~> I I I/J.,, M-=~ _-__raw water gfiizjwater syoragePUITIP +

_ treatment _river village

$11raw water apump reservoir

Q2 ' §_$i|—

1 stage pumping SCI‘l9m9(recommended, if feasible)

IRCWD 18/92

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Spring water maintains minimum water supply

/

Iringa, a ‘Tanzanian town of 80,000 people is pleasantly located at the edge of anescarpment. The citizens have a Beautiful view of the valley where Little ikuaha river ismeandering gently through cultivated maize and cassava fields. ‘This turhid river is alsothe town‘s main water supply source. C/he river water is pumped to an arfiacentconventional treatment plant, collected in a clear water tank and, in a second stage,pumped over the steep escarpment to the reservoir in the town. One of the severa[ causesfor Iringa's water shortages is the frequent hreakdown of the raw water pumps. Qhe silt-loaded river water claims its victims; ruhher seal wearouts, impeller grindoffs and shafthlock ups make the life of the station manager quite rhfficult. frequently, none of theraw water pumps are working.

fortunately, the clear water tank is also supplied hy a spring tapped across the rivervalley and conveyed hy an approzgimately 10 km long gravity pipe. ‘Iherefore, the powerfulclear water pump is operatedfor at least afew hours a day to lift the clear spring waterand alleviate the town which is suffering from this meagre supply situation.

Ohviously, the spring water supply is more reliahle not only hecause of its single pumpingstep hut also due to its hetter water quality. ‘Zhe operational difficulties of the raw waterpumps could he greatly reduced hy rehahilitation of the intake works. The intake suctionpipes hanging loosely into the river should he replaced hy agrit chamher, or preferahly hyintake filters or infiltrationgalleries which would remove a large fraction of the solidsthat reduce considerahly the life span of any pump.

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4.3 Treatment Steps

As discussed in Chapter l, surface water has to undergo a step-by-step treatment.Coarse solids and impurities are first removed by pretreatment stages; the remaining smallparticles and microorganisms are separated by the ultimate treatment step. Under special localconditions, collection and pretreatment of the raw water may be combined in a singleinstallation such as intake or dynamic filters or, alternatively, by infiltration galleries.

4.2.1 Removal of Coarse Material

Separation of coarse solids from the water is preferably carried out by a high-loadsedimentation tank (grit chamber) or by a plain sedimentation tank since sludgeremoval from such tanks is less troublesome than from roughing filters. The design valueslisted in the next Figure are applicable for the removal of coarse solid particles larger thanapprox. 50 um or 20 um respectively. '

Earth basin as sedimentation tank' pparticle " 2'6

Planremoval of particles > 50 pm:

surface load s0 = 6 m/hdetention time Td = 15 min

access forcleaning

->*“-P W

removal of particles > 20 pm:F surface load so = 0.6 m/h

_ L _ detention time Td = Zléh

w.—* -tn _. Q3Section H ~

perforated effluent l_/W ._distribution pipe trough

first ‘ti is-—prefabricatedconcrete slabs

IRCWD 19/92

The use of one sedimentation tank should be enough for a small-scale water supply scheme.The accumulated sludge can be removed during periods of low silt load. A by-pass is requiredin order to maintain operation of the treatment plant during cleaning periods. Two or moresedimentation units should be provided for larger schemes.

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4.2.2 Aeration

The water's oxygen content plays an important role in the biology of the slow sand filtrationprocess. The activity of the aerobic biomass decreases considerably if the oxygen concentrationof the water falls below 0.5 mg/l. Hence, an adequate oxygen content in the water tobe filtered is needed. Physical processes are the major mechanisms in roughing filtration.However, biochemical reactions might also occur in the prefilters, especially when the rawwater is highly loaded with organics.

Turbulent surface waters usually have a sufficient oxygen supply and, therefore, do not requireto be especially aerated. Standing water, however, can exhibit low oxygen contents, especiallywhen drawn from the bottom of polluted surface water reservoirs. Standing raw surfacewaters are preferably aerated. I

Cross - Sectionweir load:10 I/s-m

H<70cm’/ B>2/SH

/11” \

\\‘\\‘

Z

$&\\%I \Q2

‘\\\‘

IRCWD 20/92

Cascades are simple but efficient aeration devices. The installation of a submergedcascade aerator, as illustrated by above the Figure, should be realized in gravity system wheresufficient hydraulic head is available. The cascade should preferably be placed in front ofroughing filters in order to cope with a possible oxygen demand of such filters.

The different weirs used for filter conuol are an additional source of oxygen supply.

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4.2.3 Roughing Filtration as Pretreatment Step

Roughing filtration mainly separates the fine solids which are not retained by thepreceding sedimentation tank. The effluent of roughing filters should not contain more than 2-3mg/l solid matter to comply with the requirements of the raw water quality for slow sand filters.

Coarse gravel filters mainly improve the physical water quality as suspendedsolids are removed and the turbidity reduced. However, a bacteriological improvementof the water can also be expected as bacteria and viruses are solids too, ranging in sizebetween approx. 10-0.2 ttm and 0.4-0.002 um respectively. Furthermore, the specificliterature states that these organisms get frequently attached by electrostatic force to the surfaceof other solids found in the water. Hence, a removal of the solids means also a reduction of thepathogens (disease-causing microorganisms). The efficiency of roughing filtration inmicroorganism reduction might be in the same order of magnitude as that for suspended solids.e.g. an inlet concentration of 10 - 100 mg/1 can be reduced by a roughing filter to approx. l - 3mg/1. The bacteriological Water quality improvement could amount to approx.60 - 99%, or the microorganisms are reduced to approx. 1 - 2 log. Larger sized pathogens(eggs, worms) are removed even to a greater extent.

Roughing filters are used as pretreatment step prior to slow sand filters. Slottsand filtration may be omitted if the bacteriological contamination of the water to be treated isabsent or small, particularly in surface waters draining an unpopulated catchment area or wherecontamination of the water by human waste is prevented by controlled sanitation. Permanent orperiodic high silt loads in the surface water, however, might call for a physical improvement ofthe water. Excessive amounts of solids in the water result in the silting up of pipes andreservoirs. Hence, for technical reasons, roughing filtration may be used withoutslow sand filtration if the bacteriological quality level of the raw water isalready fairly acceptable, in the order of less than 20- 50 E.coli/lO() ml.

For operational reasons, at least 2 roughing filter units are normally required ina treatment plant. Manual cleaning and maintenance work may take some time during which theremaining roughing filtration unit(s) have to be operated at higher hydraulic loads. A singleprefiltration unit might be appropriate in small water supply schemes treating water orperiodically low turbidity.

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4.2.4 Slow Sand Filtration as Main Treatment Step

The substantial reduction of bacteria, cysts and viruses by the slow sandfilters is important from a hygienic point of view. Since slow Sand Filters alsoremove the finest impurities found in the water, they are placed at the end of the treatment line.The filters act as strainers since the small suspended solids are retained at the top of the filter.More important than the physical processes are the biological activities of theslow sand filter. Dissolved and solid unstable organic matter, which causes oxygendepletion or even turns to fouling processes when oxygen is absent, is oxidized by the filterbiology to stable inorganic products. The biological layer on top of the filter bed, theso-called "Schmutzdecke" is responsible for the oxidation of the organics as well as forthe separation of the pathogens. A slow sand filter will produce hygienically safe water oncethis layer is fully developed. l

Unlike roughing filters, the moment for filter cleaning of slow sand filtersisdetermined by the achievement of the maximum available headloss and not by thedeterioration of the effluent quality. This is of some advantage as the record of a hydrauliccriteria is easier than that of a quality parameter.

Further information on slow sand filtration is summarized in Annex l l. However, for detailedinformation on the design and construction of slow sand filters reference is made to differentspecial manuals (Refs. 6, 7, 8).

4.2.5 Water Disinfection

A slow sand filter with a well-developed biological layer produces a hygienicwater safe for consumption. Any further treatment such as water disinfection is.therefore, not necessary. Apart from the water quality aspect, numerous examples from manydeveloping countries reveal that a reliable disinfection by chlorine is practicallyunfeasible in small rural water supply schemes. An uninterrupted supply of mostlyimported chemicals and the accurate dosage of the disinfectant are the two main practicalproblems encountered. -

However, as regards disinfection, one has to differentiate between small (rural)and large (urban) water supply schemes. Large distribution systems with often illegalconnections present a risk of recontamination, especially if the water supply is not continuous

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but interrupted. Therefore, in large urban water supply schemes, final chlorinationof the water as a safeguard disinfection is recommended. This safeguardchlorination will kill the pathogens introduced in the water by secondary contamination either inthe distribution system or at the consumer end. Bleaching powder or a sodium hypochloritesolution are commonly used as disinfectant.

More persuasive than a preventive disinfection in rural water supply schemesis the production of an acceptable water quality and the realization of a generalhealth education programme with special training in water handling.

4.3 Water Distribution

4.3.1 Water Storage

Water treatment installations are preferably operated at a constant rate 24 hoursa day to make full use of their treatment capacity and to avoid interference of the treatmentprocess by intermittent operation. Particularly slow sand filters should be operatedcontinuously to permanently supply the biological layer with nutrients and oxygen.Roughing filters are less sensitive to operational interruptions although a careful restarting otfiltration should be observed in order not to resuspend the solids accumulated in the filter.Water supply schemes operated entirely by gravity can easily cope with the 24-hours a dayoperation requirement.However, in water supply systems requiring raw water lifting, pumpoperation is often reduced to 6 - l6 hours a day. In pumped schemes, the constructionof a raw water tank may be an economically anti technically sound optionenabling continuous operation of the treatment plant and acting additionally aspresedimendation tank.

Water storage capacity must be provided in order to compensate daily waterdemand fluctuations. In rural water supply schemes, the daily water consumption is moreor less concentrated in the morning and evening hours. Therefore, a storage volume ofapprox. 30 to 50% of the daily treatment capacity has to be provided to compensatefor the uneven distribution of the daily water demand.

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4.3.2 Distribution System

Finally, a water distribution system will have to be provided. Depending on the economicalconditions and local topography, the installation of a piped gravity system might be considered.However, water supply is also always linked with wastewater disposal. Disposalproblems will increase with improved accessibility and higher water consumptions.Furthermore, increased water demands might lead to overloaded treatment facilities and,consequently, result in a quality deterioration of the treated water. Therefore, unlimitedwater availability does not always run in parallel with an improvement of thepublic health situation. A fair equilibrium should be maintained between the quantitativeand qualitative water supply and the required water demand to achieve or keep a reasonablepublic health standard free from wastewater disposal problems. .

An important criteria for the consumer is water accessibility and not waterquality. His main interest is the walking distance between home and water point; the waterquality plays a minor role to him. Consequently, treated water and water of betterquality have to be brought nearer to the houses than the traditional watersources. For instance, treated river water as a new source is likely to be more readily acceptedif the original walking distance to the river can substantially be reduced by the installation of awater supply system..

Yet in many situations, the economical as well as the topographical conditions are against apiped gravity system. Differences in altitude can be overcome by water lifting, but pumpsrequire relatively high investment and operation costs and particularly energy - an aspect whichwill gain increased importance in the coming years. In rural water supply schemes,pumped systems should therefore be installed only after careful consideration,and be limited to special cases.

The Figure on page 45 illustrates different hydraulic layout possibilities. On the raw water side.the water flows by gravity directly to the treatment plant or, ifpumped, preferably first to a rant

water balancing tank. After passing through the treatment plant it is stored. The treated water isdistributed to the consumers either by a piped system next to their houses or it is pumpedmanually from a system of cistems located between treatment plant and village. - I

A rational combination of the discussed different aspects could result in a semi-pipedsystem equipped with handpumps. The treated water could supply by gravity differentwater cisterns located between treatment plant and village. Each cistern would act as reservoir

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and water point. The energy supplied by the consumers when operating thehandpump keeps the water supply system running at low and sustainableoperation costs. Finally, the supply level will, in this case, maintain water consumption at a

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reasonable level and hardly cause wastewater disposal problems.

The next Figure illustrates a possible layout for a water treatment plant operating without anyforeign chemical or energy input.

sand washing placePlan

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LI "-

Ill *_“@_,tAll

§ 52- i Ar ' I. i H

S9dlm6f‘ll3llOl'\ tank av, schemeQ7 “Y

(“mace bad 0'6 m/h) horizontal-flow roughing filter Sbvy sand dear wafer tank 0F dlslflbmlon bl’river ‘make (liltratlon rate 0.8 m/h) M I zmgz‘ m/h) lV°l""‘9 5° ma) cisterns equipped

‘ " 3° ' with handpumps.I Max Sectlon A-A

@’ B *1 4° C 4-1 -so -so -180

iii-""i‘3."!'§'-Eli!‘'l~i°-l t-—--1<><>——-I

E§J J51; Ieo E§\$_?/' I10080 so as 100 eotoo

El 1| /ti

300 600 l

Section B-B Section C-C

all dimensions in cm|RcwD'zt/s2

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The handpump hand[e Keeps the water suppty in operation

Fl [arge numher of irrigation canafls suppiy water to the giezira/9\danagi[ zone where cashcrops such as cotton, corn and vegetahies are grown. ‘The ferti[e soi[ and sufficient waterdrawn from the river 9\[i[e are the Ease for profitah[e agricu[ture. 9-[owez/er, since thefarmers which have settfed down a[org the irrigation canafls have a very [ow income, theyare forced to [ive in modest straw huts. flfong with the use of irrigation water, mafaria,fiifharzia and diarrhoeai diseases have spread to the popidation. ‘The CB[ue 9\(i[e HeaithProject was, therefore, [aunched to improve the health situation of the peopte [iving inthe project area.

The ahsence of infrastructure and energy as we[[ as the ergisting poverty presented severeconstraints for the improvement of the water supp[y situation. The vi[[agers usua[[y[iving in sett[ernents of 200 - 500 peop[e drew their water from the irrigation canaflswhich were poffuted fiy human e2Qcreta. ‘Zhe use of groundwater from a we[[ is aiwaysfavoured. However, since groundwater is often not avaifahte, the po[[uted water from theirrigation canals has to he treated .91 standard design was impfemented in the project area.The water drawn from the canaiffows hy gravity through a horizontai-flow roughingfitter and a sfow sandfifter into a the cfear water tan/Q 91 simpte handpump is insta[[edon top of this tank a[[owing peopfe to fetch treated water from the insta[[ation. Theoperation costs are minimai and consist essentia[[y of a rep[acement of the handpumpruhher sea[ every two months andfi[ter c[eaning carried out 6y the community twice ayear. ‘Zhe daiiy energy input of the water drawers operating the handpump Keeps thesystem running andprovides cfean water even to a non-monetary society. (Ref. 9)

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5. Roughing Filter Application

5.1 Historic Use

The natural Water treatment potential was adopted long before chemical watertreatment methods such as chlorination and flocculation were discovered and utilized. Gravel andsand used as filter media are key components in natural treatment processes. Althoughsand was able to maintain its important role since the development of the first slow sand filters at thebeginning of lastcentury, the use of roughing filters was successively replaced by chemical watertreatment processes. A comprehensive review of gravel filter application is far beyond the scope of thismanual. However, a few examples presented hereafter will document that the roughingfilter technology is an old Water treatment process used in the past and rediscovered in recentyears. e

Numerous castles and forts were constructed in Europe during the Middle Ages. They were oftenlocated at strategic important points, difficult to conquer and also to supply with water. Ingenious watersupply installations were, therefore, constructed; a good example is the former castle ofHohentrins located on top of a steep rocky reef in the Swiss Alpine valley of the river Rhine. Thepeople, who sought protection in this castleduring periods of war, depended on rainwater collected inthe yard and stored in a cistern. However, water pollution caused by man and animal in this extensivelyused area could not be avoided. Therefore, in order to treat the water, a gravel pack was installedaround the inlet of the cistern. This is probably one of the first roughing filters used to treatsurface water (Ref. lO). '

ln 1804, John Gibb constructed the first water filtration plant for a public water supply atPaisley in Scotland. In order to pretreat the muddy river water, John Gibb designed and constructedan intake filter described as follows:

"Water from the River Cart flowed to a pump well through a roughing filter about 75feet long, composed of "chipped" freestone, of smaller size near the well than at theupper end. This stone was placed in a trench about 8 feet wide and 4 feet deep,covered with "Russian matts” over which the ground was levelled." (cited from Ref.9, page 78)) -

The pretreated raw water was then lifted by a steam engine-driven pump to a place l6 feet higher thanthe river from where it flowed by gravity to the water treatment plant. This installation consisted of 3concentric rings each 6 feet wide and arranged around a central clear water tank measuring 23.5 feet in

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diameter. The water flowed in horizontal direction from the outer ting, which was used as settling basin,through the two other rings towards the centre into the clear water tank. The two inner rings containedcoarse gravel, very fine gravel or sand as filter material respectively. John Gibb applied alreadythen the multi-stage treatment approach; the intake filter, the settling basin and thegravel filter were used as pretreatment processes prior to sand filtration. Many otherwater treatment plants in England followed the example of Paisly and applied coarse gravel and slowsandfiltration. Last century, Great Britain's general water treatment practice was the use of multiplefiltration in form of roughing filters placed in front of slow sand filters. Only since 1925 were rapid sandfilters slowly introduced to increase the capacity of the slow sand filters.

Another example of roughing filter application are the Puech-Chabal filters initially constructedin France in 1899 to treat part of the water supplied to the city of Paris. The treatment schemeconsisted in the use of a series of filters and cascades to treat turbid surface water. The water flowedthrough four downflow roughing filters and one so-called prefilter before beingtreated by a finishing filter. Cascades were used to aerate the water in between the different filterstages. The filter material decreased successively in size, and the filtration rate was also reduced fromfilter to filter. The Puech-Chabal treatment scheme was largely applied in Europe. By I935, I25 of suchplants were built in France, nearly 20 in Italy and some in other countries (Ref. 10).

As time passed, the roughing filters were virtually converted into rapid or mechanicalfilters. Coagulation in combination with sedimentation was introduced as pretreatment method and.more recently, direct filtration replaced the prefilter technology. In recent years. however. theroughing filter technology has been revived in Europe through its use in artificialgroundwater recharging plants. In the early 60s, the waterworks of Dortmund, German).constructed horizontal-flow roughing filters of 50-70 m filter length which are operated at approx. I0m/h filtration rate (Ref. ll). The raw water falls over an aeration cascade, crosses a sedimentationtrough before entering into the roughing filter on the top of the gravel bed. The filter inlet zone isprogressively impounded with increasing running time and the entrance area of the water thus slowlyshifts in direction of the filter outlet. After prefiltration, the water falls over a second cascade, percolatesthrough the sand filter bed and finally reaches the aquifer. Other waterworks in Europe (e.g. inSwitzerland and Austria) followed the example of Dortmund with modified horizontal-flow roughingfilter designs.

European rivers are usually of low turbidity and filter operation is stopped during the shortperiods of high turbidity. The continuous supply of water to the consumers is guaranteed by the use oithe aquifer's water storage capacity. In tropical countries, roughing filters usually have tocope with raw water of permanent or seasonally high turbidity. In the absence of anaquifer. the water supply installations have to maintain operation throughout the year. A reliable

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operation is especially required during the rainy season when the risk of epidemic outbreaks ofdiarrhoeal diseases increases as a result of washed off faeces not properly disposed. Due to the need ofreliable and simple water treatment processes, roughing gravel filtration has received considerableattention in recent years. Studies on the design and performance of prefilters functioningunder tropical water quality conditions were and are still conducted by variousresearch groups.

5.2 Recent Development of Horizontal-flow Roughing Filters

Motivated by the simplicity of horizontal-flow roughing filters, different institutionsembarked on laboratory and field studies in order to assess the potential of horizontal-flowroughing filters in reducing the solid matter concentration of highly turbid surface water. In I977, theAsian Institute of Technology (AIT) in Bangkok, Thailand, conducted laboratory tests with aprefilter composed of7 gavel layers (Ref. I2) and later, 3 full-scale water treatment plants applying theAIT prefilter design were constructed in combination with slow sand filter units. The treatment plants.monitored for approx. half a year, revealed a good performance of the prefilters and enabled slow sandfilter runs of several months (Ref. 13). These investigations were, however, not continued and thereforemarked the end of the project in Thailand.

The University of Dar es Salaam, Tanzania, embarked on laboratory filtration tests in 1980.Initially, investigations were carried out with vertical-flow roughing filters, revealing short filter runs oia few days only. Subsequently, the horizontal-flow roughing filter concept was developed and thedesign tested with a 15-m long open channel filled with 3 gravel fractions ranging between I6-32, 8-I6and 4-8 mm in size. The laboratory tests clearly indicated that significant solid removal is achieved onlyunder laminar flow conditions as sedimentation is the predominant process in roughing filters (Ref. I4).Field tests were then conducted to assess the applicability of the horizontal~flow roughing and slow sandfilter treatment combination. The pilot plant investigations compared the filter resistance development oidifferent slow sand filters fed either with untreated or with prefiltered turbid river water. Remarkableincrease of the slow sand filter runs were achieved with prefiltration. The field tests revealed thathorizontal-flow roughing filtration combined with slow sand filtration could be a viable process schemefor turbid surface water treatment (Ref. 15).

In 1982, extensive filtration tests were conducted by the International Reference Centre forWaste Disposal (IRCWD) at the laboratories of the Swiss Federal Institute for WaterResources and Water Pollution Control (EAWAG) in Duebendorf, Swit'/.erland, over aperiod of 2 years. A model suspension of kaolin was used to investigate the mechanisms taking place inhorizontal-flow roughing filtration. Two important results of the laboratory tests established that lltc

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filter efficiency is hardly influenced by the surface properties of the filter medium and that filterregeneration can be enhanced by drainage. The results of the research are summarized in a scientificpaper (Ref. 3), and the more practical aspects on the implementation of horizontal-flow roughingfiltration are compiled in a design, construction and operation manual (Ref. 16).

With the financial support of the Swiss Development Cooperation (SDC) which alreadycofinanced IRCWD's laboratory tests, promotion and dissemination of the horizontal-flow roughing filter technology started in 1986. Under the technical assistance of IRCWD,engineers of local institutions designed full-scale demonstration plants in order to understand thetechnology and gain practical experience with the treatment process. Frequently, horizontal-flo\vroughing filters were constructed for the rehabilitation of deficient slow sand filter plants. In the past 4years, the promoted filter technology has spread to more than 20 countries and,according to IRCWD's knowledge, over 60 horizontal roughing filter plants have beenconstructed duringthis period (Ref. 17). The following chapters of this publication present thebasic information on horizontal-flow roughing filtration as well as new approaches and designsdeveloped by local engineers and practical field experience with the filter technology.

Furthermore, several institutions carried out additional studies on the horizontal-flow roughing filterprocess usually in the form of M.Sc. research work (Refs. 18, I9, 20, 21, 22). The TampereUniversity of Technology in Finland, the University of Surrey in Guildford, England,the International Institute for Hydraulic and Environmental Engineering in Delft, andthe Delft University of Technology in the Netherlands as well as the University olNewcastle upon Tyne in England conducted, among other institutions, laboratory or field testswith horizontal-flow roughing filters. Different pretreatment methods, including horizontal-flowroughing filtration, are currently field tested on a comparative basis by an extensive researchprogramme in Cali, Colombia, where the Centro Inter-Regional de Abastecimiento yRemocion de Agua (CINARA) investigates, in collaboration with the InternationalWater and Sanitation Centre in The Hague, The Netherlands, and different otherinternational technical institutions and supporting agencies, possibilities to optimize and simplify thepretreatment processes under investigation (Ref. 23).

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I was actuaffy surprised to [earn ahout your work on[y'5 years after you started yourresearch. IRCI/Vi) has heen trying to enhance information eegchange in most of its wor/flingfiefds - one ofit heing roughing fiftration. Quite a [ot of information on this topic wasafso sent to the Qep. ofCivi[ ingineering ofyour ‘University ....... ..

I woufd have appreciated receiving your information regarding your topic, progress andrestdts of your research over the [ast ffyears instead of the ahove far with the "jiua/I ca/.'/Hto finance your travef expenses. I heiieve in reciprocat information erchange!

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Main Features of a Horizontal-flow Roughing Filter (HRF)

Part 2 will describe the design, construction and operation of the horizontal-flow roughingfilter. It provides technical information on the filter layout, discusses operational andeconomic aspects and, discloses, along with the annexes, valuable information on thepractical experience with this filtration technology.

Part 2 is available as draft and has purposely not been published in order to includeadditional information presented at the workshopon Roughing Filtration. The full manualentitled "Surface Water Treatment by Roughing Filters" will appear shortly after internaland external review.

References

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Lloyd B., Pardon M., Wheeler D. The performance of slow sand filters in Peru, inSlow Sand Filtration - Recent developments in water treatment technology,Graham N.J.D. (editor), Ellis Horwood Series in Water and WastewaterTechnology, pp393-411, 1988Weehler D. The microbiological benefits of gravel prefiltration, Developing WorldWater, Grossvenor Press International, 1988Wegelin M., Boller M., Schertenleib R. Particle removal by horizontal-flowroughing filtration, Journal of Water Supply Research and Technology - AquaVol. 36, pp. 80-90, 1987Galvis G., Fernandez J. Manual de disefio, operacion y mantenimiento de filtrosgruesos dinamicos, CINARA, Octubre 1991Barrot L.P., Llloyd B.J., Graham N.J.D. Comperative evaluation of two noveldisinfection methods for small-community water treatment in developing countries,Journal of Water Supply Research and Technology - Aqua Vol. 39, pp. 396-404,1990Visscher J.T., Paramasivam R., Raman A., Heijnen H.A. Slow ‘sand filtration forcommunity water supply, IRC, Technical Paper No. 24, June 1987Hendricks D. (editor) Manual of design for slow sand filtration, AWWA ResearchFoundation, 1991 'Logsdon G. S. (editor) Slow sand filtration, ASCE Report, 1991El Basit S., Brown D. Slow. sand filter for the Blue Nile Health Project,Waterlines Vol. 5, pp 29-31, July 1986Baker M.N., The quest for pure water, AWWA, Vol 1, 1981Kuntschik O., Optimization of surface water treatment by a special filter technique,AWWA Journal, Vol. 68, pp 546-551, 1976Thanh N.C., Horizontal-flow coarse-material prefiltration, AIT, Research ReportNo. 70, 1977Monitoring and evaluation of village demonstration plants, Technical Report,Project Managing Committee and AIT, Octobre 1981Wegelin M., Mbwette T.S.A. Slow sand filter research report No. 2, Universityof Dar es Salaam, 1980 _Wegelin M., Mbwette T.S.A. Slow sand filter research report No. 3, Universityof Dar es Salaam, 1982Wegelin M., Horizontal-flow roughing filtration: A design, construction andoperation Manual, IRCWD Report No. 06/86, 1986 _Wegelin M., Schertenleib R., Boller M. The decade of-roughing filters -Development of a rural water treatment process for developing countries, Journalof Water Supply Research and Technology - Aqua Vol. 40, pp 304-316, 1991Riti M.M., Horizontal roughing filter in pretreatment of slow sand filters, Thesis,Tampere University of Technology, 1981Tilahun G.T. Direct filtration with horizontal roughing filter as pretreatment,Thesis, Tampere University of Technology, 1984Symonds Ch.N. Aspects of prefiltration concerned with the application of smallscale slow sand filtration in rural communities, University of Surrey, 1985Siripatrachai T. Physical and mathematical analysis of the performance ofhorizontal roughing filtration, International Insitute for Hydraulic andEnvironmental Engineering, Delft, 1987Brown D. Horizontal-flow roughing filtration as an appropriate pretreatmentbefore slow sand filtrattion in developing countries, Thesis, University ofNewcastle, 1988Proyecto integrado de investigacion y demostracion de metodos de pretratamientopara sistemas de abstecimiento de agua, lnforme Resumen, CINARA, Diciembre1991

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