J Water SRT—Aqua Vol. 40, No. 5, pp. 304-316,1991

I

The decade of roughing filters — development of a ruralwater-treatment process for developing countriesM. Wege l in , R. Schertenleib a n d M . Boi ler , International Reference Centre for Waste Disposal (IRCW'D) and Swiss FederalInstitute for Water Resources and Water Pollution Control (EA WAG), Ueberlandstrasse 133 CH-8600 Dübendorf, Switzerland

ABSTRACT: Slow sand filtration applied as surface water treatment is particularly effective in im-proving the bacteriological water quality. However, efficient application of the treatment process re-quires water of low turbidity, hence pre-treatment of the surface water is usually necessary. Chemicalflocculation combined with sedimentation is often inapplicable, because rural water supplies in de-veloping countries generally face serious operational problems with chemical water treatment. Pre-filtration is a simple and efficient alternative treatment process. This paper presents the concept, fieldexperience and promotion of horizontal-flow roughing filters as a viable pre-treatment process.Roughing filters combined with slow sand filters present a reliable and sustainable treatment process,particularly appropriate for developing countries.

."F

La décade des préfiltres—développement d'un système de traitementd'eau pour les zones rurales du tiers mondeRESUME: L'emploi de la filtration lente sur sable comme traitement des eaux de surface est par-ticulièrement approprié pour améliorer la qualité bactériologique des eaux. Cependant, une applica-tion efficace du procédé exige une eau de basse turbidité et, de ce fait, un prétraitement de l'eau desurface. La floculation chimique en combinaison avec la sédimentation est souvent inapplicable vuqu'un traitement chimique pose en général de sérieux problèmes aux systèmes ruraux d'alimentationen eau dans les pays en voie de développement. La préfiltration est un procédé de traitement alter-natif simple et efficace. Ce document présente la conception du procédé, les expériences sur place etla promotion des préfiltres à flux horizontal comme procédé viable de prétraitement. La préfiltrationà flux horizontal en combinaison avec la filtration lente sur sable est un procédé sûr et éprouvé, ainsique particulièrement approprié dans les zones rurales de pays en voie de développement.

I N T R O D U C T I O N

In developing countries, a large number of people areforced to use surface water, water drawn from pollutedrivers, irrigation canals, ponds, and lakes. This surfacewater, however, is a carrier of many infectious and tropicaldiseases and therefore ought usually to be treated priorto consumption. The main target of any water treatment isthe removal or inactivation of disease-causing organisms(pathogens), such as harmful bacteria, viruses, protozoalcysts, and worm eggs. Disinfection — usually by applica-tion of chlorine - and slow sand filters are the two mostwidely used treatment processes for bacteriological water-quality improvement.

Slow sand filtration (SSF) is regarded as a particularly ap-propriate technology [1] for developing countries. The SSFtechnology is described in many publications; the most re-cent for this type of application is presented in IRCs Tech-nical Paper No. 24 [2]. As surface water usually requires tobe pre-treated prior to SSF application, roughing filters havereceived considerable attention in the past decade. Studies

304

on the design and performance of prefilters to cope withhighly turbid surface water were conducted at different in-stitutes. After a brief review of prevailing technical problemsin rural water treatment, the development and world-widepromotion of the horizontal-flow roughing filter (HRF) tech-nology will be presented hereafter. Figure 1 gives an overviewof IRCWD's HRF project activities during the last decade.

THE CRUX IN R U R A L W A T E R T R E A T M E N T

Disinfection efficiency and SSF performance are stronglyinfluenced by the turbidity of the water to be treated. Tur-bidity mainly reflects the amount of solids and colloids pre-sent in the water. A large number of the micro-organismsare attached to the surface of these solids, which act as ashelter. Hence, the chemical disinfection will be impairedin turbid water. Organic solid or dissolved matter using thedisinfectant for oxidation processes will further reduce theefficiency of chemical disinfection. Consequently,adequate water disinfection is only possible with water of

The decade of roughing filters 305

phase

activities

problemidentification

evaluationsfield missions

HRF Project

periodic sequence

local sequence

preliminaryinvestigations

literature searchlaboratory tests

fieldtests

aclive project sequence

1979 • 1980

East Africa

1980-1984

UDSMEAWAG

1981-1990

promotiondissemination

pilot plant demonstrationtests projects

implementation

consultancy

post project phase

1986-1990

initially Tanzania, Africalater on among Asiaothers Sudan,

,Ghana, Colombia

Latin America

1990-

in developingcountries

Fig. 1. HRF project phases.

low turbidity or virtually free of solid and organic matter.SSF also requires a relatively clear water. Reasonable

filter operation can be expected with raw-water turbiditiesbelow 20-30 NTU. The solid matter of water with higherturbidities will rapidly clog the top of the sand bed andinterfere with the biological processes located in this part ofthe filter. The filter resistance will increase drastically, cal-ling for filter cleanings at short intervals of a few days orweeks. Such an operation will considerably impair thebiological filter activities and greatly reduce the bac-teriological quality of the treated water. Therefore, onlyraw water of low turbidity will enable SSF to produce agood-quality water quality that is safe for consumption.

Pre-treatment of surface water for the reduction of tur-bidity or solid-matter concentration is therefore required forboth discussed treatment processes, i.e. chlorination andSSF. Sedimentation, possibly preceded by chemical floccu-lation, is applied as a solid removal process. Plain sedimen-tation will remove the settleable and part of the suspendedsolids. Particles smaller than 20 u,m will hardly be sepa-rated, although this fraction might represent the bulk of sol-ids in a surface water. In such situations, sedimentation isenhanced by the addition of chemicals that destabilize thesuspension and induce flocculation of the solids.

A reliable use of chemicals in rural water supplies in de-veloping countries is, however, extremely difficult andoften bound to fail. The chemicals must frequently be im-ported and therefore require foreign currency often scarcein developing countries. Apart from the availability ofchemicals, transportation poses another problem, gener-ally pertinent to developing countries. The adequate supplyof chemicals to remote rural treatment plants is oftendifficult and unreliable. The dosage of chemicals dependson the raw-water quality and therefore needs careful super-vision. The sensible dosing equipment is exposed to the ag-gressivity of the chemicals which can attack and damage theinstallations. Finally, chemical water treatment requires

skilled personnel, often not available in rural areas. Thesefactors question a reliable and successful application ofchemicals in rural water supply schemes of developingcountries. Numerous malfunctioning or abandoned watertreatment plants supposed to use chemicals endorse thisstatement.

Consequently, chemical flocculation, a process reactingsensitively to water-quality changes and therefore difficultto operate, generally consitute an inappropriate technologyfor the considered situation. Similarly, a safe and reliablechlorination of the water remains a target difficult to attainunder rural conditions in developing countries.

In contrast to chemical water treatment, SSF offers thegreat advantage of being safe and stable, simple and reli-able. Operation is easily handled by local staff at villagelevel and does not depend on the external inputs. SSF is es-sentially a self-reliant technology largely reproducible withlocal resources. However, even SSF requires efficient andreliable pre-treatment processes for turbid surface water.Pre-filtration can close this gap.

THE MULTIPLE FILTER A P P R O A C H

Filtration is a more effective process for solid removal dueto the large filter surface area available for sedimentation,adsorption and chemical and biological activities. Soundwater treatment concepts start with the separation of coarsematter, usually easy to achieve, and end with the removalor inactivation of small solids and micro-organisms gener-ally more difficult to separate. Figure 2 shows an approximaterelationship between particle size and solids classification.It also illustrates the main application areas for differenttreatment processes. Water containing impurities of dif-ferent sizes has to undergo a step-by-step treatment. In afirst step, coarse matter, possibly present in surface water,is removed by screens, and settleable solids are retained bysedimentation tanks. In a second stage, pre-filters, known

306 M. Wegelin, R. Schertenleib and M,

10"'° 10~a 10"a 10~7 1O'S 10"5 10"4

I I I I I I

1Ä 1 pm

molecules colloids

viruses

suspended solids

| algae |

bacteria

Boiler

10"3

1 mm

i . sedimentation

bioloQical oxid ition

filtration ' ""

(organic)

• chemical oxidation and precipitation

(inorganic)aas transfer

10"2

1 cm

10"1 11 1

(leaves, wood etc.)

screening• *

Solid SizeIn m

Panicle

TreatmentProcesses

Fig. 2. Particle size and treatment process.

as roughing filters, separate the fine and light particles clas-sified as suspended solids.

Roughing filters usually consist of filter materials thatsuccessively decrease in size. The bulk of solids is separatedby the coarse filter medium, while the subsequent mediumand fine filter media have more of a polishing function.Roughing filters are able to reduce turbidity to a level thatallows a sound and efficient SSF operation. SSF as the finaltreatment stage will substantially reduce the number of themicro-organisms present in the water, retain the last tracesof solid matter and oxidize organic compounds dissolved inthe water.

In other words, roughing filters mainly act as physicalfilters and are applied to retain the solid matter. SSF arebiological filters used to improve, in particular, the bac-teriological water quality. Both filter types are of an equaltechnical level and their operation is characterized by a highprocess stability. They make full use of natural purificationprocesses, and do not require chemicals to support or sup-plement the treatment scheme. Therefore, roughing filtra-tion combined with SSF presents an appropriate processscheme for rural water treatment in developing countries.

R O U G H I N G FILTERS

The filter medium of roughing filters is composed of rela-tively coarse (rough) material ranging from 25 mm to 4 mmin size, installed in layers of different fractions. Gravel isusually used as filter material. The design and mode ofapplication of roughing filters vary considerably. The dif-ferent filter types can be classified according to their loca-tion within the water supply scheme, and with respect to theflow direction. There is therefore a distinction between in-take and dynamic filters, which form part of the water in-take structure, and the actural roughing filters, which areintegrated in the water-treatment plant. Roughing filtersare further subdivided into down-, up- and horizontal flow

filters. Figure 3 illustrates the layout of intake and dynamicfilters, whereas Fig. 4 presents the main features of thethree roughing filter types.

Intake and dynamc filters are installed next to small andnarrow river beds. Part of the river water, impounded by asmall weir, is diverted either into a filter box or into a smallcanal where a subsurface filter is installed. The river wateris filtered through different gravel layers, the top layerbeing of finest size. These two filters therefore act mainly assurface filters.

Intake and dynamic filters are not only designed for con-tinuous solids separation, but also to protect the treatmentplants from shock loads of high solid concentration. Suchpeaks rapidly clog the filter and thereby interrupt the flowof highly turbid water to the treatment plant, especiallywhen high filtration rates of approximately 10 m/h areapplied to a top layer of relatively fine gravel of about 1 -2mm in size. Intake and dynamic filters are cleaned manuallyby scouring, normally every week, the top of the filter bedwith a shovel or a rake. The accumulated solids are re-suspended and washed back to the river. In order tosafeguard the SSF from a sudden silt load, these two filtersshould preferably be used with relatively clear rivers react-ing with short turbidity peaks during rainfall. Alternatively,they might be used as a first pre-treatment step in combina-tion with roughing filters to reduce the solid load of highlyturbid water.

Roughing filters are designed to treat surface water ofhigh turbidity over prolonged periods. The water is filteredby a sequence of normally three filter fractions. Due to thedeep penetration of the solids into the filter medium,roughing filters act, in contrast to intake and dynamicfilters, as space filters and therefore possess a large silt stor-age capacity. In vertical-flow roughing filters, the availableheight of the filter medium is limited to about 1.0-1.5 m dueto structural constraints. Hence, the total length of a three-stage vertical-flow filter will amount to a maximum of 4.5 m.

The decade of roughing filters 307

Intake FilterPlan

river

intake filter

Section A - A

river

\

Plan

canal

Dynamic FilterSection A - A

canal

POO

=-*O

intake weir

Fig. 3. Layout of intake and dynamic filters.

Alternatively, the filter length of horizontal-flow rough-ing filters is theoretically unlimited, but varies in generalbetween 5 and 9 m. Roughing filters are operated at filtra-

Roughing Filters

Down-flow All1 dm

Up - flow

Horizontal - flow teSí£££&&&§l

¥ ïFig. 4. Layout of roughing filters.

tion rates ranging from 0.3 to 1.5 m/h. Up- and down-flowroughing filters can cope with raw-water turbidities of 50-150 NTU, whereas horizontal-flow roughing filters canhandle even short turbidity peaks of 500-1000 NTU on ac-count of their comparatively long filter length. The filtersare cleaned periodically by a fast filter drainage and, ifnecessary, manually by removing, washing and reinstallingthe filter material.

Recently, another pre-filter type called pebble matrixfilter has been studied in the laboratory. The down-flowfilter consists of a large (approximately 50 mm in size) peb-ble layer filled half-way with sand (specific diameter 0.4-1.0 mm). Kaolin clay suspensions of up to 5000 mg/1 werereduced by the filter to below 25 mg/1. The laboratory-scalefilter achieved filter runs of up to 116 h, with head-losses notexceeding 1.5m. The filter column was cleaned by drainageand backwash process. It is planned to field test this filtra-tion technology in different developing countries in orderto assess its suitability as a pre-treatment method for highlyturbid surface water [3].

An overview of the pre-treatment methods and currentexperience is presented in an IRC publication [4], Furtherdesign details and guidelines on roughing filters are alsosummarized in a training document [5], published by theWorld Bank. The different pre-treatment methods arecurrently field tested in parallel in Cali, Colombia, whereCINARA (Centro Inter-Regional de Abastecimiento y

308 M. Wegelin, R. Schertenleib and M. Boiler

Remoción de Agua) investigates, in collaboration with IRC(International Water and Sanitation Centre, The Hague)and other international technical institutions and support-ing agencies, possibilities to optimize and simplify the pre-treatment processes [6].

THE H R F T E C H N O L O G Y

In the past decade, horizontal-flow roughing filtration(HRF) has received greater attention than any other pre-filtration technique. The comprehensive development ofthe HRF technology through research, laboratory and fieldtests, and successful application of the treatment processhas also enhanced the development of other types of rough-ing filters. IRCWD has been strongly involved in the de-velopment and promotion of the HRF technology since1982 (see also Fig. 1). This long-term engagement enableda world-wide dissemination of the treatment process. Theconceptional layout, development, field experience, pro-cess limitation, and promotion of the HRF technology ispresented hereafter.

HRF CONCEPT AND DESIGN

Simplicity of the installation, reliability of the treatment aswell as sustainability in operation and maintenance arebasic requirements for rural water-treatment schemes ingeneral, and in particular also for new pre-treatment pro-cesses. A horizontal-flow rectangular sedimentation tankmeets these critieria. However, its efficiency in solid matterremoval is limited to easily settleable solids. Particles to beseparated have to reach, within a certain period, (e.g. thedetention time), the bottom of the sedimentation tank lo-cated at a depth of 1.50 m or more. Hence, the settlingproperties of the particles greatly determine the solid-re-moval efficiency of the tank. This efficiency increasesgreatly when the same tank is filled with filter material. Thesettling distance is now reduced from 1.50 m to a few mm.The turbid water flows through the pore system of the filterand is in extensive contact with the filter medium during its

inlet channelwith weir

outlet chamberwith drain

filter drains forhydraulic cleaning

inletchamber

Fig. 5. Main features of a horizontal-flow roughing filter (HRF).

flow. In the small pore system, the suspended solids settleon the upper surface of the filter material.

Apart from improved solid-removal efficiency, opera-tional aspects were also decisive in the design of the newpre-treatment process. Long filter runs of 2-3 months ormore, similar to those required for sound SSF operation,have to be attained by the filter even during high-turbidityperiods. The provision of a large silt-storage capacity istherefore required. A small filter resistance is an additionaloperational criteria to avoid pumping whenever possible.

All these requirements are met by HRF's, the main fea-tures of which are illustrated in Fig. 5. HRFs have thesimplest layout of all roughing filters. The raw water runsfrom the inlet compartment in horizontal direction througha series of differently graded filter material separated byperforated walls. The water level in the filter is controlledby an outlet weir or pipe, and kept beneath the filter surfaceto prevent the growth of algae. Type and amount of solidspresent in the raw water determine the filtration rate, thelength and the media size of the different filter compart-ments. Filtration rates in the order of 0.5-1.5 m/h appliedto coarse and graded filter material of 4-20 mm in size and5-9 m total length, keep the total filter resistance small, i.e.normally below 30 cm.

The shallow filter box eases the construction of HRFs,which do not require mechanical equipment for their oper-ation. The high process stability of the filter prevents oper-ational breakdowns. HRF operational and maintenance ispossible at village level — an important criterion for thesustainability of the technology.

H R F D E V E L O P M E N T

The combination of HRF with SSF is in fact not a new treat-ment concept. The first known horizontal-flow gravel filterused in a public water supply was constructed by John Gibbat Paisley, Scotland in 1804 [7]. The treatment plant con-sisted of three concentric rings arranged around a centralclear water tank. The water flew from the outer ring, usedas a settling basing, in a horizontal direction through theinner rings filled with gravel and sand, respectively, and wasfinally collected by the central clear water tank. This was anapplication of gravel filters prior to sand filters. Many otherfilter installations constructed in Great Britain in the lastcentury followed this example. Down-flow roughing filterswere used as pre-treatment in Europe. The Puech-Chabalfilter was, until the first decades of this century, one of thebest-known and extensively applied filters in France [7].

As time passed, the roughing filters were converted virtu-ally into rapid or mechanical filters. Coagulation in con-junction with sedimentation and, more recently, with directfiltration replaced the pre-filter technology. Thus, the re-spective knowledge and experience with this treatment pro-cess gradually vanished and had to be rediscovered. In the

The decade of roughing filters 309

Table 1. Exampes of roughing filters in artificial groundwater recharge plants

Suspended solids(mg/1) Filter

— length FiltrationPlant

DortniundAeschGraz

Country

W. GermanySwitzerland

Austria

River

RuhrBirs

Andritzbach

Mean

875

Max.

204020

(m)

50-701510

rate (m/h)

105-84-14

early 1960s, the waterworks of Dortmund, Germany, con-structed horizontal-flow roughing filters for an artificialgroundwater recharge plant [8]. Other water works inEurope (e.g. in Switzerland and Austria) followed theexample of Dortmund with modified designs. Salient dataof such plants are given in Table 1.

European rivers usually have a low turbidity and filteroperation is stopped during the short periods of high turbid-ity, i.e. during heavy rainfall. The continuous supply ofwater to the consumer is guaranteed by the use of theaquifer's storage capacity.

Permanent or seasonally high turbidity is a characteristicof tropical rivers. In the absence of an aquifer, surface-water treatment plants have to maintain operation through-out the year. A reliable water supply is required especiallyduring the rainy season when the risk of epidemic out-breaks of diarrhoeal diseases increases as a result ofwashed-off faeces not properly disposed.

Motivated by the simplicity of the HRF technology,different institutions embarked on laboratory and fieldstudies in order to assess the potential of HRFs to reducehighly turbid surface water to a turbidity level required bySSF. This marked the beginning of HRF introduction todeveloping countries.

L A B O R A T O R Y TESTS

Studies on the design and performance of HRF for thetreatment of highly turbid and silted surface water wereconducted at the Asian Institute of Technology (AIT),Bangkok, Thailand, at the University of Dar es Salaam(UDSM), Tanzania and by the International ReferenceCentre for Waste Disposal (IRCWD) at the Swiss FederalInstitute for Water Resources and Water Pollution Control(EAWAG), Duebendorf, Switzerland.

In 1977, AIT carried out respective laboratory tests on a5-m-long pre-filter composed of seven gravel layers. Thedifferent gravel fractions, which ranged in size from 5 to 18mm, were installed with the finest filter medium at thecentre of the filter. In spite of the good results reported byAIT [9], no reasonable arguments justify the installation ofthe smallest gravel fraction in the centre of the filter.

Nevertheless, three water-treatment plants applying theAIT pre-filter design were constructed in combination withSSF units. The treatment plants were monitored for ap-proximately half a year. The recorded data [10] reveal agood performance of the pre-filters. This enabled SSF filterruns of several months. These investigations were, how-ever, not continued and therefore marked the end of theproject in Thailand.

After evaluating the operational problems with SSFplants in Tanzania, the UDSM embarked on filtration testsin 1980. Laboratory tests were first conducted with vertical-flow roughing filters, revealing short filter runs of a few daysonly. Subsequently, the HRF design was developed as pre-sented in Fig. 5. The concept was tested in the laboratorywith a 15-m-long open channel filled with three gravel frac-tions of 16-32, 8-16, 4-8 mm in size at different filtration

REYNOLDSNUMBER Re

200 -

60403020

%¡î2

\

: \

IÍM

i.i.i

11

1

XV

\

k

turbulentflow(/?e>600)

transitionzone

laminarflow

20 40 60 80 100 . |o,TURBIDITY REMOVAL c°/ce >'°

filter materialbroken coral limestonegravel size d% :

• 4 - 8 mmx 8 -16 mm° 16-32 mma 32-64mm

vertical filter columnlength : 100 cmp : 20 cm

filtration rates :Vf = 0 . 5 - 8 m/h

kinematic viscosity v :0.9*10~6m2/s for 25 °C

Fig. 6. Influence of flow conditions on filter efficiency.

310 M. Wegelin, R. Schertenleib and M. Boiler

rates ranging between 0.5 and 8 m/h. The laboratory testsclearly indicated the sensitivity of the filter performancewith respect to flow conditions in the filter as illustrated inFig. 6. Significant solids removal efficiency is only achievedunder laminar flow conditions since sedimentation is thepredominant process in roughing filtration.

Field tests were carried out to assess the practical applica-bility of the HRF—SSF treatment combination. Figure 7summarizes the filter resistance development — an opera-tional key parameter—for different SSF fed with either un-treated river water or with water pretreated by HRF. Theinvestigations indicated that HRF combined with SSFcould be a viable process scheme for turbid surface-watertreatment [11].

20 50 70DAYS

Key: raw water untreated— pretreated with HRF

© Handeni© Wanging'ombe, SSF models

( D Wanging'ombe , big SSF

© Iringa , fine sand

© Irlnga , coarse sand

Fig. 7. Filter resistance development for different SSF inTanzania.

In 1982, extensive filtration tests were conducted byIRCWD at EAWAG's laboratory over a period of 2 years.A model suspension of kaolin was used to investigate themechanisms taking place in HRF and to elaborate designguidelines for HRF. A semi-empirical filtration model wasdeveloped and used as an efficient design tool for the evalu-ation of different filter layouts. The results of the researchare presented in a scientific paper [12], whilst the more-practical aspects of HRF implementation are compiled inthe design, construction and operation manual [13]. An im-portant result obtained by the laboratory tests reveals thatthe filter efficiency is hardly influenced by the surface prop-erties of the filter medium as shown in Fig. 8. Hence, anyinert, graded, insoluble and mechanically resistant material

1.2

1.0

ß 0.8

>oI 0.6

Q: 0.4UJ

• charcoalglass spheresgravel of quartzpumice

FILTER GRAIN SIZE dq 5,15-6mmVf = 1 m / h

10' u IO1

FILTER LOAD a [ g / i ]

Fig. 8. Filter efficiency in correlation to filter load for differentfilter material.

can be used as filter medium. This aspect could represent anessential economic factor in the implementation of HRF.Another important phenomenon observed during the filt-ration tests was the possibility of filter regeneration bydrainage. At a drainage rate of 60-90 m/h, the solid matteraccumulated in the filter is flushed to the filter bottom andout of the filter if adequate drainage facilities are provided.Filter efficiency and filter resistance are thereby restored,

FIELD E X P E R I E N C E

The HRF design guidelines derived from the laboratorytests, conducted under strictly controlled conditions, had tobe tested under field conditions. The HRF technology wasexposed to different types of raw water, to inadequate filteroperation, and to different socio-cultural behaviours dur-ing the field test phase (see also Fig. 1).

Filter efficiencies had to be monitored on a regular basis.However, local water-quality laboratories were often notavailable so that IRCWD had to design some simple andsturdy field test methods. The equipment for turbidity,filtrability, settleable solids and suspension stability werepacked into a field test kit and provided to each of the pilotplants. The filtrability test replaces the analysis of sus-pended-solids concentration, for which a well-equippedlaboratory is required. A specific volume of water to beanalysed is filtered by gravity through a filter paper and thefiltered water volume recorded in function of time. Neitherchemicals nor energy are required for the different analyses.Only filter papers as consumables have to be supplied. Theapplied test methods are described in [13].

The decade of roughing filters 311

Fig. 9. Water treatment plant of the Blue Nile Health Projectshowing HRF, SSF, clear water tank and handpump.

Some practical experience gained from pilot plants arepresented hereafter.

Blue Nile Health Project, Sudan. The rural population ofthe Gezira/Managil irrigation draws its water from smallirrigation canals with a high bacteriological contamination.

HRF Filter Material:RehanaI

Turbidity

3 fractionsof gravel

Rehana II

3 fractionsof burnt bricks

Rehana III

1 fraction of coarseburnt bricks

2 fractions of mediumand fine gravel

jmt

100

10<5

Traw.watflr

-

W/

wyy//1HRF effluent

SSF effluent

1 _Í

•7-Ji 1

i//A T

wyyBÈÈkf• 111

•_dry season

(raw water turbidity lower than 500 TU)

Turbidityunit

1000

wet season(raw water turbidity higher than 500 TU)

Fig. 10. Turbidity reduction (mean ±SD) with different HRFfilter materials in the Blue Nile Health Project's treatmentplants.

Besides malaria, diarrhoeal diseases and bilharzia are pre-dominant in this region. The surface water supplied by theBlue Nile is further characterized by high turbidities reach-ing a thousand and more turbidity units during the rainyseason. Small water supply schemes, operated by gravityand consisting of an HRF and SSF unit, a clear water tankand a handpump, as illustrated in Fig. 9, were constructedto supply villages of 200-500 inhabitants. Part of the HRFfilter medium has been replaced by broken burnt brickssince gravel and stones are scarce in the project area locatedin an alluvial zone. Figure 10 summarizes the turbidity re-duction of three plants using different filter material for theHRF. The comparable efficiency is similar for all threeplants, thereby confirming IRCWD's respective laboratoryresults. Turbidity is reduced to approximately 5-10 NTUand substantial improvement of the bacteriological waterquality is recorded. In the raw water, E. coli counts of 200to several 1000/100 ml are reduced to 10-30/100 ml in thetreated water [14,15]. The treatment plants were not oper-ated at optimal conditions as the float valves for a smoothraw-water supply were purposely set out of operation bysome of the caretakers. In order not to lose control over thetreatment plant and consequently their social influence,they run the filters in batch charges by operating the inletvalves more or less at random. Nevertheless, 30 filter plantsare in operation now, and their performance is reported tobe good. The water-quality improvement achieved is a steptowards better health conditions for the farmers in these ir-rigation schemes.

Jinxing, China. In cooperation with the Zhejiang Healthand the Anti-Epidemic Station, two pilot plants were con-structed in the Zhejiang Province in 1989. Jinxing, which isone of them, treats water from a canal grossly contaminatedby small-scale industry and heavy navigation. The treat-ment plant of 240 m3/day capacity has two lines, each con-sisting of a small tilted plate settling tank, a HRF and anSSF unit as shown in Fig. 11. Turbidity of the canal water isrelatively low, ranging between 20 and 90 NTU. The HRFsoperated at a high filtration rate of 1.7 m/h still drasticallyreduce turbidity, which is further decreased by SSF asshown in Fig. 12. The two HRF units are cleaned by drain-age every 40 days. After 5 months of operation, head-loss inthe SSFs was recorded to be only 31 cm [16]. The remark-able operational experience with the first HRF-SSFschemes in China is now attracting the interest of the localauthorities.

Mafi Kumase, Ghana. SSF operation is not only impairedby high turbidities originating from inorganic solid matter,but also by high algae concentrations. The field tests of theMafi Kumase also demonstrate the potential of HRF foralgal removal. The population of this Ghanian village hassuffered under guinea worm and biiharzia. These diseasesoriginated from a shallow lake used as a water supply

312 M. Wegelin, R. Schertenleib and M. Boiler

•4V-

Fig. 11. Aerial view of Jinxing Water Treatment Plant showingthe two treatment lines (tilted plate settling tank, HRF and SSF)clear water tank and sand-washing facility.

© © © samplingpoints

operationalconditions:

HRF

SSF

filtration rate 1.7 m/h

cleaning intervalls 40 days

0.25 m/h

>4 months

%ofsamples100

90

80

70

60

50

40

30

20

10

5 10 20 30 40 50 70 80 90 100Turbidity

Fig. 12. Operational conditions and turbidity reduction ofJinxing Treatment Plant.

source. Under local expertise [17], the community con-structed, in a self-help project, an HRF-SSF treatmentplant. The lake virtually acts as an efficient sedimentationtank and also as a potential reactor for biomass production.In this case, the HRF technology is used to reduce the algaeconcentration. A grab sample illustrated in Fig. 13 indicatesthe efficiency of HRF and SSF for algae separation. Thealgae counts found in the SSF effluent are partly explainedby the organotrophic regrowth in the treated water.Nevertheless, practical experience shows that the SSF unitshave to be cleaned every 4-6 months only. The HRF-SSF

algae size

family

species

0.5 - 7 urn

Cyanophyceae

Microcystis

7 - 50 \m

Cryptophyceae

Cryptomonas

50-130 \m

Euglenophyoeae

Euglena cf. acus

Fig. 13. Algae removal by Mafi Kumase Treatment Plant(filtration rates: HRF, 1.5 m/h; SSF, 0.25 m/h).

process combination reveals its potential also for the treat-ment of algae loaded surface water.

La Javeriana, Colombia. The treatment plant of LaJaveriana in Cali, Colombia, is a good example of the multi-ple filter approach. The treatment process scheme consistsof an intake filter, two HRF and two SSF units. The respec-tive filtration rates amount to 4.7, 1.2 and 0.15 m/h. Theperformance of the treatment plant with a capacity of 260m3/day has been extensively monitored by CINARA [18].The variation of the different water-quality parameters andtheir respective reduction by the different treatment stagesis summarized in Table 2. Figure 14 illustrates the gradualreduction of turbidity, apparent colour and faecal coliformsthrough the different treatment stages. The gradual im-provement of these three parameters is an indication of thedevelopment of biochemical processes in the differentfilters. The applied natural purification processes improvethe bacteriological water quality by three or four orders ofmagnitude and also significantly reduce the intense colourof the raw water.

Aesch, Switzerland. In the early 1970s, nine HRF units andone SSF basin were designed for the artificial groundwaterrecharge plant in Aesch. The plant, with a minimum capac-ity of 17300 m3/day, started operation in 1976 and appa-rently worked without operational problems in the firstyears. However, the recharge wells started to silt up for dif-ferent reasons, thereby endangering the long-term use ofthe plant. Possibilities for the conversion of the exisiting 15-m-long and 16.8-m-large HRFs filled with uniform gravelranging in size between 50 and 80 mm and operated at filtra-tion rates of 5-8 m/h, had to be elaborated as part of the re-habilitation study. A pilot plant consisting of two HRFunits and three up-flow roughing filters, was installed forthis purpose. The field tests proved that the shorter 6.6-m-long test filters filled with three gravel fractions (see Fig. 5)had a greater suspended solids removal efficiency than the

The decade of roughing filters 313

Table 2. Water-quality improvement by the different treatment stages for thetreatment plant La Javeriana, Cali [18]. (Number of samples = 24-26; sus-pended solids concentration and apparent colour recorded with HACH test kitDREL 5)

Raw water Intake filter Effluent of HRF SSF

Turbidity (NTU)Suspended solids(mg/1)Apparent colour(CU)Faecal coliforms(/100 ml)

Turbidity (NTU)

22±17.249±41

132±113

4800±2400

12±9.527±19

79±53

2400±1600

Apparent Colour (CU)

4.4±3.011+7

34±20

295+300

2.6±1.97.5±4.9

22+17

3.9±6.6

Faecal Col

0 30 80 90 120 150 ISO

Running tino (dayj)

0 30 60 90 120 ISO 180

Running tin» (day«)

o ao so so 120 l í o leo

Running tin» (dayt)

Fig. 14. Reduction in turbidity, apparent colour and faecal coliforms by the different treatment stages at La Javeriana TreatmentPlant.

old HRFs (see Fig. 15). The new HRF layout would evenallow an increase in the plant's capacity by a factor of 2 (in-crease of the filtration rate from 2.3 to 4.6 m/h) without asignificant reduction in solids-removal efficiency. However,filter-cleaning aspects constitute another criterion for theselection of the pre-treatment system. Under the given situ-ation, up-flow roughing filters allowing a filtration rate of0.4 m/h to meet the minimum plant capacity will probablybe installed as they showed higher performance in solids re-moval and hydraulic filter cleaning. The experience ofAesch also exemplifies that industrial countries can benefitfrom the development of appropriate technologies fordeveloping countries.

LIMITS OF THE H R F T E C H N O L O G Y

'Filtration is more an art than a science' is a saying whichimplies that the implementation of this technology is limit-less. On the contrary, the HRF technology also has limita-tions. First of all, HRF is predominantly designed for ruralwater supplies. The treatment process requires relativelylarge installations and can therefore hardly be applied inlarge urban water-supply schemes.

HRFs operate at relatively small filtration rates askingfor large installations and consequently higher specific con-struction costs per m3 design capacity. The initial costs willbe higher than those required for the construction of floccu-

314 M. Wegelin, R. Schertenleib and M. Boiler

Old Filters

Pilot Plant:

Filter 1Filter 5

— Si 14.3

#

mil! flow direction:

uP- f l o w

uP- f l o w

"p-flowhorizontalhorizontal

io 30 100Susp. Solids Cone, (mg/l) in Raw Water

Fig. 15. Reduction of suspended solids by roughing filters inAesch.

lation/sedimentation tanks. However, the operation andmaintenance costs will be lower for the HRF option, whichis an important factor with respect to the economical sus-tainability. Construction costs can be saved by minimizing

the filter length, which, however, calls for hydraulic filtercleaning at shorter intervals.

Filter regeneration by fast drainage is not yet fullyexplored. Depending on the efficiency of hydraulic filtercleaning, manual cleaning by removing the filter material,washing it and then refilling it into the filter box must there-fore be foreseen after several years of filter operation. Sub-stantial volumes of settleable solid matter should be sepa-rated by sedimentation tanks or intake and dynamic filtersprior to the application of HRF. Cleaning of these installa-tions is usually easier than the removal of large amounts ofsolids from HRFs. Furthermore, solids separation byroughing filters might be hindered by high suspension sta-bility likely in the presence of humic acids. In such a situa-tion, the treatment efficiency should be assessed by pilot-plant tests.

HRF P RO M O T I O N

In 1986, the IRCWD-managed HRF project had entered inits last phase, which involved the promotion and dissemina-tion of the HRF technology (sec also Fig. 1). New technicalapproaches are often difficult for rural inhabitants to under-stand and their advantages difficult to believe. Demonstra-tion projects on a 1 to I scale helped to overcome thesedifficulties and to gain more practical experience. Local au-thorities and institutions willing to introduce the HRF and,in some cases, also the SSF technology were identified. SSFplants requiring rehabilitation by the addition of HRFs ornew treatment schemes were selected as demonstrationprojects. Under the technical assistance of IRCWD, localstaff designed the treatment plants which were usually con-structed as self-help projects by the communities. Themonitoring of the HRF/SSF plants was facilitated by the

China

JíSs»

• direct cooperation(technical + financial support)

X technical assistance(provision of information + advice)

Papua NewGuinea

Fig. 16 Geographical distribution of the HRF demonstration projects.

The decade of roughing fitters 315

101 sand = 301 safe water/d

River Intake GritChamber

(Sedim.)

Horizontal- flowRoughing Filter

(HRF)

T

Slow SandFilter (SSF)

ClearTank

Fig. 17. Scheme of a self-reliant watertreatment plant.

?

provision of a field test kit described under the heading'Field Experience'. The HRF technology has spread tomore than 20 countries (see Fig. 16) in the past 4 years.

According to IRCWD's information, over 60 HRF plantshave been constructed during this period. This not only en-abled the development of local HRF expertise, but positiveexperience with the treatment process convinced the coop-eration partners who now strongly identify with the HRFtechnology.

IRCWD also acted as an information pool. New ap-proaches and designs developed by the local engineers werecollected and the information passed on to other coopera-tion partners through an HRF Filter Newsletter. Several in-stitutions carried out studies on the HRF process in theform of M.Sc. theses [e.g. 19-23] and further consolidatedthis technology. Additional comprehensive field studies areunder way to compare the HRF technology with other pre-treatment processes [6]. All these activities contribute tokeeping the HRF technology in the technical press, a vitalmedium for the dissemination of information.

FINAL REMARKS

The provision of safe water remains a challenge, particu-larly in developing countries facing increasing economic,institutional and socio-cultural problems. Under such con-ditions, self-reliant, substainable and community-basedwater supplies will be less affected. Slow sand filtrationmeets these criteria and is therefore recognized as a particu-larly appropriate technology for developing countries. Inthe past, however, slow-sand-filter operation was oftenhampered by inadequately pre-treated turbid surfacewater. Over the last decade, roughing filtration was redis-covered and is now used for its simple and efficient process.

Horizontal-flow roughing filtration received special at-tention due to its simplicity in construction and reliability inoperation. Horizontal-flow roughing and slow sand filtersare characterized by their high process stability, ease of op-eration and remarkable efficiency. Treatment schemes likethe one illustrated in Fig. 17 make utmost use of local re-sources and are therefore reproducible under various con-ditions. Twenty litres of gravel and 10 litres of sand used asfilter material are able to transform contaminated turbidsurface water into 30 litres of quality drinking water, awater volume often applied as design value for the daily

per-capita demand in water distribution schemes using pub-lic standposts. A large number of demonstration projects inmany developing countries are proving the viability of thepromoted treatment concept. This experience is to be usedin the next decade — the implementation decade of rough-ing filters — for the benefit of those still lacking a safe andreliable water supply.

ACKNOWLEDGMENTS

The authors wish to express their gratitude to all thosepeople who strongly supported the HRF project, particu-larly to Prof. E. Triieb, Swiss Federal Institute of Technol-ogy, Zurich and to Mr A. Hartmann, Swiss DevelopmentCooperation which cofinanced the HRF project. Specialthanks go to IRCWD's cooperation partners in the de-veloping countries. Without their esteemed work the HRFproject would never have taken root.

B I B L I O G R A P H Y

1. WHO. Technology for Water Supply and Sanitation in De-veloping Countries. Technical Report Series No. 742.WHO (1987).

2. Visscher, J.T., Paramasivam, R., Raman, A. and Heij-nen, H.A. Slow Sand Filtration for Community Water Sup-ply. IRC Technical Paper series No. 24. InternationalWater and Sanitation Centre (IRC), The Hague (1987).

3. Rajapkse, J.P. and Ives, K.J. Pre-filtration of very highlyturbid waters using pebble matrix filtration, Journal of theInstitution of Water and Environmental Management(1990).

4. IRC. Pre-Treatment Methods for Community Water Sup-ply. IRC (1989).

5. World Bank. Rural Water Supply Treatment. Submodule4.5 b of International Training Network Project. WorldBank, Geneva.

6. CINARA. Proyecto Integrado de Investigación y De-monstration de Métodos de Pretratamiento para Sistemasde Abastecimiento de Agua. CINARA/IRC, (1989).

7. Baker, M.N. The Quest for Pure Water, Vol. I. AWWA(1981).

8. Kuntschik, O. Optimization of surface water treatment bya special filtration technique. Journal of the AmericanWater Works Association 68, 546-551 (1976).

316 M. Wegelin, R. Schertenleib and M. Boiler

9. Thanh, N.C. Horizontal-flow Coarse-Material Prefiltra-tion. Research Report No. 70. AIT (1977).

10. Monitoring and Evaluation of Village DemonstrationPlants. AIT (1981).

11. Wegelin, M. and Mbwette, T.S.A. SSFResearch ReportsNos 2 & 3. University of Dar es Salaam (1980-82).

12. Wegelin, M., Boiler, M. and Schertenleib, R. Particle re-moval by Horizontal-flow roughing filtration Journal ofWater Supply Research and Technology—Aqua 36, 80-90(1987).

13. Wegelin, M. Horizontal-flow roughing filtration: a design,construction and operation manual, IRCWD Report No. 6(1986).

14. Basit, S.E. and Brown, Q. Slow sand filter for the BlueNile Health Project. Waterlines 5, 29-31 (1986).

15. Basit, S.E. Monitoring Reports Blue Nile Health Project,(1988-89).

16. Xu, X.K. Monitoring Reports for Jinxing ChenzhuangProjects. Zhejiang Health and Anti-epidemic Station(1989-90).

17. Dorcoo, K. Self-help Community Water Supply Project(1986).

18. CINARA. Proyecto Filtración Gruesa Horizontal. (1990).19 Riti, M.M. Horizontal roughing filter in pretreatment of

slow sand filters. Thesis, Tampere University of Technol-ogy (1981).

20. Tilahun, G.T. Direct filtration with horizontal roughingfilter as pretreatment. Thesis, Tampere University ofTechnology (1984).

21. Symonds, Ch. N. Aspects of Prefiltration concerned withthe Application of Small Scale Slow Sand Filtration inRural Communities, University of Surrey (1985).

22. Siripatrachai, T. Physical and Mathematical Analysis ofthe Performance of Horizontal Roughing Filtration. Inter-national Institute for Hydraulic and Environmental En-gineering, Delft (1987).

23. Brown, D. HRF as an appropriate pretreatment beforesmall SSF in developing countries. Thesis, University ofNewcastle (1988).

Forthcoming IWSA Events1991

4-5 November, Budapest, HungaryIWSA Standing Committee Workshop on Statisticsand Economics

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1992

3-6 February, Cotonou, Benin6th UADE Congress/IWSA Regional Conference

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