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CLOSE THIS BOOKSurface Water Treatment by Roughing Filters - A Design, Construction and Operation Manual (SANDEC - SKAT, 1996, 180 p.)
Part 1: General aspects of roughing filter application
VIEW THE DOCUMENT1. Historical development and experience with water treatment
VIEW THE DOCUMENT2. Water treatment concept
3. Raw water quality
VIEW THE DOCUMENT3.1 Raw water characterisation
VIEW THE DOCUMENT3.2 Catchment area
VIEW THE DOCUMENT3.3 Water quality analysis
4. Solid matter separation
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT4.1 Sedimentation
VIEW THE DOCUMENT4.2 Roughing filtration
5. Bacteriological water quality improvement
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT5.1 Slow sand filtration
VIEW THE DOCUMENT5.2 Chlorination
6. Layout of a water supply scheme
VIEW THE DOCUMENT6.1 General considerations
VIEW THE DOCUMENT6.2 Hydraulic profile
VIEW THE DOCUMENT6.3 Treatment steps
VIEW THE DOCUMENT6.4 Water distribution
7. Roughing filtration application
VIEW THE DOCUMENT7.1 Historic use
VIEW THE DOCUMENT7.2 Development of roughing filters

Surface Water Treatment by Roughing Filters - A Design, Construction and Operation Manual (SANDEC - SKAT, 1996, 180 p.)

Part 1: General aspects of roughing filter application

1. Historical development and experience with water treatment

" In the earliest days of the human race, water was taken as found. It might be pure and abundant, plentiful but muddy, scarce but good, or both scarce and bad. To get more or better water, man moved to other sources rather than transport better water to his own location or to try to improve the quality of water at hand". This cited text marks the beginning of Baker's epilogue in "The Quest for Pure Water" [1], a reference book he started compiling at the beginning of this century and which was finalised in the 1940s. Baker continues by saying " Man's earliest standards of quality were few: freedom from mud, taste and odour". However, an increase in man-made water pollution, the development of technical and public health science, as well as the consumers' greater need for clean water contributed to the development of the water purification technology.

At the beginning of the 19th century, the first water treatment plants for public water supplies were constructed in Britain and France. They generally comprised settling basins followed by gravel and sand filters. In the course of time, slow sand filters were developed as an efficient water treatment process, and used by many water authorities at the end of last century. By this time however, the Industrial Revolution came up with the "mechanical" filters as rapid sand filters were initially called. The growing water demand and the subsequent discovery of chlorine to disinfect the water enhanced the use of rapid sand filters. In 1940, there were about 2,275 rapid filter plants in the United States as opposed to about 100 slow sand filter plants. Another outstanding feature with regard to the water treatment technology was the use of aluminium and iron salts as coagulants in water treatment. Since the beginning of this century, coagulation and flocculation combined with sedimentation, rapid filtration, and final chlorination are now commonly used in water treatment. This treatment combination is now usually regarded as conventional.

Water treatment plants are either built in situ, usually as reinforced concrete structures, or installed as package plants manufactured by the water industry. Fig. 1 illustrates the extensive use of chemicals in conventional water treatment. Colloidal matter has to be destabilised by coagulants, such as aluminium sulphate or ferrous sulphate, possibly in combination with lime dosage for pH adjustment and polymers or polyelectrolytes to improve flocculation. As rapid filters do not significantly improve the microbiological water quality, chlorine has to be used as final treatment step to produce water which is safe for consumption. Finally, the numerous chemicals added may also have changed the chemical water characteristics. The treated water, which may either be corrosive or deposit-forming, could greatly harm the distribution system. Consequently, the treated water often has to be stabilised with a final dose of lime.

Conventional water treatment also requires a substantial input of energy and mechanical equipment. Frequently, the raw water has to be pumped through the different treatment stages. Flocculation requires energy inputs for hydraulic or mechanical mixing, sludge removal in sedimentation tanks is often carried out with mechanical scrapers, and rapid sand filters are backwashed for filter cleaning. Dosing pumps are necessary for adequate chemical application. In brief, conventional treatment calls for an extensive use of power-driven, mechanical and often sophisticated equipment.

Fig. 1 Operational Problems in Conventional Water Treatment Plants

A reliable and efficient operation of a conventional water treatment plant is a demanding task. A continuous supply of different chemicals must be guaranteed, spare parts of mechanical equipment must be stocked or easily available, and the treatment plant operated by well-trained and skilled personnel. The local infrastructure should support maintenance and repair of treatment plant components. However, these criteria are hardly ever met by local conditions prevailing in rural areas of developing countries.

Wagner states in the preface of the manual "Upgrading Water Treatment Plants" [2], which is the result of a WHO working group on operation and maintenance established in the 1990s: "In the majority of plants, especially in the less developed countries, much of the expensive equipment does not operate properly due to lack of understanding or disregard of maintenance and operation recommendations". Only a few plants are designed on the basis of bench and pilot plant testing. The need for careful design is most urgent in countries with the least resources. However, design studies are in fact considered a luxury rather than a necessity in these countries. The most widely encountered deficiency in water treatment is the application of coagulants to raw water. Incorrect dilution of the solution, inadequate doses and inappropriate dosing are the most common mistakes. Difficulties are also experienced with the flocculation step. Uncontrolled energy inputs result in small floes of low settleability. Sedimentation tanks are often not well-designed; short circuiting and incorrect water abstraction lead to poor clarification and overloading of the subsequent filters. These in turn cannot be backwashed properly and produce filtered water of high turbidity. Finally, poorly or inoperative chlorination equipment is commonplace in rural water treatment plants in developing countries, as the equipment usually originates from industrialised countries and, hence, foreign exchange is required to purchase these installations and spares. The described difficulties encountered with conventional water treatment will result in the production of water of erratic quality which is often unsafe for consumption.

Objectiveness demands that earlier experienced operational difficulties with slow sand filters have to be mentioned at this point. Initially, slow sand filters were developed to combat the cholera and typhus epidemics in Europe last century. On account of its simplicity and low-cost, the slow sand filter concept was then indiscriminately exported to developing countries in the early days of technical cooperation. Slow sand filters operate perfectly well with raw water of low turbidity as generally encountered in European surface waters. However, raw water quality in tropical climates can vary considerably, especially as regards turbidity and solid matter load. Therefore, this direct transfer of technology has proved inadequate. The inability of slow sand filters to sustain adequate filter runs when subject to high turbidity loads became obvious. Worldwide practical experience revealed that the slow sand filter design concept was often misunderstood, the use of pretreatment processes, such as plain sedimentation or flocculation and sedimentation, were either inefficient or unreliable as well as inappropriate, and that operation and maintenance deficiencies contributed to the poor performance of slow sand filters. In the early 1960s in Brazil, for example, the communities were not adequately trained in slow sand filter operation, thus causing a high failure rate of the slow sand filters [3]. In Cameroon on the other hand, slow sand filters were operated adequately twenty years ago. However, due to the raw water quality deterioration caused by progressive deforestation of the catchment areas, these filters faced increasing operational difficulties which required treatment plant rehabilitation [4]. Finally, an evaluation of the performance of four slow sand filter plants carried out in India in 1993 revealed that its current design, construction and operation, including source protection, is far from being satisfactory and often leads to poor filter performance [5].

Successful projects call for a multidisciplinary approach requiring various types of inputs. Sociocultural, institutional, and natural conditions must be considered along with financial and technical aspects. The synthesis of all these inputs lead to appropriate and sustainable solutions. This manual focuses mainly on technical aspects and gives answers to perhaps the least difficult problems. From the technical viewpoint, development of the roughing filter technology has contributed towards an efficient and reliable slow sand filter operation appropriate for rural water supply schemes in developing countries.

Photo 1 Compact Plant An Example of Conventional Water Treatment

Photo 2 Roughing and Slow Sand Filter - An Alternative Treatment Option

A Blue "White Elephant"

A Blue "White Elephant"

William, the driver of the project car, and I were heading north of the capital of a country in West Africa. The midday sun was beating down on the paved road, the air was vibrating and I felt drowsy from the heat. We were nearing Ndikinimeki, a small administrative centre of the province. Suddenly, I spotted a few dark blue dots in a banana plantation about 150 m off the main road. I ordered William to stop the car at once, which he did some 30 m further on. We drove back to a small path leading to the plantation where some people were waiting for transport to Ndikinimeki I asked them to see the treatment plant manager but he was in town.

The first 100 m we drove to the treatment plant, but had to leave the car next to a bridge and walk the last 50 m. The main gate was locked, so we climbed over it and stood on a plot which had originally been the treatment plant premises but had now been partly converted into farmland. Nice banana trees were growing on the fertile and humid soil located along the river.

The treatment plant consisted of about seven large ship containers standing on small concrete foundation blocks. All the containers were painted blue and had large doors. We climbed on one of the containers to study the treatment scheme. The plant was apparently designed as conventional treatment scheme comprising prechlorination, aeration, coagulation, pH correction, flocculation, sedimentation, rapid sand filtration, pH control, and disinfection. However, wafer was not flowing through the different treatment stages. Only the cascades and the sedimentation tanks were partly filled with rain water which had collected during the wet season and had ended a few weeks ago.

We rejoined the ground and tried to fight our way through the vegetation and pipe fittings scattered all over the plot. We managed to reach one of the side doors which we opened and were horror-stricken by what we saw. Corroded dosing pumps were still on the containers, some were falling to pieces, electric cables from the switchboard were hanging loosely from wall to wan and, in the far end of the container, we discovered a pair of sandals focally called flip flop. As we opened another container used as storeroom for the chemicals, a few lizards disappeared through corroded holes and two meagre bags of alum sulphate were lying in a corner. The last container contained the general switchboard. Two red bulbs were still burning and the small display indicated 004 382 pumping hours. Hence, the plant was about two years in operation if water was pumped for six hours a day.

William was rather angry as we left this place of "quick money". He realised that this represented a big loss to his country. He calculated that with the same amount of money about fifty sturdy roughing and slow sand filter plants could have been constructed, providing some income to local contractors as well as a good and durable investment for public welfare. At this point, we passed a large European-style villa with blue window shutters, located in a large lawn and enclosed in high walls ...

2. Water treatment concept

Water Treatment is usually a complex process which is often bound to fail if the objectives are not defined, the raw water properties not closeIy examined and the treatment methods inadequate. With a clear treatment concept, including a reasonable appreciation of the raw water characteristics and seasonal variations of the water quality, logically combined with the most appropriate treatment processes, failures can be avoided.

Fig. 2 Solid Matter Content in Surface Water

Fig. 3 Multiple Barrier Water Treatment Concept

A bucket filled with turbid river water, as illustrated in Fig. 2, often contains floating matter, such as debris of wood, leaves and grass, fine and coarse sand, which has settled at the bottom, and some fine suspended matter in the form of silt and clay particles or algae. However, harmful microorganisms, carriers of so many infectious diseases transmitted by consumption or contact with polluted water, cannot be detected with the naked eye. The size of such organisms, such as protozoa, bacteria and viruses, range within a few micrometers (1 mm is a thousandth of a millimetre) or even less. Removal or inactivation of these pathogenic organisms should, however, be given first priority in any water treatment concept. A difficult task, considering their small size and possibly low concentration in such a large volume of water. Slow sand filtration and chlorination are thus the two most widely used treatment processes, as they are capable of improving, in particular, the microbiological water quality.

The efficiency of chlorination and slow sand filtration is strongly influenced by the level of turbidity of the water to be treated. Turbidity mainly reflects the amount of fine suspended solids present in the water. A large number of microorganisms, tired of swimming around, attach themselves like "boat people" to the surface of these solids. The solids protect the microorganisms from the deadly chlorine. In slow sand filters, the pathogens will triumphantly observe how the fine particles block the sand surface. Hence, an efficient use of chlorine and slow sand filters is only possible with a low water turbidity virtually exempt from sol id matter.

As illustrated in Fig. 3, water has to undergo a step-by-step treatment, especially if it contains differently sized impurities. The first and easiest step in sound water treatment schemes is coarse solids separation. Finer particles are separated in a second pretreatment step and, finally, water treatment will end with the removal or destruction of small solids and microorganisms. These different pretreatment steps will contribute to reducing the pathogenic microorganisms. The '´boat-people" or pathogens attached to the surface of suspended solids will get stranded when the solids are separated. Some of the microorganisms floating in the water might also get pushed to the surface of the treatment installations and adhere to biological films. Solid matter and microorganisms, therefore, face a multitude of treatment barriers. Since treatment efficiency of each barrier increases in the direction of flow, it becomes increasingly difficult for the impurities to pass through each subsequent treatment barrier.

Surface water treatment thus requires generally at least two treatment steps as shown in Fig. 4. The first step, also called pretreatment, concentrates mainly on the removal of solids. Screens, grit chambers, sedimentation tanks, gravel and coarse sand filters are typically used as pretreatment units. The second step, commonly considered as main treatment, is used especially to remove or destroy the remaining microorganisms and the last traces of solid matter. Slow sand filtration and chlorination are the most commonly applied treatment processes in this second step.

Fig. 4 Surface Water Treatment in Two Stages

3. Raw water quality

3.1 Raw water characterisation

Surface water must generally be treated before it is used as drinking water as it is highly exposed to natural and man-made pollution. The extent of treatment depends, however, on the degree of water pollution to be assessed prior to designing any treatment facility. The design of a rural water treatment plant is based mainly on the following important water quality parameters:

· turbidity
· true colour
· solids concentration
· degree of faecal pollution

Quite often, however, hardly any information is available on the surface water quality of a raw water source meant for a rural water supply system. In such a case, the following preliminary surface water quality assessment steps can be used:

· sanitary inspection of the catchment area
· water quality analysis of the raw water

Reference [6] contains a detailed description of these two main rural water quality assessment steps. The information obtained through a sanitary inspection is more of a qualitative or descriptive nature and reflects the long-term situation of an assessed water course. The results of a water analysis present a quantitative assessment of the examined water source, and might only reflect the actual water quality at the time of sampling. Both methods complement each other, however, a thorough sanitary inspection of the catchment area often provides a more reliable and practical method of risk identification and general water quality assessment. Several water analyses have to be carried out to determine extent, duration and frequencies of water quality fluctuations. However, such information is rarely available prior to treatment facility design. Water quality analysis is often performed at a later stage to monitor only the performance of constructed treatment plants.

Detailed information on raw water quality will ease filter design. Nevertheless, accurate prediction of filter performance is hardly possible due to the complexity of filter processes.

3.2 Catchment area

An overall characterisation of the catchment area and its hydrology, along with a sanitary inspection of the area, can provide relevant information on the raw water quality. The specific characteristics of the catchment area, such as climate, hydrogeology, topography, vegetation, as well as human and animal activities greatly influence the qualitative and quantitative levels, as well as the surface water variations. Total rainfall and its annual distribution, together with soil conditions and topography, are significant criteria influencing the natural characteristics of a flowing surface water. Human activities, (deforestation, agriculture and settlements) in the catchment area will induce qualitative and quantitative changes in the natural regime of the surface water.

Turbidity level and suspended solids concentration are often correlated with the seasonal fluctuations of a river discharge. The size of the catchment area usually influences the period of high discharges; short heavy storms normally affect the discharge of small highland rivers to a greater extent than of large lowland streams. Inspection of the river bed and its embankments will certainly provide first-hand information on flow characteristics of the river. Closer inspection of the bed sediments and embankments will supply some details on the type of solids carried by the river at different periods of discharge. Information provided by the locals will focus more on frequency and length of turbidity peaks rather than on absolute turbidity levels, which can only be determined with measuring equipment.

Faecal pollution is not visible in a water sample. Even clear and pleasant water may carry harmful and disease-causing microorganisms. Population density, wastewater disposal practice and general public health condition will influence the bacteriological quality of a surface water. This quality varies widely, e.g. a highland river draining a well-protected, unpopulated area has probably a low public health risk level when used as drinking water, whereas a surface water draining wastewater from a slum area without proper sanitation facilities will certainly have an extremely high public health risk level even when used as washwater. Points of surface water pollution have to be detected by a sanitary inspection of the catchment area. Source protection is the first step in water treatment. Hence, remedial actions must be taken when such pollution points are identified. A survey of the public health condition is necessary to assess the presence of endemic diseases. Such a survey might also determine the need to improve the situation with the construction of a water supply system and, particularly, with the installation of water treatment facilities. Nevertheless, surface water remains unprotected and is, therefore, permanently exposed to human and animal faecal contamination and other man-made pollution. As a result it will generally have to be treated before it is used as drinking water.

Water Treatment Starts in the Catchment Area

Water Treatment Starts in the Catchment Area

Jacob, caretaker of Guzang's water supply scheme fore more than 20 years, points to the barren hills of the watershed. His sunny nature becomes serious and he looks quite demoralised. The situation has changed considerably since the water project was inaugurated. Formerly, the raw water was tapped from a small clear river which was well-protected by a dense forest. A sedimentation tankard two slow sand filters were installed right from the beginning to treat the raw water. Operation of these installations did not pose any problems in the first few years. It then became increasingly more difficult and, for the past three years, slow sand filter operation has become very cumbersome. Now the filters have to be cleaned every two weeks, which leads to water shortages in the village. The community is blaming the caretaker for this state of affairs, however Jacob always tries to do his level best to supply water to the growing number of villagers. This increase in population puts great pressure on the available land, which is rapidly transformed from water reservation areas to agricultural plots. plots. Over the years, the community has expanded into the water catchment area, and deforestation, careless farming and grazing methods have negatively affected quantity and quality of the small river.

The delegation from the District Office is aware of Jacob's dilemma and has promised to tackle the problem from two sides: as immediate solution, roughing filters will replace the sedimentation tank, however, in the long run, Guzang's water supply can only be secured by a more comprehensive protection programme of the catchment area. Farmers in the watershed will not be sent away from the area but motivated to change to improved land use methods, such as agroforestry and pasture improvement. Treatment plant rehabilitation and watershed conservation are essential to ensure a more sustainable water and food supply to Guzang.

3.3 Water quality analysis

In rural areas, the main surface water treatment objective is to improve its bacteriological quality. Drinking water should not contain any pathogenic organisms, which are often difficult to detect analytically. Therefore, the bacteriological water quality is analysed for faecal indicators. The bacteria used for such analysis are faecal coliforms, Escherichia coli and faecal streptococci present in large concentrations (10 - 1,000 million conform bacteria are found in 1 gram of faeces) in the faeces of humans and warm-blooded animals. If waters contain faecal indicators, pathogenic microorganisms are also considered to be present.

Faecal conform analyses are performed either by the membrane filtration technique or by the multiple tube method. Field test kits (e.g. manufactured by DelAgua Ltd. [7] ) are available and generally use the membrane filtration technique. The multiple tube method is often applied in central laboratories. The use of field test kits requires some basic training in test procedures, initial supervision of field analysis and, at a later stage, correct and careful handling during routine work. To obtain reliable data, the analysis of faecal coliforms should be carried out by specially trained people.

Type and amount of solid matter is the second most important aspect in surface water characterisation. Expensive and very sensitive laboratory equipment has been developed for the analysis of size, shape and concentration of solid particles. However, such equipment is hardly available nor necessary for the design of treatment facilities. Even the standard routine method of determination of the suspended solids concentration is often not possible as it requires a highly accurate scale, a vacuum pump and a drying furnace installed in an air-conditioned room. Such equipment is often unavailable or has fallen into disrepair. Hence, determination of the physical characteristics of the solids, to be separated by adequate treatment processes, requires sturdy and simple field test methods.

The physical characteristics of the solid matter can be assessed by different simple analytical methods easily applied by any treatment plant operators. These simple tests are described in Annex 1 and include the following:

- turbidity test by means of a simple tube
- determination of the settleable solids volume with a test cone
- determination of the filterability by means of a filter paper
- suspension stability test using a vessel and turbidity readings
- solid classification test using a common bottle
- particle size characteristics by sequential membrane filtration

Chemical water quality parameters should be determined on a case by case basis if water pollution levels caused by hydrogeological conditions, agriculture or industry are likely to occur. Simple field test equipment, as described in [8, 9], could be used for preliminary chemical water quality assessment. Especially, manganese, true colour and water aggressivity are important parameters which need to be examined. Furthermore, the amount of dissolved organic matter should be determined as it will greatly influence the extent of biological activity and oxygen demand in the filters.

4. Solid matter separation

Let us now examine the first treatment step; i.e., the separation of solid matter. We might be confronted with a great variety of solids as observed in our bucket filled with turbid river water. The variety, illustrated in Fig. 5, is greatly dependent on the type of surface water and whether natural purification processes can separate part of the solids or possibly generate undesirable particulate matter by organic growth. Natural purification should largely be integrated into the treatment design when determining surface water source and intake location.

Fig. 5 Solid Matter Content for Separation

Sedimentation and filtration processes are mainly applied for solid matter separation. These shall be discussed in detail in the next two sections.

Yet, let us focus first on the peculiarities of the various types of surface water and their impact on the different solids in the raw water:

· The properties of the drained catchment area and the characteristics of the surface water influence the type and concentration of solid matter in the raw water. Flow velocity and rate of erosion determine the amount of settleable solids carried by the water. Flowing and still surface waters greatly differ with respect to the encountered type of solid matter. The turbulent flow of a water course may carry coarse settleable solids, which settle in gently flowing or impounded surface water. Algae found in ponds and lakes contribute to the suspended solids concentration of the water.

· Flowing surface water is often subjected to drastic quantitative and qualitative changes. The annual rainfall distribution influences the seasonal surface water fluctuation mainly with regard to turbidity and solids concentration. Flowing surface water will usually carry settleable solids at varying concentrations during different periods of time. During the dry season, small upland rivers are generally low in turbidity, however, they can exhibit high short-term turbidity peaks during heavy rainfalls. Larger lowland rivers may be of moderate turbidity throughout the year but with relatively long periods of increased turbidity levels.

· In still surface water, amount and type of solid matter change only gradually in the course of a year. In fact, the large volume of stored water in lakes, reservoirs and ponds preconditions the water quality. Coarse inorganic particles settle at the bottom of the receiving water body, light organic solid debris tend to float on the water surface, and dissolved organic matter may be transformed by photosynthetic processes to algae and plankton. Hence, each still water source acts as a first pretreatment step since the incoming and stored water is exposed to natural purification. As a result, impounded water is generally characterised by smaller water quality fluctuations. This higher stability of the raw water quality usually facilitates treatment plant operation.

· Flowing surface water carries solids of different sizes, such as coarse sand and silt to fine clay. Due to the irregular flow conditions, the solids are unevenly distributed over the cross section of a river bend. Coarse solids drift towards the outer side of the bend whereas the fine solids are washed to the inner side of a river bend and form a silting zone. Selecting an appropriate location for the intake structure contributes to reducing the levels of fine particles which are difficult to remove in treatment processes. The intake should, therefore, be placed at the outer or erosion side of a river bend in order to reduce the abstraction of fine matter and to avoid the silting of intake works.

· Surface water can also carry coarse floating matter which may block or even damage part of the water supply installations. The undesirable material is thus retained right from the beginning either by screens or by a scumboard. Fixed screens (e.g. a coarse screen followed by a finer one) are most commonly used to avoid blockage and excessive headlosses.

In short, if surface water is used as raw water source in a water supply scheme, preference should be given to still water provided excess amounts of algae or colour do not create special treatment problems. Natural purification processes reduce in particular the solid matter concentration by sedimentation, and smaller water quality variations often decrease and simplify the required degree of treatment. Flowing surface water frequently exhibits rapid water quality changes which render water treatment more difficult.

4.1 Sedimentation

Small pebbles or sand particles will undoubtedly settle in still water. This process, called sedimentation, is dependent on the physical properties of the solid matter and water. The settling velocity is influenced by density, size and shape of the particle, as well as by viscosity and hydraulic conditions of the water. Stilling basins and sedimentation tanks are quite efficient in removing relatively heavy and coarse solids, such as sand and silt particles. Inorganic matter larger than about 20 mm (0.02 mm) can usually be removed by plain sedimentation and without the use of chemicals.

Stilling basins can often be installed in small rivers. As shown in Fig. 6, a small weir is placed in the water course to raise the water depth and to reduce the flow velocity. Easily settleable matter can now be separated in the backwater of the weir equipped with a small gate to ease periodic removal of the settled material. The intake of the water supply scheme may be integrated into the sidewall of the weir, in a zone with sufficient water current to achieve removal of floating matter retained by the scum-board.

Sedimentation tanks are either rectangular, square, or circular in shape. The tanks are operated on a continuous or intermittent basis. In continuously operated tanks, the flow direction is either horizontal or vertical. In circular tanks, the flow pattern is complex, and the conditions are unstable in vertically operated tanks. Therefore, rectangular tanks operated on a horizontal flow and continuous basis are recommended for rural water supply schemes.

Fig. 6 Layout and Design of a Stilling Basin

Fig. 7 Layout and Design of a Sedimentation Tank

Sedimentation tanks separate finer solids, such as silt, clay and part of the suspended solids. The raw and turbid water enters on one side of the tank and is evenly distributed over the entire tank cross section. The solids then settle under laminar flow conditions to the tank bottom, and the clarified water is abstracted uniformly over the full width on the opposite side of the tank. In order for the particles to separate, each solid particle has to overcome a settling distance equal to the tank's depth, e.g. around 1 to 3 m. The accumulated sludge is periodically removed from the tank bottom. The solids removal efficiency of a sedimentation tank depends mainly on the hydraulic surface load, tank depth, and retention time. Some general design values fore sedimentation tank are given in Fig. 7, however, they should be chosen according to the settling characteristics of the solids. These can be determined in a sedimentation test using a transparent test tube; for additional information consult Annex 1. The recorded time necessary to attain a certain clarification level in the test has to be multiplied by a factor three to allow for unfavourable flow conditions in a full-sized tank. Low surface loads should be applied with raw water of poor settling properties, and in small plants with variable operating conditions.

Even properly designed and operated sedimentation tanks will separate only part of the suspended solids. With the help of coagulants, such as alum or iron salts, suspensions can be destabilised. The small particles lose their repulsive force, cluster together and coalesce to larger floes of improved settling characteristics. Coagulants are extensively used in conventional water treatment systems. However, the flocculation/sedimentation process is already an advanced treatment technique requiring qualified personnel and well-equipped facilities; both scarce in rural areas of developing countries. 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 reliable supply of chemicals to remote treatment plants is yet another stumbling-block. Dosage is also an art in itself, as it must be adapted to the varying raw water quality and thus requires professional water quality monitoring. Accurate and sensitive dosing instruments are attacked by the corrosive action of the chemicals. Chemical water treatment calls for skilled personnel trained in water quality monitoring, dosage adjustment, as well as in maintenance and repairs of dosing equipment. Finally, use of chemicals often greatly increases operating costs. In practice, rural water supplies often face considerable problems with chemical water treatment. A reliable and successful application of chemical flocculation is, therefore, rather unrealistic for many small water supply schemes. The chemical coagulation and sedimentation process applied in conventional water treatment schemes for separation of suspended solids and colloidal matter will generally fail in rural water supply schemes and is therefore not recommended.

To conclude, it can be said that stilling basins and sedimentation tanks are quite efficient in removing coarse and easily settleable solids. They are used as preliminary treatment step, especially to treat raw water drawn from running water courses containing high solids concentrations. In rural water supply schemes, use of chemicals to enhance sedimentation by flocculation is difficult and, therefore, quite unreliable.

4.2 Roughing filtration

The water quality of contaminated surface water can be improved significantly when filtered through gravel and sand layers. Therefore, favourable hydrogeological conditions allow polluted and turbid river water to be drawn as clear and safe groundwater from a shallow well located next to a river. However, local soils are quite often impervious for lack of gravel and sand layers. Nevertheless, why should nature's excellent purification capacity be ignored just because of unfavourable hydrogeological conditions at the site of a new water supply scheme? Let us then copy nature and construct an artificial aquifer by filling a sedimentation tank with gravel. As illustrated in Fig. 8, the solids removal efficiency of such a tank will drastically increase due to the greatly reduced settling distance in the gravel material. In other words, the fine solids crossing an ordinary sedimentation tank have to overcome a vertical settling distance of 1 to 3 m before coming into contact with the tank bottom. The same solids flowing through a filter will fortunately touch the gravel surface already after a few millimetres. Thus, filtration becomes a more effective process for solids removal since the settling distance is drastically reduced by the filter material. Presence of a small pore system and large internal filter surface area enhances sedimentation and adsorption, as well as chemical and biological activities.

Design and application of prefilters vary considerably. The different filter types are classified according to their location within the water supply scheme, their main application and flow direction. Intake and dynamic filters, which often form part of the water intake structure, differ from actual roughing filters which are generally located at the water treatment plant. As illustrated in Fig. 9, roughing filters are further subdivided into down, up and horizontal-flow filters. Finally, vertical-flow filters can be classified according to the manner in which the gravel layers are installed. The different gravel fractions of roughing filters "in series" are installed in separate compartments, while those of roughing filters "in layers" are placed on top of each other in the same compartment.

Fig. 8 Particle Removal in a Sedimentation Tank and a Roughing Filter

Roughing filters usually consist of differently sized filter material decreasing successively in size in the direction of flow. The bulk of the solids is separated by the coarse filter medium located next to the filter inlet. The subsequent medium and fine filter media further reduce the suspended solids concentration. The filter medium of a roughing filter is composed of relatively coarse (rough) material ranging from about 25 to 4 mm in size. Gravel is generally used as filter material. Significant solids removal efficiencies are only achieved under laminar flow conditions since sedimentation is the predominant process in roughing filtration. Therefore, roughing filters are operated at small hydraulic loads, which have been defined as flow rate Q divided by the filter area A perpendicular to the direction of flow. Filtration velocity, synonymous with hydraulic load, usually ranges between 0.3 and 1.5 m/h. The coarse filter material and the small hydraulic load limit filter resistance to a few centimetres.

Fig. 9 Classification of Prefilters

Filter cleaning is carried out manually and hydraulically depending on the pattern of solids accumulated in the filter. Intake and dynamic filters separate the solids mainly at the inlet zone of the filter and, thus, act as surface filters. These filters are therefore manually cleaned by scouring the top of the filter bed with a shovel or rake. Compared to intake and dynamic filters, roughing filters act as space filters on account of the deep penetration of the solids into the filter medium. The accumulated solid matter is periodically flushed out of roughing filters by hydraulic filter cleaning. If necessary, these filters can be cleaned manually by excavating the filter material from the filter compartment, washing and refilling it into the filter boxes.

Sedimentation is the main process in roughing filtration. It is responsible for solids separation from the water as observed in laboratory tests conducted with roughing filters [10, 11, 12, 13,14]. The filter acts as multi-storage sedimentation basin since it provides a large surface area to accumulate the settled matter. As shown in Fig. 10 and illustrated in Photo 3, the deposits are retained on top of the collectors where they grow to dome-shaped aggregates with advanced filtration time. Part of the small heaps drift to the filter bottom when the loosely accumulated aggregates become unstable. In horizontal-flow roughing filters, this drift regenerates filter efficiency of the upper gravel layers and allows accumulation of a considerable amount of retained material at the filter bottom. Depending on the organic characteristics of the raw water, other processes such as biological oxidation or adsorption of solid matter at the slimy filter surface may occur. Under these circumstances, enhanced aggregation and consolidation of deposits have been reported [12]. This poses inherent difficulties during hydraulic cleaning and filter regeneration.

Fig. 10 Solid Removal in a Horizontal-flow Roughing Filter

Filter regeneration can be enhanced by filter drainage. The loosely accumulated aggregates collapse and are washed down to the filter bottom if the water table in the filter is lowered. Part of the accumulated solids can be flushed out of the filter with high filter drainage rates and adequate installations.

Photo 3 Accumulation of Kaolin in a Horizontal-flow Roughing Filter after 24h, 100h, and 300h of Filter Operation

5. Bacteriological water quality improvement

The water in our bucket is now clear but still unsafe for consumption. The turbid river water has changed its appearance as the solid matter has been separated by the pretreatment processes discussed in the foregoing chapter. The water has lost its brownish tinge and turned into a clear and pleasantly looking liquid. However, the water is still not as pleasant and safe as it looks. As schematically shown in Fig. 11, disease-causing pathogenic microorganisms are usually not visible to the naked eye of the consumer who could get a severe attack of diarrhoea a few hours after drinking this water. Hence, the pretreated water still needs further treatment for final removal or inactivation of pathogens. Slow sand filtration and chlorination are the two most commonly applied treatment processes for bacteriological water quality improvement.

Fig. 11 Microorganisms for Separation

5.1 Slow sand filtration

Slow sand filtration plays a key role in rural water treatment. Design and application of this treatment process is well-documented in the available literature [15, 16, 17]. Since slow sand filters reduce the number of microorganisms present in the water, they improve the bacteriological water quality. In addition, fine organic and inorganic matter is separated, and the organic compounds dissolved in the water are oxidised. Since the effluent of a well-designed and operated slow sand filter is virtually free from pathogenic microorganisms, water treated by such a slow sand filter is safe for consumption. Furthermore, a comparative evaluation [18] of slow sand and rapid sand filter efficiencies revealed that slow sand filters are more efficient in the removal of several commonly occurring pesticides. In contrast, they were found to be poorer than coagulant-assisted rapid sand filters for the removal of dissolved organic carbon and organic colour. However, slow sand filtration is one of the most efficient processes for the production of hygienically safe drinking water with a possibly small bacterial regrowth.

The slow sand filter technology copies nature. The sand layers of aquifers convert unsafe surface water into 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 no longer endangers human health. These natural purification processes are also used by the slow sand filters - a technology which was introduced last century. At that time, Europe was struck by cholera epidemics, which forced the waterworks to take quick action. The advantages of slow sand filtration were then discovered. This water treatment technique proved to be efficient against water-borne diseases and, in combination with other public health improvements, these epidemics were eradicated in Europe. Numerous water supplies in industrialised countries are still using slow sand filters. Thames Water supplies for instance two thirds of London's population with slow sand filter treated surface water drawn from the River Thames which carries a very high percentage of sewage effluent from upstream settlements during drought years. This is a tribute to the efficiency and reliability of the slow sand filter technology.

The layout of slow sand filters is simple and straightforward. As shown in Fig. 12, a slow sand filter contains an open box filled with a sand layer of a depth of about 0.8 to 1.0 meter. The upper part of the filter box is filled with water flowing by gravity through the sand bed. The filtered water is then collected by an underdrain system and conveyed to the clear water tank. The well-graded sand of the filter bed is relatively fine; i.e., its effective size ranges between 0.15 and 0.30 millimetre, but recent field experience revealed that also somewhat coarser sand can be used [4].

Fig. 12 Layout and Design of a Slow Sand Filter

Slow sand filter operation is easy and reliable. Slow sand filters are usually operated at 0.1 to 0.2 m/h filtration rates. Consequently, an area of 1 m² sand produces about 2.5 to 5 m³ of water per day. The flow rate is preferably controlled at the filter inlet, and the water level is maintained at a minimum level above the sand bed by means of a weir or effluent pipe located at the filter outlet. Effective biological treatment can only be achieved if a reasonably steady throughput is maintained. Therefore, a 24-hour operation is recommended as it makes maximum use of the available filter installations. The initial filter resistance of a clean sand filter ranges between 0.20 and 0.30 meter. The headloss gradually increases with progressive filtration time. The sand filter has to be cleaned when filter resistance amounts to about 1 meter.

Slow sand filters act mainly as surface filters. The water quality changes at the surface of the sand bed, in the so-called "Schmutzdecke" and, to a lesser extent, in the first 20 to 30 centimetres of the send bed. A thin layer on the surface of the sand bed, formed by retained organic and inorganic matter, and a large variety of biologically active microorganisms, are responsible for the physical, chemical, and biological improvement of the water. This thin biological layer must first develop in a new slow sand filter. The initial ripening period normally requires two to four weeks. Cleaned filters will regain their full biological activity within two to four days, provided shut down time for filter cleaning is short; i.e., not more than 6 - 12 hours.

Filter cleaning must be carried out once the supernatant water has reached its maximum permissible level; i.e., when maximum filter resistance of about 1 meter is attained for the designed filtration rate. Filter cleaning starts with drainage of the supernatant water and dewatering of the top part of the sand bed. Subsequently, the biological skin and 1 to 2 centimetres of sand are removed from the sand bed as shown in Photo 4. Resanding is possibly performed after removal of the top sand layer. Thereupon, filter operation is immediately restarted to avoid disrupting biological filter activity more than is necessary. The filter bed is refilled with water introduced via the under-drainage system. This drives the air out of the pores of the sand and completely saturates the filter bed. Normal operation is then reassumed by opening the inlet valve and adjusting the filtration rate.

Well-operated slow sand filters should at least achieve more than 1 to 3 months of filter runs. The term "filter run" is defined as the time between two subsequent filter cleanings. In order to realise such long filter runs, slow sand filters have to be supplied with relatively clear water. Reasonable filter operation can only be expected with inlet water turbidities below 20 to 30 NTU. Higher turbidities, with consequently higher solids concentrations, will rapidly clog the sand surface and interfere with the biological processes. Hence, it is strongly recommended that surface water is pretreated prior to slow sand filtration.

Photo 4 Cleaning of a Slow Sand Filter

Design deficiencies will cause problems to slow sand filters. In the past, several slow sand filter plants in developing countries have faced operational problems or had to be closed down. Serious design faults, inadequate operation and poor water quality supplied to the slow sand filters are the main reasons for the problems and failures experienced. As illustrated in Fig. 13, a lack of flow control equipment, inadequate pipe installations, a soiled and poorly graded sand which does not conform to the recommended size, or missing water level control systems, are the most common design errors encountered. Random filter operation under variable and often too high filtration rates by insufficiently trained caretakers, are generally the causes of inadequate filter efficiency.

Poor quality raw water, inadequately pretreated, also contributes to poor slow sand filter performance. Frequently, slow sand filters are directly fed with raw water or are often combined with inefficient or inappropriate pretreatment processes. Slow sand filters usually face serious operational problems when chemical flocculation and sedimentation are used as pretreatment. The local caretaker might not be able to control flocculation as it is an unstable pretreatment process difficult to operate. Light floes often get washed onto the slow sand filters, or a lack of chemicals greatly reduces the solid removal efficiency of the sedimentation tank. Premature, rapid filter clogging and frequent filter cleaning are the resulting consequences. Therefore, efficient pretreatment of the surface water, such as for instance by roughing filters, is necessary to avoid serious operational difficulties with slow sand filters. Small slow sand filter units receiving raw water of moderate turbidity can also be protected by layers of non-woven synthetic filter fabrics [19,20] or by a layer of gravel [21 ] installed on top of the sand bed.

Fig. 13 Common Design Faults of Slow Sand Filters

In summary, slow sand filtration can thus be regarded as a safe, stable, simple and reliable treatment process. Filter construction makes extensive use of local material and skills. Filter operation neither requires sophisticated mechanical parts nor the use of chemicals. Construction, operation and maintenance of the filters are easy and require only limited skills. However, adequate filter operation is only possible with raw water of low turbidity; i.e., virtually free of solid matter. Pretreatment of surface water is therefore necessary. In combination with adequate pretreatment methods, slow sand filtration is considered a most appropriate water treatment technology for developing countries.

Defective Slow Sand Filter Next to the Cemetery

Defective Slow Sand Filter Next to the Cemetery

The photograph is self-explaining. From the slope of a steep valley in the Andean highlands we can see two slow sand filter units filled with chocolate coloured water, a large heap of sand deposited on the soil next to the structure and, slightly further down, the cemetery of the village whose population is supplied by the water of these defective filters. Mist is climbing from the valley and will soon engulf this gloomy vision .....

In 1985, DelAgua evaluated the 18 treatment plants in two departments of the Andean country. Two of the plants had inoperative rapid sand filters. The study also revealed that all the slow sand filter and disinfection units had major deficiencies and operating problems. Technical and institutional problems were responsible for these failures. The main technical problems were associated with the flow control and raw water quality. Absence of a flow control at the raw water intake caused unstable or intermittent filter operation. The highly turbid and contaminated raw water was not adequately pretreated and led to short filter runs and operational problems. Consequently, filter efficiency was considerably reduced and, according to the survey, more than half of the plants could reduce only marginally or not at all turbidity and bacterial contamination. As regards institutional aspects, the caretakers and administrative committees had not received adequate training in treatment plant operation and maintenance. The users were not given professional supervisory support from the responsible national authorities which had no incentive to providing a reliable water supply. The described problems were tackled by a rehabilitation and technology transfer programme for rural water treatment. Effective and appropriate pretreatment processes, such as roughing filtration, were introduced, and institutional development as well as community education were supported.

To ensure a reliable and sustainable treatment plant operation, appropriate treatment processes and local development of technical and managerial skills are required.

5.2 Chlorination

Chlorination aims at destroying or, at least, inactivating harmful microorganisms, such as pathogenic bacteria, viruses, and cysts present in the water. Chlorine is a strong oxidant, which not only reacts with the enzymes vital to the metabolic processes of living cells, but is also responsible for other chemical reactions. Dissolved organic matter, for instance, depletes by fast chemical reaction the available chlorine that will then be unavailable for water disinfection. Or chlorine reacts with nitrogen to form the more stable chloramines often purposely generated by the addition of ammonia to the water so as to cope with any type of pollution problems in the distribution system.

The advantages why chlorination is widely used in water treatment practice are the following:

- Chlorine is a strong disinfectant when applied to low water turbidity with a small dissolved organic content.

- Residual chlorine content is extremely simple to determine by calorimeters, which is not the case for other disinfection processes such as ozone or UV radiation.

- Since chlorination installations are relatively small, they do not require large civil engineering structures and their investment costs are relatively low.

- Chlorine is often applied as a safeguard (especially in the form of stable chloramines) against secondary water pollution. Although small quantities of chlorine may deal with minor contaminations resulting from incorrect water handling at household level, they will never be able to combat heavily contaminated water caused by cross-connections or wastewater infiltration in intermittently operated water supply systems.

Numerous disadvantages of chlorination, however, question the application of this water treatment process in rural water supply schemes. Chlorination is associated with the following problems:

- Chlorination requires a reliable water treatment system. It is neither applicable to turbid water nor to water of high organic content.

- With inadequately pretreated water, chlorine forms by-products (e.g. trihalomethanes) that are considered carcinogenic.

- Chlorine is usually an unstable and corrosive chemical that loses its disinfecting power during storage, and attacks the delicate installations in the dosing room.

- Dosing equipment and chemicals must often be imported, which leads to a foreign currency demand and high operating and maintenance costs.

- Consumers frequently refuse to drink chlorinated water for reasons of taste and odour.

Accurate chlorine dosage is essential to attain efficient disinfection. Only partial disinfection is achieved with chlorine dosages lower than the chlorine demand of the water. Water containing a too high chlorine concentration might not be accepted by the consumers, as chlorinated water has a distinct odour. A strong smell develops when chlorine reacts with ammonia to form chloramines. People often reject chlorinated water even when chlorine is carefully handled and dosed at low concentrations.

Chlorine is available in gaseous, solid, and liquid form. Chlorine gas is extremely toxic, difficult to handle and, therefore, usually inappropriate for rural water supplies. Chlorinated lime, commonly known as "bleaching powder", calcium hypochlorite powder, or sodium hypochlorite solution, also called "Javel water", are used as chlorine derivatives. Since a chlorine solution is preferably added to the water, chlorinated lime and calcium hypochlorite should be dissolved in the water to a stock solution containing usually 1 to 3 percent active chlorine. Chlorine solutions require careful preparation; i.e., it is extremely dangerous to spill water on dry hypochlorite. Fig. 14 summarises the different chlorine applications. Please note that adequate disinfection is attained not only with a sufficient chlorine concentration C (mg/l), but also requires an appropriate contact time T (min) as the inactivation of mircroorganisms is dependent on the product C x X.

A constant rate of chlorine solution is added to the water by dosing devices. The relatively small doses of chlorine call for accurate dosing equipment. These are, however, exposed to the corrosive action of the chlorine and often get damaged. The dose has to be adjusted to the chlorine demand of the water to be disinfected. In practice, a limited chlorine dosage adjustment is possible, e.g. on a day-to-day basis. Reliable water treatment prior to chlorination is consequently necessary. Adequate water disinfection is generally feasible only with water virtually free of solid and organic matter.

Fig. 14 Application of Chlorine

A reliable supply of chlorine is often difficult to obtain. Chlorine must be purchased on a regular basis as its unstable nature does not allow lengthy storage. Chlorine generally has to be imported and, thus, requires foreign currency; an often scarce amenity in developing countries. In addition, these countries usually face other difficulties, such as communication and transportation problems. Finally, chemical water treatment requires skilled personnel often unavailable in rural areas. All these aspects are a stumbling block to a reliable and efficient chlorine application and, more generally, to the use of chemicals in rural water supply schemes in developing countries. This observation is endorsed by numerous cases of malfunctioning or abandoned chemical water treatment installations.

Since conventional disinfection methods are generally unsuccessful in small rural water supply schemes, simple, robust and easily maintained low-cost, reliable methods of disinfection are thus necessary.

New water disinfection techniques have already been developed and field-tested [22]. The use of iodine instead of chlorine bound onto resins housed in a cartridge is but one alternative. By placing this cartridge in the water, the microorganisms are rendered non-viable by oxidative reaction with iodine. Compared to chlorine, iodine does not react so easily with organic compounds in the water. However, this disinfection method requires further development before it can be used on a larger scale, especially with regard to fixing the iodine on an adequate supporting material. Furthermore, dosing of iodine must be well-controlled - at high dosage, it may pose a health hazard, particularly to pregnant women.

The use of an electrolytic cell which produces an oxidising gas when an electrical current is passed through a saturated sodium-chloride solution is a second water disinfection alternative. The Moggod method ("mixed oxidant gas generated on demand") requires salt, water and electric current to produce a strong oxidising gas. This method, however, is sensitive to the use of normal salt as it creates substantial problems when associated with calcium and magnesium deposits on the membrane. Further investigations on the nature of the gas produced and on operational aspects regarding the use of low-quality salt, are necessary before this disinfection method can be recommended for wider use.

The described methods suggest different processes rather than real disinfection alternatives to replace or produce chlorine at the site. Other processes (e.g. the MIOX method) are being developed and field-tested. A comprehensive description of chlorination and alternative disinfection methods is beyond the scope of this manual, however, reference is made to the relevant literature [23, 24, 25].

To conclude it can be said that an efficient and reliable disinfection with chlorine requires pretreated water virtually free of solid and organic matter. The use of chlorine in rural water supply schemes often creates enormous problems and is, therefore, frequently bound to fail as documented by numerous treatment plants. Furthermore, the rural population often rejects chlorinated water. Thorough technical, institutional and sociocultural investigations are necessary before chlorination is introduced in rural water treatment.

Chlorinated Water "Not for Drinking"

Chlorinated Water "Not for Drinking"

One could almost smell the paint on the recently constructed public standpost. Its design differs from the many other thousands used around the world. The local standpost uses water siphons with flexible tubes. A floating valve maintains the water at a constant level in the closed steel cylinder. This prevents leaks or broken down taps due to frequent public use. However, the interest of our group, composed of representatives of the foreign consultant and supplier and led by a Desk Officer of the national water company and the local Director of the water supply, was not only restricted to this special standpost design but to the entire water supply system. The system visited was the first of four which had just started operation. Construction of an additional twelve schemes was under discussion. All the water supply schemes were identical in design; i.e., a surface water intake, water treatment consisting of pre-chlorination, pH-control, aeration, coagulation with alum sulphate, flocculation, tilted plate settlers, rapid sand filtration, and final disinfection, as well as a clear water pumping station supplying the reservoir and distribution scheme. A module system for the treatment plant allowed rapid and efficient construction. However, we had some doubts whether the river water draining dense and unpopulated woodlands would require such extensive treatment in this location. The abstracted raw river wafer was quite clear even during our visit in the rainy season. Nevertheless, the water, which ran through the different treatment stages, was still treated with chemicals.

Two girls passing by the standpost were asked by the Desk Officer of the water company if they enjoyed the new water supply. Their answer was unexpected and also depressing as they declared that the water supply was used for many other purposes than for drinking. On account of its artificial and strange taste, the distributed wafer was not consumed by them, their families nor by the villagers. This is why they still draw water from the nearby river.

6. Layout of a water supply scheme

6.1 General considerations

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

· which raw water source should be used for the water supply scheme?
· if treatment is necessary, what type of treatment scheme should be favoured?
· 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 future water supply scheme. The different local water sources have to be evaluated with respect to their quantity, quality and accessibility. The future water demand must be covered by the selected source with the best possible water quality, and located as close as possible to the supply area.

Fig. 15 Layout Possibilities of water supply Schemes Using Surface Water

Since water treatment is usually the most difficult element in any water supply scheme, it should be avoided whenever possible. The general statement that no treatment is the best treatment especially applies to rural water supply schemes which generally exhibit a poor infrastructural and institutional framework to adequately maintain water treatment facilities. The use of better water quality sources is, therefore, an alternative which will always have to be taken into serious consideration. If no other alternative is available, rural water treatment must concentrate on improving the bacteriological water quality by locally sustainable treatment processes.

Water distribution systems depend on the type of water source used, on the topography, and on the provided supply service level. Individual water supplies, e.g. rainwater harvesting and shallow groundwater wells equipped with hand pumps usually do not need piped supply systems. Treated surface water, however, is normally distributed by a piped system. A suitable topography often allows the installation of a gravity system which will improve reliability and supply continuity. Since pumped water supply schemes depend on the reliable supply of energy and spare parts, they are very susceptible to temporary standstills. Finally, the service level of water supply strongly governs water demand. Water usage increases drastically with the provided service level, e.g. public standpost, yard connection, multiple tap house connection. Water supply is always interlinked with wastewater disposal. The health situation of a community supplied with treated water does not necessarily improve, especially if public health and wastewater disposal issues are neglected. The main components necessary to significantly improve the public health situation of a community are therefore a reliable and safe water supply, an adequate waste disposal system and a comprehensive hygiene education programme.

As schematised in Fig. 15, surface water has to be collected, treated and stored before it reaches the consumer. These activities can be met by different water supply layout options. Figs. 15 and 16 only illustrate some arrangement examples.

6.2 Hydraulic profile

Selection of the hydraulic profile is a basic criteria when planning a water supply scheme. First choice must be given to gravity supply systems since they guarantee reliable operation at low running costs. Schemes, which integrate the use of handpumps, are given second choice. The installation of mechanically driven pumps should be chosen as last option and only applied in special cases where a reliable and affordable energy supply is guaranteed, including the infrastructure for pump maintenance and repair work. Hydraulic rams making use of the potential energy of a large water volume to pump a small fraction of this water volume to a higher level [26] may be an appropriate option where surface water gravity is available and water volume abundant. Under special local conditions, collection and pretreatment of the raw water may be combined in a single installation such as infiltration galleries.

Fig. 16 General Layout of Pumped Water Supply Schemes

Water treatment plants should, whenever possible, be operated by gravity and with a free water table to minimise water pressure on the structures. The total headloss through the treatment plant will amount to 2 or 3 m. In general, any type of water lifting, except through handpumps, should be avoided as the supply of energy and sophisticated spare parts is generally unreliable. If water lifting is absolutely necessary for topographical reasons, the number of pumping steps must be limited. As illustrated in Fig. 16, a one-stage pumping scheme should be chosen for raw water to be pumped to an elevated site where the treatment plant and reservoir are located. Such a one-stage pumping scheme has greater advantages over a two-stage scheme as it increases its reliability by a factor of 2. Moreover, the risk of flooding in lowland areas can often not be excluded entirely. Protecting a high-lift pumping station against floods is easier than a full-sized treatment plant. However, a two-stage pumping system is unavoidable for a piped supply on a flat area devoid of natural elevation and in case of serious raw water quality fluctuations, e.g. heavy sediment loads during the monsoon. In such a situation, installation of a low lift raw water pump is recommended. It may consist of an irrigation unit of low efficiency but of simple repair to limit high lift pumping for treated water and protect impellers and seals from damage. Hence, high lift pumps should be used for treated water or raw water pumped from infiltration galleries or similar intake systems.

Fig. 17 Treatment of Surface Water

Spring for Minimum Water Supply

Iringa, a town in East Africa of 80,000 inhabitants is pleasantly located at the edge of an escarpment. The citizens have a beautiful view of the valley where the Little Ruaha river is gently meandering through maize and cassava fields. This turbid river is also the main water source of the town. The river water is pumped to an adjacent conventional treatment plant, collected in a clear water tank and, in a second step, lifted over the steep escarpment to the reservoir located in the town. Iringa often faces water shortage, mainly due to the frequent breakdown of the raw water pumps. The silt-loaded river water claims its victims in the form of rubber seal wear outs, impeller grind offs and shaft blockages, which put a great strain on the plant manager. Quite frequently, none of the raw water pumps are working.

Fortunately, a gravity pipe conveys wafer to the clear wafer tank from a tapped spring 10 km across the river valley. The powerful clear water pump can therefore at least be operated for a few hours a day to lift the clear spring water to the poorly supplied town.

The spring water supply is obviously more reliable not only for its single pumping step, but also for its better water quality. Rehabilitation of the intake could significantly reduce the operational difficulties of the raw water pumps. The intake suction pipes hanging loosely in the river should be replaced by a grit chamber, or even better by intake filters or infiltration galleries, which would remove a large fraction of the solids that considerably reduce the life of any pump.

6.3 Treatment steps

As discussed in Chapter 2, surface water has to undergo a step-by-step treatment. Coarse solids and impurities are first removed by pretreatment, whereas the remaining small particles and microorganisms are separated by the ultimate treatment step. Under special local conditions, raw water collection and pretreatment may be combined in a single installation, such as intake or dynamic filters or, alternatively, by infiltration galleries. Fig. 17 illustrates different schemes for surface water treatment. The required water treatment scheme is mainly dependent on the degree of faecal pollution, characteristics of the raw water turbidity and on the available tvpe of surface water.

6.3.1 Removal of Coarse Material

Separation of coarse solids from the water is preferably carried out by a high-load sedimentation tank (grit chamber) or by a plain sedimentation tank, since sludge removal from such tanks is less troublesome than from roughing filters. Simple sedimentation tanks can be designed according to the layout and guidelines given in Fig. 7, or constructed as earth basins as illustrated in Fig. 18.

Use of one sedimentation tank should be sufficient for a small-scale water supply scheme. The accumulated sludge can be removed during periods of low silt load. A bypass is required to maintain operation of the treatment plant during cleaning periods. In order not to interfere too much with normal operation of larger water treatment plants, two or more sedimentation tanks operating in parallel should be provided to allow cleaning, maintenance and repair of one tank.

6.3.2 Aeration

The water's dissolved oxygen content plays a key role in the biology of the slow sand filtration process. The activity of the aerobic biomass decreases considerably if the oxygen concentration of the water falls below 0.5 mg/l. Furthermore, nitrification of ammonia is associated with a significant consumption of oxygen, e.g. 1 mg NH4-N/I requires 4.5 mg O2/l. Hence, an adequate oxygen content in the water to be filtered is of prime importance. Physical processes are the main mechanisms in roughing filtration. However, biochemical reactions might also occur in the prefilters, especially if the raw water contains high organic loads.

Fig. 18 Design of an Earth Basin as Sedimentation Tank

Fig. 19 Layout and Design of an Aeration Cascade

Since turbulent surface waters are generally well oxygenated, they do not require additional aeration. Still water, however, can exhibit low oxygen contents, especially when drawn from the bottom of polluted surface water reservoirs. Multi-level drawoffs are recommended as intake structures for stratified water bodies to allow abstraction of best raw water quality. However, stagnant raw surface waters are preferably aerated.

Cascades are simple but efficient aeration devices. A submerged cascade aerator, as illustrated in Fig. 19, should be installed in gravity systems with sufficient hydraulic head. The cascade should preferably precede filters to meet the possible oxygen demand. The different weirs, used for flow control, are an additional source of oxygen supply.

6.3.3 Roughing Filtration as Pretreatment

Roughing filtration mainly separates the fine solids which are not retained by the preceding sedimentation tank. The effluent of roughing filters should not contain more than 2-5 mg/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 they remove suspended solids and reduce turbidity. However, a bacteriological water improvement can also be expected as bacteria and viruses are solids too, ranging in size between about 10 - 0.2 mm and 0.4 - 0.002 mm respectively. Furthermore, according to the specific literature [27], these organisms get frequently attached by electrostatic force to the surface of other solids in the water. Hence, a removal of the solids also means a reduction of pathogens (disease-causing microorganisms). The efficiency of roughing filtration in microorganism reduction may be in the same order of magnitude as that for suspended solids, e.g. an inlet concentration of 10 - 100 mg/l can be reduced by a roughing filter to about 1 - 3 mg/l. The bacteriological water quality improvement could amount to about 60 - 99%, or the microorganisms are reduced to about 1 - 2 log. Larger sized pathogens (eggs, worms) are removed to an even greater extent.

Roughing filters are used as pretreatment step prior to slow sand filters. Slow sand filtration may not be necessary if the bacteriological contamination of the water to be treated is absent or small, particularly in surface waters draining an unpopulated catchment area, or where controlled sanitation prevents water contamination by human waste. However, physical improvement of the water may be required with permanent or periodic high silt loads in the surface water. Excessive amounts of solids in the water lead to the silting up of pipes and reservoirs. For technical reasons, roughing filtration may therefore be used without slow sand filtration if the raw water originates from a well-protected catchment area and if it is of bacteriologically minor contamination; i.e., in the order of less than 20 50 E. coli/100 ml.

For operational reasons, at least two roughing filter units are generally required in a treatment plant. Since manual cleaning and maintenance may take some time, the remaining roughing filtration unit(s) will have to operate at higher hydraulic loads. A single prefilter unit may be appropriate in small water supply schemes treating water of periodically low turbidity.

6.3.4 Slow Sand Filtration as Main Treatment

The substantial reduction of bacteria, cysts and viruses by the slow sand filters is important for public health. Slow sand filters also remove the finest impurities found in the water. For this reason 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. However, the biological activities of the slow sand filter are more important than the physical processes. Dissolved and unstable solid organic matter, causing oxygen depletion or even turning to fouling processes during the absence of oxygen, is oxidised by the filter biology to stable inorganic products. The biological layer on top of the filter bed, the so-called "Schmutzdecke", is responsible for oxidation of the organics and for the removal of the pathogens. A slow sand filter will produce hygienically safe water once this layer is developed.

Unlike roughing filters, the time for slow sand filter cleaning is determined by maximum available headloss level, and not by deterioration of effluent quality. This offers some advantages as recording of a hydraulic criteria is easier than measuring water quality parameters.

Further information on slow sand filtration is summarised in Annex 3, and detailed information on design and construction of slow sand filters is provided by different technical manuals [15, 16,17] and proceedings [28, 29, 30].

6.3.5 Water Disinfection

Water from a slow sand filter with a well-developed biological layer is hygienic and safe for consumption. Any further treatment, such as disinfection is, therefore, not necessary. As documented by numerous examples in many developing countries, provision of a reliable chlorine disinfection system in small rural water supply schemes is often not practicable. A regular supply of mostly imported chemicals, and accurate dosage of the disinfectant, are the two main practical problems encountered.

However, as regards disinfection, one has to differentiate between small (rural) and large (urban) water supply schemes. Large distribution systems with often illegal connections present a risk of recontamination, especially if the supply of water is intermittent. In large urban water supply schemes, final water chlorination is recommended as a safeguard. However, residual chlorine will be too low and contact too short to deal with serious contamination introduced by infiltration of highly contaminated shallow groundwater in intermittently operated water supply systems. In rural water supply system, implementation of a general health education programme with special emphasis on correct water handling is a more effective measure than preventive d is infection.

An example of a water treatment plant operating without any foreign chemicals or energy inputs is illustrated in Fig. 20. The pipe layout of this 60 m³/d capacity plant provides the necessary flexibility to run the plant uninterruptedly also during the required cleaning and maintenance activities.

6.4 Water distribution

6.4.1 Water Storage

To make full use of the treatment capacity and to avoid interference of the treatment process by intermittent operation, water treatment installations should preferably be operated uninterruptedly on a 24-hour basis. Particularly slow sand filters should be operated continuously to provide the biological layer with a permanent supply of nutrients and oxygen. Roughing filters are less sensitive to operational interruptions, although careful restarting of filtration should be observed in order not to resuspend the solids accumulated in the filler. Water supply schemes, operated entirely by gravity, can easily handle a 24-hour operation. However, pump operation is often reduced to 6 -16 hours a day in water supply systems requiring raw water lifting. In pumped schemes, construction of a raw water tank may offer an economically and technically sound option since it enables continuous operation of the treatment plant and also acts as presedimendation tank. Fig. 21 illustrates possible installations for a controlled and constant raw water supply of the treatment plant.

Water storage capacity must be provided to compensate for daily water demand fluctuations. In rural water supply schemes, daily water consumption occurs more or less in the morning and evening hours. Therefore, a storage volume of at least 30 to 50% of the daily treatment capacity should be provided to compensate for the uneven daily water demand distribution.

6.4.2 Distribution System

Water accessibility and not so much water quality is the most important criteria for the consumer as his main concern is the walking distance between his home and the water point. Consequently, treated or better quality water has to be brought nearer to the homes than the traditional water sources. Treated river water as a new water source is likely for instance to be more readily accepted if the original walking distance to the river can be reduced substantially by the installation of a water supply system.

A water distribution system will therefore have to be constructed. The service level of a piped system is dependent on the economic situation construction costs of a distribution system normally amount to 50 - 70% of the total investment costs of a water supply scheme, including a water treatment plant. Gravity schemes should be installed whenever possible. In many instances, however, topography is unfavourable and differences in altitude must be overcome by water lifting. Pumps require, however, relatively high investment and operating costs, spare parts and, particularly, energy, an aspect which will, in future, gain increased importance. In rural water supply schemes, pumped systems should therefore be introduced only after careful consideration and in exceptional cases.

Fig. 20 Example of a Water Treatment Layout

Fig. 15 on page Vl-1 illustrates different hydraulic layout possibilities. On the raw water side, the water flows by gravity directly to the treatment plant or, if pumped, preferably first to a raw water balancing tank. After passing through the treatment plant it is stored in a reservoir and later distributed to the consumers by a piped gravity scheme close to the houses. In a semi-piped scheme, the water flows by gravity through the treatment plant into the reservoir equipped with handpumps, or, as an extended alternative, the reservoir is connected to a system of cistern located between treatment plant and village. Treated water is now supplied by gravity to these cisterns equipped with handpumps. Each cistern acts as reservoir and water point.

Such distribution systems may increase sustainability and reliability of a water supply as the energy supplied by the consumers when operating the handpump keeps the water supply system running at low operating costs and at village maintenance level. The proposed system of storage tanks equipped with handpumps can best control excess water usage, prevent contamination and avoid wastewater disposal problems.

However, the consumer may require higher service levels than the aforementioned "handpump option". On the one hand, higher service levels run parallel with increased water consumption and wastewater disposal problems, on the other, collection of water charges may become easier if the distribution level is shifted from public to individual supply.

Concerning the different service levels, the following per capita daily water demand values are generally used:

supply with public handpumps

q = 15 - 25 I/c.d

supply with public standpipes

q = 20 - 30 I/c.d

supply with yard connections

q = 40 - 80 I/c.d

supply with multiple tap house connections

q = 80 - 120 I/c.d

The effective q values for the supply with public handpumps or standpipes are greatly influenced by transport distance, ranging from a few dozen to 300 and more metres. For yard and house connections, water use will be influenced by the level and manner in which the water charges are levied (e.g. as a monthly lump sum or on an effectively used water volume basis recorded by water metres). Furthermore, use of drinking water for backyard garden irrigation leads to an enormous water demand and should therefore be prohibited.

Fig. 21 Raw Water Supply and Flow Control

The Handpump Handle Keeps the Water Supply Operational

The Handpump Handle Keeps the Water Supply Operational

A large number of irrigation canals supply water to the Gezira/Managil zone where cash crops, such as cotton, corn and vegetables are grown. The fertile soil and sufficient water drawn from the river Nile are the base for profitable agriculture. However, the income of the farming community, which has settled along the irrigation canals in modest straw huts, is very low. Since malaria, bilharzia and diarrhoeal diseases are also widespread among the population, the Blue Nile Health Project was launched to improve the health situation of the people living in the project area.

Absence of infrastructure, energy and low income of the population placed severe constraints on the water supply improvement scheme. The villagers, which live in settlements of 200 - 500 people, drew their water from the irrigation canals contaminated by human excreta. Use of groundwater drawn from a well has always been favoured. However, since groundwater was often unavailable, the polluted irrigation canal water had to be treated.

In the implemented standard design of the project area, the water flows by gravity from the canal through a horizontal-flow roughing filter and a slow sand filter into the clear water tank. A simple handpump was installed on top of this tank to allow the drawing of treated water from the installation. The operating costs are kept minimal and mainly used for new handpump rubber seals which have to be replaced every two months and for filter cleaning carried out by the community twice a year. The daily energy input of the water drawers at the handle of the handpump keeps the system running and provides clean water even to an underprivileged society.

7. Roughing filtration application

7.1 Historic use

The natural water treatment potential was adopted long before chemical water treatment methods, such as chlorination and flocculation, were discovered and applied. Gravel and sand used as filter media are key components in natural treatment processes. Although sand was able to maintain its important role since the development of the first slow sand filters at the beginning of the last century, the use of roughing filters was successively replaced by chemical water treatment processes. A comprehensive review of gravel filter application is far beyond the scope of this manual. However, a few examples presented hereafter will document that the roughing filter technology is an old water treatment process used in the past and rediscovered in recent years.

Numerous castles and forts were constructed in Europe during the Middle Ages. They were often located at strategically important points, difficult to conquer and also to supply with water. Ingenious water supply installations were therefore constructed. A good example is the former castle of Hohentrins located on top of a steep rocky reef in the Swiss Alpine valley of the river Rhine. During periods of war, the people who sought protection in this castle depended on rainwater collected in the yard and stored in a cistern. In this extensively used area, it was, however, not possible to avoid water pollution caused by man and animal. Therefore, in order to treat the water, a gravel pack was installed around the inlet of the cistern. This is probably one of the first roughing filters used to treat surface water [31].

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

"Water from the River Cart flowed to a pump well through a roughing filter about 75 feet long, composed of "chipped" freestone, of smaller size near the well than at the upper end. This stone was placed in a trench about eight feet wide and four feet deep, covered with '´Russian mats" over which the ground was levelled." (cited from [1]).

The pretreated raw water was then lifted by a steam engine-driven pump to a place 16 feet higher than the river from where it flowed by gravity to the water treatment plant. This installation consisted of three concentric rings each six feet wide and arranged around a central clear water tank measuring 23.5 feet in diameter. The water flowed in horizontal direction from the outer ring, which was used as settling basin, through the two other rings towards the centre into the clear water tank. The two inner rings contained coarse and very fine 9 ravel or sand as filter material respectively. John Gibb applied, already then, the multi-stage treatment approach; i.e., the intake filter, the settling basin and the gravel filter were used as pretreatment processes prior to sand filtration. Many other water treatment plants in England followed the example of Paisley and applied coarse gravel and slow sand filtration. In the last century, the general water treatment practice in Great Britain comprised the use of multiple filtration in form of roughing filters placed in front of slow sand filters. It was only in 1925 that rapid sand filters were slowly introduced to increase the capacity of slow sand filters. In the US, however, the clay content in the raw water prohibing adequate slow sand filter operation was one reason for developing rapid sand filters at the turn of the century.

Puech-Chabal filters, constructed in France in 1899 to treat part of the water supplied to the city of Paris, are another example of roughing filter application. The treatment scheme consisted of a series of filters and cascades to treat turbid surface water. The water flowed through four downflow roughing filters and one so-called prefilter before being treated by a finishing filter. Cascades were used to aerate the water in between the different filter stages. The filter material decreased successively in size, and the filtration rate was also reduced from filter to filter. The Puech-Chabal treatment system was used extensively in Europe. By 1935,125 plants were built in France, nearly 20 in Italy and some in other European countries [1].

After some time, the roughing filters were virtually converted into rapid or mechanical filters. Coagulation, combined with sedimentation, was introduced as a pretreatment method and, more recently, direct filtration (coagulation, flocculation and solids removal are carried out in filter units only) replaced the prefilter technology. In recent years, however, the roughing filter technology has been revived in Europe through its use in artificial groundwater recharging plants. In the early 1960s, the waterworks of Dortmund, Germany, constructed horizontal-flow roughing filters of 50-70 m filter length which are operated at about 10 m/h filtration rate [32]. The raw water falls over an aeration cascade, crosses a sedimentation trough before entering the roughing filter at the top of the gravel bed. The filter inlet zone is progressively impounded with increasing running time, and the entrance area of the water thus slowly shifts in direction of the filter outlet. After prefiltration, the water falls over a second cascade, percolates through the sand filter bed and finally reaches the aquifer. Other waterworks in Europe (e.g. in Switzerland and Austria) followed the example of Dortmund with modified horizontal-flow roughing filter designs as shown in Fig. 22.

European rivers usually exhibit low turbidity, however, filter operation is stopped during the short periods of high turbidity. A continuous supply of water to the consumers is guaranteed by the use of the aquifer's water storage capacity. In contrast to filter plants in moderate climates, roughing filters in tropical countries usually have to handle raw water of permanent or seasonable high turbidity. Since aquifers are often unavailable due to unfavourable hydrogeological conditions, the water supplies have to draw the water directly from surface water, treat it and supply it to the consumer throughout the year and even during periods of extremely poor raw water quality. Reliable operation is especially required during the rainy season, at the beginning of the wet period when the risk of epidemic outbreaks of diarrhoeal diseases increases as a result of rain washing poorly disposed faecal material into surface waters, and later on to cope with heavy sediment loads when the faecal pollution may be reduced by high dilution. Efficient and reliable water treatment is nevertheless also required in the dry season when surface waters in arid areas may discharge poorly diluted wastewater. The need for reliable and simple water treatment processes initiated the development of roughing gravel filtration which received considerable attention in recent years. Studies on design and performance of prefilters functioning under tropical water quality conditions have been, and are still being, conducted by various research groups.

Fig. 22 Different Layouts of Horizontal-flow Roughing Filters

7.2 Development of roughing filters

Motivated by the simplicity of horizontal-flow roughing filters, different institutions em barked on laboratory and field studies in order to assess the potential of horizontal-flow roughing filters in reducing the solid matter concentration of highly turbid surface water. In 1977, the Asian Institute of Technology (AIT) in Bangkok, Thailand, conducted laboratory tests with a prefilter composed of seven gravel layers [33]. Three full-scale water treatment plants applying the AIT prefilter design were later constructed in combination with slow sand filter units. The treatment plants, monitored for about half a year, revealed a good performance of the prefilters and enabled slow sand filter runs of several months [34]. These investigations were, however, discontinued and, therefore, marked the end of the project in Thailand. Since 1979, the Pan American Centre for Sanitary Engineering (CEPIS/PAHO) conducted an experimental programme that concluded in a comprehensive review and design manual introducing/he roughing filter technology [35].

The University of Dar es Salaam, Tanzania, embarked on laboratory filtration tests in 1980. Initially, investigations on vertical-flow roughing filters revealed short filter runs of a few days only. Subsequently, the horizontal-flow roughing filter concept was developed and the design tested with a 15-m long open channel filled with three gravel fractions ranging in size from 16-32, 8-16 and 4-8 mm. The laboratory tests clearly indicated that significant solids removal is achieved only under laminar flow conditions, as sedimentation is the predominant process in roughing filtration [36]. Field tests were then conducted to assess the applicability of the horizontal-flow roughing and slow sand filter treatment combination. The pilot plant investigations compared the developed filter resistance of different slow sand filters fed either with untreated or with prefiltered turbid river water. A significant increase in slow sand filter runs was achieved with prefiltration. The field tests revealed that horizontal-flow roughing filtration combined with slow sand filtration could be a viable system for turbid surface water treatment [37].

From 1982 to 1984, extensive filtration tests were conducted by the Department of Water and Sanitation in Developing Countries (SANDEC), formerly IRCWD, at the laboratories of the Swiss Federal Institute for Environmental Science and Technology (EAWAG) in Duebendorf, Switzerland. A model suspension of kaolin was used to investigate the mechanisms of horizontal-flow roughing filtration. Two important laboratory test results established that filter efficiency is hardly influenced by the surface properties of the filter medium, and that filter regeneration can be enhanced by drainage. The results of the research are summarised in a scientific paper [38], and the more practical aspects on implementation of horizontal-flow roughing filtration are compiled in a design, construction and operation manual [39]. In a collaborative effort, the University of Surrey, the DelAgua Organisation and CEPIS/PAHO developed and implemented vertical roughing filtration in Peru in 1985. Implementation and evaluation of horizontal roughing filters [12] were extensively supported by SANDEC in subsequent years.

Financially supported by the Swiss Development Cooperation (SDC), which already cofinanced SANDEC's laboratory tests, promotion and dissemination of the horizontal-flow roughing filter technology started in 1986. Under the technical assistance of SANDEC, engineers of local institutions designed full-scale demonstration plants in order to study this technology and gain practical experience with the treatment process. Frequently, horizontal-f low roughing filters were constructed in order to rehabilitate deficient slow sand filter plants. In the past ten years, the promoted filter technology has spread to more than 20 countries and, according to SANDEC's knowledge, over 80 horizontal roughing filter plants have been constructed during this period [40]. Fig. 23 indicates the countries where these filters have been constructed. Basic information on roughing filtration, as well as new approaches and designs developed by local engineers and practical field experience with the filter technology are presented in the following chapters of this publication.

Furthermore, several institutions conducted additional studies, usually in the form of postgraduate research work [41, 42, 43, 44, 45, 11, 12], on the horizontal-flow roughing filter process. The University of Dares Salaam, Tanzania, the Tampere University of Technology in Finland, the University of Surrey in Guildford, England, the International Institute for Hydraulic and Environmental Engineering in Delft, the Delft University of Technology in the Netherlands, and the University of Newcastle upon Tyne in England, as well as the University of New Hampshire in Durham, USA conducted, among other institutions, laboratory or field tests with roughing filters. Furthermore, laboratory tests on pebble bed filtration were carried out at the Imperial College in London, England [46].

Different pretreatment methods, including horizontal-flow roughing filtration, are currently field tested on a comparative basis by an extensive research programme in Cali, Colombia, where the Instituto de Investigacin y Desarrollo en Agua Potable, Saneamiento Bsico y Conservacin del Recurso Hdrico (CINARA) investigates, in collaboration with the International Water and Sanitation Centre (IRC) in The Hague, The Netherlands, and different other international technical institutions and supporting agencies, the potential to optimise and simplify pretreatment processes [47].

Fig. 23 Geographical Distribution of Horizontal-flow Roughing Filter Use

"Hot Water" for Filter Promotion

Sadig, Manager of the Water Supply Programme, can be proud of the progress of his project. A map on the wall behind his desk is covered with small pins indicating the location of the filter projects. All the pins are arranged along irrigation canals used as raw water source for the water supply systems of the small villages. The programme has started three years ago and since then over 30 schemes have been put into operation. A success which is not shared by the progress of other development projects carried out in the region.

The pins on the map were of different colours; i.e., red, green and blue depending on the year of construction. The pins were not evenly arranged on the map. Some were scattered along the canals, but an important number of pins were clustered around a limited area of the map. All but one pin were green and blue, the single red pin was marked "Hariga", the name of the village.

Neither the project office, nor the workshop or the training centre were located near Hariga. Nonetheless, this spot seemed to be the focal point of the project. A large smile covered the manager's face when he revealed the secret of Hariga. The village was well-known to everybody for its illegal production of alcohol. Customers from neighbouring villages came to purchase the distilled alcohol at night and the purchase was quite often combined with cheerful social activities. Since distillation was not yet quite refined, the alcohol had to be diluted with clear water produced by the recently installed filter. The nocturnal customers saw the treatment plant, enjoyed the crystal clear water and wanted a similar installation in their village.

Sadig deliberately selected Hariga as demonstration site to introduce the new treatment process, as Hariga means hot water in the local language .....