13.2 Filter material

The filter material should have a large specific surface to enhance the sedimentation process taking place in the roughing filter, and high porosity to allow the accumulation of the separated solids. Generally speaking, any inert, clean and insoluble material meeting the above two criteria can be used as filter medium. Filtration tests revealed that neither the roughness nor the shape or structure of the filter material have a great influence on filter efficiency [10]. The following material could therefore be used as filter media:

· gravel from a river bed or from the ground

· broken stones or rocks from a quarry

· broken burnt clay bricks

· plastic material either as chips or modules (e.g. used for trickling filters) may be used if the material is locally available. Attention has to be paid to the uplift forces of the water

· possibly burnt charcoal, although there is a risk of disintegration when cleaning the filter material, it should only be considered in special cases (e.g. for removal of dissolved organic matter)
· possibly coconut fibre, however, due to the risk of flavouring the water during long filter operation, it should be used with care.

Every Day and Every Kilogram Count Every Day and Every Kilogram Count I was about to go for lunch when the phone in my office rang. Peter, a former working colleague, was at the other end. With the same enthusiasm as in critical situations, I was informed that his office was contracted by the Disaster Relief Unit to design and construct a water supply system for a resettlement camp in East Africa. The camp should host 20,000 refugees and the infrastructure should consist of simple shelters, a feeding centre, a small hospital, water and sanitation facilities to be set up as soon as possible. Peter needed technical advice on the design of the water treatment plant which had to meet special construction criteria; i.e., installation time and weight of building material had to be kept to a minimum. After the call, I had my lunch in the train heading towards the capital where I met Peter in his office early afternoon to discuss the water supply project. The raw water had to be pumped from a large irrigation canal whose water quality was unknown but rather turbid looking. Cement is rather expensive and often scarce in that country. We sat between drawings, reports and computers and looked at each other waiting for a stroke of genius. Meanwhile, the water of a small biotope in the back garden was reflecting the sunlight of the warm summer afternoon. Peter suddenly got enthusiastic again and proposed to use the "biotope construction technique". The rest of the design was completed within a short time. Two weeks later, Toni volunteer of the Disaster Relief Unit and construction foreman, was sitting in a cargo aircraft heading for Africa. The aircraft also carried a number of large plastic sheets and different plastic fittings which were unavailable in the country of destination. Soon after his arrival, Toni started construction of the treatment plant with about 100 casual labourers, and within six weeks the pumping station, two sedimentation tanks, four horizontal-flow roughing filters and the treated water reservoir were set up. The tanks and filters were designed as earth basins with inclined walls and earth dams made of bags filled with the excavated soil. The basins were then coated with the prefabricated plastic linings. Perforated pipes were laid in the roughing filters to allow hydraulic filter cleaning. Trial operation of the treatment plant revealed a satisfactory efficiency; i.e., the raw water turbidity of 1,000 2,000 NTU was reduced to half of its initial value by the sedimentation tanks, and turbidity in the effluent of the roughing filters was recorded at 5 - 20 NTU. The infrastructure of the refugee camp was then handed over to the local Commissioner for Refugees. However, political disputes about implementation and use of the camp started among the countries involved soon thereafter. Finally, 5,000 refugees settled almost two years after completion of the camp and reminded Toni of an experience he had during construction. A local foreman had told him: "You have a watch but I have the time .... ".

Gravel is the commonly used filter material but it was replaced by broken burnt bricks in the horizontal-flow roughing filters constructed by the Blue Nile Health Project in the Sudan [55], by palm fibre called "ijuk" in a roughing filter project in Indonesia [45], and by plastic material in laboratory tests at the University of Newcastle in England [45]. Table 6 shows that filter efficiency of bricks and plastic as filter material is similar to gravel with respect to turbidity reduction. The filter filled with palm fibre has a better respective performance compared to the gravel filter. Greater porosity (92% versus 37%), responsible for the reduction of the effective flow velocity, is certainly an explanation for this observation. However, since use of the palm fibre caused a considerable drop in the dissolved oxygen concentration, odour and taste problems could occur. Therefore, more detailed investigations are always necessary before palm fibre or any other alternative material is used on a long-term basis.

Roughing filters are usually composed of three filter fractions ranging in size from coarse to fine. The coarse and a large amount of the finer suspended solids are removed by the first filter pack. Since a large pore volume is required in this part of the filter, a coarse filter material is best installed over a considerable filter length. The subsequent filter material is finer and the packs installed over a shorter filter length. The last filter fraction of limited length should assume only a polishing function as it is supposed to remove the last traces of the finest suspended solids found in the water.

Table 6 Relative Filter Performance with Different Filter Material (parallel tests)

*only filled in first filter compartment

Since filter efficiency increases with decreasing filter material, one is tempted to use the smallest possible filter material or even to omit the larger filter material and to install only one the finest - filter medium. However, the roughing filter technology requires coarse filter material as denoted by its name. The finest filter material should not be smaller than about 4 mm to ease hydraulic filter cleaning. Filter material which is too coarse, however, has a smaller filter efficiency and would therefore require a longer filter length to achieve the same turbidity reduction. As already illustrated by Fig. 31, use of at least two or generally three different filter material sizes will result in an economic filter design and adequate filter operation.

Table 7 Guidelines on Size and Length of Filter Material for Different Types of Raw Water

Table 7 lists some general guidelines on size and length of different filter fractions. These guidelines should not be followed too rigidly. Practical aspects, such as e.g. availability of specifically graded material from a quarry, are more important. If adequately graded filter material is not available, gravel at the construction site can also be sieved through wire meshes or perforated steel plates used as sieves.

During the initial development phase of the horizontal-flow roughing filter technology, the Asian Institute of Technology (AIT) in Bangkok recommended the installation of six to eight small gravel layers [58]. Gravel size should subsequently be reduced from 20 to 2.5 mm and thereafter increased again to 25 mm. There is no reason why the smallest gravel fraction should be located in the centre of the filter bed since the following gravel packs have, by nature, a smaller removal efficiency.

The filter media used for roughing filters has to be clean and free from organic material. It is therefore important to wash the aggregates thoroughly in order to remove all loose and dirty material from the surface of the filter media. If this recommendation is not followed, the effluent quality of the roughing filter will be poor and result in rapid clogging of the slow sand filter.

The different filter fractions should be separated from each other to avoid mixing of the aggregates during manual filter cleaning. The filter material of an upflow roughing filter in layers is preferably separated by a plastic wire mesh. The different filter media of roughing filters in series are separated by the different filter boxes, and perforated separation walls are used for that purpose in horizontal-flow roughing filters. Burnt brick or cement block walls with open vertical joints are best suited for separating the filter fractions. The total area of the open joints should ideally amount to 20 to 30% of the total filter cross section and be equally distributed over the entire cross section to maintain an even flow throughout the horizontal-flow roughing filter. Prefabricated perforated bricks or blocks (e.g. holes F 3 cm, spacing 5 x 5 cm) or loose rubble could be installed instead of open joints. Simultaneous filling of filter material in layers must be observed with loose or weak separation wall structures.