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<Sourcebook of Alternative Technologies for
Freshwater Augumentation PART B - ALTERNATIVE TECHNOLOGIES 1. TECHNOLOGIES GENERALLY APPLICABLE TO ISLAND STATES 1.2 Water Quality Improvement Technologies 1.2.2 Slow Sand Filtration Technical Description A distinguishing feature of slow sand filters is the presence of a thin layer, called the schmutzdecke, which forms on the surface of the sand bed and includes a large variety of biologically active microorganisms. A slow sand filter comprises approximately 1.2 m depth of fine sand supported on two or three gravel layers. It is a very simple and effective technique for purifying surface water. It will remove practically all of the turbidity from the water as well as most of the pathogens without the addition of chemicals. Slow sand filters can frequently be constructed largely from locally-available materials. The effective size of the sand used in slow sand filters is about 0.2 mm, but may range between 0.15 mm and 0.35 mm, and with a coefficient of uniformity of between 1.5 and 3.0. In contrast, the range of effective size for the rapid sand filters described in the preceding section is 0.35 mm to 1.0 mm, with a coefficient of uniformity of 1.2 to 1.7. The walls of the filter can be constructed of concrete or stone. Sloping walls dug into the earth, supported or protected by chicken wire reinforcement, and lined with sand or a sand-bitumen mixture could be a cost-effective alternative to concrete. Inlets and outlets should be provided with controllers to keep the raw water level and the filtration rate constant. Bottom drains consist of a system of manifolds and lateral pipes (Figure 10). The filtration rate usually employed in developing countries is between 2.5 and 6.0 m3/m2/day. Higher rates may be used, but should be tested to ensure that the higher rates yield a good quality product water. The system should be designed for flexibility. Highly turbid water may need some form of pre-treatment such as settling or rough filtration. Home filters using the principles of slow sand filtration can be constructed using a 200 l drum. Such systems are a common way of treating water for household drinking. The drum has a layer of 3 mm sized gravel on the bottom surrounding the outlet pipe, 600 mm of fine sand on top of the gravel, and a space for the inlet water. An overflow and a lid to prevent mosquito breeding and algal growth should be provided. Often large stones are placed on top of the sand to prevent the flow from the inlet pipe from scouring the sand. This type of filter is often used in rainwater systems (see above) to remove sediment before the rainwater is conveyed to a storage tank.
Figure 10. Simple drawing of slow sand filter (Schultz, 1984). Extent of Use Slow sand filters are often used in both urban and rural water supply systems on larger islands. On La Digue, Seychelles, the installation of slow sand filters has proven to be the most cost-effective method for the treatment of raw water. Water supplied to Apia and part of the west coast of Upolu, Western Samoa, is treated using slow sand filters which work well under normal flow conditions. In Rarotonga, Cook Islands, water from many of the stream sources is treated with small versions of a slow sand filter, while, in French Polynesia, slow sand filters have been operated successfully in small communities supplied by stream catchments on the high islands. A small-scale slow sand filter unit has been designed and operated in a 500 mm steel pipe. Operation and Maintenance Filter cleaning, by scraping off a 25 mm surface layer of sand, is required at intervals of between 30 and 100 days depending on the turbidity of the water being filtered. Trials in India found that, at a turbidity of 10 NTU, the filter could be run for up to 90 days, whereas, at a turbidity of 30 NTU, the filter run time would be reduced to 30 days. Filter surface fabrics have been developed for slow sand filters to improve and ease the cleaning process. However, because aliquots of the sand are removed in the cleaning process, the sand layer will need to be topped up from time to time. The depth of sand should not be allowed to become less than 600 mm. Topping up can be done using washed sand, removed during previous filter cleanings, or fresh sand. Pathogen removal is achieved within the biological slime layer (schmutzdecke) which forms on the surface of the sand. This will take a few days to re-form after each cleaning and there will be a slight decline in the performance of the filtration system during this time; usually, this decline is not significant. Level of Involvement The most attractive aspect of slow sand filtration is its simplicity of operation and control. After a short training period, the operation of the system can be mastered by a local caretaker. Costs Slow sand filter costs can be low if appropriate local materials are available. For large slow sand treatment plants, the land area required for constructing the system may add significantly to the cost. A slow sand filter may take up to five times the equivalent land area of a rapid sand filtration plant. Hence, the technology is best adopted by small communities where land cost may not be a problem. Because slow sand filters usually require no chemicals or energy inputs, the capital and operational costs can be very low in comparison to rapid sand filters. Maintenance costs will include minor repairs to the filters, and replacement of the sand washed out or removed during the scraping of the silt from the filter surface. Other maintenance costs relate to the replacement of the few moving parts present in the filter system. The costs will be higher in pumped schemes. Examples of specific costs for small islands are not available. Effectiveness of the Technology This is a proven technology that is very effective in removing both suspended materials and bacteria. However, the improvement in water quality brought about by slow sand filtration will differ from place to place due to raw water quality, sand grain size, rate of filtration, temperature, and the oxygen content of the water. An indication of the purification effect of a slow sand filter is summarized in Table 2 and shown in Figure 11. TABLE 2. Performance of Slow Sand Filters.
Suitability Slow sand filtration is normally used for the treatment of surface water supplies only, and, thus, would not be applicable to many of the small low-lying islands where surface waters do not exist. Slow sand filtration is most suited for use with gravity operated, surface water supply systems, or systems with large, clear water storage facilities, as the filters require a continuous flow of water.
Figure 11. Effect of sand size on removal of total coliform bacteria in slow sand filtration (Visscher, et al., 1987). Advantages The cost of construction is low, and its simplicity of design and operation means that slow sand filters can be built and used with limited technical supervision. Little special pipework, equipment, or instrumentation is needed, and the labour required for maintenance can be unskilled as the major labour requirement is in cleaning the beds, which can be done by hand. Imported materials and equipment is usually negligible and no chemicals are required. Likewise, power is not required if a gravity head is available, and there are no moving parts or requirements for compressed air or high-pressure water. Variations in raw water quality and temperature can be accommodated, provided turbidity does not become excessive, and overloading for short periods does no harm. Compared to rapid sand filtration, there is a net savings of water as large quantities of backwash water are not required. Disadvantages Slow sand filtration units require large land areas for plants treating large flows (about five times that of rapid sand filtration plants). Clogging may occur if the source water is excessively turbid or if certain (filamentous) types of algae are present in the raw water. Pre-treatment with roughing filters or settling tanks may be necessary if such clogging occurs frequently. Also, toxic chemical contamination of the raw water may affect the biological surface layer (this could be a good indication of water source problems!) Cultural Acceptability Land ownership issues could become a problem, but there are no known cultural concerns associated with the use of this technology. Further Development of the Technology Although this technology is well known, there have been many new developments in slow sand filtration. In association with roughing filters, slow sand filters have been used very effectively for cleaning very turbid pond water for small village, community water supplies in West Bengal, India. Small-scale applications of this technology could benefit small islands which need low technology surface water treatment technologies. Information Sources American Water Works Association 1971. Water Quality Treatment. Third Edition. McGraw-Hill Book Company, New York. James M. Montgomery Consulting Engineers, Inc. 1985. Water Treatment Principles and Design. John Wiley and Sons, New York. Pescod, M.B., H. Abouzaid, and B.B. Sundaresan 1986. Slow Sand Filtration: A Low Cost Treatment for Water Supplies in Developing Countries. World Health Organization, Geneva. Pickford, J. 1991. The Worth of Water, Technical Briefs on Health, Water and Sanitation with an Introduction by John Pickford. Intermediate Technology Publications, London. Smethurst, G. 1992. Basic Water Treatment for Application World-Wide, Second Edition. Thomas Telford, London Schulz, C.R. and D.A. Okum 1984. Surface Water Treatment for Communities in Developing Countries. John Wiley and Sons, New York. UNICEF [United Nations International Children's Emergency Fund] 1992. Proceedings of the Workshop on Sanitary Protection and Upgradation of Traditional Surface Water Sources for Domestic Consumption. All India Institute of Hygiene and Public Health, Calcutta. Visscher, J.T., R. Paramasivam, A. Raman, and H.A. Heijnen 1987. Slow Sand Filtration for Community Water Supply. International Water and Sanitation Centre, The Hague.
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