Evaluation of the performance of natural fabrics for slow sand filter protection.

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University of Dar es Salaam
The treatment potential of slow sand filtration is demonstrated by the fact that many cities in Europe (Paris, Zurich, Amsterdam, London etc.) as well as in Asia (Thailand, Singapore and South Korea) still use it either as a secondary filtrate step or as the main filtration process in small water works. The use of SSF in the developing World is also well documented. In these countries, it is considered to be particularly suitable for rural water supply schemes in view of the ease and simplicity of operation and maintenance. The main factor limiting the applicability of SSF is its short filter run times which place a big demand in terms of labour and effort in filter cleaning even if the process is mechanized. Research into the use of non-woven synthetic fabrics in the United Kingdom have shown that it is possible to concentrate the majority of impurities in the NWF layers thus protecting the sand bed so that an ultimate extension of the filter run time is obtained. In Tanzania, however, due to the inavailability of NWF, research into the use of locally available natural fabrics for SSF protection has been necessitated. This study is aimed at investigating to what extent, the natural fabrics, coir and kapok can protect the SSF Unit particularly the household SSF. During this study, laboratory and field tests were conducted at a filtration rate ranging from 0.08 - 0.24m/hr. These tests were conducted on inlet water and filtrates from SSF units for water quality improvement/changes. Parameters monitored included: head loss, Temperature, pH, Turbidity, Bacteriological quality, conductivity, TDS, D.O. Apparent colour, suspended solids. Results showed that maximum removal efficiencies of the household SSFs were obtained at a filtration rate of 0.16m/hr. It was also found that it is possible to concentrate the majority of impurities in the fabric layers such that routine maintenance involved cleaning of the fabrics only at the end of the run. It was also shown that lower rates of head loss development could be achieved by optimization of the thickness of the fibres. As a result of this, filter run time extension by a factor of up to to 1.4 was attained. With regards to Kapok fibre, even though it was possible to extend filter run time and produce fairly good water quality it was found unsuitable for long term protection of the filter. This is because with continued microbiological degradation, penetration of cellulose into the top few cms of the underlying sand can occur and if not curtailed it could damage the filtration mechanism of the SSF. Coir fibre on the other hand afforded good protection to the SSF as shown by the results and the degradation rate was minimal. It is thus recommended that coir and other locally available vegetable fibres with high lignin content be further explored for use in SSF protection
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Water purification, Filtration, Fabric filters, Tanzania
Sanyaolu, B. O. (1998). Evaluation of the performance of natural fabrics for slow sand filter protection. Master dissertation, University of Dar es Salaam. Available at (