The potential for the use of roughing-slow sand filtration systems in Zambia
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There is growing concern in most developing countries regarding the use of technologies that are inappropriate to local conditions. In the area of treating surface water to drinking water quality, consideration of alternative treatment methods is receiving great attention particularly with regard to small and medium community water supplies. In Zambia, surface water treatment to potable water is mainly by conventional methods. A recent evaluation of these methods revealed that operation and maintenance problems associated with the chemical pretreatment stage were rampant, particularly in small and medium community water supplies.To date, there have been hardly any studies on alternative methods. The current trend in the water sector is largely inclined towards addressing the rehabilitation of water supply systems so that private sector participation is facilitated. One of the sector principles, however, is to consider alternative treatment methods to alleviate current problems.The combination of roughing and slow sand filtration systems has emerged to be an appropriate alternative to conventional methods in most small and medium community water supplies. Roughing filtration as a pre-treatment method neither requires expert supervision nor chemicals. However, current studies elsewhere reveal that adequate design guidelines for roughing filters are not yet fully established. Slow sand filtration, as the main and final filtration stage, is excellent in producing potable water. However, new applications of slow sand filtration require pilot testing to ascertain their suitability. Operation and maintenance needs of roughing and slow sand filtration systems are reported to be simpler and economical compared to conventional systems. Nevertheless, for new applications, and where local experience is lacking, this can only be ascertained through pilot studies. The principal objective of the study was to evaluate the potential of a combination of roughing filtration and slow sand filtration systems for small and medium community water supplies in Zambia (using local materials) as alternatives to conventional systems. A pilot plant encompassing up-flow roughing filtration in layers and slow sand filtration processes was designed, constructed and investigated. Local filter media were used for the filtration processes. The pilot plant treated Kafue River water and high turbidity simulated raw water.The use of simulated raw water was inevitable since the investigation period did not cover the rain season when high turbidity raw water is common. The characteristics of the actual Kafue iver water during the period of investigation were: daily average turbidity < 5 NTU, total suspended solids < 5 mg/1, faecal coliforms < 200 FC/100 ml. Those of the simulated raw water were: daily average turbidity < 300 NTU, total suspended solids < 2000 mg/L and faecal coliforms < 4000 FC/100 ml. The performance of the pilot plant was evaluated by analyzing the quality of the filtrates. Roughing filters were operated at filtration rates ranging from 0.4 to 1.25 m/h, while slow sand filters were run at an average filtration rate of 0.24 m/h. Up-flow roughing filters in layers managed to pre-treat raw water to quality suitable for slow sand filtration, by significantly reducing the levels of turbidity, total suspended solids and faecal coliforms. There was no significant difference in performance, with respect to turbidity and suspended solids removal, of the roughing filters by varying filtration rates from 0.4 to 1.25 m/h. However, the removal of faecal coliforms was slightly lower at 1.25 m/h. The final slow sand filtrates showed acceptable turbidity levels (<1 NTU). However, faecal coliform levels occasionally exceeded the less than 1 FC/100 ml recommendation by the World Health Organization. Hence, slow sand filtrates may still require disinfection to guarantee potable water supply. Because of the sufficient pre-treatment provided by roughing filtration, slow sand filters were characterized by longer filter-runs than those reported for slow sand filters applied in Zambia, and elsewhere where chemical pre-treatment methods are used. The operation and maintenance of the pilot plant was easy, simpler and economical, managed by a local, compared to reported operation and maintenance requirements of conventional systems. It was therefore concluded that the use of roughing and slow sand filtration systems has great potential for small and medium communities in Zambia. Local materials can readily be utilized to construct these systems. The systems are able to treat raw water of high turbidity to potable water without the use of pre-treatment chemicals. Operation and maintenance procedures are relatively easy and can even be met at community level management. The results of the study provide the first basis for designing roughing and slow sand filtration systems in Zambia based on local practical investigations.
- Engineering