DR. NDIBA PETER KURIA

Filtration accounts for a substantial portion of water treatment cost. The dual media filter, by operating at a higher filtration rate than the conventional rapid sand filter, can reduce the cost of filtration considerably. However, use of the dual media filter in Kenya is limited by lack of suitable material for use as coarse media. In this study, pilot plant filtration tests have been carried out to investigate the performance of crushed coconut shell as coarse media in dual media filters. Filtration rate, size of crushed coconut shell, relative depth of media and backwash requirements have been investigated. A dual media filter with equal depths of 1.20 rom effective size crushed coconut shell and 0.42 romeffective size sand was found to give the best performance. The filter was found to operate at 2.4 times the filtration rate of the conventional rapid sand filter while maintaining the same effluent quality and length. of filter runs as the rapid sand filter. The filter also required a smaller percentage of filtered water for backwashing than the rapid sand filter. Durability tests on crushed coconut shell media have indicated that the media would be durable against microbiological degradation while in service. Scrapping off the less compact surface layer of the shell was found to improve the durability of the media