Browsing by Author "Makokha, Mary"

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    Investigation of the effect of the hydraulic design of the underdrainage system on the cleaning efficiency of downflow roughing filters (DRF) for drinking water treatment.
    (University of Dar es Salaam, 2001) Makokha, Mary
    DRF are usually cleaned hydraulically by fast filter drainage. To date, no scientifically derived criteria for assessment of the cleaning efficiency of the underdrainage system of the DRF has been developed. In reviewing the DRF hydraulic system, this study used both the theoretical analysis followed by the experimental rigs to determine the effect of the hydraulic design on the cleaning efficiency of the DRF. Theoretical analysis was done using the method of successive steady states to the solution of unconfined flow using the Dupuit Forcheimer approximations as in ground water flow. The experimental rigs were used to determine the filter efficiency using the filter coefficient (lcm-1). From the theoretical model it was established that varying the position of the drains from the filter bottom substantially affected the efficiency of the underdrainage system. Available water table head is proportional to the volume of water drained. As the length of influence increased the amount of discharge reduced. Hydraulic cleaning was done in 3 drainage cycles. The varying head depended on the falling water table, which influenced the initial drainage velocity. When a head of 0.39m was used, the initial drainage velocity was 40.4 m/h and when a head of 0.3m was used in the experiments the resulting velocity was 30.12 m/h. From both the rig-based experiments and the theoretical model it was established that cleaning with raw water inlet valve open did not provide enough head to effect the cleaning process but instead resulted into unnecessary loss of washwater. Therefore hydraulic cleaning with raw water inlet valve closed was strongly recommended.