Improving slow sand filtration through sand depth optimization and lead removal using zeolite: case of Kamuli water treatment plant

Abstract

Globally, there are increasing challenges related to physiochemical and bacteriological contamination often met in provision of safe water for human consumption. Eventually, increasing access to portable water ranked high on the United Nation’s 2030 agenda. This study focused on improving slow sand filter through optimization of the filter medium depth and the use of zeolite to remove lead from water. Kamuli water treatment plant was considered as the case study. In characterizing raw water from the dam as intake, the minimum and maximum turbidity values recorded during the four months were 3 and 103 NTU before treatment, 1 and 48 NTU were recorded after treatment, respectively. The existing Kamuli water treatment plant exhibited turbidity removal efficiencies in the range of 0 – 26%. To optimize the filter medium for turbidity removal, four different sand depths including 400, 650, 900 and 1150mm were considered in a down scaled physical system setup. Synthetic turbid water with turbidity levels from 5 NTU to 120 NTU were considered to characterize the turbidities recorded from the raw water. The different set ups of sand depths were individually evaluated for system performances. The 400mm and 650mm depths had average removal efficiencies of 81.8% and 85.7%, respectively. However, the 900mm and 1150mm had removal efficiencies of 90% and 93.7%, respectively. Different models in terms of exponential, logarithmic, linear and polynomial functions were used to describe the variation of final turbidity after raw water filtration with sand depths. The models were assessed in terms of mean squared error (MSE) and the cost of the sand to achieve the required 5 NTU based on the World Health Organization guideline for turbidity. The values of MSE for exponential, logarithmic, linear and polynomial models were 0.4758, 0.4078, 0.6535 and 0.3848, respectively. The corresponding costs of sand to achieve the optimal depth based on the models were 2,880, 3,300, 3,235 and 2,915 Uganda shillings, respectively. For a given contact time, efficiency of lead removal increased with increasing mass of zeolite added to water. This was true especially for contact time greater 40 minutes. For instance, the efficiencies of lead removal using zeolite of 1, 3, 5 and 7 grams at a contact time of 80 minutes were 81.8, 90.8, 92.7 and 100%, respectively. However, at a 40-minute contact time, the lead removal efficiency increased as zeolite mass was varied from 1g to 5g and thereafter it decreased. Thus, the optimal removal of lead was at contact time of 40 minutes using zeolite mass of 5g with removal efficiency of 98%. This study therefore, demonstrated potential of zeolite in lead removal.