Innovative technology for reducing the finished water turbidity during sand filtration

Sand filtration efficiency in treating low turbidity water greatly depends on the electrostatic interactions of both the surface of influent particles and that of sand grains or the deposited matter on the grains. This article describes two innovative filtration techniques to increase the particle separation efficiency: (1) coagulant-coated filter medium by enhancing the electrical potential of the surface of the filter medium; and (2) coagulant dosing in the influent by controlling the electrical potential of particles entering the filter layer. From the results of the various filtration experiments using a pilot plant, these two techniques were found to be very effective in reducing the effluent water turbidity during an initial stage of a filter run, and from the start to the end of a run, respectively. It should be also noted that the effectiveness of each method increased, as the amount of coagulant PACl used in the experiment increased. Moreover, in the filtration experiments using these two methods simultaneously, substantially higher removal efficiency of approximately 3 log (99.7%) was realized, resulting that the finished water turbidity was accordingly reduced to 0.004 mg/L.