Hydraulic flocculation with up-flow roughing filters for pre-treatment of surface water prior to conventional rapid sand filtration / Hydraulisk flockning med uppströmsförfilter för förbehandling av ytvatten före konventionell snabbsandfiltrering
Hydraulic flocculation has been used in many different ways in drinking water treatment for many years. In this paper, the results of experimental work using an up-flow roughing filter for hydraulic flocculation prior to treatment with conventional rapid sand filtration are presented. The objective was to evaluate optimum flocculation conditions with up-flow roughing filtration and the quality of formed suspensions with respect to their filterability. River water was used for the experiments. Turbidity removal, head losses development and velocity gradients in the roughing filter were the parameters used to evaluate the effectiveness of pilot plant processes. Overall turbidity removal in the pilot plant was between 84 % and 97 %. Removal efficiencies in the up-flow filter were between 18 % and 73 % and at the rapid sand filter between 77 % and 92 %. Both units operated under positive pressure. Irrespective of operational conditions established, G-values between 45–190 s–1 were attained in the up-flow filter. Best performances were attained when the up-flow filter was operated at the lowest filtrations velocities and alum doses. Overall, the up-flow filter performed both as particle aggregation and separation unit and the quality of formed suspensions was suitable for removal by rapid sand filtration. The method can therefore provide a rather versatile technique for pre-treatment of turbid water prior to conventional rapid sand filtration.