Evaluation of two-stage filtration systems in drinking water treatment

Abstract

Two parallel pilot plants, PP1 and PP2, each consisting of a two-stage filtration system, were evaluated; the first stage of PP1 consisted of an upflow coarse sand filter while that of PP2 consisted of an upflow gravel filter. Both pilot plants had a rapid downflow sand filter as a second stage. System pretreatment performance was determined based on the efficiency of filtering surface raw water previously coagulated with aluminium sulphate. Filter run lengths were longer in PP1 than in PP2. Coagulant doses were determined and controlled by using a bench-scale sand filter, the dominant mechanism being adsorption-destabilization with partial charge neutralization. Results of turbidity, apparent colour, total iron and manganese concentrations in the final effluent in both pilot plants were shown to meet Brazilian standards. The quality of effluents from both systems showed no variation and thus the two filtration systems were equivalent. Conclusively, both systems were found to be technically feasible processes for water clarification and compare favourably with conventional clarification systems of flocculation, sedimentation and filtration when treating raw water with turbidity lower than 20 NTU and true colour below 50 HU.