Effects of backwash water recirculation in drinking water treatment - A case study on coagulation, flocculation, and sedimentation performance in Sweden

Abstract

The purpose of this thesis was to investigate the impact of recycling spent filter backwash water (SFBW) in coagulation, flocculation and sedimentation processes in a Swedish drinking water treatment plant. The study focused on interpreting these effects based on dissolved organic carbon (DOC), turbidity reduction, and visual floc characteristics. These values were measured through laboratory experiments, in which raw water was blended with SFBW. Results showed that by blending raw water and 5% – 40% SFBW, the DOC removal efficiency remained unchanged across all samples. However, the final DOC concentrations decreased, as the level of organics in SFBW was lower compared to raw water, suggesting a dilution effect. Turbidity measurements showed low values across all samples (0-3 FNU). Moreover, the addition of SFBW to the samples led to an increase in floc density. Due to variations in SFBW quality and flow, the implementation of an equalization tank was suggested as a useful intermediate step to improve water quality and allow potential emergency discharges to the sewer. This study is especially relevant for surface water treatment plants with SUVA values around 2.5 and a filtrate backwash system operating filtrate water with a pH of around 6. These findings suggest that the SFBW is feasible for optimal recycling process, though further research is recommended including pilot-scale testing.