Effects of inlet water flow and potential aggregate breakage on the change of turbid particle size distribution during coagulation-sedimentation-filtration (CSF): Pilot-scale experimental and CFD-aided studies

Abstract

Considering the limitations of using treated water turbidity as the primary water-quality indicator in water treatment plants, pilot-scale experiments and Computational Fluid Dynamics (CFD) simulations were conducted to examine how inlet flow rate and potential shear-induced aggregate breakage affected the evolution of coagulated turbid particles and overall performance of coagulation-sedimentation-filtration (CSF). With increasing flow rates (5–8 m³/h), settled water turbidity and total particle number both increased, with the highest rate (8 m³/h) reducing the removal efficiency of larger-sized turbid particles (> 15 μm) after sedimentation. Although sand filtration achieved about 99 % turbidity and 97 % particle removal efficiency at all flow rates, higher flow rates caused severe filter clogging. Regarding the influence of mixing speed, settled water turbidity and total particle number first increased and then decreased (with the peaks at 51 rpm), and a certain degree of shear-induced breakage appeared to improve the removal percentage of UV₂₅₄ after sedimentation and reduce the head loss rise in sand filtration process. Moreover, CFD-based discussion highlighted that higher inlet flow rates intensified particle mixing and collisions in the flocculation tank, while excessive mixing speeds increased local shear forces and energy consumption near the impeller, worsening filter clogging during filtration.