The destabilizing effect of particle concentration in inclined settlers

Abstract

Water scarcity has required constant water recycling, leading to a decline in water quality, further exacerbated by high concentrations of fine particles that reduce the efficiency of solid-liquid separation systems. Inclined settlers offer a viable secondary treatment option for high-turbidity water. Effective design requires understanding of operational conditions, geometry, and suspension properties. Using OpenFOAM, computational fluid dynamics simulations were performed for a continuous inclined countercurrent conduit to assess the influence of inlet particle concentration on efficiency, exploring various Surface Overflow Rates (SOR) and inclination angles. The results show that the steady state in which the flow settles is strongly dependent on the particle concentration. For very low particle concentrations, the flow is mostly stationary with little to no resuspension of particles. Increasingly unstable regimes are observed to emerge as the inlet concentration increases, leading to increased particle resuspension. Instabilities arise from overhanging zones at the tip of the suspension, generating recirculation zones that enlarge the resuspension region and induce entrainment within the bulk suspension. Shear instabilities become noticeable at large particle concentrations, further increasing resuspension. Different regimes were identified, influenced by the SOR and the inclination angles. Additionally, a Reynolds number characterizing these systems is proposed alongside a scale analysis. The findings highlight particle concentration as a critical parameter in inclined plate settler design.