A secondary circular settling tank (SCST) with low hydraulic load was numerically analyzed. A transient, three-dimensional model was employed to evaluate flow in a pilot plant. Three phases were considered: clean water was a continuous phase; sludge was a dispersed phase and an air layer under atmospheric conditions was considered above the surface of the water at the top of the tank. Height and diameter of the baffle located in the tank inlet was studied in order to reduce the resuspension of sludge particles. High velocity magnitude and turbulent kinetic energy cause resuspension of solids in the center of the bottom of the tank. With the sludge outlet closed, after 30 min of simulation there was an increase of approximately 1.0% in sludge concentration. The results provided detailed insight into the hydrodynamic flow within the SCST. Keywords Secondary circular settling tank • Computational fluid dynamics • Multiphase flow • Wastewater treatment Abbreviations C D Drag coefficient H Tank height (m) Interfacial force p Pressure (Pa) r Volume fraction for the α-phase t Time (s) t q Particle drop time (s) U Velocity vector v t Terminal velocity of the particle (m/s) Greek letters Turbulence dissipation rate (m 2 /s 3) k Turbulent kinetic energy (m 2 /s 2) Dynamic viscosity (kg/(m s)) t Turbulent viscosity Density (kg/m 3) Turbulent frequency (1/s) Subscripts Indicates the phase in the flow
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