Three-dimensional simulation of a secondary circular settling tank: flow pattern and sedimentation process

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.