Influences of Density on Circular Clarifiers with Baffles

Abstract

The effects of the inlet densimetric Froude number on the velocity and suspended solids concentration fields in secondary circular clarifiers are investigated by application of a verified numerical model. The model consists of a set of conservation equations for continuity and momentum, turbulent kinetic energy k, dissipation of turbulence energy E, and solids concentration. The densimetric Froude number as determined from typical tank geometry, solids loading and flow, varied from 0.078 to 1.052. The inlet density waterfall was studied to determine its effect on the settling zone flow pattern and tank performance. It was found, in both physical and numerical models, that the density waterfall entrained large volumes of the ambient fluid. The entrainment compensating flow rate increased inversely with the densimetric Froude number. The strength of the bottom density current, the upward flow in the withdrawal zone, and the recirculation all increased with a decreasing densimetric Froude number, largely due to the entrainment into the density waterfall. The increase in the upward flow in the withdrawal zone resulted in a degradation in the clarifier performance. There was generally good agreement between the model simulations and available field measurements and physical model data.