Dynamic Model for Zone Settling and Compression in Gravity Thickeners

Abstract

A model was developed for predicting the solids concentration profile in the zone-settling and compression regimes of a gravity thickener. This dynamic one-dimensional model arises from a force balance across a settling layer and retains the effective solids pressure component of that balance. A function relating effective solids pressure to solids concentration and the rate of change of solids concentration is proposed from consideration of the hydrodynamic stress transmittance. The final expression contains four parameters, two for the zone-settling velocity component, which are determined from batch settling tests, and two new parameters, which are determined by regression from solids concentration profile data. The model is applied to steady-state secondary clarifiers and provides good agreement with literature data for both laboratory and field-scale clarifiers. Unlike other methods, this model directly predicts the solids concentration profile in both the zone-settling and compression regimes for various operating conditions with one set of sludge-specific parameters.