Effects of Gravity Currents in Circular Secondary Clarifiers

Abstract

A determining factor for the performance of secondary settling tanks is the occurrence of gravity currents. Experiments were carried out in a laboratory model of a circular, center‐feed settling tank. The gravity currents were scaled using the internal Froude number criterion, and the settling velocity was scaled on the basis of kinematic similarity. The activated sludge was represented by kaolinite. A flow pattern with three layers of different densities and different flow directions was observed. A simple one‐dimensional analytical model taking into account the influence of flocculation was developed to predict the effluent concentration. It was found that gravity currents may result in a layered flow pattern reducing short circuiting. This flow pattern thus seems to promote settling rather than to deteriorate it as is generally assumed.