Improved Design of Final Settling Tanks

Abstract

The flow and density fields in final settling tanks of biological treatment plants with a rectangular plan are accounted for by extensive observations in a prototype basin. The time-averaged flow and density fields can be summarized as follows: for an inlet structure located at the bottom, a density current of variable concentration develops. Because its flow is supercritical behind the inlet wall and due to energy dissipation, a density hydraulic jump is generated that is responsible for macroturbulence and local resuspension. The hydrodynamics of the density current, the basin end zone, and the surface effluent layer are described and a generalized approach is presented. The flow in a final settling tank may be improved with a number of modifications that can be added to existing designs or included easily in future designs. Modifications refer mainly to the inlet structure, the density current, and the outlet channels. A preliminary design procedure is presented and examples outline the computational procedure.