Author(s): Ketan Madane; Peter Leonard; Sean Mulligan
Linked Author(s): Ketan Madane, Sean Mulligan
Keywords: Mixing time CFD Oxidation ditch
Abstract: Mixing in wastewater treatment biological reactors is essential as it facilitates biological and chemical processes in the treatment process. Quantifying mixing intensity and the time to achieve complete mixing in the reactor will be useful to develop models to describe various processes in the reactor mathematically. This work aims to quantify the effect of different geometries on the mixing time. Three typical activated sludge reactor geometries which consist of circular, square, and rectangular shapes were considered. Constant fluid volume and power input were maintained across all three geometries to quantify the effect that the geometry has on the mixing time. Computational fluid dynamics (CFD) simulations were conducted to simulate the flow and mixing within each case where turbulence was modeled with the SST\ k-\omega model. In these reactors, the activated sludge and water phases were modelled using a species transport model to quantify mixing and the time required to achieve a complete mixture of activated sludge and water in the reactor was noted. A fan model was used to model the propeller of the submersible mixer. The geometry of the reactors was meshed using Ansys Mosaic meshing technology. Transient flow simulations were conducted to calculate mixing-time based on the coefficient of variance (COV). For the same power input and volume, the square tank exhibited the lowest mixing time compared to rectangular and circular reactors.
Year: 2025