EFFECT OF WASTEWATER TREATMENT BIOLOGICAL REACTOR GEOMETRY ON REQUIRED MIXING INTENSITY

Mixing in wastewater treatment biological reactors is essential as it facilitates biological and chemical processes. Quantifying mixing intensity and the time to achieve complete mixing in a reactor will be helpful in developing models to describe various biological and chemical processes in the reactor mathematically and in improving the energy efficiency of reactor operation. This work therefore aims to quantify the effect of different geometries on the mixing time and efficiency which can serve as a new design methodology for future designs and assessments. Three typical activated sludge reactor geometries were considered, consisting of circular, square, and rectangular shapes. Constant fluid volume and power input were maintained across all three geometries to quantify geometric effects on the mixing time. Computational fluid dynamics (CFD) simulations were conducted to simulate the flow and mixing within each case, where turbulence was modelled with the SST k-ω Model. Initially, the activated sludge and water phases were modelled in these reactors using a species transport model to quantify mixing and the time required to achieve a complete mixture of activated sludge and water. However, as the activated sludge has physical properties different from water (density and viscosity), using an Euler-Euler approach was considered prudent. Therefore, the Euler-Euler mixture model was used to study the mixing. The 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) and mixing intensity (MI) where thresholds were used to establish when insufficient mixing was taking place in the reactor. The square tank exhibited the lowest mixing time for the same power input and volume compared to rectangular and circular reactors. The effects of energy density/mixing intensity on the coefficient of variance (COV) was also studied for differing volumes, where minimum thresholds to achieve good mixing were established.