A comprehensive hydrodynamic analysis of a full-scale oxidation ditch using Population Balance Modelling in CFD simulation

Abstract

This work exhibits the importance of the experimental validation when full-scale computational fluid dynamics (CFD) models are developed to provide a detailed analysis of the spatial variations in 3D of the fluid flow inside aerated tanks. Single-phase and two-phase CFD models were performed to study the fluid behaviour carefully by means of the velocity profiles and the aeration pattern in a full-scale oxidation ditch. Air holdup , bubble size distribution and interfacial area density were calculated by polydisperse models where Population Balance Model (PBM) was governed by break-up and coalescence; the free-surface approach allowed the CFD model to describe the threedimensional effect of bubbly plumes in large scales in detail. Tracer tests were carried out to obtain the flow pattern and the hydraulic distribution of the flow into two wastewater treatment lanes in order to define the boundary conditions for the model correctly. Despite the difficulty of performing velocity measurements of the fluid in 3D, with and without air bubbles, these provided essential information to validate the CFD model. From this analysis, several simulations were performed to improve the hydrodynamics and the operation of the process by relocating the propellers.

CFD Simulation of Aeration and Mixing Processes in a Full-Scale Oxidation Ditch

“…Two-equation models are reliable and successful models for practical engineering purposes [16]. In full-scale WWTP studies, they have a wide range of applicability [2,7,9,11,21]. In this simulation setup, the SST turbulence model used for the continuous fluid, whereas the Dispersed phase zero equation model applied to the dispersed one.…”

“…In recent years, several two-phase flow CFD simulations have been performed for full-scale OD configurations. The results show that it is possible to predict flow pattern accurately with correct CFD model development [3,7,9,11]. Therefore, simplifications in the computational geometries are still preferred and widespread approach in CFD modelling because of less computational time.…”

“…Therefore, the department of computational fluid dynamics in Helmholtz-Zentrum Dresden-Rossendorf is working on multiphase models, mostly gas-liquid flows to provide reliable simulation for the design, optimisation and safety analysis of medium and large applications [8]. Despite the rapid development of computer technology, dynamic simulation of multiphase flow in a full-scale OD requires extremely high computational resources and long computational time [3,9]. The computational challenges make considerable simplifications necessary for geometries and operating parameters such as impeller modelling and using a single bubble mean diameter, respectively.…”

Study on Propellers Distribution and Flow Field in the Oxidation Ditch Based on Two-Phase CFD Model

“…Key aspects of ODs include their simplicity, low sludge production, and confidence in the market, including operation and maintenance processes [3]. Therefore, they are one of the most widely used methods for activated sludge biological treatment [4]. Despite the existing research recognizes the critical role played by OD in the treatment plant's production effectiveness and economics, old and inefficient technology still prevails in many OD systems; thus, studies suggest their modernization will enhance the flow field and reduce costs and environmental impact [5][6][7][8].…”

“…The results of liquid velocity and volume variation formed the basis for the optimization of OD. In the study of Climent et al [4], a two-phase CFD model was investigated to improve the hydraulic characteristics in the OD, which provides a further understanding of the interaction between liquid behavior and phase motion. However, there is a limited understanding of the mechanism of the flow field characteristics influenced by the propeller's arrangement and sludge concentration.…”