Biological reactor retrofitting using CFD-ASM modelling

Abstract

In recent years, the interest on modelling activated sludge (AS) systems by means of Computational Fluid Dynamics (CFD) techniques has significantly increased. This work shows a successful case study combining CFD hydrodynamics and biokinetic modelling. The hydrodynamics is analysed by using the Reynolds-averaged Navier-Stokes equation for incompressible non-Newtonian fluids and SST turbulence model. Biokinetics has been included into the CFD as transport equations with source and sink terms defined by the Activated Sludge Model nº1 (ASM1). Furthermore, a strategy for reducing the computational cost while maintaining accuracy of the results of these calculations has been proposed. This strategy is based on a two-step solver configuration and the definition of a variable timestep scheme. The resulting CFD-ASM approach permits a proper evaluation of denitrification in the anoxic tanks as well as the reproduction of nitrate and readily biodegradable substrate distributions. To demonstrate the strength of the proposed CFD-ASM, it has been used to evaluate the operation of a full-scale AS system and optimize its performance through changes in the biological reactor anoxic zone. The original configuration has been retrofitted and modified after detecting intrinsic defects on the fluid behaviour within the tank. This study has been assessed by analysing hydrodynamics in detail and validating the simulation results with tracer tests and flow velocity measurements. Substantial variations on the Residence Time Distribution have been confirmed when modifying the internal elements of the tank configuration: the wall-bushing and the stirrer positioning. As a result of this work, an influential short circuiting was corrected improving hydrodynamics and increasing mean residence time, all favouring denitrification efficiency. Outcomes of this study show the benefit of CFD when applied to AS tanks.

Methodology for Energy Optimization in Wastewater Treatment Plants. Phase II: Reduction of Air Requirements and Redesign of the Biological Aeration Installation

“…However, given the nature of the solids in the mixed liquor, with very low sedimentation rates, and the uncertainty about the minimum velocity value needed to keep the solids in suspension, the volume with an absolute velocity below 0.15 m/s and 0.05 m/s was In order to know what was happening inside the anoxic chamber, a series of experimental tests and studies based on computational fluid dynamics (CFD) were carried out using the Ansys v13.0 software as a simulation tool. These tests were focused on studying the hydrodynamics (the study of current lines and of velocity isovolumes) and the study of residence time distribution (RTD) [34][35][36][37][38][39][40][41].…”

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

“…The mixed liquor was modelled as an incompressible non-Newtonian Herschel-Bulkley fluid. The fitting parameters [23] were 0 = 0.00883 ; K= 0.01932 ; =0.6262. Turbulence was modelled by using the Shear Stress Transport model [24] with automatic near-wall treatment.…”

CFD simulation of a novel anaerobic-anoxic reactor for biological nutrient removal: Model construction, validation and hydrodynamic analysis based on OpenFOAM®

“…Regarding hydrodynamic analysis, combination of RTD and CFD for reactor analysis and optimization has been previously applied for different reactors (Brannock et al 2010a-b, Climent et al 2018, Le Moullec et al 2008, Pereira et al 2011, Plascencia-Jatomea et al 2015, Terashima et al 2009). The present work follows the validation performed by many of them regarding the hydrodynamic field, which is only based on RTD tests (Brannock et al 2010a-b, Plascencia-Jatomea et al 2015, Terashima et al 2009.…”

“…Different approaches can be undertaken for a correct representation of the impeller: being MRF (Multi Reference Frame) approach (Bai et al 2008, Bridgeman 2012, Renade 2002, Wu et al 2009 or momentum source approach common ones. The latter is a classical simplification that allows a significant computational resource saving and has been widely and successfully used in wastewater field, all of them carried out with commercial codes (Brannock 2003, Climent et al 2018, Climent et al 2019, Rehman 2016.…”