Chemical Engineering and Processing 40 (2001) 187 – 194 Global and local mass transfer coefficients in waste water treatment process by computational fluid dynamics A. Cockx a, *, Z. Do-Quang a , J.M. Audic a , A. Line ´ b , M. Roustan b a Lyonnaise des Eaux -CIRSEE, 38 rue du Pre ´sident Wilson, 78230 Le Pecq, France b INSA-GPI -LIPE, Complexe Scientifique de Rangueil, 31077 Toulouse Cedex 4, France Received 7 June 1999; received in revised form 8 November 1999; accepted 31 May 2000 Abstract Gas – liquid mass transfer is a currently used process in the waste water treatment industry. The objective today is to optimize the design and have a better control on the operation of these processes by using the CFD code ASTRID in two-phase flow reactors. A source term which represents interfacial mass transfer is introduced, accounting for two-phase flow hydrodynamics, it permits to predict accurately dissolved gas concentration in reactors such as laboratory airlift internal loop reactor and full scale activated sludge basin. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Mass transfer; Simulation; Waste water treatment; Gas – liquid; Aeration www.elsevier.com/locate/cep 1. Introduction Gas – liquid reactors are used in different waste water processes to achieve high biological treatment level. The need of oxygen for major pollutant removal induces an enhancement of oxygen mass transfer. The reactor’s efficiency depends mainly on the interfacial area be- tween the dispersed phase (bubbles) and the continuous phase (water). A major issue of bubbly flow reactors is to accurately predict their hydrodynamics. Computa- tional fluid dynamics (CFD) code adapted to such two-phase flows are available. In the present study, ASTRID CFD code is used to simulate gas distribution and gas – liquid velocities for airlift internal loop reactor and activated sludge basin. ASTRID is a commercial code developed by ‘Electricite ´ de France’ (EDF) based on the two-fluid model in bubbly flow [1,2]. Hydrodynamic modeling of the airlift internal loop reactor has been validated in a previous study, by comparison of numerical simulations to local measure- ments of liquid velocity (with the particle image velocimetry technique, PIV) and longitudinal distribu- tions of gas fraction [3]. The numerical tool had al- lowed to obtain reliable predictions for the hydrodynamic efficiency of the airlift reactor (axial dispersion coefficient, RTD,…). The next step predicts mass transfer efficiency of this contactor. Using the classical concept for mass transfer coefficient K L a, it is proposed to estimate local and global gas – liquid mass transfer by numerical simulation. Given the physical modeling of momentum and mass transfer previously validated, the ASTRID code can be used to study the hydrodynamics and mass transfer in different kinds of reactors, such as laboratory scale airlift reactor or full scale ozonation reactor [4] and full scale activated sludge basin (present study). In the first part of the paper, the relation between global and local mass transfer is recalled. In the second part, the CFD code is validated in terms of global mass transfer coefficient in a laboratory scale airlift reactor (volume 0.7 m 3 ). In the third part, the model is applied to analyze mass transfer efficiency of a full scale acti- vated sludge basin (volume 15 000 m 3 ). 2. Local and global mass transfer On a given reactor scale, mass transfer between phases is a global evolution of the concentration versus * Corresponding author. Tel.: +33-1-34802336; fax: +33-1- 30536207. E-mail address: arnaud.cockx@lyonnaise-des-eaux.fr (A. Cockx). 0255-2701/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0255-2701(00)00138-0