Chemical Engineering Journal 146 (2009) 1–5 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Modelling a full scale UASB reactor using a COD global balance approach and state observers I. L ´ opez ∗ , L. Borzacconi Engineering Faculty, University of the Republic, J. Herrera y Reissig 565, Montevideo, Uruguay article info Article history: Received 5 December 2007 Received in revised form 24 April 2008 Accepted 6 May 2008 Keywords: AM2 model Anaerobic digestion Asymptotic observer Coefficients determination Decoupled parameter estimation abstract The AM2 model proposed by Bernad et al. [O. Bernad, Z. Hadj-Sadok, D. Dochain, A. Genovesi, J.P. Steyer, Dynamical model development and parameter identification for an anaerobic wastewater treatment pro- cess, Biotechnol. Bioeng. 75 (4) (2001) 424–438] was developed as a simple model to simulate wastewater anaerobic digestion. In order to apply this model to a full scale UASB reactor, global COD balance was done. This allows us to determine the stoichiometric coefficients. Using an asymptotic observer biomass con- tent estimation was done without knowledge about the reaction kinetics. Then, a decoupled parameter estimation procedure was followed to determine the evolution of specific growth coefficients. The model and parameters validation was done using experimental data of methane production. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Since the last decade, dynamical modelling of anaerobic diges- tion has become an active research area. An important development was the IWA ADM1 [2]. This structured model includes multi- ple steps describing biochemical and physicochemical processes, involving at least 26 dynamics state variables and many parame- ters. Although the complexity of anaerobic processes is reflected in the ADM1 model, the direct application for modelling and con- trol purposes is difficult. The identification of model parameters in real conditions is virtually an impossible task. Other simple mod- els were proposed to model anaerobic processes with a reduced set of state variables and parameters [1,3–6]. Although simple models do not represent the complexity of real process, parameter identi- fication and model validation are more straightforward than with ADM1. Generally, these models are implemented based on com- pletely stirred tank reactor (CSTR) conditions. Few authors have considered other hydrodynamic behaviours [7–9]. The AM2 model proposed by Bernard et al. [1] is a two-step (acidogenesis–methanisation) mass-balance model. In the first step, the acidogenic bacteria (X 1 ) consume the organic substrate (S 1 ) and produce volatile fatty acids (VFA, S 2 ) and CO 2 (and more bacteria). Next, methanogenic population (X 2 ) consumes VFA and produce methane and CO 2 (and more micro-organisms). The bio- ∗ Corresponding author. Tel.: +598 2 7114478; fax: +598 2 7107437. E-mail address: ivanl@fing.edu.uy (I. L ´ opez). logical reactions are k 1 S 1 → X 1 + k 2 S 2 + k 4 CO 2 (1) k 3 S 2 → X 2 + k 5 CO 2 + k 6 CH 4 (2) where S 1 represents the concentration of complex organic sub- strate (expressed as gCOD/L), S 2 is the concentration of the VFA (expressed as mmolHAc/L), X 1 and X 2 are the concentrations of aci- dogenic and methanogenic populations (as gVSS/L). The reaction rates are, respectively: r 1 =  1 X 1 (3) r 2 =  2 X 2 (4) where  i (in d -1 ) are the specific growth rates of both micro- organisms types. The other state variable considered is the inorganic carbon concentration (C). Additional model assumptions are: acid–base equilibriums and phase equilibriums, it is consid- ered that inorganic carbon is constituted by CO 2 and bicarbonate (B), and total alkalinity is composed by bicarbonate alkalinity and VFA. In normal pH conditions VFA are completely dissociated; methane is slightly soluble and it is released instantaneously, and CO 2 follows Henry’s law. CSTR behaviour is assumed for the liquid phase. In order to incorporate the effect of solids retention in the reactor, the authors introduce the ˛ parameter, which represents the solid fraction that leaves the reactor. Then, equations of the dynamical model are dX 1 dt = [ 1 () - ˛D]X 1 (5) 1385-8947/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2008.05.007