Robust regulation of anaerobic digestion processes L. Mailleret*, O. Bernard* and J. P. Steyer** * COMORE, INRIA, 2004 route des Lucioles, BP 93, 06902 Sophia Antipolis, France (E-mail: ludovic.mailleret@sophia.inria.fr ; olivier.bernard@sophia.inria.fr) ** LBE, INRA, avenue des Étangs, 11100 Narbonne, France (E-mail: steyer@ensam.inra.fr) Abstract This paper deals with the problem of controlling anaerobic digestion processes. A two-step (i.e. acidogenesis-methanization) mass balance model is considered for a 1 m 3 fixed bed digester treating industrial wine distillery wastewater. The control law aims at regulating the organic pollution level while avoiding washout of biomass. To this end, a simple output feedback controller is considered which regulates a variable strongly related to the Chemical Oxygen Demand (COD). Numerical simulations assuming noisy measurements first illustrate the robustness of this control procedure. Then, the regulating procedure is implemented on the considered anaerobic digestion process in order to validate and demonstrate its efficiency in real life experiments. Keywords Anaerobic wastewater treatment; continuous bioreactor; control; real life experiment Introduction Wastewater treatment using anaerobic processes is a very promising re-emerging tech- nology which presents extremely interesting advantages compared to the classical aerobic treatment (Pavlostathis, 1994; Mata-Alvarez et al., 2000): it has a high capacity for degrad- ing concentrated and resilient substrates (plant residues, animal wastes, food industry wastewater, etc…), produces very little sludge, requires little energy and it can become profitable with the use of biogas (methane) combustion for cogeneration. But in spite of these advantages, the anaerobic treatment plants are still rare at the industrial scale, probably because they are known to become easily unstable under some circumstances like variations of the process operating conditions and because they need a high level of expertise to be operated. Nevertheless, these drawbacks can be overcome by associating a control procedure to enhance the stable performance of the wastewater treatment operation via feedback control and to connect the process through the Internet to an expert center to ensure its perenniality: this is the goal of the IST European project TELEMAC. The first step in this remote advanced management strategy is therefore to have a dynamic model of the process, which is required for the design of control algorithms. The dynamic modelling of anaerobic digestion has been an active research area over the past three decades. Andrews (1968) introduced the Haldane model to characterise growth inhibition, which can emphasise the process instability, i.e. the biomass washout via the accumulation of acids. A model with a single bacterial population was then proposed (Graef and Andrews, 1974). The main modelling studies have been since then extended and detailed by other authors in order to get closer to the complexity of the process (Mosey, 1983; Costello et al., 1991a, 1991b; Batstone et al., 2000; IWA Task Group, 2002). It results in detailed models of the anaerobic digestion process that include several bacterial populations and several substrates. As a consequence, these models are difficult to calibrate and to use for control purposes. However, simpler models based on mass balance considerations (Bastin and Dochain, 1990) circumvent this difficulty by locating the biological lack of knowledge in dedicated terms, namely the reaction rates. The use of such models for control design has been proved to be more effective than detailed Water Science and Technology Vol 48 No 6 pp 87–94 © IWA Publishing 2003 87 Downloaded from https://iwaponline.com/wst/article-pdf/48/6/87/423621/87.pdf by guest on 05 November 2018