On-line diagnosis and uncertainty management using evidence theory––experimental illustration to anaerobic digestion processes Laurent Lardon, Ana Punal, Jean-Philippe Steyer * Laboratoire de Biotechnologie de l’Environnement, INRA, Avenue des etangs, 11100 Narbonne, France Available online Abstract The on-line diagnosis is a key requirement in biological processes. This is particularly true in the case of wastewater treatment processes due to the composition of media, the requirements of operating conditions and the wide variety of possible disturbances that necessitate careful and constant monitoring of the processes. Moreover, because only partial information is available in an on- line context and because of the technical and biological complexities of the involved processes, specific characteristics are required for diagnosis purposes. Several approaches like quantitative model based, qualitative model based and process history based methods were applied over the years. This paper present a methodological framework based on evidence theory to manage the fault signals generated by conventional approaches (i.e., residuals from hardware and software redundancies, fuzzy logic based modules for process state assessment) and to account for uncertainty. The advantages of using evidence theory like modularity, detection of conflict and doubt in the information sources are illustrated with experimental results from a 1 m 3 fixed bed anaerobic digestion process used for the treatment of industrial distillery wastewater. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Fault detection and isolation; Diagnosis; Evidence theory; Biological process; Anaerobic digestion 1. Introduction More than their control, the diagnosis (i.e., detection, isolation and analysis) of faults occurring in biological processes has become a challenging research area. In- deed, several types of disturbances like influence of the inoculate, contamination of the media, presence of toxic in the feeding line, fouling of sensors, can be present even in normal operational conditions. These disturbances can largely affect the process operation and damage the quality of the end products. Moreover, these distur- bances can be either sudden or slow and they can be related to normal or faulty process operation provoking real or apparent deviations from the normal operation. Hence, there is a clear need for advanced supervisory control (i.e., gathering on-line control and diagnosis) in order to keep the system performance as close as pos- sible to optimal. This is particularly true for biological processes with environmental purposes like WasteWater Treatment Plants (WWTPs) where the state of the ‘‘living’’ part of the system is to be closely monitored together with large possible disturbances occurring on any part of the systems. In the present study, anaerobic digestion has been chosen as an illustrative example of biological WWTPs. Anaerobic digestion (AD) is a serie of biological processes that take place in the absence of oxygen and by which organic matter is decomposed and converted into biogas, a mixture of mainly carbon dioxide and methane, microbial biomass and residual organic mat- ter. Several advantages are recognised to AD processes when used in WWTPs: high capacity to treat slowly degradable substrates at high concentrations, very low sludge production, potentiality for production of valu- able intermediate metabolites, low energy requirements and possibility for energy recovery through methane combustion. AD is indeed one of the most promising * Corresponding author. Tel.: +33-468-425-151; fax: +33-468-425- 160. E-mail addresses: lardonl@ensam.inra.fr (L. Lardon), punal@ ensam.inra.fr (A. Punal), steyer@ensam.inra.fr (J.-P. Steyer). 0959-1524/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jprocont.2003.12.007 Journal of Process Control 14 (2004) 747–763 www.elsevier.com/locate/jprocont