A pilot scale anaerobic fixed-bed process containing a high biomass concentration was studied. The liquid mixing efficiency was good and independent of operating conditions, thanks to the production of biogas. A model integrating two interconnected reactors was developed to account for hydrodynamics. A biokinetic model of anaerobic digestion was then adapted to take into account the hydrodynamic characteristics. This model can easily predict the process during any destabilisation of the influent flow and it also improves an understanding of the process. A model analysis shows that the efficiency of the biological reactions is limited by the Q(2) parameter that characterises the substrate transfer into the biofilm. Its potential is illustrated in a case of a partial unclogging of the process. This model can be used as a diagnostic tool in the event of process failures, as well as a framework for guiding process recovery. (c) 2004 Elsevier Ltd. All rights reserved.