In a previous study, a hybrid bioreactor comprised of a bubble column bioreactor section and a biofilter section was successfully applied to the treatment of benzene. In order to design and optimize the bioreactor system for actual use in the field, simple but effective mathematical models of the two-stage system were required. Since the liquid phase in the bubble column bioreactor section was well mixed, a CSTR (continuously stirred tank reactor) model was adopted for this section, with benzene removal by both air stripping and biodegradation being considered in the model equations. The gaseous benzene degradation in the biofilter section was described using a PFR (plug flow reactor) model. The combined model was validated through independent experiments, and the simulation results were in a good agreement with measured data. (c) 2006 Elsevier Ltd. All rights reserved.