The modelling of aerobic biodegradation of toluene and the associated biofilm growth in a fixed biofilm system is presented. The model includes four biomass fractions, three dissolved components, and seven processes. It is assumed that part of the active biomass is composed of filamentous bacteria which grow relatively fast and detach easily, leading to a biomass growth delayed with respect to substrate degradation. The non-filamentous bacteria inside the biofilm also degrade toluene but with a slower rate compared to the filamentous bacteria. Because the nonfilamentous bacteria do not detach, they are primarily responsible for the biofilm growth. The active biomass decays into biodegradable and "inert" dead biomass which is hydrolysed into soluble products at two different rates. These products are partly degradable by the biomass and constitute the endogenous respiration. The dynamic growth phase with toluene degradation was successfully modelled as well as the decay phase when toluene addition was turned off. In addition to this, modelling of toluene removal and oxygen consumption versus toluene concentration in the reactor was performed. This required consideration of inhibition of substrate degradation due to the combination of high oxygen and toluene concentrations.
