Removal of hazardous organic compounds from waste gases in co-current gas-liquid down-flow trickle-bed bioreactors can result in bed clogging as a result of biomass accumulation. Such biomass growth permanently reshapes the bed pore structure and leads to progressive bed obstruction often accompanied by an increase in pressure drop. A predictive dynamic model linking two-phase hydrodynamics to the space-time distribution of bioclogging and biokinetics in trickle-bed bioreactors for waste gas treatment was developed on the basis of the volume-average mass, momentum, and species balance equations coupled with classical diffusion/ bioreaction equations to describe biofilm evolution. The model, which includes effects of liquid holdup/layer and biomass loss (via biomass decay and physical shearing), could be very helpful for determining the operating conditions that reduce biological clogging. Toluene degradation using biodegrading microbes immobilized on diatomaceous earth biological support media was chosen for a case study to illustrate the influence of biomass accumulation on bioreactor hydrodynamics.