A dynamic analysis of a continuous stirred tank bioreactor with biofilm was performed. The existence of gas, liquid, and biofilm were taken into account. The proposed heterogeneous model of such bioreactor takes into account dynamic biofilm growth and interphase transfer of substrates and biomass for a double-substrate aerobic process. Simulations were performed to investigate the influence of important process parameters, i.e., toxic substrate concentration in the feed stream, detachment rate coefficient, mean residence time of the liquid and aeration intensity, on dynamic properties of the bioreactor. Dynamic behavior at conditions of anoxia of microorganisms were shown. A method was proposed to reduce bioreactor start-up time significantly. The paper presents a mathematical model of the bioreactor that uses a discrete model of biofilm growth based on the theory of cellular automata. Dynamics of the bioreactor based on the continuous and discrete biofilm model was compared. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 1818-1829, 2017