The paper concerns start-up of a continuous stirred tank bioreactor with biofilm for aerobic microbiological degradation of a toxic organic compound. Method for the bioreactor start-up was proposed, which is based on using a proportional controller for the control by flow rate. Numerical simulations were performed using a heterogeneous mathematical model of the bioreactor, which takes into account three phases: gas, liquid and biofilm. It was shown that using the proposed strategy results in obtaining even over 8-fold decrease of time, after which a stationary, low concentration of a toxic compound is obtained. Moreover, it decreases the risk of anoxia of microorganisms related to considerable decrease in dissolved oxygen concentration, which takes place during a standard start-up method. The efficiency of the proposed method was shown on the example of aerobic degradation of phenol for different values of the substrate concentration in the feed stream and for different values of mean residence time of the liquid in the bioreactor. (C) 2018 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.