The work concerns the biodegradation of toxic organic substrate in a cascade of two bioreactors with partial separation and recirculation of biomass. Heterogeneous model of fermentation broth was used for modelling of interphase oxygen transfer and dual-substrate kinetics was assumed. Quantitative results were obtained for phenol biodegradation. It was shown how the intensity of aeration affects the basic stationary properties of the cascade and the control of the process analysed.