In this paper a convenience of using the airlift fluidized bed bioreactor and fluidized bed bioreactor for waste water purification have been analysed. On the base of results of the mathematical modelling the conditions for efficient degradation have been investigated. The criterion of this was to achieve a high conversion in device of real size in real process time. For solving of the mathematical model the method of orthogonal collocation with STIFF integration routine have been used.