This is a preliminary study of hybrid fluidised-bed bioreactors considered the hydrodynamic models and their comparison. In this type of bioreactor, there are two characteristic components which determine their work mode. One part of the bioreactor works as a twophase, gas-liquid, air-lift bioreactor. The second part is a two-phase, liquid-solid, fluidisedbed bioreactor. This type of construction provides high biomass concentration and low shear forces which influence biofilm. Two different types of construction of hybrid fluidised-bed bioreactors were proposed: with external or internal draft tube. Two different mathematical models are needed to design and analyse the operation of these devices.