A new reactor, the biofilm airlift suspension extension (BASE) reactor, was developed for nitrification and denitrification of wastewater. It consists of an internal-loop airlift reactor, appended with an additional downcomer: the extension. The internal airlift part of the BASE reactor consists of an ideally mixed pool of liquid, air bubbles and solid particles. The extension is free of air bubbles, and particles and liquid move downward in plug flow. Overpressure in the headspace is used to control the liquid velocity in the extension, independent of the air flow rate, changing the recirculation ratio between the internal compartment and the extension. A three-phase model was developed to describe the hydrodynamic behaviour of the BASE reactor. Using the model a full-scale BASE reactor was designed. Then a scale-up/scale-down approach was applied to design and construct a pilot-scale BASE reactor (height 6 m, volume 1.7 m(3)), to validate the hydrodynamic model. The model predicts the measurements in a two-phase system (water and air) well.