Tilt: biofilm airlift suspension extension (BASE) reactor is a three-phase reactor, developed for nitrification and denitrification. It consists of an internal-loop airlift reactor (with liquid, air bubbles and biofilm particles) extended with an external concentric downflowing fluidised bed (with liquid and biofilm particles, the extension). Previously a model was developed, which describes the hydrodynamic behaviour of the reactor. A pilot-scale reactor was constructed (height 6 m, volume 1.7 m(3)), and for two phases (water and air) successful validation of the model was reported. In this paper validation has been extended to three-phase systems with either polystyrene or silver sand. The hydrodynamic trends which were observed could be predicted by the model, however the gas hold-up was overestimated at high gas velocities and solid hold-ups (more than 20%). Due to the special design of the reactor the liquid velocity in the extension can be manipulated by the overpressure in the headspace, therewith controlling the nitrogen removal efficiency.