For design purposes a simple model to predict the hydrodynamic behaviour of a three-phase internal airlift reactor is developed. The model predicts liquid circulation, gas hold-up and minimum gas supply rates for solids suspension. The reactor type considered has an internal riser and is not equipped with a gas disengagement area, resulting in a high downcomer gas flow rate. Its purpose is waste water treatment using biofilm particles. First it is recognized that several flow regimes may be distinguished, necessitating different models. Modelling for the gas recirculation regime is straightforward based on the momentum balance combined with a simple assumption for the gas hold-up that is possible for these type of airlift reactors with unrestricted gas carryover into the downcomer. The predictive model is compared successfully with a pilot-scale reactor (400 1) and a full-scale reactor (284 m(3), both with a draught tube height of about 12 m) containing up to 250 g/l solid particles with superficial gas supply rates up to 0.1 m/s.