Balsalt (ρ=2760 kg.m−3, d=0.245 mm) was used as the solid carrier in a pilot-scale airlift internal-loop reactor for wastewater treatment (AILRW), which was filled with tap water in this study, to investigate the regimes of fluidization and sedimentation. Solid sediment was observed even when the liquid velocity in the riser was much higher than the terminal settling velocity of the particles. Circulation cannot be maintained if the superficial liquid velocity in the downcomer ud. is below a definite value (blocking velocity ub) . In a dimensionless model, ub is related to the bulk volume of carrier solids added (Vt), the available space at the bottom (Va) and the gap size at the bottom (a0) according to ub2/(a0.g) = 0.04(Vt/Va) −0.036.

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