Abstract: A device for the introduction of a liquid in a medium containing granulate material for the fluidization of this material, intended more particularly for equipping reactors such as those used for the biological treatment of aqueous effluents, characterized in that it is made of a part, bearing on the bottom of the reactor, the upper portion of said part cooperating with the liquid injection tube so as to create a flow with a very small turbulence of the liquid at the tube outlet and the shape of which being such that it provides, at all points of the apparatus, without dead zones, the setting in movement of the material to be fluidized.