The Fig. 5 presents the horizontal hydraulic gradient Ih vs. Vmix for different mixtures of Alumina beads of diameter 6 mm and 15 mm. Contrary to what one might think a priori, the pressure drop of the mixtures is not a simple linear combination of the pressure drop of each bead size: for a 50% of 6 mm mixture (mixture 1) the pressure drop curve coincides with that of the 15 mm beads.