The operational range for a gas-liquid-solid fluidized bed aerobic biofilm reactor with a low-density biomass support is limited: on one side, by a minimum fluidization gas velocity ugf; on the other side, by a critical gas velocity ugcr. In this study, the values of ugf and ugcr were determined for a reactor in which polypropylene particles of density 910 kg/m3 were fluidised downwards by an upward cocurrent flow of gas and liquid. The treatment of industrial wastewaters was also investigated to determine the optimal air velocities ugm for comparison with the ugf and ugcr.The values of ugf and ugcr were in the range 1.58 x 10-3 - 14.27 x 10-3 and 5.37 x 10-3 - 58.59 x 10-3 m/s, respectively. Hence, the values of ugcr were approximately 4 times larger than the corresponding ugf. The values of ugm were 0.021, 0.025 and 0.036 m/s for the initial values of chemical oxygen demand 26,790; 33,210 and 41,640 mg/l, respectively. Thus, the values of ugm were approximately 3.5 times larger than the corresponding ugf. On the other hand, the values of ugm were about 65% of the corresponding ugcr.The application of low-density particles in a reactor allowed the control of a biomass loading in the apparatus. After change in (Vb/VR) at a set ug, the steady-state mass grown on particles was achieved after approximately 4 days of operation. With change in ug at a set (Vb/VR), the new steady-state biomass loading occurred after the culturing for about 2 days and the loading was constant over the entire period of experimentation for this ug.
