The aerobic treatment of wastewaters was investigated in the inverse fluidised bed biofilm reactor (IFBBR) in which polypropylene particles of density 910 kg/m(3) were fluidised by an upward cocurrent flow of gas and liquid. Measurements of chemical oxygen demand (COD) versus residence time t were performed for various ratios of settled bed volume to bioreactor volume (V-b/V-R) and air velocities u to determine the optimal operating parameters for a reactor, that is, the values of (V-b/V-R), u and t for which the largest reduction in COD occurred. The largest COD removal was attained when the reactor was controlled at the ratio (V-b/V-R) = 0.55 and an air velocity u = 0.021 m/s. Under these conditions, the value of COD was practically at steady state for times greater than 25 h. In the wastewater treatment conducted in a reactor optimally controlled at (V-b/V-R) = 0.55, u = 0.021 m/s and t = 25 h, a decrease in COD from 27,650 to 450 mg/l was obtained, that is, approximately a 98% COD reduction was achieved. The pH was controlled in the range 6.5-7.0 and the temperature was maintained at 28-30 degrees C. The biomass loading in a reactor depended on the ratio (V-b/V-R) and an air velocity u. In the cultures cultivated after change in (V-b/V-R) at a set u, the steady-state mass of cells grown on the particles was achieved after approximately 3 days of operation. With change in u at a set (V-b/V-R), the new steady-state biomass loading occurred after cultivation for about 2 days. (c) 2005 Published by Elsevier B.V.