The biological wastewater treatment was investigated in the inverse fluidised bed reactor (IFBR) in which polypropylene particles of density 910 kg/m(3) were fluidised by an upward flow of gas. Measurements of chemical oxygen demand (COD) versus residence time t were performed for various ratios of settled bed volume to reactor volume (V-b/V-R) and air velocities u(g). The largest COD removal was attained when the reactor was operated at the ratio (V-b/V-R)(m) = 0.55 and an air velocity u(gm) = 0.024 m/s. Under these conditions, the value of COD was practically at steady state for times greater than 30 h. Thus, these values of (V-b/V-R)(m), u(gm) and t can be considered as the optimal operating parameters for a reactor when used in treatment of industrial wastewaters. A decrease in COD from 36,650 to 1950 rng/l, i.e. a 95% COD reduction, was achieved when the reactor was optimally controlled at (V-b/V-R)(m) = 0.55, u(gm) = 0.024 m/s and t = 30 h. The pH was controlled in the range 6.5-7.0 and the temperature was maintained at 28-30 degrees C. The biomass loading was successfully controlled in an IFBR with support particles whose matrix particle density was smaller than that of liquid. The steady-state biomass loading depended on the ratio (V-b/V-R) and an air velocity u(g). In the culture conducted after switching from the batch to the continuous operation, the steady-state biomass loading was attained after approximately 2-week operation. In the cultures conducted after change in (V-b/V-R) at a set u(g), the steady-state mass of cells grown on the particles was achieved after about 6-day operation. For a set ratio (V-b/V-R), the biomass loading depended on u(g). With change in u(g) at a set (V-b/V-R), the new steady-state biomass loading occurred after the culturing for about 2 days. (C) 2008 Elsevier B.V. All rights reserved.