The treatment of phenolic wastewater was investigated in a gas-liquid-solid fluidised bed bioreactor containing polypropylene particles of density 910 kg m(-3). 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 values of (V-b/V-R) and a for which the largest reduction in COD occurred. Optimal operation, corresponding to the largest COD removal, was attained when the bioreactor was controlled at the ratio (V-b/V-R) = 0.55 and an air velocity u = 0.036 m s(-1). Under these conditions, the value of COD was practically at steady state for times greater than 50 h. At this steady state, only about 50% COD removal was achieved in the treatment of a `raw' wastewater (no mineral salts added), whereas in the operation with wastewater enriched in nutrient salts approximately 90% COD removal was attained. The following amount of mineral salts (mg dm(-3)): (NH4)(2)SO4-500; KH2PO4-200; MgCl2-30; NaCl-30; CaCl2-20; and FeCl3-7, when added to wastewater before treatment, was sufficient for biomass growth. The application of low density particles (used as biomass support) in a bioreactor allowed the control of biomass loading in the apparatus. In the cultures conducted 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 6 days of operation. With change in a at a set (V-b/V-R), the new steady state biomass loading occurred after culturing for about 2 days. Phenolic wastewater was successfully treated in a bioreactor. In the operation conducted in a bioreactor optimally controlled at (V-b/V-R) = 0.55, u = 0.036 m s-1 and t = 50 h, conversions greater than 99% were achieved for all phenolic constituents of the wastewater. Conversions of about 90% were attained for other hydrocarbons. (c) 2005 Society of Chemical Industry.