The operating parameters for a gas-liquid-solid fluidised bed bioreactor with a low density (matrix particle density smaller than that of water) biomass support were investigated. In particular, the ratio of bed (settled) volume to bioreactor volume (V b /V R ) and the air velocity U, for which the largest COD removals from brewery wastewater's were achieved, were determined for a bioreactor with KMT R (Kaldnes Miljotechnologi AS) particles (made of polypropylene) used as biomass support. Measurements of COD versus time were performed in a semi-continuous mode, that is, the air was continuously supplied to a bioreactor and the wastewater was introduced batch-wise at the beginning of experiments conducted for a set U.
It was found that the lowest values of COD at the end of operation and hence the largest COD removals were obtained when a bioreactor was operated at the ratio (V b /V R ) m = 0.50 and the air velocity U m = 0.018 m/s. Therefore, these values of (V b /V R ) m and U m can be considered as the optimal operating parameters for the bioreactor with KMT R particles when used in treatment of brewery wastewaters. The pH was controlled in the range 6.5-7.0 and the temperature was maintained at 28-30 • C.
In the treatment of a 'raw' (no nutrients were added) wastewater, about 70% COD removal was achieved. In the operation with the wastewaters enriched in the nutrients, approximately a 90% COD removal was attained. The following amounts of the nutrient salts (mg/l): (NH 4 ) 2 SO 4 -500, KH 2 PO 4 -200, MgCl 2 -30, NaCl -30, CaCl 2 -20 and FeCl 3 -7 added to wastewater before treatment, were sufficient for biomass growth.
It was possible to achieve the steady-state biomass loading in a bioreactor with the KMT R biomass support. Particle-particle and particle-wall collisions sloughed off the excess biomass. Clogging and channelling of the bed were eliminated through the intensive motion of the particles.