A study has been carried out on the operating parameters that influence the biodegradation of petroleum and brewery wastewaters, with a low-density biomass support. The biodegradation rate of a mixture of two wastes was compared with that of the separate wastes. A low aspect ratio reactor was employed, and this made it possible to operate at low superficial gas and liquid velocities. The gas distributor used created a fluid flow pattern similar to that of a draft tube, which enhanced axial mixing. At a particles loading of 12% (v/v), the optimum superficial gas velocity was 2.5cm/s for the mixture. The interstice structure of the biomass-support particles, improved microbial attachment due to the resulting large surface area. There was a low biomass concentration when petroleum wastewater was treated alone, however, for a mixture of petroleum and brewery wastewaters, an increase in the concentration was observed. There was a higher gas hold up in the mixture than in the petroleum wastewater, but lower than in the brewery wastewater. An improved biodegradation was achieved when a mixture of brewery and petroleum wastewaters was treated, and this gave an indication that nutrient deficient wastes can be treated together with phosphate and nitrate rich food industry wastewaters.