The biological treatment of a tri-component mixed waste gas system in BRC1 and BRC2 biofilters packed with rock wool-compost media was studied. The model gases were NH3, H2S and toluene. The gases were fed initially at about 50-55 ppm each. H2S was found to have the shortest start-up while toluene had the longest. Under two different NH3:H2S:toluene concentration ratios of 250:120:55 and 120:220:55 (in ppm) for BRC1 and BRC2, the removal efficiencies of NH3, H2S and toluene were found to be affected by their respective loading rate. On the other hand, toluene removal was observed to be inhibited at H2S concentration of 220 ppm as well. Almost complete removal of NH3 and H-2 S was achieved when loading rate was applied up to 16.14 g-NH3/(m(3) bed h) and 36.09 g-H2S/(m(3) bed h), respectively. The maximum elimination capacity for NH3 was determined to be 23.67 g-NH3/(m(3) bed h) at 78.6% removal efficiency and for H2S, 38.50 g-H2S/(m(3) bed h) at 68.1% removal efficiency. The maximum toluene elimination capacity was 30.75 g-toluene/(m(3) bed h) at 87.9% removal efficiency when the concentration of NH3:H2S:toluene was 250:120:55 in BRC1, and was 16.60g-toluene/(m(3) bed h) at 45.5% removal efficiency when the concentration of NH3:H2S:toluene was 120:220:55 in BRC2. The pressure drops along both columns were low and the ratio of bed compactions over biofilter height was observed to be less than 0.02. (c) 2007 Elsevier B.V. All rights reserved.