This study evaluates the use of a mixture of compost, sludge and pieces of hard plastics as a biofilter medium to remove ammonia from a waste gas stream through a bench-scale biofilter column. This study investigated the effects of operational parameters such as inlet concentration, loading rate, retention time. pressure drop and variations of pH and alkalinity in the packing material on the performance of the biofilter. After a start-up period with an average inlet concentration of ammonia of about 51 ppm(v) corresponding to a loading rate of 2.15 g NH3/m(3) It and an empty bed residence time of 60 s. the biofilter reached a removal efficiency of more than 97.9% by day 10. The maximum elimination capacity of 9.85 g NH3/m(3) It was achieved at a loading rate of 9.86 g NH3/m(3) h, corresponding to an inlet concentration of about 236 ppm(v), with the outlet concentration of NH3 increasing for higher inlet concentrations. The pH and alkalinity of the bed medium decreased due to nitrate formation but there was no need to control them synthetically. The concentration of ammonium and nitrate in the bed medium decreased and increased, respectively. Under steady-state conditions, the number of nitrifying bacteria increased from an initial 5.6 x 10(4) cell/g wet material in the bed medium to about 2.8 x 10(8) cell/g wet material. Biological removal and nitrification were the dominant processes in ammonia removal. The maximum pressure drop during the experiment was 12 mm H2O for each meter of column. The minimum retention time that the system could attain at an average loading rate of <= 9.45 g NH3/m(3)h was 30 s. (c) 2007 Elsevier B.V. All rights reserved.