The effect of ammonium concentration on the microbial population and performance of a biofilter operated at methane concentrations of 0.3% (v/v) was studied. In a range of ammonium concentration from 0.05 to 0.5 gN-NH4+/L, the removal efficiency, carbon dioxide production rate, biomass production, ammonium conversion and nitrate production rate were determined. The middle biofilter section microbial population was analyzed using denaturing gradient gel electrophoresis (DGGE). DNA sequence analysis revealed that the bacteria colonizing the middle biofilter section belonged essentially to 4 physiological groups: methanotrophs, methylotrophs. nitrifying and denitrifying bacteria. Ammonium concentration affected the presence of bacteria from each group and the interactions among them defined the biofilter performance. In the range of ammonium concentrations tested, the methane removal efficiency decreased from 70 to 13%, the carbon dioxide production rate from 25 to 7.5 g/(m(3) h), and the mean of dry biomass content in the packed bed from 12.8 to 6.7 g biomass/kg filter bed. The nitrate production rate increased with ammonium concentration; however, it presented negative values at nitrate concentrations around 0.5 gN/L This suggests that denitrification occurred due to favourable conditions for the growth of denitrifiers. An analysis of methane biodegradation at different layers of the biofilter shows that the methanotrophic and nitrifying activities were more important in the upper section of the biofilter. (c) 2011 Elsevier B.V. All rights reserved.