Two filter beds, one composed of inorganic material and the other of mature compost, have been tested for their comparative performance in the biofiltration of methane. The main aim of these tests was to measure the importance of nutrient nitrogen concentration when provided to the bacteria in the form of nitrate ions. The results of this work have revealed that, in a biofilter of 18 L capacity, and fed with 4.2 L/min of gas mixture in which the methane concentration was maintained between 7000 and 7500 ppmv, the inorganic bed proved to be more effective than the organic one. Indeed, the influence of the nitrogen concentration in the nutrient solution is very obvious in the inorganic bed and the optimum level is observed at similar to 0.75 g of nitrogen/L, corresponding to a conversion of 41 % and an elimination capacity of 29.2 g/m(3)/h. With the mature compost-based bed, a maximum conversion of 19% only was obtained, also under a nitrogen concentration of 0.75 g/L. The production Of CO2, the bed temperature and its pressure drop were also examined in this study. The results also show that the inorganic filter bed, in which Methylocystis parvus was the main active bacteria, operates at both a higher and a more uniform temperature, and provides for the lowest rate of bed clogging. (c) 2005 Elsevier B.V. All rights reserved.