Studies of odour-control pilot-scale biofilters at a rendering plant were conducted for five years. The biofilters contained different sizes of crushed pine bark or a mixture of zeolite and crushed bark, and treated the exhaust gases from direct-fired meal dryers. The exhaust gases were odorous and contained significant smoke. The odour concentration of the rendering process air ranged between 50,000 and 307,200 OU m(-3). Odour-removal performance measurements of the biofilters were undertaken on five occasions using forced-choice dynamic-dilution olfactometry. Biofilter odour-removal efficiencies of between 80% and 99% were measured at various influent odour concentrations and air loading rates. There was no obvious deterioration in performance of these biofilters between various sampling times in the five year study period. The biofilters also reduced the "offensiveness" of the odour. The fine crushed bark biofilter generally reduced odour concentration more efficiently than the coarse bark biofilter. The additions of zeolite to the bark medium in the biofilter had little effect on the odour-removal performance. An increase in air loading rate produced only a very small decrease in odour-removal performance. The pilot-scale biofilters had smoke removal efficiencies between 71% and 100%. Finely crushed bark removed smoke more effectively than coarsely crushed bark. Drainage from the biofilters contained significant concentrations of pollutants, suggesting that controlled leaching has potential to remove accumulated substances in biofilter media from rendering gas emissions and increase the longevity of a biofilter system. (c) 2005 Elsevier Ltd. All rights reserved.