Landfill leachate pollutants were treated in a biofilter filled with a mixture of aged refuse and slag, and the performance was compared with those reactors filled solely with either medium. Cultural counting method showed that bioreactor filled with slag had the highest amount of nitrifying bacteria, while polymerase chain reaction-denaturing gradient gel electrophoresis method showed that reactor filled with both media had the highest bacterial community diversity. Particle size distributions measurement showed that slag contained less fine particles than aged refuse, which provided better permeability. The reactor containing both media exhibited a high efficiency in removal of pollutants, and a higher resistance to shock loading and low temperature compared with single-medium reactors. It also overcame both the poor permeability of aged refuse filling and the low bacteria diversity of slag. The study shows that a mixture of aged refuse and slag as a new biofilter medium for leachate pollutant removal is technically viable. (C) 2010 Elsevier Ltd. All rights reserved.