A traditional packed bed biofilter was operated as a CSTR by using internal recycle and combined with water content control. Sufficient headspace mixing eliminated interparticle gas phase concentration profiles in the compost bed. A hydrophilic membrane, hydraulically connecting the compost to a water chamber, maintained under vacuum, controlled the water content in the compost. Changing the matric potential from -20 to -300 cm H2O decreased the elimination capacity (EC) of toluene by 50%. The reactor design allowed easy manipulation of dissolved components in the liquid phase while controlling the unsaturated water content in the compost. Nitrogen was identified as limiting the biomass growth for the particular composts tested. The addition of either ammonia or nitrate increased the steady state EC significantly (7 -> 76 g m(-3) h(-1)). The EC versus toluene concentration profile was easily generated and displayed a typical mass transfer-limited system up to a concentration between 100 and 200 ppm, followed by a biomass-limited system with increasing toluene concentration. This reactor permitted a better exploration of environmental influences on biofiltration performance than the traditional long column biofilter. (C) 2009 Elsevier B.V. All rights reserved.