Toluene degradation in the recycle liquid of biotrickling ®lters for air pollution control

Pollutant degradation in biotrickling ®lters for waste air treatment is generally thought to occur only in the bio®lm. In two experiments with toluene degrading biotrickling ®lters, we show that suspended microorganisms in the recycle liquid may substantially contribute to the overall pollutant removal. Two days after reactor start up, the overall toluene elimination capacity reached a maximum of 125 g m h, which was twice that found during prolonged operation. High biodegradation activity in the recycle liquid fully accounted for this short-term peak of pollutant elimination. During steady-state operation, the toluene degradation in the recycle liquid was 21% of the overall elimination capacity, although the amount of suspended biomass was only 1% of the amount of immobilized biomass. The results suggest that biotrickling ®lter performance may be improved by selecting operating conditions allowing for the development of an actively growing suspended culture.