The response of bio®lters to varying periods of starvation and to changes in inlet concentrations of a mixture of toluene and xylene have been tested in laboratory-scale bio®lters packed with a mixture of Perlite and compost. These results have been mathematically modelled taking the emission of carbon dioxide as a proxy for kinetics measurements. The use of CO 2 is a more practical approach than that of kinetics based upon batch experiments on pure cultures. A simpli®cation of Zarook's method, our model produced good outlet predictions given small changes in the inlet concentration of toluene and xylene. But for more stressful situations, like the resumption of the feed after periods of starvation, the use of carbon dioxide proved to be inappropriate as an indicator of the biomass activity, greatly overestimating bio®lter performance. This suggested either the occurrence of cryptic growth (as a result of the stress in¯icted on the biomass) or perhaps the utilisation of the compost as a carbon source.

Styrene vapors from contaminated air were eliminated using long-term adapted mixed microbial culture inoculated on four perlite packed biofilters (serial arrangement, up-flow configuration). During start-up the inlet concentration of styrene rose from 175 to 1300 mg/m3 of total carbon. The total actual residence time in the four biofilters was 24 s. Styrene was successfully degraded by the microbial population in the biofilter. An average of 66% of eliminated styrene was transformed to CO2. The removal efficiency of the pollutant was, after 18 d of start-up, nearly 85% at an organic load of 170 g/m3 per h. The concentration profiles along the bed height were linear for various pollutant inlet concentrations. The total amount of microorganisms in analyzed biomass from the biofilters was about 10(9) per gram of dry packing mass. The moisture content was around 80% in all biofilters.

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