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BACKGROUND: Evaluation of the biodegradation interactions between styrene and acetone, two typical paint solvents and vapor phase pollutants differing in water solubility and biodegradability, was conducted both in a trickle bed reactor (TBR) and a biofilter (BF). The loading rate experiments were performed by increasing the acetone concentration in the inlet air while keeping the styrene concentration and loading rate constant.

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.

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Steady-state performances of a trickle bed reactor (TBR) and a biofilter (BF) in loading experiments with increasing inlet concentrations of polar solvents, acetone, methyl ethyl ketone, methyl isobutyl ketone and n-butyl acetate, were investigated, along with the system's dynamic responses. Throughout the entire experimentation time, a constant loading rate of aromatic components of 4 g(c)·m(-3)·h(-1) was maintained to observe the interactions between the polar substrates and aromatic hydrocarbons. Under low combined substrate loadings, the BF outperformed TBR not only in the removal of aromatic hydrocarbons but also in the removal of polar substrates. However, increasing the loading rate of polar components above the threshold value of 31-36 g(c)·m(-3)·h(-1) resulted in a steep and significant drop in the removal efficiencies of both polar (except for butyl acetate) and hydrophobic components, which was more pronounced in the BF; so the relative TBR/BF efficiency became reversed under such overloading conditions. A step-drop of the overall OL(POLAR) (combined loading by polar air pollutants) from overloading values to 7 g(c)·m(-3)·h(-1) resulted in an increase of all pollutant removal efficiencies, although in TBR the recovery was preceded by lag periods lasting between 5 min (methyl ethyl ketone) to 3.7 h (acetone). The occurrence of lag periods in the TBR recovery was, in part, due to the saturation of mineral medium with water-soluble polar solvents, particularly, acetone. The observed bioreactor behavior was consistent with the biological steps being rate-limiting.