The growth dynamics of a toluene-degrading biofilm was investigated under aerobic and under nitrate-reducing (anoxic) conditions. The study was performed with a continuously fed biofilm reactor (biodrum system). Under denitrifying conditions, the maximum toluene degradation in the reactor was achieved during the exponential growth phase of the biofilm. Under aerobic conditions, however, the maximum toluene degradation was reached during the lag phase of biofilm growth. Meanwhile, a simultaneous increase of the suspended biomass concentration in the bulk occurred concomitant with the toluene removal. The comparison of the structure of the aerobic and anoxic biofilms indicated that the anoxic biofilm was smooth and regular whereas the aerobic biofilm showed a lot of irregularity and a filamentous structure on top of the biofilm. Both the aerobic and the anoxic growth were modelled using the computer programme BIOSIM from EAWAG. The models considered active and inactive biomass. The active biomass was growing on toluene as sole carbon source, whereas the inactive biomass was composed of polymers and dead cells unable to degrade toluene. Model simulations showed that the biofilm activity was mainly concentrated in the top-layer of the biofilm under both aerobic and anoxic conditions.

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