Our fundamental studies of the kinetics of growth and substrate removal by aerobic and anaerobic biofilms have shown that the process comprises six phases : the latent, dynamic, linear, decrease, stabilization and detachment phases. During these experiments we also observed:

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    steady state functioning in the liquid bulk from the end of the dynamic phase. At this point a very thin pseudo-thickness of biofilm was observed (50 µm maximum).

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    steady state functioning relative to the observed biofilm mass, reached in the stabilization phase, the corresponding thickness being generally several hundred microns.

To explain this phenomenon we suggest a new biofilm modelling hypothesis, based on physiological aspects, which consists of defining two types of bacteria : active bacteria (Ma) responsible for substrate removal and characterized by a specific growth rate (µo), and inert or deactivated bacteria (Md) which play no role in the removal process but are responsible for the observed accumulation of biofilm. Using this hypothesis, it is possible to modelize the dynamic and linear phases of growth of total biofilm dry matter (Mb) and carbon substrate removal kinetics.

This model enables the exponential growth rate (µo), the accumulation rate (K) and the maximum quantity of active bacteria (Ma)max to be calculated. In another series of experiments, we studied the influence on these parameters of several factors which affect growth, such as the carbon substrate concentration provided by the feed (So) and the dissolved oxygen for the aerobic biofilms.

The results demonstrate that the biological constants are strongly dependent on (So). The same is true for the volumetric substrate removal rate(kov), which shows that the process always depends on the reaction. Thus we have established that the substrate metabolization reaction occurs at the biofilm-liquid interface, and that it is preferable to use thin biofilms for an attached culture industrial process.

This has been done by optimizing the surface and volume properties of new granular materials called OSBG (Optimized Support for Biological Growth). Initial results, notably for three-phase fluidized bed carbon removal, show that it is possible to eliminate very high carbon loading (10 to 15 kg TOD.m3.day−1) under very stable conditions with a very small quantity of active biomass (0.5 D.W. m). In addition, excess sludge production is relatively low and respirometric studies performed in situ with a gas phase mass spectrometer confirm the very high catabolic activity of thin biofilm.

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