Etude descriptive fondamentale et modelisation de la croissance d'un film biologique—I. Etude descriptive fondamentale de la croissance d'un film biologique

Abstract

Optimization of fixed biomass biological reactors depends notably upon a better knowledge of growth and degradation substrate kinetics. Previous authors working on the subject of biofilms have introduced the concept of “active thickness” which is the thickness beyond which the essential growth substrates becomes a limiting factor. The “active thickness”, in general, is evaluated with mathematical models, associating diffusional transport with more or less complex kinetics. In the framework of this particular research, the purpose was to verify the principal hypotheses of previous models by investigating, in particular, the evaluation of the biofilm (structure) and its purifying capacities as a function of time. For this purpose, a particular ring reactor was conceived (Fig. 1) and, observation and characterization methods of the biofilm were formulated (Fig. 2), taking into account different methodologies proposed by the literature (Table 3). The experimental results obtained demonstrated that the growth of an anaerobic biological film follows six stages (Fig. 7): a latent phase corresponding to the implantation of small dispersed bacterial colonies (Fig. 8) and an accelerated or dynamic phase in which the colonies multiply and spread until the total occupation of the support. At the same time, substrate degradation and product elaboration rates tend towards maximum values, which indicates the achievement of a stationary regime corresponding to the maximum active biomass quantity, while the total biomass of the biofilm ( M b) continues to grow. To explain this phenomenon, two types of bacterium constituting the biofilm should be distinguished (Fig. 10): active bacteria ( M a) responsible for the degradation of the substrate; inactive bacteria ( M d) no longer involved in the degradation of the substrate but still conserving certain enzymatic activities. To model the phenomenon, the foundation of a new model was proposed associating the intrinsic growth kinetic of the active bacteria, the inactivation kinetic by confined effect and the accumulation of toxic products: (dM a/dt) acc = μ 0M a − ki·I·M a a This expression can be rearranged in two other forms: (dM a/dt) acc = μ 0M a(1 − (M a)/(M a) max ) and (dM a/dt) acc = μ 0M aβ(A 0− a/A 0) . The latter translates the influence of the available fixation surface (biological space concept); the linear growth phase corresponding to the accumulation, at a constant rate, of the inactive biomass, and the linear evolution of the biofilm thickness with time, while the active biomass stays constant and maximal [(d M a/d t) acc = 0]; the decreasing phase followed by a stabilization phase and finally the detachment of the biofilm, all three linked to the physical phenomena depending principally on the hydrodynamics of the reactor, which becomes the most dominating action compared to those of the biological process. From a practical point of view, the accelerated growth phase is fundamental, as it translates the essentials of the biological process. On the other hand, it should be noted that in the course of this phase, the active biological mass is not distributed in a uniform film, but appears as a juxtaposition of microcolonies, and, in fact, the observed thickness constitutes an apparent outer layer. On the contrary, in the other phases, the biofilm notion seems correct, but for an aerobic system, the appearance of a filamentous outgrowth is however observed. On the other hand, activity measurement in terms of ATP did not signify the biofilm's potentiality in degrading the substrate. It is thus necessary to modelize the phenomenon in order to determine the intrinsic degradation rate of this substrate. This will be the purpose of part II.