Compartment

The biofilm:A biofilm is a gel-like aggregation of microorganisms and other particles embedded in extracellular polymeric substancs. A biofilm contains water inside it, but its main physical characteristic is that it is a solid phase. A biofilm normally is anchored to a solid surface called the substratum on one side and in contact with liquid on its other side. Frequently, a mass-transfer boundary layer is included between the bulk liquid and the biofilm itself. Thus, following figure illustrates a biofilm having four compartments: the substratum, the biofilm itself, the boundary layer, and the bulk liquid outside of the biofilm. While it is complex even for a homogeneous biofilm morphology, we assume the biofilm surface is flat and all material below the maximum biofilm thickness as part of the biofilm components, and they have a constant density.

the mass-transport boundary layerExperimental observations clearly indicate strong concentration gradients for solutes just outside the biofilm when these solutes are utilized or produced by the microorganisms in the biofilm. Consequently, the solute concentrations at the biofilm surface and in a completely mixed bulk liquid often are significantly different.So we introduce the mass-transport boundary layer,which is a hypothetical layer of liquid above the biofilm and in which all the resistance to mass transport of dissolved components outside the biofilm occurs.

The bulk liquid: In our experiments, the bulk liquid is large compared to the biofilm. So the simplest way seems to consider it as a boundary condition of the biofilm compartment and specify the concentrations of dissolved. However, dissolved components can exchange between the biofilm and the bulk liquid, and it has a profound impact on the concentrations in the bulk liquid. Thus we include the bulk liquid not only as a boundary condition, but also as a separate, completely mixed compartment, varying according to the inflow, outflow, and the exchanges with the biofilm.

The substratum: In our basic model, the substratum is a separate compartment and impermeable. So it does not have much effect on the biofilm system. However in some bioreactor, the substratum may be permeable, or include organic solids that are biodegraded by attached microorganisms.