ABSTRACT
The modeling of multiple events within a biofilm is a good deal more complex than their modeling in suspended growth systems. There are many reasons for this. First is the necessity to consider both transport and reaction simultaneously. We have already seen how that is handled for a single electron donor and electron acceptor provided in the bulk fluid. Extension of those concepts to multiple donors and acceptors in the bulk fluid is not complicated; it just increases the number of differential equations that must be solved. However, it must be recognized that when an electron acceptor such as nitrite-N or nitrate-N is generated within the biofilm, transport can occur in either or both directions from the point of generation, depending on the concentration gradient established. This, too, must be considered in the equations, complicating them somewhat. Another event that complicates the modeling of multispecies biofilms is the competition between the various types of bacteria for the electron acceptor. Autotrophs require molecular oxygen as their electron acceptor and heterotrophs will use it in preference to nitrate-N and nitrite-N when it is present. We saw earlier, however, that heterotrophs have a lower half-saturation coefficient for oxygen than autotrophs do. This means that heterotrophs can lower the oxygen concentration within the biofilm to the point that autotrophs cannot grow. This puts the autotrophs at a disadvantage and limits the region in which they can grow. Perhaps the most important complication arises, however, from the competition for space within the biofilm. In a suspended growth system the biomass is distributed uniformly and is lost from the system in proportion to its concentration. In other words, all of the biomass has the same retention time. This is not true in an attached growth system. Rather, biomass grows outward from a solid support and is removed by detachment at the liquid-biofilm interface. This displacement toward the interface must be considered in any multispecies model. In addition, as we saw in Figure 15.6, the distribution of biomass is not uniform throughout the biofilm. This means that different types of bacteria have different residence times in the biofilm, and the model must be structured to consider this as well.
