ABSTRACT
Although a packed tower is mechanically simpler than most suspended growth systems, it is more difficult to model for a number of reasons. First, both the electron donor and the electron acceptor must be transported into the biofilm for reaction, as discussed in Chapter 15. This means that the models for substrate removal are much more complicated. Second, although flow is generally assumed to be in thin sheets in most models, in reality, the flow patterns are quite complex over all of the media types. Third, various types of bacteria must compete for nutrients and space in the biofilm, as discussed in Section 15.4, rather than being homogeneously distributed as in a suspended growth culture. Fourth, the biofilm will not be evenly distributed over all of the media; rather, the distribution will depend on the both the flow patterns
As a consequence of the various factors in the preceding paragraph, no consensus models yet exist for biofilm processes that are comparable to International Association on Water Quality (IAWQ) activated sludge model (ASM) Nos. 1 and 2 for suspended growth processes. Models exist that account for transport and reaction in the manner discussed in Chapter 15, but they are generally limited to a single limiting nutrient and consider fluid flow to be in thin sheets with no intermixing. At the other extreme, models exist that seek to account for the complexity in fluid flow through packed towers, but they use limiting case solutions for transport and reactions, thereby limiting their generality, as discussed in Section 15.2.4. As a consequence, it is currently impossible to use mechanistic modeling to investigate the theoretical performance of packed towers with the same degree of confidence that modeling was used in Part II to investigate suspended growth systems. Nevertheless, much can be learned from models that idealize the hydraulic regime as thin film flow, as long as one views their results qualitatively rather than quantitatively. Consequently, we will use such a model herein to learn how certain factors influence packed tower performance and then will consider other important aspects not included in the model.
