Biofilms are very complex, both physically and microbiologically. In fact, they are so complex that it is impossible to fully explore all aspects of them in the space available here. Therefore, those interested in a more detailed explanation of their properties should consult the treatise edited by Characklis and Marshall" and the reviews by Costerton et al.y and Lazarova and Manem.:'
The basic conceptualization of a biofilm system is shown in Figure 15.1. The biofilm grows attached to a solid support, which is usually impermeable, although it need not be. In this book, only impermeable supports will be considered. The solid support may be natural material, such as rock in old trickling filters, or it may be synthetic, such as the plastic packing in modern ones. Furthermore, it may range in configuration from corrugated sheets, such as in packed towers, to small particles, such as in fluidized beds. In general, the biofilm can be divided into two zones, the base film and the surface film. Both contain an assemblage of microorganisms and other particulate material bound together by a matrix of extracellular polymers. Those polymers, which are excreted by the microorganisms, are thought to be the same as
Figure 15.1 Conceptualization of a biofilm system. The base film and the surface film constitute the biofilm. (After Characklis and Marshall. )
the polymers involved in biofiocculation (see Section 10.2.1). The base film consists of a structured accumulation, with well-defined boundaries. Transport in the base film has historically been viewed as being by molecular processes (diffusion), although, as we will see later, that view is changing. The surface film provides a transition between the base film and the bulk liquid, and transport within it is dominated by advection. The relative thicknesses of the base and surface films depend largely on the hydrodynamic characteristics of the system, but also on the nature of the microorganisms in the biofilm. Consequently, one biofilm may have almost no surface film whereas another may be entirely surface film. There is normally relative motion between the biofilm and the bulk liquid, with the one moving depending upon the configuration of the attached growth proccss. For example, in packed towers the bulk fluid moves down over the biofilm in a thin sheet, whereas in a rotating disc reactor the biofilm support moves through the bulk liquid. In either case, however, mass transfer from the bulk fluid to the biofilm depends on the hydrodynamic regime. Finally, some biofilm systems contain a gas phase that provides oxygen or serves as a sink for gaseous products.
Most mathematical models of biofilm systems consider the surface film to be negligible, and therefore consider only the base film. Furthermore, unless they are specifically trying to model a variety of events such as carbon oxidation, nitrification, and denitrification, they usually reflect a single-species biofilm. Figure 15.243 shows such a biofilm. The bacterial cells can be seen to be suspended within a polymeric matrix, much the way fruit is held in a Jell-O™ salad. From such pictures the concept developed that the transport of substrates, nutrients, electron acceptor, etc. to and from the bacteria within the biofilm is by molecular diffusion alone.18 Transport
Figure 15.2 A Iransmission electron micrograph of a Pseudomonas aeruginosa biofilm consisting almost entirely of a base film. (From P. A. Wilderer and W. G. Characklis, Structure and function of biofilms. In Structure and Function of Biofilms, W. G. Characklis and P. A. Wilderer, eds. Wiley, New York, pp. 5-17, 1989. Reprinted by permission of John Wiley & Sons, Inc.)
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