ABSTRACT
The biochemical operations considered so far have all employed suspended growth cultures of microorganisms. However, as discussed in Section 1.2.3, attached growth bioreactors have been used extensively in environmental engineering practice. As the name implies, microorganisms in such bioreactors grow attached to a solid surface rather than being freely suspended in the wastewater undergoing treatment. That surface may be fixed in space with the wastewater flowing over it in thin sheets, such as in a packed tower, it may rotate about an axis, thereby moving through the fluid in the bioreactor, as in a rotating disc reactor, or it may be in the form of small particles that are held in suspension by the upward flow of water, as in a fluidized bed bioreactor. In all cases, however, the key distinguishing characteristic is that the microorganisms live in a biofilm attached to a surface. This means that the electron donor, the electron acceptor, and all other nutrients must be transported to the microorganisms within the biofilm by diffusional and other mass transport processes. It is the necessity to consider the combined effects of mass transport and reaction that makes the modeling of biofilm systems different from and more complicated than the modeling of suspended growth systems. In this chapter, we briefly examine the structure of biofilms and then review the ways in which the combined effects of transport and reaction are considered during modeling.
