ABSTRACT
Attachment of microbial cells to the walls of bioreactors is not an uncommon phenomenon. The usual assumption in most applications involving continuous stirred tank bioreactor systems is that the flow rate is fast enough that wall growth can be neglected. However, in many cases, micro-organisms attached to solid surfaces might form a film ranging from a monocellular layer to a thick layer of slime [250]. A biofilm, for instance, consists of microbial cells attached to a solid surface that are usually embedded in a matrix of organic polymers produced by the extracellular polymeric substances (EPS). A good example is the nitrification process where extracellular polymers are excreted and embeded in a slime matrix [99, 138]. The modeling of microbial growth and wall attachment is an important issue. Besides operational problems caused by microbial fouling of the internal surfaces of the bioreactor, a more-or-less loose wall growth leads to the enrichment of cells in the bioreactor and could have important ramifications on the stability characteristics of the unit. It should be noted that the ability of bacteria to attach to surfaces is not only of interest to bioreactors. This issue is of considerable interest to many industries, including medical, food industry, and membrane-based processes. In the latter applications, the development of biofilms on the surface of membrane-based separation systems such as those used in the desalination process is an important issue, since biofouling can lead to the deterioration or even the failure of the separation process.
