The Role of Microbial Films in Fermentation

Microbial mass can be said to occur in one of two states; as 'flocs' frcely suspended in an aqueous medium, or as a 'film' adhering to a support surface. Industrial fermenter configurations, at least since the development of deep-tank fermentation, have almost invariably been based on the use of microbial flocs, and the occurrence of a visible microbial film during fermentation is generally avoided. Exccptions include the 'trickling filter' in the waste water treatment industry, 'vinegar' towers, animal tissue culture and microbial leaching. Under the stimulus of increasing industrialization, aerobic fermenters evolved from configurations based upon surface growth at the gas—liquid interface to the use of suspended cultures in stirred-tank arrangements. The application of this latter configuration to continuous operation is subject to a number of basic disadvantages, particularly 'wash-out' and the difficulties inherent in the provision of an appropriate environmental history for the microbial mass. These disadvantages can to some extent be overcome with fermenter configurations based upon the use both of microbial films and flocs. The kinetics of microbial films are now sufficiently well understood, at least for growth-associated systems, to justify inclusion in the conventional theories for both batch and continuous fermentation. Such theoretical treatments illustrate some of the advantages of using microbial films in terms of increased productivity, absence of wash-out, etc., as well as suggesting the magnitude of the surface area: fermenter volume ratios necessary in particular cases. NOMENCLATURE A dimensionless parameter (defined by equation 4) area of support surface per unit liquid volume L 1 area of biologically active area per unit liquid L— 1 volume B dimensionless parameter (defined by equation 4) C limiting substrate concentration ML3 C1 limiting substrate concentration fed to a fermenter ML C* limiting substrate concentration at interface between ML3 microbial mass and adjacent solution limiting substrate concentration entering second ML3 stage of a tubular fermenter volumetric flow-rate to a fermenter L3T specific growth rate 279