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This bioreactor was used to investigate the use of discontinuous rotation for bed temperature control. Each time the bed temperature reached 34°C, a 60 s rotation period was triggered, with several clockwise and anticlockwise rotations, at rates of 4 to 6 rpm. Although it was possible to control the temperature of the 1-kg bed using this strategy (Fig. 8.6(b)), it is unlikely to be effective at large scale.

Kalogeris et al. (1999) developed a bioreactor that is a variation of a rotating-drum bioreactor (Fig. 8.7). In this bioreactor the substrate bed is held within a 10-L perforated cylinder that can be rotated. This perforated cylinder is inside a larger water-jacketed solid-walled cylinder through which air is passed. This bioreactor did work well for the cultivation of thermophilic organisms, but heat removal from the bed is unlikely to be sufficient for the cultivation of mesophiles, for two reasons. Firstly, the air blown into the headspace region will preferentially flow past the surface of the bed rather than through the bed itself (it is for this reason that this bioreactor is classified as a group III bioreactor). Secondly, there is no intimate contact between the bed and the water jacket; a layer of process air separates them. This type of operation was later adapted for an SSF process in which a nutrient medium was placed in the bottom of the bioreactor and nylon sponge cubes were regularly wetted with this nutrient medium as the inner perforated drum rotated at 3 rpm (Dominguez et al. 2001). The system was used for ligninolytic enzyme production by Phanerochaete chrysosporium.

Roller bottle systems are useful for testing, at laboratory scale, a number of different treatments for a process intended to be performed in a rotating-drum bioreactor. Figure 8.8 indicates one possible way in which a roller system can be constructed. Note that the direct introduction of air into the headspace of each individual bottle is complicated, although not impossible. In the majority of cases it would be more likely for each bottle simply to have a perforated lid, with a passive exchange of gases between the headspace and the surrounding air.

static outer cylinder (the walls static outer cylinder (the walls

Bioreactor Perforated Cylinder

Fig. 8.7. General design principles of the Group III bioreactor used by Kalogeris et al. (1999) and Dominguez et al. (2001). The bottom of the bioreactor of Dominguez et al. (2001) was filled with a liquid nutrient medium up to the level shown by the dotted line. In the case of the bioreactor of Kalogeris et al. (1999) there was no liquid held by the outer cylinder. Adapted from Dominguez et al. (2001) with kind permission of Elsevier

Fig. 8.7. General design principles of the Group III bioreactor used by Kalogeris et al. (1999) and Dominguez et al. (2001). The bottom of the bioreactor of Dominguez et al. (2001) was filled with a liquid nutrient medium up to the level shown by the dotted line. In the case of the bioreactor of Kalogeris et al. (1999) there was no liquid held by the outer cylinder. Adapted from Dominguez et al. (2001) with kind permission of Elsevier

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