L

penetrative hyphae thin liquid film with "biofilm hyphae"

aerial hyphae moist particle biofilm = cells + water

Fig. 2.4. The phases present within an SSF bioreactor. (a) Macroscale view; (b) Micro-scale view. From left to right the diagrams represent uninoculated substrate, the growth of a filamentous fungus and of a unicellular organism, such as a yeast or bacterium; (c) Greater detail of the microscale, showing a transverse section through the particles

• allowing space for particle movement in mixed bioreactors;

• bringing air to the bed surface in cases where the air is circulated through the headspace rather than being blown forcefully through the bed (such as the rotating and stirred drum bioreactors described in Chap. 8).

The bed is the site of the bio-reaction, that is, the site where the microorganism grows. It will be seen in more detail in the microscale view in the next section. As Fig. 2.5 shows, it is quite possible to have macroscale temperature, moisture, and gas concentration gradients across the bed, these resulting from mass and heat transfer processes that will be discussed in Chap. 4.

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