Where Has this Description Led Us

This qualitative analysis of the physical nature of SSF bioreactor systems and the phenomena occurring within them has led us to the situation where we can say:

• There is a very wide array of phenomena occurring, and many of these can potentially limit growth. The limiting factor will depend on the microbe-substrate system, the bioreactor used and how it is operated, and the stage of the fermentation;

• The system is highly complex. It is so complex that this chapter, although it describes most of the important processes, finds it difficult to give a clear picture of the interactions between all the phenomena that are occurring simultaneously. The best approach to understanding the complexity of the interactions is to combine the various phenomena within a mathematical model;

• The inter-particle diffusion processes can be considered intrinsic to SSF, and there is little that can be done to affect them in the way that the bioreactor is operated. For example, little can be done to prevent O2 from being exhausted within a small depth below the surface of substrate particles, and therefore O2 limitation for at least some of the biomass can be considered to be an intrinsic characteristic of SSF systems;

• In bioreactor operation, the best that we can do to minimize overheating and O2 limitations is to provide a gas environment close to the substrate particle that will lead to high rates of heat removal and high rates of oxygen supply. The operating variables available to try to achieve this are the rate at which air is supplied, the state (temperature, degree of saturation etc.) in which the air is supplied, and the frequency with which the bed is agitated.

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