0 200 400 600 800 1000

0 200 400 600 800 1000

SRT, hrs

Figure 6.15 Effect of SRT on the optimal aeration fraction (dots) and the range of near optimal aeration fractions (lines) in a CSTR operated with intermittent aeration. The influent biodegradable COD = 200 mg/L and the influent nitrogen concentration = 30 mg/L. The dissolved oxygen concentration was fixed at 2.0 mg/L when the aeration was on and 0.0 mg/L when it was off. (Adapted from Batchelor.s)

6.5.3 Closure

The important point to gain from the preceding is that engineers can exert control over the environment in biochemical reactors, thereby allowing processes to occur in a single system that would not otherwise occur together. This suggests that the engineer has considerable latitude in system design. The complexity of the interactions, however, makes it impossible to intuitively predict the outcome of all possible systems that the engineer might conceive. This, in turn suggests why it is necessary to work with models like those in Table 6.1. Through their application, engineers can explore large numbers of possible bioreactor systems to see how system layout and environment influence the outcome of the possible reactions.

It is apparent that the number of options available during design of a single CSTR is very limited. Thus, most biochemical operations employ reactors with spatial gradients in them, usually through use of reactors with large length to width ratios, but also through use of compartmentalized reactors. Since both can be modeled as tanks in series systems, in the next chapter we will apply ASM No. 1 to study the performance of a number of such systems.

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