Performance Of A Cstr As Predicted By Model

The major value of the models in this chapter is as an aid to understanding how CSTRs behave under a variety of conditions. We saw during derivation of the equations that the most important operational variable is the SRT. Consequently, we will first use graphs generated with the parameter values in Table 5.2 to see how SRT influences bioreactor performance. Next, we will investigate the impact of adding inert and microbial solids to the influent to a CSTR. Finally, we will consider the impact of the values of the kinetic parameters and stoichiometric coefficients on bioreactor performance by considering situations representative of both high and low temperatures.

5.4.1 Effect of SRT

We see in Chapter 10 that the use of sedimentation as the means of biomass separation places both upper and lower limits on the SRT allowable in an operating bioreactor. However, to fully demonstrate the potential impact of SRT on bioreactor performance, the figures presented here were generated without regard to those limits. Consequently, most operating systems will not experience the broad range of conditions exhibited in the figures.

Table 5.2 Kinetic Parameters, Stoichiometric Coefficients, and System Variables Used to Generate Figures in Section 5.4

Symbol Units Value

Ks mg/L as COD 50 Y,, mg biomass COD formed/mg COD removed 0.60

fn mg debris COD/mg biomass COD 0.20

'Unless spceitied othcrwisc.

Figure 5.3 shows the effect of SRT on the soluble substrate concentration. For the parameter values in Table 5.2, the minimum SRT is 2.24 hours, and at that SRT the effluent substrate concentration is equal to the influent concentration. This SRT is also called the washout point for the bioreactor because biomass can no longer exist in it. As the SRT is increased, however, growth can be established in the bioreactor and substantial substrate removal occurs even when the SRT is low. For example, an SRT of slightly more than four hours is all that is required to reduce

Figure 5.3 shows the effect of SRT on the soluble substrate concentration. For the parameter values in Table 5.2, the minimum SRT is 2.24 hours, and at that SRT the effluent substrate concentration is equal to the influent concentration. This SRT is also called the washout point for the bioreactor because biomass can no longer exist in it. As the SRT is increased, however, growth can be established in the bioreactor and substantial substrate removal occurs even when the SRT is low. For example, an SRT of slightly more than four hours is all that is required to reduce

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