Figure 5.9 Effect of the inlluent biomass concentration relative to the intluent substrate concentration on the minimum substrate concentration attainable in a CSTR. Ss„„„ is expressed as a fraction of the S.,,,,,,, value attainable in a similar bioreactor receiving no intluent biomass.
mg/L with an active fraction of 0.76 if it were treating a wastewater with the characteristics in Table 5.2. What would be the fate of the excess biomass from that bioreactor if it were sent for treatment to another CSTR in which the ratio of the SRT to the HRT ratio remains fixed at 10? In other words, the SRT is increased by increasing the HRT proportionally. Since the concentration of soluble substrate in the waste biomass stream is negligible, Eqs. 5.64-5.68 describe the performance of the CSTR receiving the waste biomass and the results of their use are shown in Figure 5.10. There it can be seen that because of the buildup of debris in the bioreactor, the total biomass concentration will not go to zero as the SRT is increased, but will approach a limit, although the active biomass will become quite small. Furthermore, it can be seen that there is a point of diminishing return with regard to further increases in SRT because the active biomass declines rapidly at first, but then more slowly as the SRT is increased further. This is characteristic of the first order expression chosen to depict decay. It should be remembered that the model used assumes that debris is totally inert, whereas it will undergo some destruction given sufficient time, as discussed in Sections 2.4.2 and 3.3.1. Thus, it should be recognized that the residual stable biomass concentration will probably be less than that depicted by the model. Just as with the CSTR whose performance was depicted in Figure 5.6, in an aerobic process the destruction of biomass occurs at the expense of oxygen. Thus, the oxygen requirement is the mirror image of the total biomass curve. For simplicities sake, the influent flow rate to the bioreactor was taken as 1.0 L/hr, making the mass input rate of biomass equal to 3100 mg/hr as COD. Thus, it can be seen that about 50% of the oxygen demand of the influent biomass must ultimately be satisfied at longer SRTs. In other words, the final residual solids are highly stabilized.
The SRT of a CSTR is the primary control variable available to a designer or operator; however, it is not the only factor affecting the performance of such a bioreactor. Examination of the equations for the performance of a CSTR reveals that the values of each of the kinetic parameters and stoichiometric coefficients will influence them as well. The primary effects of (1,, and Ks are on the substrate concentration. A higher value of (1H and a lower value of Ks allow the biomass to grow faster at a given substrate concentration, thereby giving a lower reactor substrate concentration for any given value of the SRT. The Monod parameters also exert a strong effect on the minimum SRT, so that organisms with high |iH and low Ks values can grow in CSTRs with short SRTs. The effect of the Monod parameters on biomass concentration is strongest at short SRTs where the effect on the substrate concentration is strongest. They have almost no effect at longer SRT values, however. In contrast to the Monod parameters, the primary effect of the decay coefficient is on the biomass concentration and the oxygen requirement at longer SRTs. A high decay coefficient means that the bioreactor will be more efficient in oxidizing the substrate to carbon dioxide; consequently, the biomass concentration will be low and the oxygen requirement high. This effect will be especially pronounced at long SRTs. Changes in the true growth yield will also primarily affect the biomass concentration and the oxygen requirement. High yields will result in more biomass, but the culture
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