Figure 5.4 Effect of SRT on the observed yield and the total mass of biomass (as COD) in a CSTR receiving a soluble substrate. Kinetic parameters and stoichiometric coefficients are listed in Table 5.2.
debris). Thus, if the reactor volume were 2 L the concentration would be 10 g/L as COD, whereas if the volume were 4 L the concentration would be 5 g/L. The dashed curve is important because it demonstrates clearly that the mass, and thus the concentration, of biomass in a reactor of volume V increases as the SRT is increased. It is this increase in the mass of organisms that allows more substrate to be removed as the SRT is increased, even though the HRT is kept the same; more biomass can accomplish more in the same available time. A fixed mass of organisms is required to accomplish a given amount of substrate removal; only the concentration is influenced by the bioreactor volume.
The solid curve in Figure 5.4 is important because it illustrates the decrease in the observed yield that occurs as the SRT of a bioreactor is increased. This is because of the increased importance of decay as the SRT is increased, as indicated in Eq. 5.28. Only when the SRT is very short and the biomass is growing very rapidly will most substrate utilization go for growth, allowing the observed yield to approach the true growth yield, which was 0.60 for this case. For all other situations a significant amount of energy must be expended for maintenance and other purposes associated with decay, thereby lowering the observed yield.
Figure 5.4 presents the total mass of biomass, but we know that biomass decay will decrease the active fraction through the build up of biomass debris as the SRT is increased. Figure 5.5 shows that effect. At low SRT, when the specific growth rate is high, the impact of decay will be small so that little debris will be generated, making the active fraction large. As the SRT is increased, however, the build up of biomass debris in the bioreactor becomes significant and the active fraction drops, until at high SRTs only a small percentage of the biomass is actually contributing
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