Figure 5.3 Effect of SRT on the concentration of soluble substrate (as COD) in a CSTR receiving a soluble substrate. Kinetic parameters and stoichiometric coefficients are listed in Table 5.2.
the substrate concentration from 500 to 50 mg/L as COD. This demonstrates an important characteristic of biological reactors: they are able to achieve substantial removal of soluble substrate at very short SRTs. However, incremental removal of substrate declines sharply as the SRT is increased, although such increases make the bioreactor more stable. For example, compare the differences in substrate concentration resulting from a 10% change in SRT at SRTs of 4, 40, and 400 hours. The minimum attainable substrate concentration (SSmm) for the parameter values used to generate Figure 5.3 is 0.76 mg/L as COD and examination of the graph shows that the value is approached very slowly. When methods like COD and biochemical oxygen demand (BOD) are used to measure effluent substrate concentration on full scale systems, the SRT generally has little measurable effect beyond certain values. That is simply because the potential change is small compared to the error associated with the test method.
The dashed curve in Figure 5.4 shows the impact of SRT on the total mass of biomass (as COD) in the CSTR. It will be recalled from Eqs. 5.24 and 5.13 that the biomass concentration depends on the HRT whereas the substrate concentration is independent of it. Thus, the HRT must be considered to convert the mass values in Figure 5.4 into concentrations. However, since the influent flow rate was fixed for the simulations, as shown in Table 5.2, consideration of different HRTs is equivalent to consideration of different bioreactor volumes. Thus, the total biomass concentration in a CSTR with a volume of V liters being operated at a particular SRT can be obtained by dividing the mass associated with that SRT by V. For example, when the SRT is 100 hr, the bioreactor contains 20 g COD of total biomass (active plus
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