SRT, hr

Figure 5.8 Effect of 50 mg/L (as COD) of active biomass in the influent to a CSTR on the soluble substrate concentration (as COD) in the reactor. Kinetic parameters and stoichiometric coefficients are listed in Table 5.2.

lines, thereby introducing biomass with the feed, can prevent the expected response and lead to error in the determination.

Another effect of influent biomass discussed previously is to reduce Ssm,„. the minimum attainable substrate concentration from a single CSTR. It was seen in Eqs. 5.56 and 5.57 that the degree of reduction depends on the magnitude of the influent biomass concentration relative to the influent substrate concentration and this effect is illustrated in Figure 5.9. For the parameter values given in Table 5.2, SSll,m in the absence of influent biomass is 0.76 mg/L as COD, yet the presence of influent biomass can decrease that value significantly, as shown in the figure. This fact may be useful as engineers seek to reduce the concentrations of specific pollutants to very low levels. For example, although industrial wastewater treatment systems generally receive influent from several production areas, one may be the primary source of a targeted pollutant. If that waste stream were pretreated in a small bioreactor without biomass recycle prior to discharge to the main bioreactor, it would do two things: (1) provide a source of bacteria capable of degrading the targeted pollutant coming from the other production areas, and (2) reduce the concentration of the targeted pollutant in the influent to the main bioreactor. The combined effect of these two contributions would be to make the influent biomass to substrate concentration ratio large for the targeted pollutant, thereby allowing the main bioreactor to achieve a lower effluent substrate concentration than would be possible otherwise.

Re-examination of Figures 5.4 and 5.5 reveals that a CSTR with an SRT of 200 hr and an HRT of 10 hr would have a total biomass concentration of 3100

to 1.00

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