SRT hr

Figure 5.7 Effect of UK) mg/L (as COD) of influent inert organic solids on the active fraction of the biomass in a CSTR receiving a soluble substrate. Kinetic parameters and stoichiometric coefficients are listed in Table 5.2.

Comparison of the two curves reveals that only moderate amounts of inert solids in the influent to a CSTR can decrease the active fraction to less than 50%, especially at longer SRTs. As a result, final settlers and pumps for the recycle of biomass must be made larger to handle solids which contribute nothing to the process. Thus, it is generally more economic to reduce the concentration of inert solids prior to biological reactors.

As discussed in Section 5.2.3, one significant effect of having biomass in the influent to a CSTR is to prevent washout, thereby allowing substrate removal to occur at SRTs below the normal minimum. This is illustrated in Figure 5.8, where the impact on the soluble substrate concentration of having 50 mg/L as COD of active biomass in the influent is shown. The most dramatic effect is at SRTs near the minimum. Instead of having a discontinuity at the point of washout like the curve for the bioreactor without biomass in the influent, the concentration in the bioreactor receiving biomass slowly approaches the influent concentration as the SRT is made smaller and smaller. Under those conditions the microorganisms are growing and removing substrate at a very rapid specific rate, but the residence time in the bioreactor is too short to allow more complete removal to occur. Of course, higher concentrations of biomass in the influent will allow more substrate to be removed at short SRTs. The main importance of the effect illustrated in Figure 5.8 is as an explanation of why washout does not occur in circumstances where it might be expected. This is especially important during laboratory studies in which investigators attempt to measure (1 by observing the SRT at washout. Contamination of feed

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