SRT hr

Figure 5.5 Effect of SRT on the active fraction of the biomass in a CSTR receiving a soluble substrate. Kinetic parameters and stoichiometric coefficients are listed in fable 5.2.

to substrate removal. Nevertheless, the amount of active biomass continues to increase as the SRT is increased, as can be seen by multiplying the active fraction from Figure 5.5 times the mass of biomass from Figure 5.4. As a consequence, increases in SRT are generally worthwhile, although a point of diminishing return will be reached. One price associated with a high SRT is the cost of moving inactive biomass continually around the system, yet little can be done about it because it is impossible to separate active biomass from debris. In fact, it is possible that accumulated debris contributes to improved settling properties at increased SRT.

A benefit associated with increased SRTs is that less excess biomass must be disposed of because more of it is oxidized through decay, maintenance energy needs, etc. as discussed above for Y,,^. This effect is illustrated in Figure 5.6. When the SRT is small, even though Yohv is high, substrate removal is incomplete, as shown in Figure 5.3, and little excess biomass is synthesized. However, as the SRT is increased beyond the minimum SRT, more excess biomass is generated because of increased growth and substrate removal, all with a relatively high observed yield. As the SRT is increased further, the effluent substrate concentration becomes small relative to the influent concentration so that the term Ss(l — Ss becomes essentially constant. This occurs at an SRT of about 10 hr for the parameter values used to generate the graphs. Beyond that point, further increases in SRT increase the importance of decay, causing Y„,,s and the net production of biomass to decline as shown by the decreasing mass which must be wasted. Those same events determine the shape of the oxygen consumption curve. At very short SRTs, substrate removal is incomplete and decay is of little importance, with the result that most of the electrons

Figure 5.6 Effect of SRT on the biomass wastage rate (as COD) and oxygen requirement in a CSTR receiving a soluble substrate. Kinetic parameters and stoichiometric coefficients are listed in Table 5.2.

SRT, hr

Figure 5.6 Effect of SRT on the biomass wastage rate (as COD) and oxygen requirement in a CSTR receiving a soluble substrate. Kinetic parameters and stoichiometric coefficients are listed in Table 5.2.

available in the influent substrate are either associated with the effluent substrate or the biomass formed. Thus, relatively little oxygen is required. As the SRT is increased slightly to the point where substrate removal is essentially complete (SRT = 10 hr) but decay is not yet important, all of the oxygen use is associated with substrate removal and biomass growth, i.e., energy for synthesis. Further increases in the SRT result in more decay, and almost all of the increased oxygen requirement as the SRT is increased past 10 hr is associated with that. In other words, the decreased mass of excess biomass associated with longer SRTs is at the expense of an increased oxygen requirement. This suggests that the choice of SRT is often governed by the relative costs of supplying oxygen versus disposing of excess sludge.

Although Figures 5.3-5.6 were developed with parameter values represenlative of aerobic growth of heterotrophic bacteria, it is important to recognize that the shapes of the curves are representative of microbial growth in general, including aerobic growth of autotrophs and anoxic growth of heterotrophs. All that is necessary is an appropriate change in electron donor or acceptor and adjustment of the parameter values.

5.4.2 Effects of Influent Solids

In Section 5.2.2 it was seen that the impact of inert solids in the influent to a CSTR is to reduce the active fraction of the MLSS. This effect is illustrated in Figure 5.7. The solid curve is the same as the one in Figure 5.5 whereas the dashed one represents the case in which 100 mg/L as COD of inert solids are added to the influent.

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