Figure 6.9 Effects of SRT and nitrification on the ML,SS concentration and oxygen requirement in a CSTR with (h\/t = 20. Parameter values are given in Table 6.3 and the influent conditions are given in Table 6.6. The value of was set equal to zero to eliminate nitrification in one case. The dissolved oxygen concentration was lixed at 4.0 mg'L.

Table 6.6 Characteristics Considered to Be Representative of a United States Domestic Wastewater Following Primary Sedimentation



Inert particulate organic matter Slowly biodegradable substrate Readily biodegradable substrate Oxygen

Soluble nitrate nitrogen

Soluble ammonia nitrogen

Soluble biodegradable organic nitrogen

Particulate biodegradable organic nitrogen


35.0 mg/L as COD 150.0 mg/L as COD 115.0 mg/L as COD 0.0 mg/L as O, 0.0 mg/'L as N 25.0 mg/L as N 6.5 mg/L as N 8.5 mg/L as N 5.0 mM./L

tion with the differences in biomass yield, means that similar amounts of oxygen are required by the two groups of microbes. This can be seen clearly in Figure 6.9b. Failure to consider this effect during bioreactor design will lead to an inability to transfer sufficient oxygen, with resulting impairment of nitrification performance, as discussed previously.

Because of the important effect of nitrification on the oxygen requirement in a bioreactor, there are circumstances in which it would be advantageous to be able to calculate it explicitly. This can be done by using the simple model in Chapter 5 with minor modification. For the simple decay model, an equation analogous to Eq. 5.33 can be used by recognizing that the factor 1.0 must be replaced with 4.57, as discussed in Section 6.1.1:

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