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SRT, days

Figure 6.6 Effect of SRT on the performance of nitrification in a CSTR receiving a wastewater containing inhibitory compounds. (Adapted from Bridle et al ")

j_i_i_1_1_1_._1_l

approach 100% more gradually. Both effects are illustrated in Figure 6.6. which was obtained with an industrial wasted

Another important characteristic of nitrifying bacteria is their extreme sensitivity to the dissolved oxygen concentration, which necessitated the inclusion of a term for oxygen in the rate expression in Table 6.1. Examination of the values of K„ for autotrophs and heterotrophs in Table 6.3 shows that K„ A is much larger than K,,.H. This means that as the dissolved oxygen concentration is decreased, the term S<>/ (K() + Su) in the rate expression for autotrophs will become small more rapidly than the corresponding term in the rate expression for heterotrophs. Consequently, autotrophs will be affected by decreases in the oxygen concentration much more drastically than will heterotrophs. The importance of dissolved oxygen concentration to the growth of autotrophs is illustrated in Figure 6.7 where the minimum SRT required for their growth is plotted as a function of the oxygen concentration. The curve was generated with Eq. 5.16, with adjustment of |iA for the effects of oxygen as given by the rate expression in Table 6.1. The parameter values from Table 6.3 were used, as indicated in the figure. Examination of the figure shows that oxygen concentrations above 2.0 mg/L have little effect on the minimum SRT, and it is seldom necessary to maintain the concentration in excess of that value to get satisfactory nitrification. However, oxygen concentrations below 2.0 mg/L begin to have a strong effect and those below 0.5 mg/L have a drastic effect. A low dissolved oxygen concentration also diminishes the percent nitrification that can ultimately be achieved. Consequently, care in the specification of the oxygen transfer system is an important component of the design of a suspended growth bioreactor in which nitrification is to occur.

Nitrifying bacteria are also very sensitive to temperature, as reflected by the temperature coefficients in Section 3.9.2. Because |1A is small even at 20°C. low temperatures require bioreactors to have very long SRTs in order for nitrifying bacteria to grow. This is illustrated in Figure 6.8 which was generated with Eq. 5.16

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