ABSTRACT
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 K0 for autotrophs and heterotrophs in Table 6.3 shows that Ko.A is much larger than Ko,H· This means that as the dissolved oxygen concentration is decreased, the term S0 / (K0 + S0 ) 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 µA 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.
