A review of the literature concerning the nitrification process shows diverse opinions regarding the reaction rate equation for the nitrification process. Several rate equations have been proposed. Each stems from different assumptions, and different results have therefore been obtained. A review of these equations is presented in Table 3.3.
Knowles, Downing and Barrett (1965) and Downing (1968), were among the first to attempt to quantify nitrifying bacteria in waste water treatment plants. They all used the Monod Model of population dynamics proposed by Monod in 1942, which is similar to the Michalis-Menten relationship for enzyme reactions.
Huang and Hopson (1974) reviewed four different reaction rate equations (see
Table 3.4) to determine the appropriate equation. From the initial ammonia-nitrogen concentration and the contact time studies, the nitrification process was shown to follow a zero-order reaction.
The Monod Model used to describe the kinetics of biological growth of either Nitrosomonas or Nitrobacter is the standard expression used in formulating the rate equation:
where n = growth rate of micro-organisms, in day"1.
Hmax = maximum growth rate of microorganisms, in day"1.
Ks n = saturation constant = substrate concentration, mg/l, at half the maximum growth rate. SN = growth limiting substrate concentration, mg/l expressed as NH4+ - N.
When the reaction rate is independent of the substrate concentration, the reaction rate can be considered as a zero order reaction. This results from a high substrate concentration which leads to a maximum growth rate, indicating that no diffusional limitations exist.
When the reaction is directly proportional to the substrate concentration then the reaction can be considered as first order and the rate of reaction would be directly governed by the ambient ammonia concentration.
The saturation constant Ksn is temperature dependent, as will be discussed in section 3.8. As the maximum growth rate of Nitrobacter is considerably higher than the maximum growth rate of Nitrosomonas, and as the Ks n values for both organisms are less than 1 mg/l NH4+ -N at temperatures below 20° C, nitrite does not accumulate in large amounts in biological treatment systems under steady-state conditions.
Table 3.7 and Fig. 3.4 presents values for Ks for both nitrifying species as found under different environmental conditions.
Integrated rate law ds_ dt
Plot needed to give a straight line
Slope of the straight line
[product] versus t
Figure 3.1 Graphical representations showing a) Monod kinetics; b) Tranformation of Michaelis-Menten Kinetics to the Lineweaver-Burk Plot; c) Zero Order kinetics and d) First Order kinetics.
Nitrosomonas and Nitrobacter are both sensitive to their own and each others substrate. Tables 3.5 and 3.6 show that wide ranges of ammonia and nitrite ion concentrations can be oxidized by the nitrifiers. Different conditions can account for the apparent discrepancies. Normal ammonia and nitrite ion concentrations in domestic waste waters are not in the inhibiting ranges. Substrate and product inhibition, however, are of significance in the treatment of industrial and agricultural wastes. Table 3.19 show the ammonium nitrogen and nitrate nitrogen concentration range for Nitrobacter inhibition as function of pH.
It would be desirable for the process of nitrification to be a reaction having zero-order kinetics at least to low concentrations (< 5 mg/l) as the rate would be constant and unaffected by the substrate concentration.
Mateles et at. (1965) showed that while the Monod Model for microbial growth was useful for steady-state cultures, its application in predicting the dynamic behaviour of chemostats has limitations.
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