In previous sections, the effects of ammonia level, temperature, pH, and dissolved oxygen on the nitrification rate have been presented. In all practical systems, these parameters influence the nitrification rate simultaneously. Chen (1970) showed that the combined effect of several limiting factors on biological growth can be introduced as a product of a Monod-type expression.
Taking this approach for nitrification, the combined kinetic expression for nitrifier growth would take the following form (EPA 1975):
where: n = maximum nitrifier growth rate at temperature T and pH less than 7,2.
Using specific values for temperature, pH, ammonia and oxygen, from Tables shown in the EPA (1975), the following expression results for pH less than 7,2 for Nitrosomonas and is valid for temperatures between 8 °C and 30 °C:
LI = 0,47*(e°'098,(t:-15)) * (1-0,833 (7, 2-pH)) *-—-*-——
In equation (3.36) the first term in brackets allows for the effect of temperature. The second term in brackets considers the effect of pH. For pH less than 7,2 the second quantity in brackets is taken to be unity. The third term in brackets is the Monod expression for the effect of the ammonia nitrogen concentration. Similarly, the fourth term in brackets accounts for the effect of DO on the nitrification rate.
Equation (3.37) has been adopted for illustrative use. When other reliable data become available, equation (3.37) can be modified to suit particular circumstances.
If the ammonia removal rate is defined as in equation (3.36), then equation (3.38) can be written as follows:
The biggest problem in the analysis of rate data for microbial nitrifying bacteria, with or without heterotrophic bacteria, is the estimation of nitrifier concentration for determination of the specific growth rate n, the yield coefficient Yn and the saturation constant Ks n.
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