Inhibition and Toxicity

There are several forms of inhibition and toxicity that may occur during nitrification (Table 18.1). Inhibition is a temporary, short-term (acute) or long-term (chronic) loss of enzymatic activity. Toxicity is the permanent loss of enzymatic activity or irreversible damage to cellular structure.

Although nitrifying bacteria can overcome (acclimate) to inhibition by repairing damaged enzyme systems, chronic inhibition may significantly lower the rate of reproduction of nitrifying bacteria, resulting in a ''washout'' of the population through the loss of bacteria in the secondary effluent or sludge wasting. Also nitrifying bacteria obtain only a relatively small amount of energy from the oxidation of ammonium ions and nitrite ions, and unfortunately, sufficient energy is often not available for sustained or repeated acclimation. Therefore relatively small increases of inhibitory wastes can cause dramatic changes in the growth of nitrifying bacteria.

Nitrifying bacteria cannot acclimate as often to an inhibitory event as can organotrophs. This is due to the fact that organotrophs obtain a relatively large amount of energy from the oxidation of cBOD, and it is energy that permits them to repair damaged enzyme systems. Therefore nitrifying bacteria are more ''sensitive'' to inhibitory wastes than organotrophs. Thus significant loss of nitrification occurs before significant loss in efficiency of cBOD removal.

Due to the relatively small amount of energy available for acclimation, nitrifying bacteria are ''sensitive'' to very low concentrations

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