Autotrophic nitrifiers are aerobic microorganisms oxidizing ammonia via nitrite (Eq. 18) to nitrate (Eq. 19). Organisms catalyzing nitrification (Eq. 18) belong to the genera Nitrosomonas, Nitrosococcus, Nitrosolobus, Nitrosospira, and Nitrosovibrio, organisms catalyzing nitration (Eq. 19) include, e.g., members of the genera Nitrobact-er, Nitrococcus, and Nitrospira.
Ammonia oxidation to nitrite or nitrite oxidation to nitrate are energy-yielding processes for the autotrophic growth of the nitrifying bacteria. CO2 is assimilated via the Calvin cycle. Since the positive redox potential of the oxidizable nitrogen compounds is not low enough to form NADH2 for CO2 reduction, NADH2 must be formed by an energy-consuming reverse electron transport. For this reason, the growth yield of nitrifying bacteria is low. Nitrosomonas sp., e.g., must oxidize 30 g NH3 to form 1 g of cell dry mass (Schlegel, 1992).
The oxidation of ammonia by nitrifiers is initiated by an energy-neutral monooxy-genase reaction that yields hydroxylamine (Eq. 20):
Then hydroxylamine is further oxidized, presumably via nitroxyl (Eq. 21) to nitrite (Eq. 22), which is the energy-yielding reaction during nitrification. For microbial oxidation of 1 mg NH+-nitrogen to nitrite, 3.42 mg O2 is required, and for the oxidation of 1 mg NO--nitrogen to nitrate (Eq. 19), 1.14 mg O2.
Due to their slow growth autotrophic nitrifiers cannot successfully compete with heterotrophic bacteria for oxygen. In a highly loaded activated sludge system, the autotrophic nitrifiers are overgrown by the heterotrophic sludge flora, which consume the oxygen. Ammonia oxidation starts only if the BOD5 concentration in the wastewater is <110 mg L-1 (Wild et al., 1971). During nitrification of ammonia the alkalinity of the wastewater increases slightly, due to CO2 consumption for autotrophic growth (pH increase), but in a counter reaction it drastically decreases due to nitric acid formation from ammonia (pH decrease from above neutral to acidic). If the buffer capacity of the wastewater is weak, the pH drops far below 7 and thus prevents further nitrification by autotrophic nitrifiers (Rheinheimer et al., 1988).
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