Biological nitrification oxidizes nitrogen from ammonia (NH+) to nitrate (NOT). The overall reaction is a two-step process. In the first step, ammonia is oxidized to nitrite (NOT) and mediated by the genus Nitrosomonas. The second step converts nitrite to nitrate, controlled by the genus Nitrobacter. The conversional process is as follows:
However, these autotrophic bacteria do not use all the ammonia for energy. Viable biomass utilize ammonium for the required nitrogen source during cell synthesis. The overall stoichiometry of biological nitrification is as follows (U.S. EPA 1993):
Equation 7.39(3) theoretically gives three important parameters. First, 4.34 mg of O2 are needed to oxidize 1 mg NH+-N. This amount agrees fairly closely with the sum of the two preceding equations (4.57 mg O2/mg NH+-N). Secondly, 7.07 mg of alkalinity (as CaCO3) are consumed per mg of NH+-N nitrified. Finally, 0.13 mg of viable bio-
mass are produced per mg NH+-N converted. These parameters vary according to process operation and conditions, i.e., sludge age, organic loading, pH, and temperature.
Nitrification warrants treatment system adjustments and can occasionally be a difficult process to achieve reliably. The aerobic autotrophs responsible for nitrification are more sensitive to ambient conditions, toxics, and inhibitors than the competing carbonaceous heterotrophs (Sharma and Ahlert 1977; Sutton, Murphy, and Jank
1974). Additionally, some confusion exists over the biological kinetics during typical wastewater treatment conditions (Charley, Hooper, and McLee 1980).
As shown in the stoichiometry, nitrification requires more oxygen than just the amount required by carboneous heterotrophs for carbon oxidation (BOD removal). A longer sludge age must be maintained due to the slower growth rate of autotrophic nitrifiers. Also, if sufficient alkalinity is not present in wastewater, the pH can drop substantially. This low pH limits the extent of nitrification.
Table 7.38.1 shows typical process parameters for single-stage nitrifying systems.
Biological denitrification reduces nitrate (NO-) to nitrogen gas (N2), nitrous oxide (N2O) or nitric oxide (NO). This nitrogen removal process is the one most widely used in municipal wastewater treatment (Water Pollution Control Federation 1983). Denitrifying organisms are primarily facultative aerobic heterotrophs that can use nitrate in the absence of DO. Many genera of bacteria are capable of denitrification: Achromobacter, Bacilus, Brevi-bacterium, Enterobacter, Micrococcus, Pseudomonas, and Spirlilum (Davies 1971; Prescott, Harley, and Klein 1990). Several conditions enhance the amount of biological den-itrification: nitrate, a readily available carbon source, and a low DO concentration. Low DO is the most critical condition since denitrification is simply several modifications of the aerobic pathway used for BOD oxidation (U.S. EPA
The stoichiometric reaction describing this biological reaction depends on the carbon source involved as follows (Randall, Barnard, and Stensel 1992):
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