Nitrification

Nitrification is a microbial process that converts ammonia into nitrite and ultimately into nitrate. Ammonia in wastewater comes primarily from two sources: intense use of nitrogen-rich fertilizers such as urea and organic nitrogen from proteins. The deamination of organic nitrogen and hydrolysis of urea under urease results in ammonia (Equations 2.6 and 2.7):

urease

Urea + 2H2O b (NH4 + )2CO32-Amino acid + 0.502 b R-CO-COOH + NH4 for oxidative deamination and Amino acid + 2H b R-CH2 -COOH + NH4

for reductive deamination (Equation 2.8).

Wastewaters from the fishery, meat, and poultry industries contain substantial amounts of proteins. By the time these proteins reach the collection facilities of the wastewater treatment plants, most of them have been converted into peptides and amino acids by extracellular proteolytic enzymes and ultimately into ammonia.

In nitrogen-rich wastewater streams, only a small amount of the nitrogen is removed from wastewater through conventional heterotrophic activity by incorporating nitrogen into the microbial biomass. There is a parallel in phosphorus-rich wastewaters. If left untreated, the nitrogen or/and phosphorus in discharged effluents will cause eutrophication (a form of photo-autotrophic activity) of the receiving waters, which will gravely disrupt the aquatic ecosystem.

The nitrification process in biological wastewater treatment, i.e., the use of a limited group of autotrophic nitrifying bacteria to convert ammonia into nitrite and eventually nitrate, is often used in the so-called advanced phase of a wastewater treatment scheme if the concentration of ammonia in wastewater streams is high enough to warrant the treatment. Nitrification is a two-step process: (a)ammonia is first converted into nitrite by a group of bacteria called Nitrosomonas and (b) further conversion of nitrite leads to nitrate by another group of bacteria named Nitrobacter. Other genera may also be involved in the nitrification process; for example, Nitrospira, Nitrococcus, Nitrosogloea, and Nitrosocystis have been identified to participate in oxidizing ammonia in nitrification (Belser, 1979). Most nitrifying bacteria are autotrophic and utilize carbon dioxide as the carbon source. For oxidation of ammonia, the biochemical reaction is expressed as the following (Equation 2.9);

Here again, we use C5H7O2N to represent the composition of microorganisms.

For oxidation of nitrite, the reaction expression is written as the following (Equation 2.10):

NH4+ + 5CO2 + 10NO2- + 2H2O b 10NO3- + C3H7O2N + H+

These equations allow the amount of chemicals required for the processes to be calculated.

The growth of nitrifying bacteria is represented by Monod Kinetics (Equation 2.11):

Table 2.2. Common values for various Monod kinetic constants applicable to the nitrification process (Hultman, 1973).

Monod Kinetic Constant Value

where ^ is the specific growth rate of the nitrifying bacteria, is the maximum specific growth rate, ks is the saturation constant, and S is the residual concentration of the growth-limited nutrient.

In the two-step nitrification process, conversion of ammonia into nitrite is the limiting reaction; thus, it is more convenient to model nitrification on the ammonia-nitrite step, i.e., on the specific growth rate of Nitroso-monas, ^NS (Equation 2.12):

where [NH4 — N] is the ammonia concentration expressed in terms of nitrogen in wastewater in the reactor. The data for ks of NH4, the maximum specific growth rate, ^mNS, and yield in nitrification can be found in Table 2.2 (Hultman, 1973). Once the observed yield constants for NH4— and NO2— conversions are known, various calculations can be done with Equations 2.9 and 2.10.

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