Nitrifying Bacteria

Nitrogen (N) is an essential nutrient of all living organisms. In the amino group (—NH2) nitrogen is in the -3 oxidation state or valence and is incorporated into amino acids such as glycine (CH2NH2COQH) (Figure 10.1). Amino acids are used by organisms to build proteins. Proteins then are used to build structural materials, enzymes, and genetic materials. Amino acids, proteins, and compounds built with proteins are organic nitrogen compounds.These compounds are found in fecal waste and food waste discharged to municipal wastewater treatment plants.

While most amino acids are simple in structure and soluble in water, proteins are complex in structure and insoluble in water. Proteins are colloids and possess a relatively large surface area and remain suspended in water.

Within the sewer system, simple amino acids undergo deamination through bacterial activity (Figure 10.2). Deamination results in the release of amino groups and the production of reduced nitrogen. There are two forms of reduced nitrogen: ammonia (NH3) and ammonium ion or ionized ammonia (NH4+). The quantity of each form of reduced nitrogen that is produced is pH-dependent (Figure 10.3).

Ammonia is toxic and is released to the atmosphere from the sewer system or biological treatment unit through turbulence (aeration or mixing action). Ionized ammonia is nontoxic and is used by bacteria as their preferred nitrogen nutrient. At pH values lower than 9, most of the reduced nitrogen is present as ionized ammonia.

Another significant organic nitrogen compound is urea (H2NCONH2). Urea is a major component of urine that is discharged to municipal wastewater treatment plants. Urea undergoes hydrolysis within the sewer system through bacterial activity. Hydrolysis is the addition of water to a compound resulting in the "splitting of the compound into at least two simpler compounds. Hydrolysis of urea results in the production of reduced nitrogen (Equation 10.1).

FIGURE 10.1 Glycine (aminoethanoic acid). Glycine is the simplistic amino acid. Like all amino acids, glycine possesses a carboxyl group (—COOH) and an amino group (—NH2). The carboxyl group is located on a terminal carbon unit, while the amino group is located on the carbon unit immediately adjacent to the carboxyl group.

FIGURE 10.1 Glycine (aminoethanoic acid). Glycine is the simplistic amino acid. Like all amino acids, glycine possesses a carboxyl group (—COOH) and an amino group (—NH2). The carboxyl group is located on a terminal carbon unit, while the amino group is located on the carbon unit immediately adjacent to the carboxyl group.

Cysteine

Deaminase enzyme

NH2 ^-Amino group

NH3 -Ammonia

NH4+ —Ionized ammonia

FIGURE 10.2 Deamination of the sulfur amino acid, cysteine. When cysteine is "attacked" by the deaminase enzyme, the amino group (—NH2) is removed from cysteine. The liberated amino acid quickly forms ammonia; depending upon the pH of the aqueous environment, the ammonia may be ionized to form ionized ammonia.

The influent to municipal wastewater treatment plants contains organic nitrogen compounds and inorganic nitrogen in the form of ionized ammonia. Approximately 60% of the nitrogenous wastes to the treatment plants is in the organic forms, and approximately 40% of the nitrogenous wastes is in the inorganic form. Most municipal wastewater treatment plants have an influent, ionized ammonia concentration of 25-30mg/liter.

Additional nitrogenous wastes that may enter municipal wastewater treatment plants include polymers that are used at wastewater treatment plants and industrial wastewaters (Table 10.1). Some of these industrial wastewaters contain not only ionized ammonia and organic nitrogen compounds but also nitrite (NO2-) and nitrate (NO3-).

Percent NH3 Percent NH4+

Percent NH3 Percent NH4+

FIGURE 10.3 Distribution of ammonia and ionized ammonia in the mixed liquor.

FIGURE 10.3 Distribution of ammonia and ionized ammonia in the mixed liquor.

TABLE 10.1 Industrial Discharges of Ionized Ammonia, Nitrite Ions, and Nitrate Ions

Nitrogenous Compound

TABLE 10.1 Industrial Discharges of Ionized Ammonia, Nitrite Ions, and Nitrate Ions

Industrial Discharge

NH+

NO2-

NO3-

Automotive

X

Chemical

X

Coal

X

Corrosion inhibitor

X

Fertilizer

X

Leachate

X

Leachate (pretreated)

X

X

Livestock

X

Meat

X

Meat (flavoring)

X

Meat (preservative)

X

Ordnance

X

Petrochemical

X

Pharmaceutical

X

Primary metal

X

Refineries

X

Steel

X

X

X

Tanneries

X

TABLE 10.2 Factors that Prevent Nitrification in the Sewer System

Absence of free molecular oxygen or low concentration of free molecular oxygen Insignificant population of nitrifying bacteria

Presence of soluble cBOD produced through fermentation that inhibit nitrifying bacteria Short hydraulic retention time

The presence of nitrite and nitrate in the sewer system is infrequently observed and is an indicator of an industrial discharge. Nitrite and nitrate are not produced in the sewer system (Table 10.2).The production of nitrite and nitrate occurs during nitrification in the activated sludge process.

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