Biological Reduction Of Nitrate

Denitrification is one of two forms of nitrate reduction. When nitrate is reduced through bacterial activity, oxygen is removed from nitrate. Facultative anaerobic bacteria reduce nitrate to degrade soluble cBOD when free molecular oxygen is not available. This is referred to as denitrification or dissimilatory nitrate reduction, because the nitrogen in nitrate is not incorporated into cellular material; that is, nitrogen leaves the bacterial cell in molecular nitrogen (N2) and nitrous oxide (N2O) (Figure 11.2).

The Reduction Nitrate

FIGURE 11.2 Dissimilatory nitrate reduction. In the absence of free molecular oxygen or presence of an oxygen gradient, facultative anaerobic bacteria remove nitrate from the bulk solution to degrade soluble cBOD. The nitrogen in the nitrate is never incorporated into new cellular material. The nitrogen in the nitrate leaves the cell as molecular nitrogen (N2) and nitrous oxide (N2O).

FIGURE 11.2 Dissimilatory nitrate reduction. In the absence of free molecular oxygen or presence of an oxygen gradient, facultative anaerobic bacteria remove nitrate from the bulk solution to degrade soluble cBOD. The nitrogen in the nitrate is never incorporated into new cellular material. The nitrogen in the nitrate leaves the cell as molecular nitrogen (N2) and nitrous oxide (N2O).

endoenzymes cBOD + NO3-

endoenzymes

C5H7O2N

Endoenzyme Bacteria

FIGURE 11.3 Assimilatory nitrate reduction. In the absence of ionized ammonia, bacteria remove nitrate from the bulk solution for use as a nitrogen nutrient. The nitrogen in the nitrate is incorporated into new cellular material.

C5H7O2N

FIGURE 11.3 Assimilatory nitrate reduction. In the absence of ionized ammonia, bacteria remove nitrate from the bulk solution for use as a nitrogen nutrient. The nitrogen in the nitrate is incorporated into new cellular material.

While dissimilatory nitrate reduction or denitrification is used for respiratory purposes (degradation of soluble cBOD), assimilatory nitrate reduction is used to provide bacterial cells with the nitrogen nutrient (Figure 11.3). When present, ionized ammonia (NH4+) is used as the nitrogen nutrient by bacterial cells for the synthesis (assimilation) of cellular material. In order for bacterial cells to used nitrogen, nitrogen must be in the -3 valence or oxidation state. In ionized ammonia, nitrogen exists in the -3 oxidation state and is readily available for bacterial use; that is, it is preferred.

When ionized ammonia is no longer available, nitrate (NO3-) is used as the nitrogen nutrient. However, nitrogen exists in the +5 oxidation state in nitrate. Therefore, when nitrate is used as the nitrogen nutrient, oxygen is removed from nitrate inside the bacterial cell and hydrogen is added to the nitrogen. By removing oxygen and adding hydrogen, the nitrogen is reduced to a -3 oxidation state, and ionized ammonia is formed. The nitrogen in the newly formed ionized ammonia is then incorporated (assimilated) into new cellular material; that is, nitrogen does not leave the cell. This form of nitrate reduction is referred to as assimilatory nitrate reduction.

Dissimilatory nitrate reduction and assimilatory nitrate reduction can occur simultaneously in an activated sludge process. In the presence of soluble cBOD and nitrate ions and the absence of free molecular oxygen and ionized ammonia, nitrate is used to degrade soluble cBOD (denitrification) and provide the nitrogen nutrient.

Denitrification proceeds in a step-by-step reduction from nitrate to molecular nitrogen (Equation 11.6). There are five nitrogenous molecules and four biochemical steps involved in denitrification. The molecules are nitrate (NO3-), nitrite (NO 2),nitric oxide (NO),nitrous oxide (N2O), and molecular nitrogen (N2).Nitrate always is an initial energy substrate (Table 11.3), while nitrite may be an initial sub-

TABLE 11.3 Energy Substrates, Intermediates, and Final Products of Denitrification

Nitrogenous Compound

Formula

Energy Substrate

Intermediate

Final Product

Nitrate ion

NO3-

X

Nitrite ion

NO2-

X

X

Nitric oxide

NO

X

Nitrous oxide

N2O

X

Molecular nitrogen

N2

X

strate or an intermediate compound. Nitric oxide and nitrous oxide are intermediate compounds, while molecular nitrogen is the final product.

Most facultative anaerobic bacteria denitrify nitrate to molecular nitrogen. However, some facultative anaerobic bacteria lack critical enzymes to denitrify nitrate to molecular nitrogen. For example, some bacteria such as Escherichia coli denitrify nitrate to nitrite and stop. In addition, adverse operational conditions also permit the production of intermediate compounds by facultative anaerobic bacteria.

Although there are four biochemical steps involved in denitrification, there are only two energy-yielding steps. These reactions are the reduction of nitrate (Equation 11.7) and the reduction of nitrite (Equation 11.8). These two reactions can be combined and presented as an overall energy yielding reaction (Equation 11.9).

6NO3- + 2CH3OH denit rifying bacteria > 6NO2- + 2CO2 + 4H2O (11.7)

6NO2- + 3CH3OH denitrifying bacteria > 3N2 + 3CO2 + 6OH- (11.8)

6NO3- + 5CH3OH denit rifying bacteria > 3N2 + 5CO2 + 7H2O + 6OH- (11.9)

The overall energy yielding reaction produces alkalinity in the form of hydroxyl ions (OH-) and bicarbonate ions (HCO3-). Bicarbonate ions are produced when carbon dioxide dissolves in the wastewater to form carbonic acid (H2CO3), which dissociates to form bicarbonate ions. Approximately 50% of the alkalinity lost during nitrification is returned to the process through denitrification.

Much of the energy obtained from the degradation of cBOD is used for cellular synthesis—that is, the production of new bacterial cells (C5H7O2N) or sludge (Equation 11.10). Because less carbon from the cBOD is assimilated into new bacterial cells when nitrate is used to degrade the cBOD as compared to the use of free molecular oxygen, more carbon dioxide is produced when cBOD is degraded with nitrate. Much of the carbon dioxide that is produced does not dissolve in the wastewater.

Nitrate + cBOD denit rifying bacteria > cells (sludge) + water + carbon dioxide (11.10)

Although five gases are produced during denitrification, only three gases escape to the atmosphere from the wastewater (Table 11.4). The majority of gases that are

TABLE 11.4 Fate of Gases Produced Through Denitrification

Formula

Fate

Molecular nitrogen Carbon dioxide

Nitrous oxide Ammonia Nitric oxide

N2 CO2

Insoluble in wastewater Leaves as escaping bubbles

Although soluble in wastewater, forms carbonic acid/bicarbonate alkalinity

Some leaves as escaping bubbles Insoluble in wastewater Leaves as escaping bubbles Converted to NH4+ at pH values <9.4 NH4+ dissolves in wastewater Not usually released from bacterial cell Does not accumulate produced consists of molecular nitrogen and carbon dioxide. Often, these gases alone with nitrous oxide become entrapped in floc particles and contribute to set-tleability problems in the secondary clarifier and thickener before the gases escape to the atmosphere.

Denitrification occurs between the oxidation-reduction potential (ORP) or redox values of +100mV and -100 mV and is influenced by three significant operational factors. These factors are pH, temperature, and nutrients. The optimal pH range for denitrification is from 7.0 to 7.5, while depressed activity for facultative anaerobic bacteria occurs at pH values <6.0 and >8.0.

Denitrification is biologically mediated and occurs more rapidly with increasing wastewater temperature. Additionally, increasing wastewater temperature results in less wastewater affinity for dissolved oxygen, which contributes to more rapid denitrification. Denitrification is inhibited at wastewater temperatures <5°C.

Nitrogen and phosphorus are critical major nutrients for the degradation of soluble cBOD and the growth of bacterial cells (sludge). Adequate quantities of these nutrients for the degradation of soluble cBOD with free molecular oxygen are considered to be present when the mixed liquor effluent of an aeration tank contains these residual quantities for nitrogen and phosphorus:

• 1.0mg/liter NH4+ or 3.0mg/liter NO3- for nitrogen

Since the degradation of soluble cBOD with nitrate produces less bacterial growth than with free molecular oxygen, these values for nitrogen and phosphorus are adequate for the effluent filtrate from a denitrifying tank.

Denitrification occurs whenever an anoxic condition exists. The anoxic condition can occur by design or accident. Designed anoxic conditions consist of (1) the use of a denitrification tank to satisfy a total nitrogen discharge limit and (2) the use of an anoxic period or zone to improve treatment plant performance. Accidental anoxic conditions are most commonly observed in secondary clarifiers.

Aeration tank Denitrification tank Secondary clarifier

Aeration tank Denitrification tank Secondary clarifier

Denitrification Tanks

Return activated sludge (RAS)

FIGURE 11.4 Denitrification tank.

Return activated sludge (RAS)

FIGURE 11.4 Denitrification tank.

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  • benito
    What is use of C5H7O2N cells?
    3 years ago

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