Nitrogenous Compound Formula
Nitrate ion NO3
Nitrite ion NO3
Nitric oxide NO
Nitrous oxide N2O
oxide. Some denitrifying bacteria release all three intermediates during denitrification, while other bacteria release two, one, or none of the intermediates.
In the presence of adequate organic carbon or cBOD and absence of free molecular oxygen, biological denitrification can occur. Adequate organic carbon is considered to be a soluble cBOD to nitrite ion and nitrate ion ratio of approximately 3:1.
There are two bacterial, energy yielding steps or reactions involved in denitrification that not only provide the bacteria with energy but also release molecular nitrogen to the atmosphere. These two energy-yielding reactions and the overall energy-yielding reaction are provided in Equations (24.3), (24.4), and (24.5). These equations use methanol as the cBOD source.
6NO33 + 3CH3OH ! 3N2 + 3CO2 + 3H2O + 6OH" (24.4)
6NO3 + 5CH3OH ! 3N2 + 5CO2 + 7H2O + 6OH" (24.5) (overall energy-yielding reaction)
Because energy obtained through aerobic respiration of cBOD is greater than the energy obtained through anoxic respiration of cBOD, denitrifying bacteria prefer aerobic respiration or the use of free molecule oxygen to degrade cBOD. Therefore, in the presence of a high dissolved oxygen concentration (> 1.0 mg/l), denitrifying bacteria activate their enzymatic machinery for using free molecular oxygen and deactivate their enzymatic machinery for using nitrite ions and nitrate ions. However, the energy obtained from anoxic respiration compares well with aerobic respiration (Equations 24.6 and 24.7).
Glucose + 6O2 ! 6CO2 + 6H2O + 686 kcal (24.6) (aerobic respiration)
Glucose + 4.8NO" + 4.8H+ ! 6CO2 + 2.4N2 + 8.4H2O + 636 kcal (24.7) (anoxic respiration)
Like aerobic respiration, denitrification allows a complete oxidation of the organic substrate (cBOD) to carbon dioxide. In aerobic respiration, free molecular oxygen serves as the final electron carrier molecule. In anoxic respiration, nitrite ions or nitrate ions serve as the final electron carrier molecule.
Approximately 25% of the cBOD degraded under anoxic respiration is used for cellular synthesis or sludge production (Equation 24.8). The quantity of cells or sludge produced under aerobic respiration is greater due to the large quantity of energy obtained through aerobic respiration as compared to anoxic respiration.
NO" + 1.8CH3OH + H+ ! 0.065C5H7O2N* + 0.47N2 + 0.76CO2 + 2.44H2O (24.8) (* new cells or sludge)
The hydroxyl ion (OH") and some of the carbon dioxide produced during denitrification are returned to the activated sludge process as alkalinity. This return is important because much alkalinity is lost in the activated sludge process during nitrification. Approximately 50% of the alkalinity lost during nitrification is returned during denitrifi-cation.
The compounds resulting from a biochemical reaction are known as products (Equation 25.1). When denitrification occurs, the denitrifying bacteria form several gaseous products. These products formed during denitrification include molecular nitrogen, carbon dioxide, nitrous oxide, ammonia, and nitric oxide.
The majority of the gases formed and released by denitrifying bacteria consists of molecular nitrogen and carbon dioxide. Molecular nitrogen is insoluble in wastewater and leaves the treatment process as escaping bubbles.
Although carbon dioxide is soluble in wastewater, some of the carbon dioxide released by denitrifying bacteria leaves the treatment process as escaping bubbles, if denitrification is severe and carbon dioxide is produced rapidly. Bicarbonate alkalinity is formed when carbon dioxide dissolves in the wastewater.
During intense and sudden episodes of denitrification, two sizes of bubbles can be observed escaping a treatment process. These bubbles are molecular nitrogen and carbon dioxide. Molecular nitrogen is the smaller bubble.
Nitrous oxide, or laughing gas, also is produced and released by denitrifying bacteria. The production and release of nitrous oxide usually occurs under strongly fluctuating conditions or when denitri
fying bacteria are adversely affected. The amount of nitrous oxide released is relatively small. Nitrous oxide is insoluble in wastewater and leaves the treatment process as escaping bubbles.
Minute amounts, if any, ammonia and nitric oxide are produced. Both end products are highly toxic to denitrifying bacteria. Ammonia released to the wastewater dissolves to form ammonium ions. Nitric oxide is not ordinarily released from the bacterial cells and does not accumulate in the wastewater. How denitrifying bacteria cope with the presence of nitric oxide is not known.
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