Introduction to Denitrification

The term "denitrification" was first used in France in 1886 to describe the use of nitrate ions by some bacteria to degrade substrate. The bacterial use of nitrate ions (and nitrite ions) to degrade substrate actually evolved before the use of free molecular oxygen.

Wastewater denitrification describes the use of nitrite ions or nitrate ions by facultative anaerobes (denitrifying bacteria) to degrade cBOD. Although denitrification often is combined with aerobic nitrification to remove various forms of nitrogenous compounds from wastewater, denitrification occurs whenever an anoxic condition exists. Therefore denitrification can promote favorable operational conditions or can contribute to operational problems.

Facultative anaerobes make up approximately 80% of the bacteria within an activated sludge process. These organisms have the enzymatic ability to use free molecular oxygen, nitrite ions, or nitrate ions to degrade cBOD. Facultative anaerobes prefer and use free molecular oxygen when it is available. The use of free molecular oxygen provides the bacteria with more energy for cellular activity, growth, and reproduction than does the use of nitrite ions or nitrate ions.

Bacterial degradation of cBOD is "respiration." Respiration may be aerobic (oxic) or anaerobic. Aerobic respiration occurs when free molecular oxygen is available and is used to degrade cBOD, such as glucose (C6H12O6) (Equation 22.1).

Anaerobic respiration occurs when free molecular oxygen is not available and another molecule is used to degrade cBOD. Molecules other than free molecular oxygen that can be used to degrade cBOD include nitrite ions and nitrate ions. The molecule used for the degradation of cBOD is dependent on its availability, the presence of other molecules, and the enzymatic ability of the bacterial population. If nitrite ions or nitrate ions are used to degrade cBOD, such as a five-carbon sugar, this form of respiration is termed "anoxic" (Equation 22.2).

During anoxic respiration, nitrite ions and nitrate ions are reduced (oxygen removed from the ions) through several biochemical steps or reactions. The principle gaseous end product of the biochemical reactions is molecular nitrogen.

Anoxic respiration or denitrification is termed "dissimilatory" nitrite or nitrate reduction, because nitrite ions and nitrate ions, respectively, are reduced to from molecular nitrogen. The nitrogen in the nitrite ions or nitrate ions is not incorporated into cellular material, the nitrogen in the ions is loss to the atmosphere as a gas.

Nitrification does not remove nitrogen from the wastewater, it simply transforms it from ammonium ions to nitrate ions. Denitrifi-cation removes nitrogen from the wastewater by converting it to insoluble gases that escapes to the atmosphere. Besides molecular nitrogen, nitrous oxide (N2O) is produced during denitrification from nitrite ions and nitrate ions. This nitrogen-containing gas is insoluble in wastewater and escapes to the atmosphere.

When nitrite ions and nitrate ions are reduced to ammonium ions inside the bacterial cell, the nitrogen in the ammonium ions is incorporated into cellular material. This reduction of nitrogen is termed "assimilatory" nitrite or nitrate reduction.

Assimilatory nitrite reduction and assimilatory nitrate reduction do not remove nitrogen from the wastewater.

Although several groups of organisms are capable of denitrification, including fungi and the protozoa Loxodes, most denitrifying organisms consist of facultative anaerobic bacteria. The bacteria that denitrify are known by several names including denitrifiers, hetero-trophs, and organotrophs.

Denitrifying bacteria degrade cBOD using nitrite ions and nitrate ions in the absence of free molecular oxygen. The bacteria degrade cBOD in order to obtain energy for cellular activity and carbon for cellular synthesis (growth and reproduction).

A relatively large number of genera of facultative anaerobes are capable of denitrification (Table 23.1). Most denitrifiers reduce nitrate ions via nitrite ions to molecular nitrogen without the accumulation of intermediates. However, some denitrifiers lack key enzyme systems to denitrify completely, and the lack of these enzyme systems does permit the production and accumulation of intermediates.

Although there are numerous genera of denitrifying bacteria, all denitrifying genera do not contain large numbers of species, and all denitrifying bacteria do not respire similarly. The genera Alcaligenes, Bacillus, and Pseudomonas contain the largest number of denitrifying bacteria.

Many genera of denitrifying bacteria can use nitrite ions or nitrate ions to degrade cBOD, some genera such as Enterobacter and Es-cherichia can use only nitrate ions. Other genera such as Alcaligenes can use only nitrite ions. The use of nitrate ions in this manner is known as nitrate respiration, while the use of nitrite ions is known as

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