Nitrogen Assimilation

In the activated sludge process nitrogen is utilized or assimilated by bacteria as a nutrient for several critical cellular functions (Table 10.1). Approximately 14% of the dry weight of most bacterial cells is nitrogen. Without the proper quantity and type of nitrogen, bacterial activity would be inhibited, and improper wastewater treatment would occur.

For any nutrient to enter a bacterial cell, the nutrient must be dissolved in the bulk solution surrounding the cell. Those nutrients that pass through the cell wall and cell membrane by following a concentration gradient of higher concentration outside the cell to lower concentration inside the cell are preferred (Figure 10.1). Preferred nutrients that move by a concentration gradient or diffusion are ''readily available'' nutrients; that is, they are immediately available for use and require no expenditure of energy to diffuse into the cell.

Nitrogen may enter bacterial cells as a component of a variety of organic and inorganic molecules or ions (Table 10.2). Most nitrogenous wastes, whether simple or complex in structure, can be enzyma-tically converted to ammonium ions. When ammonium ions are no longer available for use as the nitrogen nutrient, nitrate ions are used next. In order to use nitrate ions as the nitrogen nutrient, nitrate ions must be reduced to ammonium ions; that is, oxygen must be removed from the nitrate ions, and hydrogen added to the nitrogen to form ammonium ions inside the bacterial cell (Figure 10.2). Nitrogen always enters biosynthetic pathways (processes for producing cellular material) in an inorganic form and an oxidation state of —3.

TABLE 10.1 Cellular Functions of Nitrogen

Function

Example

Structural material for cellular growth

Pepidogylcans (cell wall components)

Structural material for enzymes

Cellulase, nitrogenase, urease

Transfer of genetic material

Nucleic acids

Ammonium ions are the preferred nutrient source of nitrogen. These ions are water soluble, enter the cell by diffusion, and are in the oxidation state of —3. Most organic sources of nitrogen, such as amino acids, contain nitrogen in the —3 oxidation state, but these sources must be degraded to yield ammonium ions. When ammo-

Figure 10.1 Preferred or readily available nutrients. Whenever bacterial cells use nitrogen for the synthesis of amino acids or proteins, nitrogen is incorporated into these organic-nitrogen compounds in the —3 oxidation state or valence. Ammonium ions (NH+) contain nitrogen in the —3 oxidation state. Ammonium ions are highly water soluble, very motile in water, and diffuse or move quickly from an area of higher concentration to an area of lower concentration. Also the bacterial cells need not use any energy to bring the ammonium ions into the cell. Therefore, when nitrogen is needed and ammonium ions are available, ammonium ions are used before any other form of nitrogen. Nitrate ions (NO—) and nitrite ions (NO—) are used as a nitrogen nutrient after the ammonium ion concentration surrounding the cell as been exhausted. Nitrogen is in the +5 oxidation state in nitrate ions and +3 oxidation state in the nitrite ion. Although less time and energy are required by the bacteria to convert+3 nitrogen (nitrite ion) to —3 nitrogen than to covert +5 nitrogen (nitrate ion) to —3, the +3 nitrogen in the form of nitrite ions is highly unstable, meaning it quickly converts to nitrate ions through nitrification.

Figure 10.1 Preferred or readily available nutrients. Whenever bacterial cells use nitrogen for the synthesis of amino acids or proteins, nitrogen is incorporated into these organic-nitrogen compounds in the —3 oxidation state or valence. Ammonium ions (NH+) contain nitrogen in the —3 oxidation state. Ammonium ions are highly water soluble, very motile in water, and diffuse or move quickly from an area of higher concentration to an area of lower concentration. Also the bacterial cells need not use any energy to bring the ammonium ions into the cell. Therefore, when nitrogen is needed and ammonium ions are available, ammonium ions are used before any other form of nitrogen. Nitrate ions (NO—) and nitrite ions (NO—) are used as a nitrogen nutrient after the ammonium ion concentration surrounding the cell as been exhausted. Nitrogen is in the +5 oxidation state in nitrate ions and +3 oxidation state in the nitrite ion. Although less time and energy are required by the bacteria to convert+3 nitrogen (nitrite ion) to —3 nitrogen than to covert +5 nitrogen (nitrate ion) to —3, the +3 nitrogen in the form of nitrite ions is highly unstable, meaning it quickly converts to nitrate ions through nitrification.

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