Nitrogen

The element nitrogen (N) can be found in a large number of organic and inorganic compounds. Organic compounds contain the element carbon (C) and the element (H) (Table 2.1), while inorganic compounds may contain the element carbon or the element hydrogen (Table 2.2). Nitrogen is incorporated into organic compounds and inorganic compounds due to its ability to easily form chemical bonds with other elements such as carbon, hydrogen, and oxygen (O). When elements bond together, compounds are formed.

Organic compounds that contain nitrogen are considered "organic-nitrogen" compounds. An example of an organic-nitrogen compound is urea (NH2CONH2). Urea is a major chemical component of urine. Although fresh, domestic wastewater is rich in urea, this compound degrades quickly in the sewer system through bacterial activity. In the sewer system, a large diversity of bacteria adds water to urea. The addition of water to urea ''splits'' the compounds into ammonium ions and carbon dioxide (CO2). The ''hydrolysis'' of urea, namely, the addition of water by bacteria to split a compound into smaller compounds, results in the release of ammonium ions in the sewer system.

Examples of inorganic compounds that contain nitrogen and are of concern to wastewater treatment plant operators include ammonium ions, nitrite ions, and nitrate ions. These ions are the most important nitrogenous compounds that should be monitored for cost-effective operations, permit compliance, and process control for activated sludge processes that are and are not required to nitrify.

TABLE 2.1 Examples of Organic Compounds

Name of

Chemical Formula

the Compound

of the Compound

Acetic acid

CH3COOH

Ethyl alcohol

CH3CH2OH

Glucose

C6H12O6

Isopropyl alcohol

CH3CHOHCH3

All elements are made of atoms that contain a large and positively charged nucleus that is surrounded by small and negatively charged electrons (Figure 2.1). The electrons orbit the nucleus and may be shared between elements. When electrons are shared between elements, chemical bonds and chemical compounds are formed (Figure 2.2). For example, one atom of nitrogen and three atoms of oxygen share electrons. The sharing of electrons forms chemical bonds to produce one molecule of NOj. Although the nitrogen atom and the oxygen atoms shared the electrons, the oxygen atoms pull the electrons more closely to their nuclei. Because the electrons are pulled closely to the oxygen atom, the charge on the nitrogen atom becomes less negative, perhaps positive.

When oxygen is added to nitrogen, the nitrogen atom undergoes oxidation; that is, the nitrogen atom loses electrons. Oxidation of nitrogen results in a decrease in negative charge or an increase in positive charge due to the loss of electrons. Nitrification is the addition of oxygen to nitrogen. Nitrification is the oxidation of nitrogen.

When oxygen is removed from nitrogen, the nitrogen atom undergoes reduction; that is, electrons are returned to the nitrogen atom or the nitrogen atom gains electrons. Reduction of nitrogen results in a decrease in positive charge due to the gain of electrons. Denitrifi-cation is the removal of oxygen from nitrogen. Denitrification is the reduction of nitrogen.

TABLE 2.2 Examples of Inorganic Compounds

Name of

Formula of

the Compound

the Compound

Ammonia

NH3

Carbon dioxide

CO2

Copper sulfate

CuSO4

Mercuric chloride

HgCl2

\_J Neutron

^^ Electron

Figure 2.1 The atom. The atom contains three basic components, the proton, the neutron, and the electron. The proton and neutron are grouped together at the center or core of the atom and give the atom its weight. The neutron has no charge, while the proton is positively charged. The electron spins around the core of the atom and is negatively charged. If the number of electrons and protons in the atom are equal, the atom has a neutral charge. However, when atoms share electrons, the atoms may become positively or negatively charged depending on the number of shared electrons that spin around the core of each atom.

Nitrification and denitrification are chemical reactions that occur inside living cells or bacteria. Because these chemical reactions occur in living cells, they are considered ''biochemical'' reactions.

With all biochemical reactions there are starting compounds or re-actants and final compounds or products (Equation 2.1). In some biochemical reactions intermediate compounds may be formed (Equation 2.2). Intermediate compounds usually do not accumulate. However, under appropriate conditions, intermediate compounds may accumulate; for example, nitrite ions are intermediate compounds that may accumulate under appropriate operational conditions. The accumulation of nitrite ions is undesired.

Figure 2.2 Atoms sharing electrons. When a nitrogen atom and an oxygen atom share electrons, the electrons orbit the core of the oxygen atom more than the core of the nitrogen atom. Therefore the nitrogen atom becomes positive in charge (oxidized), while the oxygen atom becomes negative in charge (reduced; that is, its charge is lowered or reduced).

Reactants ! Products (2.1)

Reactants ! Intermediates ! Products

The charge on the nitrogen atom is called its oxidation state or valence, for example, +3 for nitrogen within the ammonium ion. Because changes in the oxidation state of nitrogen can be biologically mediated, that is, caused by biochemical reactions, changes in the oxidation state of nitrogen can occur in the activated sludge process. Therefore nitrogen may undergo nitrification (oxidation) and denitri-

TABLE 2.3 Nitrogenous Compounds Produced during Nitrification and Denitrification

Nitrogenous Compound

Chemical Formula

Oxidation State of N

Nitrate ion

no3

+5

Nitrite ion

NOg

+3

Nitric oxide

NO

+2

Nitroxyl

NOH

+1

Nitrous oxide

N2O

+1

Molecular nitrogen

N2

0

Hydroxylamine

NH2OH

31

Ammonia

NH3

33

Ammonium ion

NH+

33

fication (reduction) in an activated sludge process. Numerous oxidation states of nitrogen in the activated sludge process are presented in Table 2.3.

It is the —3 oxidation state of nitrogen in the ammonium ion that is preferred by the bacteria in the activated sludge process as their nitrogen nutrient. In this oxidation state in the ammonium ion, nitrogen is incorporation or assimilated into cellular material (C5H7O2N). Nitrate ions and nitrite ions can be used as a nutrient source for nitrogen. However, nitrate ions and nitrite ions are used only after ammonium ions are no longer available and the oxygen on each ion is removed and ammonium ions are produced. The production of ammonium ions inside the bacterial cells ensures the presence of a —3 oxidation state for nitrogen.

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