WATER QUALITY - Water Quality Engineering in Natural Systems - page 16

significant amounts of oxygen. There are several forms
of nitrogen that can exist in water bodies, including
organic nitrogen (e.g., proteins, amino acids, and urea),
ammonia-nitrogen ( NH + and NH 3 ), nitrite-nitrogen
( NO − ), nitrate-nitrogen ( NO − ), and dissolved nitrogen
gas (N 2 ). Total Kjeldahl nitrogen (TKN) is the sum
of organic nitrogen and ammonium nitrogen (i.e.,
TKN = organic-N + ammonia-N). For water in contact
with the atmosphere, the most fully oxidized state of
nitrogen is +5, and oxidation of nitrogen compounds
proceeds as follows:

in agricultural soils, thus leading to requirements for
fertilization. Nitrate-nitrogen commonly originates in
runoff from agricultural areas with heavy fertilizer
usage, whereas organic nitrogen is commonly found in
municipal wastewaters. Excessive nitrate in ground-
water is of concern if the aquifer is to be used as a
drinking water supply, since nitrate can pose a health
threat to infants by interfering with oxygen transfer in
the bloodstream. Excessive nitrate levels can also be a
concern in transitional waters such as estuaries, where
nitrogen is commonly the limiting nutrient in the bio-
logical growth of algae and other weeds.
under anoxic conditions, the nitrate-nitrogen ion
becomes the electron acceptor in the organic matter
oxidation reaction. This reaction, called denitriication ,
can be represented as (Davis and Masten, 2004;
Schindler, 1985)

Organic nitrogen O + → NH nitrogen O

NO nitrogen O
NO ni

− +

2 3 2

→ − +

(2.14)

2 2

→ − trogen

3

In aquatic environments, microorganisms break down
organic nitrogen to release ammonia in a process called
ammoniication or deamination , and ammonia (NH 3 ) is
transformed to NO 3 -nitrogen in a process called nitrifi-
cation . Ammonification can occur in sediments, water,
and soils. Depending on the pH of the water, nonionized
ammonia (NH 3 ) and ammonium ions ( NH + ) will exist in
an equilibrium according to the relation

− +

(2.17)

5 CH O NO + 4 + 4 H → 5 CO + 7 H O N + 2

2 3 2 2 2

where n CH 2 O represents a form of organic carbon, and
several forms of organic carbon (e.g., dissolved methane
from anaerobic decomposition in sediments) may serve
as the source of energy in this reaction. The denitrifica-
tion process described by Equation (2.17) represents a
loss of nitrogen from the water since the nitrogen gas
produced volatilizes into the air. Denitrification is per-
formed by facultative anaerobes, such as fungi, which
can flourish in anoxic conditions.
Nitrogen continuously cycles in the aquatic environ-
ment, although the rate is temperature controlled and
thus seasonal. Aquatic organisms incorporate available
dissolved inorganic nitrogen into proteinaceous matter.
Dead organisms decompose, and the nitrogen is released
as ammonia ions and then converted to nitrite and
nitrate, where the process begins again. If a surface
water lacks adequate nitrogen, nitrogen-fixing organ-
isms can convert nitrogen from its gaseous phase to
ammonia ions.

(2.15)

NH OH

+

+

−

NH H O +

4 3 2

At pH 7 or below, most of the ammonia nitrogen will
be ionized as ammonium, while at pH levels greater
than 9, the proportions of nonionized ammonia will
increase. The nonionized ammonia is toxic to fish, while
the ionized ammonium is a nutrient to algae and aquatic
plants and also exerts dissolved oxygen demand. The
nonionized ammonia is a gas that will mostly volatilize
from water, and water-quality standards typically regu-
late the total ammonia nitrogen ( NH NH
4

+ + ). At
normal pH values, ammonia-nitrogen occurs in the
ammonium form ( NH + ), and because of the positive
charge, it is readily adsorbed by negatively charged
(organic and clay) soil particles.
Ammonium ions are converted to nitrate and the
combined reaction, called nitrification, can be written in
the form

3

2.3.4.2 Phosphorus. Phosphorus-bearing minerals
typically have low solubility, and thus most surface
waters naturally contain very little phosphorus. Phos-
phorus is normally present in watersheds in extremely
small amounts and commonly originates from wastewa-
ter discharges, household detergents, and agricultural
runoff associated with fertilizer application and concen-
trated livestock operations. untreated domestic waste-
water contains 5-15 mg/l of phosphorus, concentrations
more than two orders of magnitude greater than those
desired in healthy surface waters (<0.02 mg/l). Thus,
significant phosphorus removal is commonly required
as part of the wastewater treatment process.

NH

+

+ 2 O → NO + 2 H H O

+

+ (2.16)

4 2 3 2

Stoichiometrically, the oxygen requirement for the
overall nitrification reaction (Eq. 2.16) is 4.56 mg of O 2
per milligram of NH + .
Plants take up and utilize nitrogen in the form of
ammonia or nitrate, which are typically in short supply