Environmental Engineering Reference
In-Depth Information
where the equilibrium relationship is given by
+
NH
H
3
K
=
+
NH
4
(14.16)
2729 92
.
pK
=−
log
K
=
0 09018
.
+
10
T
a
where
K
is the equilibrium constant,
T
a
is the absolute temperature (kelvin), and the concentrations
are molar. By substituting the Equation 14.16 into a mass balance for the total ammonia, the fraction
unionized as a function of the temperature and the hydrogen ion concentration can be computed
from (Figure 14.24):
1
f
=
u
+
H
(14.17)
1
+
K
The importance of this is that unionized ammonia may be toxic, as will be described in Section 14.4.1.
14.5.1.5 Productivity and Respiration
Nitrogen is also an essential component of biomass. Thus, it is taken up by autotrophs and introduced
back into the water column via their excretions and death (Figure 14.22). The preferred form is
ammonia, although autotrophs may use any of the dissolved inorganic nitrogen (DIN) forms. Some
autotrophs, notably some cyanobacteria, can ix atmospheric nitrogen so that they can use dissolved
N
2
as a nitrogen source when other forms are unavailable.
The nitrogen requirements of autotrophs are commonly estimated based on some assumed stoi-
chiometry, or composition of organic matter, such as (Redield et al. 1963; Chapra 1997)
100
gD: C:
40
7200
mgN:
1000
mgP:
1000
mgA
(14.18)
where g
X
is the mass of element
X
(g) and mg
Y
is the mass of element
Y
(mg). The terms D, C, N, P,
and A refer to dry weight, carbon, nitrogen, phosphorus, and chlorophyll-a, respectively.
The general concept is that the ratios of the components can be used to determine which of the
components limit growth. For many lakes and reservoirs, the two nutrients that primarily control
1
pH
=
10
0.8
0.6
pH
=
9
0.4
0.2
pH
=
8
0
0
5
10
15
20
Te mperature (°C)
25
30
35
40
45
FIGURE 14.24
Fraction of ammonia that is unionized as a function of pH and temperature.
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