Environmental Engineering Reference
In-Depth Information
EXAMPLE 4.15
12
10
At a critical location in a stream, the ammonia-nitrogen
concentration is estimated to be 4.10 mg/L. If the pH
at this location is 7.8 and the temperature is 20°C,
what is the expected concentration of undissociated
ammonia?
8
NO 3 -N
6
NH 3 -N
4
NO 2 -N
Solution
organic-N
2
From the given data: c a = 4.10 mg/L, pH = 7.8, T = 20°C,
and T a = 273.15 + 20 = 293.15 K. Hence, [H + ] and K are
given by
0
0
5
10
15
20
25
distance downstream (km)
Figure 4.9. nitrogen species downstream of wastewater
discharge.
2729.92 = 0.09018
2729.92
293.15
pK
=
0.09018
+
+
=
9.403
T
a
quality concerns. For example, the (unionized) ammonia
species is particulary toxic to fish, as well as other aquatic
organisms, and the nitrate species in drinking water has
toxic effects on infant humans. The species model yields
the total concentration of ammonia, which in molar con-
centration terms is related to the other species of
ammonia by
K
=
10
pK
=
10
9.403
=
3.954 10
×
10 mol/L
[
H
+
]
=
10
pH
=
10
7.8
=
1.585 10
×
8
mol/L
.
Denoting the concentration of unionized ammonia as
c ua , Equation (4.105) gives
c
c
[
NH
NH
]
[
H
+
]
1.585 10
3.954 10
×
×
8
a
3
T
[
NH
]
T =
[
NH
+
]
+
[
NH
]
(4.101)
=
= +
1
= +
1
=
41.1
3
4
3
[
]
K
10
ua
3
where [nH 3 ] T , [ NH + ], and [nH 3 ] are the molar concen-
trations of total ammonia, ionized ammonia, and union-
ized ammonia, respectively. The dissociation reaction of
ammonia can be expressed as
Therefore, the concentration of unionized ammonia is
estimated as 4.10/41.1 = 0.0998 mg/L.
+
+
(4.102)
NH
NH H
+
4
3
4.4.4.2  Photosynthesis,  Respiration,  and  Benthic 
Oxygen Demand.  The Streeter-Phelps analysis assumes
that the oxygen demand on a stream is caused by the
BoD of a contaminant discharge and that this oxygen
demand is supplied by the Do in the stream that is in
turn resupplied by atmospheric aeration. Besides bio-
chemical oxygen demand and reaeration, other sources
and sinks of oxygen that are distributed along rivers
include photosynthesis, respiration of photosynthetic
organisms, benthic oxygen demand, and BoD from dis-
tributed (diffuse) sources along the river.
and the ratio of reactant to product is given by the dis-
sociation constant, K , where
+
[
NH H
NH
][
]
3
K =
(4.103)
+
[
]
4
The dissociation constant, K , is related to the absolute
temperature, T a (K), such that (Emerson et al., 1975)
2729.92
pK
=
0.09018
+
(4.104)
T
a
Photosynthesis and Respiration. Photosynthesis by phy-
toplankton and periphyton, particularly algae, contrib-
utes oxygen to the water during (bright) daylight hours,
and respiration removes oxygen from the water at night
and on very cloudy days. Algal biomass in streams and
shallow lakes is frequently dominated by periphyton
(Wetzel, 2001). The process of photosynthesis can be
represented by the reaction
where pK = −log 10 ( K ). Equations (4.101) and (4.103)
can be combined to give the following relationship
between the molar concentration of unionized ammonia,
[nH 3 ], and total ammonia, [nH 3 ] T ,
1
[
NH
]
=
[
NH 3
]
T
(4.105)
3
+ K
1 [
+
H /
]
where hydrogen ion concentration, [H + ], can be deter-
mined from the pH, since pH = −log[H + ].
CO H O
+
+ →
C H O O
(
)
+
(4.106)
2
2
2
2
 
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