Environmental Engineering Reference
In-Depth Information
and Equation (2.3) gives the corresponding dis-
solved oxygen as
portion to the atmospheric pressure. At 20°C, the
saturation concentration in Denver (7.5 mg/l) is
18% less than in Miami (9.1 mg/l).
10 754
.
2140 7
.
−
2
ln
DO
=
ln
DO
−
S
1 764 10
.
×
−
+
S
sat
Since DO is inversely proportional to temperature, cool
waters typically contain higher levels of dissolved
oxygen than warm waters, and consequently, aquatic life
in streams and lakes is usually under more oxygen stress
during the warm summer months than during the cool
winter months. The minimum dissolved oxygen level
needed to support a diverse aquatic ecosystem is typi-
cally on the order of 5 mg/l. The levels of fish tolerance
to low dissolved oxygen stresses vary, for example,
brook trout may require about 7.5 mg/l of dissolved
oxygen, whereas carp can survive at 3 mg/l. As a rule,
the more desirable commercial and game fish require
higher levels of dissolved oxygen.
T
T
2
a
a
10 754
293 15
.
2140 7
293 15
.
=
ln( . )
9 1
−
4 52 1 764 10
.
.
×
−
2
−
+
.
.
2
=
2 18
.
which yields
DO
S
=
e
2 18
.
=
8 8
.
mg/L
.
Therefore, increasing the chloride concentration
from 0 to 2500 mg/l reduces the saturation concen-
tration of dissolved oxygen from 9.1 to 8.8 mg/l, a
reduction of approximately 3%.
(d) The impact of atmospheric pressure on dissolved
oxygen concentration is given by Equation (2.4).
In this case, DO
sat
= 9.1 mg/l,
P
= 83.4 kPa = 83.4/
101.325 = 0.823 atm, and
P
wv
is given by Equation
(2.5) as
2.3.2 Biochemical Oxygen Demand
Bacterial degradation oxidizes organic molecules to
stable inorganic compounds, and
biochemical oxygen
demand
(BOD) is the amount of oxygen required to
biochemically oxidize organic matter present in water.
Aerobic bacteria that are responsible for BOD make
use of dissolved oxygen in reactions similar to the fol-
lowing involving glucose (C
6
H
12
O
6
):
3840 70
.
216961
ln
P
wv
=
11 8671
.
−
−
2
T
T
3840 70
20
.
216961
20
=
11 8671
.
−
−
= −
722 57
.
2
C H O
+
6
O
→
6
H O CO
+
6
(2.8)
6
12
6
2
2
2
which yields
Accordingly, 6 moles of oxygen are consumed for every
mole of glucose. Waste discharges that contain signifi-
cant amounts of biodegradable organic matter have
high BOD levels and consume significant amounts of
dissolved oxygen from receiving waters, thereby reduc-
ing the level of dissolved oxygen and causing adverse
impacts on aquatic life. If the organic matter is protin-
aceous, then nitrogen and phosphorus are also released
as a result of the decomposition process. Biodegradable
organic wastes commonly associated with oxygen con-
sumption in surface waters include human and animal
excrement, food wastes, and organic residuals from
industrial operations, such as paper mills and food-
processing plants.
BOD measures the mass of oxygen consumed per
unit volume of water and is usually given in mg/l. The
wastewater from industrial operations, such as pulp
mills, sugar refineries, and some food-processing plants,
may easily have 5-day BOD values as high as several
thousand milligrams per liter. In contrast, raw sewage
typically has a 5-day BOD of about 200 mg/l. A classi-
cal BOD curve is illustrated in Figure 2.4a, where the
BOD is composed of carbonaceous BOD (CBOD) and
nitrogenous BOD (NBOD).
P
wv
≈ 0
atm
and
θ
is given by Equation (2.6) as
θ=
0 000975 1 426 10
.
−
.
×
−
5
T
+
6 436 10
.
×
−
8
T
2
=
0 000975 1 426 10
.
−
.
×
−
5
(
20
)
+
6 436 10
.
×
−
8
(
20
)
2
=
0 000716
.
Substituting into Equation (2.4) gives
P
P
wv
1
−
(
1
−
θ
P
)
DO DO
P
=
P
sat
(
1
−
P
)(
1
−
θ
)
wv
[
1 0 1 0 000716 0 823
1 0 1 0 000716
−
](
−
.
×
.
)
=
9 1 0
. (
.
823
)
(
−
)(
−
.
)
=
7 5
.
mg/L
This result indicates that the saturation concentra-
tion of dissolved oxygen decreases roughly in pro-
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