Environmental Engineering Reference
In-Depth Information
tion (2.1), on which the values given in Table 2.2
are based. The saturation concentration of oxygen in
water is affected by the presence of chlorides (salt),
which reduce the saturation concentration by
about 0.015 mg/l per 100 mg/l chloride at low tem-
peratures (5-10°C) and by about 0.008 mg/l per
100 mg/l chloride at higher temperatures (20-30°C)
(Tebbutt, 1998). The following equation is recommended
to account for the effect of salinity on DO
sat
(APHA,
1992):
20°C in Miami, where atmospheric pressure is 101 kPa,
with the saturation concentration in Denver, where
atmospheric pressure is 83.4 kPa.
Solution
(a) Equation (2.1) gives DO
sat
in terms of the absolute
temperature,
T
a
, where
T
a
= 273.15 + 20 = 293.15 K.
Hence, Equation (2.1) gives
5
7
1 575701 10
.
×
6 642308 10
.
×
10 754
.
2140 7
.
ln
DO
sat
= −
139 34411
.
+
−
ln
DO
=
ln
DO
−
S
1 764 10
.
×
−
2
−
+
S
sat
T
T
2
T
T
2
a
a
a
a
1 243800
.
×
10
10
8 621949 10
.
×
11
+
−
(2.3)
T
3
T
4
a
a
5
7
1 575701 10
293 15
.
×
6
.
642308 10
293 15
×
where DO
S
is the saturated dissolved oxygen concentra-
tion (mg/l) at salinity
S
(ppt). For high-elevation
streams and lakes, the barometric pressure effect is
important, and the following equation is used to quan-
tify the pressure effect on the saturated dissolved oxygen
concentration:
= −
139 34411
.
+
−
.
(
.
)
2
1 243800 10
293 15
.
×
10
8 621949 10
.
×
11
+
−
(
.
)
3
(
293 15
.
)
4
=
2 207
.
Therefore,
P
P
wv
1
−
(
1
−
θ
P
)
DO DO
P
=
P
(2.4)
DO
sat
=
e
2 207
.
=
9 1
.
mg/L
sat
(
1
−
P
)(
1
−
θ
)
wv
This is the same value of DO
sat
for freshwater given
in Table 2.2.
(b) According to Equation (2.2),
where DO
P
is the saturated dissolved oxygen concentra-
tion at pressure
P
(atm),
P
wv
is the partial pressure of
water vapor (atm), which can be estimated using the
relation (lung, 2001)
468
31 5
468
31 5 20
DO
sat
=
=
=
9 1
.
mg/L
.
+
T
.
+
3840 70
.
216961
2
ln
P
wv
=
11 8671
.
−
−
(2.5)
This is the same value (9.1 mg/l) as that given in
Table 2.2 and calculated using Equation (2.1). Since
Equation (2.2) is supposed to agree with Equation
(2.1) within 0.03 mg/l, the calculated result is
expected.
(c) The impact of salinity on the saturation concentra-
tion of dissolved oxygen is given by Equation (2.3),
and the relationship between chloride concentra-
tion,
c
, and salinity,
S
, in seawater is given by
T
T
where
T
is the temperature (°C), and
θ
is an empirical
constant given by
−
5
−
8
T
2
(2.6)
θ=
0 000975 1 426 10
.
−
.
×
T
+
6 436 10
.
×
EXAMPLE 2.1
(2.7)
S
= 1 80655
.
c
(a) Compare the saturation concentration of dissolved
oxygen in freshwater at 20°C given by Equation (2.1) to
the value given in Table 2.2. (b) How do these values
compare with the saturation concentration given by
Equation (2.2)? (c) What would be the effect on the
saturation concentration of dissolved oxygen if saltwa-
ter intrusion causes the chloride concentration to
increase from 0 to 2500 mg/l? (d) Compare the satura-
tion concentration of dissolved oxygen in freshwater at
where
S
and
c
are in parts per thousand. In
the present case,
c
= 2500 mg/l = 2.5 kg/m
3
= 2.5/
1000 = 0.00250 = 2.50 ppt, where the density of
water is taken as 1000 kg/m
3
. Applying Equation
(2.7) to estimate the salinity gives
S
=
1 80655 2 50
.
( .
)
=
4 52
.
ppt
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