Environmental Engineering Reference
In-Depth Information
7.3.3 Depth of Anoxia
L
=
V c
×
=
( .
6 4 10
×
4
)( .
3 27 10
×
−
4
)
=
20 9
.
kg/yr
a
r
r
When a lake is used as a detention pond for stormwater
management, the effective volume of the lake includes
only the aerobic portion, and therefore there is a need
to estimate the volume of the lake that is likely to be
anaerobic. In these cases, aerobic conditions can be
assumed to exist where the concentration of DO exceeds
1 mg/L. Using a data set containing 426 sets of measure-
ments of Secchi disk depth, chlorophyll
a
, TP, and depth
of anoxia, defined as DO concentrations less than 1 mg/L,
Harper and Baker (2007) derived the following relation-
ship for waterbodies in Central and South Florida:
Accounting for the assimilation of TP in the the lake
gives the net loading,
L
0
, as
L
=
(
1
−
r L
)
=
(
1 0 8 20 9
−
. )(
. )
=
4 18
.
kg/yr
0
a
This mass of TP will be distributed within the lake
volume and the lake outflow. Assuming that the lake
inflow and outflow are approximately equal, the mean
pond concentration, TP, is given by
L
V V
d
=
3 035
.
SD
−
0 004979
.
TP
+
0 02164
.
c
(7.18)
0
TP
=
a
b
+
r
L
where
d
a
is the depth of anoxia (m), SD is the Secchi
disk depth (m), TP is the TP (
µ
g/L), and
c
b
is the biomass
concentration (
µ
g Chl
a
/L). values of SD and
c
b
can be
related to TP using the following relationships derived
for Florida lakes (Harper and Baker, 2007)
4 18
64 000 35 300
4 21 10
42 1
.
=
,
+
,
=
.
×
−
5
kg/m
3
=
. µ
g/L
ln
c
b
=
1 058
.
ln
TP
−
0 934
.
(7.19)
Using Equations (7.18-7.20), the biomass (Chl
a
) con-
centration,
c
b
, the Secchi disk depth, SD, and the depth
to anoxia,
d
a
, are given by
24 24 0 3041
6 063
.
+
.
c
b
SD
=
(7.20)
.
+
c
b
Calculation of the depth of anoxia using Equations
(7.18-7.20) is useful in determining an appropriate
depth of an artificial lake to be used as a detention pond
in a stormwater management system, since the effective
volume of such lakes includes only the aerobic portion.
Proper functioning of these artificial lakes usually
require that they have minimum detention times on the
order of 2-4 weeks, where the detention time is equal
to the effective lake volume divided by the average
outflow rate.
c
b
=
exp[ .
1 058
ln
TP
−
0 934
.
]
=
exp[ .
1 058
ln(
42 1
. )
−
0 934
.
]
=
20 6 µ
.
g/L
24 24 0 3041
6 063
.
+
.
c
24 24 0 3041 20 6
6 063 20 6
.
+
.
(
. )
b
SD
=
=
=
1 1
.
4 m
.
+
c
.
+
.
b
d
=
3 035
.
SD
−
0 004979
.
TP
+
0 02164
.
c
a
b
=
3 035 1 14
.
( .
)
−
0 004979 42 1
.
(
. ) +
0 02164 206
.
(
)
=
3 70
. m
EXAMPLE 7.9
Therefore, the depth to anoxia should not exceed around
3.70 m for the entire lake to be effective in removing
TP. If a lake depth greater than 3.70 m is selected, then
artificial mixing of the lake is an option.
A lake is to be used as part of the stormwater manage-
ment system in a proposed residential development.
Under design conditions, annual runoff from the devel-
opment into the lake is estimated to be 6.4 × 10
4
m
3
, and
the TP mean concentration in runoff from this type of
land use can be taken as 0.327 mg/L. Detention time
requirements dictate that the volume of the lake be
3.53 × 10
4
m
3
, and it can be assumed that the lake will
assimilate 80% of the TP loading. Estimate the maximum
depth of the lake for aerobic conditions to exist within
the entire lake.
7.4 THERMAL STRATIFICATION
In large reservoirs, the thermal balance is usually domi-
nated by heat exchange with the atmosphere at the
water surface, with most of this direct heat exchange
occurring in the upper layers of the reservoir. If mixing
within the reservoir is limited, this mode of heat
exchange typically leads to significant thermal stratifica-
tion. Thermal stratification in lakes can have a pro-
nounced effect on water quality, since temperature
has a significant influence on the rates of chemical and
Solution
From the given data:
V
r
= 6.4 × 10
4
m
3
,
c
r
= 0.327 mg/L
= 3.27 × 10
−4
kg/m
3
,
V
L
= 3.53 × 10
4
m
3
, and
r
= 0.8.
Hence, the annual TP load on the lake,
L
a
, is given by
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