Environmental Engineering Reference
In-Depth Information
MoS, a common approach is to allocate a fixed percent-
age, typically 10%, of the TMDL to the MoS, although
it is sometimes reasonable to assume that the MoS
varies with flow condition. Whatever approach is used
to estimate the margin-of-safety load,
L
MoS
(MT
−1
), the
load allocation (LA) amounts must satisfy the following
relation for all flow rates:
these discharges are required by regulations to meet the
FC water-quality standard of 200 CFU/100 mL. Assum-
ing an MoS equal to 10% of the TMDL, determine the
LA for nonpoint sources of FC.
Solution
From the given data:
Q
p
= 0.2 m
3
/s and
c
std
= 200 CFU/
100 mL. Assuming that the point-source wastewater dis-
charges meet the FC water-quality standard, then the
WLA is given by
N
=
+
∑
L
TMDL
L
+
L L
+
(4.161)
pi
np
u
MOS
i
=
1
where
N
is the number of distinct point-source loads.
Fortunately, in most cases, the point-source LAs can be
set based on applicable permits and regulations that
govern these discharges, the TMDL is known from the
LDC,
L
MoS
can be determined directly from the TMDL
using the percentage method, and hence the loads
L
np
and
L
u
can be allocated as the residuals after the
known loads are subtracted. In cases where the TMDL
is calculated for the entire length of a stream (which is
commonly done), then
L
u
= 0 in Equation (4.161). If
the sum of the source loads are less than the TMDL
under all flow conditions, then the stream is not
impaired. Load allocations to point sources are com-
monly referred to
wasteload allocations
(WLAs), and
allocations to nonpoint sources are commonly referred
to as LAs.
WLA
=
Q c
×
8.64 10
×
8
=
(0.2)(200) 8.64 10
×
×
8
p std
=
3.46 10
×
10
CFU/d
(4.162)
The MoS is 10% of the TMDL, hence
MOS
= 0.1(
TMDL
)
(4.163)
and the TMDL is defined as
TMDL WLA LA MOS
=
+
+
(4.164)
where LA is the nonpoint source LA. Combining Equa-
tions (4.162-4.164) gives the LA as
LA
=
0.9(
TMDL
) 3.46 10
10
−
×
CFU/d
(4.165)
EXAMPLE 4.24
Since the TMDL is known (it is the LDC), the nonpoint
source LA can be calculated directly using Equation
(4.165) as shown in Table 4.9.
The components of the TMDL are shown in Figure
4.17.
The FC LDC, where a stream is impaired relative to the
FC standard, is as shown in Table 4.8.
The total point source discharge from wastewater
treatment plants within in the stream is 0.2 m
3
/s, and
TABLE 4.8. Load Duration Curve for Fecal Coliform
Exceedance (%)
Load (×10
10
CFU/d)
Load (×10
10
CFU/d)
Load (×10
10
CFU/d)
Exceedance (%)
Exceedance (%)
0.0
1520
35.0
54.7
70.0
19.6
2.5
377
37.5
50.7
72.5
18.4
5.0
290
40.0
46.6
75.0
17.3
7.5
232
42.5
42.7
77.5
16.2
10.0
193
45.0
39.7
80.0
14.9
12.5
159
47.5
36.7
82.5
13.6
15.0
136
50.0
33.8
85.0
12.6
17.5
118
52.5
31.3
87.5
11.6
20.0
106
55.0
28.9
90.0
10.7
22.5
94.6
57.5
26.9
92.5
10.0
25.0
84.1
60.0
25.1
95.0
9.19
27.5
75.4
62.5
23.6
97.5
8.16
30.0
67.3
65.0
22.2
100
3.46
32.5
60.3
67.5
21.0
-
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