Environmental Engineering Reference
In-Depth Information
Q
w
L
w
+
Q
s
L
s
(Q
w
+
Q
s
)
L
o
=
,
(6.90)
where
Q
w
and
Q
s
arevolumetricflowratesofwastewaterandstreamwater,respectively.
The initial oxygen deficit
Δ
o
is given by
Q
w
C
O
2
+
Q
s
C
s
O
2
Q
w
+
Q
s
,
Δ
o
=
C
O
2
−
(6.91)
where
C
O
2
and
C
s
O
2
are respectively the oxygen concentration (mg/L or kg/m
3
)inthe
wastewater and the stream water just upstream of the discharge location. The three
parameters of significance obtained from the above analysis are
Δ
,
Δ
c
, and
t
c
(or
y
c
).
This information gives the maximum possible pollutant concentration at the discharge
point. To utilize the Streeter-Phelps equation, one has to estimate values for
k
d
and
k
r
with precision.
k
d
is determined by obtaining the BOD at two known locations (a and b)
in a stream and using the equation
U
log
L
a
.
y
k
d
=
(6.92)
L
b
The value of
k
r
is obtained from an equation such as that of O'Connor and Dobbins:
k
r
d
−
1
=
3.9
U
H
3
1
/
2
,
(6.93)
where
U
is the stream velocity (m/s) and
H
is the mean depth (m). The constant in the
equation for
k
r
corrects for the bed roughness in a stream that affects the stream velocity.
k
r
is affected by the presence of algae (that affect
C
O
2
through diurnal variations from
C
o
2
*
Δ
0
Δ
c
Clean water
Zone of recovery
C
o
2
min
Zone of initial deficit
Zone of deaeration
t
c
t
or
y
FIGURE 6.18
A typical Streeter-Phelps oxygen-sag curve.
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