Geoscience Reference
In-Depth Information
Using Equation 23.84,
(
)
3
2
3
2
Gdpp
=
−
pg
/
µ
=
( .
0 090
)
×
0 999
.
×
( .
26809
−
.
99
)
×
981
/ (.
0 0113
)
=
9405
n
eq
s
Compute
U
mf
using Equation 23.83:
µ
33 7
.
µ
(
)
−
05
.
U
=
×
1135 69
.
+
0 0408
.
G
mf
n
pd
pd
eq
eq
0113
0 999
0
)
−
33 700113
0 999
.
×
.
(
05
.
=
×
1135 69
.
+
0 0408
.
×
9405
.
×
0 090
.
.
×
0 090
.
=
0 660
.
cm/s
Compute backwash rate. Apply a safety factor of 1.3 to
U
mf
as backwash rate:
Backwash rate
=× =
1.3
0.660 cm/s
0.858 cm/s
3
2
cm
cm
L
1000 cm
1
3.785
gal
L
cm
60 s
min
2
0 858
.
×
×
× ×××
929
×
=
12.6 pgm/ft
2
3
2
⋅
s
ft
23.6.17 h
orizontal
W
ashWater
t
roughs
Wastewater troughs are used to collect backwash water as well as to distribute influent water dur-
ing the initial stages of filtration. Washwater troughs are normally placed above the filter media
in the United States. Proper placement of these troughs is very important to ensure that the filter
medium is not carried into the troughs during the backwash and removed from the filter. These
backwash troughs are constructed from concrete, plastic, fiberglass, or other corrosion-resistant
materials. The total rate of discharge in a rectangular trough with free flow can be calculated by
using Equation 23.85:
Q = C
×
w
×
h
1.5
(23.85)
where
Q
= Flow rate (cfs).
C
= Constant (2.49).
w
= Trough width (ft).
h
= Maximum water depth in trough (ft).
■
EXAMPLE 23.101
Problem:
Troughs are 18 ft long, 18 in. wide, and 8 ft to the center with a horizontal flat bottom.
The backwash rate is 24 in./min. Estimate (a) the water depth of the troughs with free flow into the
gullet, and (b) the distance between the top of the troughs and the 30-in. sand bed. Assume a 40%
expansion and 6 in. of freeboard in the troughs and 6 in. of thickness.
Solution:
Estimate the maximum water depth (
h
) in the trough:
v
= 24 in/min = 2 ft/60 s = 1/30 fps
A
= 18 ft × 8 ft = 144 ft
2
Q
=
V
/
A
= 144/30 cfs = 4.8 cfs
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