Geoscience Reference
In-Depth Information
Using Equation 23.85,
w
Q
=
=
1.5 ft
15
.
15
.
Cwh
=
249
.
wh
=
(
)
23
/
[
]
23
/
hQ
249
.
w
=
4 8
. ((.
24915
×
.)
=
1 18
.
ft (approx. 14 in.
=
1.17 ft)
Determine the distance (
y
) between the sand bed surface and the top troughs:
Freeboard = 6 in. = 0.5 ft
Thickness = 8 in. = 0.67 ft (bottom of trough)
y
= 2.5 ft × 0.4 + 1.17 ft + 0.5 ft + 0.5 ft = 3.2 ft
23.6.18 F
ilter
e
FFiCienCy
Water treatment filter efficiency is defined as the effective filter rate divided by the operation filtra-
tion rate as shown in Equation 23.86 (AWWA and ASCE, 1998):
==
−
R
R
UFRV UBWV
UFRV
e
o
E
(23.86)
where
E
= Filter efficiency (%).
R
e
= Effective filtration rate (gpm/ft
2
).
R
o
= Operating filtration rate (gpm/ft
2
).
UFRV
= Unit filter run volume (gal/ft
2
).
UBWV
= Unit backwash volume (gal/ft
2
).
■
EXAMPLE 23.102
Problem
: A rapid sand filter operates at 3.9 gpm/ft
2
for 48 hours. Upon completion of the filter run,
300 gal/ft
2
of backwash water is used. Find the filter efficiency.
Solution:
Calculate the operating filtration rate (
R
o
):
R
o
= 3.9 gpm/ft
2
× 60 min/hr × 48 hr = 11,232 gal/ft
2
Calculate the effective filtration rate (
R
e
):
R
e
= 11,232 gal/ft
2
- 300 gal/ft
2
= 10,932 gal/ft
2
Calculate the filter efficiency (
E
) using Equation 23.86:
E
= 10,932/11,232 = 97.3%
23.7 WATER CHLORINATION CALCULATIONS
Chlorine is the most commonly used substance for disinfection of water in the United States. The
addition of chlorine or chlorine compounds to water is called
chlorination
. Chlorination is consid-
ered to be the single most important process for preventing the spread of waterborne disease.
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