Science of Water : Concepts and Applications

384 The Science of Water: Concepts and Applications
distance between the top of the troughs and the 30-in. sand bed. Assume 40% expansion and 6 in. of
freeboard in the troughs and 6 in. of thickness.
Solution:
Step 1. Estimate the maximum water depth, h, in the trough.
Using Equation 10.86:
1
v 24 in. min 2ft60s 30
A
fps
2
18 ft 8 ft 144 ft
QVA14430 cfs
4.8 cfs
1.5
Q2.49 wh , w1.5
ft
⎛ Q ⎞
h
⎝
⎜
2.49w⎠
⎟
23
⎡ 4.8 ⎤
⎢
⎣(2.491.5)
⎥
⎦
2 3
1.18 ft (or approximately 14 in. 1.17
ft)
Step 2. Determine the distance (y) between the sand bed surface and the top troughs.
Freeboard 6in. 0.5
ft
Thickness8in. 0.67 ft (the bottom of trough)
y 2.5 ft 0.41.17 ft 0.5 ft 0.5
ft
3.2 ft
FILTER EFFICIENCY
Water treatment fi lter effi ciency is defi ned as the effective fi lter rate divided by the operation
fi ltration rate as shown in Equation 10.87 (AWWA and ASCE, 1998).
where
E = fi lter effi ciency, %
Re = effective fi ltration rate, gpm/ft2 Ro = operating fi ltration rate, gpm/ft2 UFRV = unit fi lter run volume, gal/ft2 UBWV = unit backwash volume, gal/ft2 Re
UFRV UBWU
E
R UFRV
o
(10.87)