Page 416 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Rapid Filtration 371

Example 12.10 Enumerate Protocol for Air–Water 440 mm
t(rise of water) ¼
Backwash in ‘‘Collapse Pulsing’’ Mode 73 mm=min
¼ 6 min
Given
A filter bed is sized 4.27 m 6.10 m (14 ft 20 ft) and uses e. Terminate air when the water level is about 160
an air–water backwash in accordance with the procedures mm (6 in.) from the weir crest, that is, after 6 min
f. Increase the water backwash rate to cause about
outlined by Amirtharajah and Trusler (1982). The back-
0.20 bed expansion for say 10 min, calculated by
wash troughs weir crests are 760 mm (30 in.) above the
Equations 12.52 through 12.71 as incorporated
media. Assume the media is sand and d 10 ¼ 0.46 mm,
in Table CD12.7
depth 760 mm (30 in.).
g. Terminate the backwash slowly so that the por-
Required
osity of the bed approaches a minimal level (see
Determine: (1) the backwash rates for air and water, (2) Trussell and Chang, 1999; Trussell et al. 1999)
determine a recommended backwash sequence, (3) esti- 3. Backwash volume:
mate the volume of backwash water used, and (4) estimate
the compressor power for the air wash.
Backwash water volume
Solution
3
3
1. Backwash rates: From Figure 12.37a, select a coord- Volume to start 1.88 m =min (200 ft or
inate pair with low backwash rate and higher air of air-wash: 3 min ¼ 5.64 m 3 1500 gal)
rate, as recommended by Amirtharajah (1984), for Volume during 1.88 m =min (400 ft or
3
3
example, air-wash: 6 min ¼ 11.28 m 3 3000 gal)
3
3
Volume during 1.88 m =min (400 ft or
v backwash: 6 min ¼ 11.28 m 3 3000 gal)
0:30;
3
3
v mf Total volume: 1.88 m =min (1000 ft or
3
2
3
2
Q(air) 1:7m =min=m (5:6 standard ft =min=ft ) 6 min ¼ 28.20 m 3 7500 gal)
Calculate HLR (or ‘‘v’’), 4. Compressor power:
From (1)
HLR 0:30 v mf 3 3
Q(air) ¼ 44:3m =min (1:8ft =min)
¼ 0:30 14:54 m=h
2
¼ 4:36 m=h(1:8 gpm=ft ) Table CDD.5 calculates P, given the data inputs,
¼ 73 mm=min (Ex12:10:1) Q a , p 2 , p 1 , that is,
P ¼ Q a p 1 (k=(k 1)[(p 2 =p 1 ) (k 1) =k 1] (D:75)
Calculate Q,
Comments
Q ¼ HLR A(filter)
The calculated power, P, must be increased by an effi-
2
2
3
¼ 4:36 m =h=m (4:27 6:10 m ) ciency factor, for example, 0.67 to calculate the compres-
3
¼ 112:76 m =h sor power. The power required by the electric motor is the
compressor power increased by another efficiency factor,
3
¼ 1:88 m =min
for example, 0.67. Calculate P using the spreadsheet;
p 1 (absolute pressure at compressor intake) and p 2 (absolute
Calculate Q(air), pressure on discharge side of compressor). To determine p 1 ,
the elevation, and atmospheric pressure is used as a rule (or
Q(air) Q(air) A(filter) subtract losses if the intake pipe is long or if there are
3
2
2
¼ 1:7m =h=m (4:27 6:10 m ) obstructions). To determine p 2 , start with absolute atmos-
pheric pressure at the filter, and calculate water depth,
3
3
¼ 44:3m =min (1:8ft =min) bubble pressure, orifice pressure loss, pipe friction losses
and any other losses, back to the compressor intake; p 2
2. Backwash sequence (Amirtharajah, 1984): must be high enough to overcome these losses. All of this
a. Lower water level in filter to media surface can be seen most easily in terms of a pneumatic-grade-line
3
b. Begin water backwash at 1.88 m =min (500 (similar to an hydraulic-grade-line, i.e., HGL).
gal=min)
c. When the bed is flooded by80–160 mm (3–6in.) of
water, which requires about 3 min from top of 12.5 OPERATION
media, begin introducing air slowly to an air flow,
3
3
Q(air) ¼ 44.3 m =min (1.8 standard ft =min) Operation has many facets, such as performing the functions
d. Time to reach water level 160 mm (6 in.) from of the filter cycle, monitoring performance, ensuring that
the weir crest, that is, 760 mm 2 160 mm ¼ equipment and instruments function properly, maintaining
440 mm (30 12 ¼ 28 in.), is records, relating to the public, providing security, managing