Page 415 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

370 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

160 while the first pocket collapses. The first air pocket collapses due
8 to sand being thrown up into the pocket from below due to air
140
flow below. Within this dynamic system, the media grains slip
7 and slide against each other resulting in abrasion and detachment
120
Air of particles (Hewitt and Amirtharajah, 1984, p. 592).
6
100 v (scfm/ft 2 ) The particular combinations of v=v mf and v(air) in which
Sand 5 ‘‘collapse pulsing’’ is observed is a ‘‘locus-of-points’’ defined
v (m/h) 80 4 empirically, such as seen in Figure 12.37 for three sands, that
is, d 10 ¼ 0.46, 0.64, and 0.88, respectively. The best fit rela-
Anthracite
tionship, based on observations, is
60
3
v
40 Water 15 v mf ¼ 0:49 0:119 v(air) (12:59)
Anthracite
10
20 Sand v (gpm/ft 2 ) where
5 v is the backwash water velocity (m water=m filter bed
2
3
0 0 area=s)
1.0 1.5 2.0 2.5 v(air) is the air flow per unit of filter bed area (m air=m 2
3
d (mm) filter bed area=s)
10
FIGURE 12.36 Velocities for simultaneous air-water backwash. Referring to Figure 12.37, for air–water flow combinations
(Adapted from Cleasby, J.L., Backwash and underdrain consider-
below-the-line, air channeling predominates; this region is not
ations, unpublished paper for short course at Colorado State Univer-
effective. On-the-line, the air–water backwash is effective
sity on design of filtration systems, June, 1991.)
because air transfers momentum to the sand grains, which,
in turn, is dissipated both by shear and by random collisions
12.4.4.10 Collapse Pulsing with other sand grains. In addition, the upward backwash
The ‘‘collapse pulsing’’ mode of media cleaning was considered velocity reduces the grain-to-grain pressure that is sufficient
to be most effective based on visual and film observations of to facilitate displacement by the air bubbles and permits
bubble-media behavior. Based on observations, ‘‘collapse puls- relative movement between grains. For operating points
ing’’ occurs at certain combinations of v=v mf and v(air) in the above-the-line, the media-water matrix begins to behave as
subfluidization range of backwash (Amirtharajah, 1984; Hewitt if fluidized and the cleaning effectiveness is reduced and
and Amirtharajah, 1984). This mode is characterized by the media loss may be appreciable (Hewitt and Amirtharajah,
following description. The air flow moves up through the 1984). Media loss may be minimized by (1) operating
media via air channels, eventually forming air pockets. As
along-the-line, (2) using larger media, for example, d 10 (sand)
more air flows into a given pocket, another channel forms 0.88 mm, (3) locate the crest of the backwash troughs at least
above the pocket and air flows up to form yet another pocket 760 mm (30 in.) above the media surface.

0.5 0.5
y=0.49–0.11942x y = 0.49 – 0.0364x 2.8
Air flow as bubbles-“collapse-pulse” 0.8 Air flow as bubbles: collapse-pulsing
(and effective cleaning) (and effective cleaning)
0.4 0.4 2.4
0.7
0.6 0.3 Air flow in channels 2
v(backwash)/v mf (ineffective cleaning) 0.5 v(m/h) v (backwash)/v mf (ineffective cleaning) Minimum media loss 1.6 v (gpm/ft 2 )
0.3
Air flow in channels
0.4
Conditions
0.2
Sand 1: d 60
Sand 1: d 60 =0.62 mm Minimum media loss 0.3 0.2 Conditions = 0.62 mm 1.2
Sand 2: d =0.86 mm Sand 2: d 60 = 0.86 mm
60
Sand 3: d 60 =1.54 mm Sand 3: d = 1.54 mm 0.8
60
0.1 18 and 30 in. sand 0.2 0.1 18 and 30 in. sand
depths for each size depths for each size 0.4
v mf =5.96 gpm/ft 2 0.1 v mf = 5.96 gpm/ft 2
0.0 0 0.0 0
0 1 1 2 2 3 1 2 3 4 5 6 7
2
2
3
(a) v(air) (m /min/m ) (b) v(air) (SCFM/ft )
FIGURE 12.37 Air–water combinations for collapse pulsing in backwash. (a) Metric units. (b) U.S. Customary units. (Adapted from
Hewitt, S.R. and Amirtharajah, A., J. Environ. Eng., ASCE, 110(3), 601, 1984.)