Page 484 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Cake Filtration 439

TABLE 14.3
a
Tank-Septum Data for Dry Cake Discharge (Durco Catalog, 2009)
Septum Area Tank Dia. Leaf Spacing b Tank Volume Weight
Leaves
3
2
2
(m ) (ft ) (mm) (in) (#) (mm) (in) (m ) (gal) (kg) (lb)
4.35 46.8 914 36 6 102 4 0.79 210 1021 2250
5.79 62.4 914 36 8 75 3 0.79 210 1066 2350
7.24 78.0 914 36 10 102 4 1.05 278 1134 2500
9.42 101.4 914 36 13 76 3 1.05 278 1202 2650
11.14 120 1219 48 8 102 4 1.74 461 1860 4100
15.33 165 1219 48 11 76 3 1.74 461 1942 4280
18.11 195 1219 48 13 102 4 2.21 584 1987 4380
20.90 225 1219 48 15 76 3 2.21 584 2064 4550
28.20 303.6 1524 60 12 102 4 3.44 909 2130 4645
32.90 354.2 1524 60 14 76 3 3.44 909 2155 4750
37.60 404.8 1524 60 16 102 4 4.16 1100 2220 4895
47.00 506 1524 60 20 76 3 4.16 1100 2291 5050
56.93 612.8 1829 72 16 102 4 5.68 1500 2405 5300
78.27 842.6 1829 72 22 76 3 5.87 1550 2643 5825
92.50 995.8 1829 72 26 76 3 6.43 1700 2813 6200
120.97 1302.2 1829 72 34 76 3 7.95 2100 3176 7000
a
Durco Filters by Ascension Industries, North Tonawanda, NY, 2009.
b
Spacing is distance center-to-center between leaves. The cake thickness is assumed to be 38–95 mm leaf spacing.

14.3.1.2 System Components Q(septum tank) is the flow of water septum assembly in a
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Those components of a diatomite filtration system that given filter housing (m =s)
make it work as a technology include the filter housing, t(tank) is the time duration for volume of slurry in body-
septum assembly, tanks for pre-coat and body feed, metering feed tank to last (s)
pumps, flow meters, pressure gages, pipes, valves, control
system software, etc. Calculation of tank size, V(tank) BF , depends upon the magni-
tudes of the variables in Equation 14.9. Generally, operation
14.3.1.2.1 Tanks for Pre-Coat and DE Body Feed will be served best if t(tank) 48; since C(body feed) and
Q(septum tank) are established prior to this stage of design,
The volume sizing for both pre-coat tank and the body-feed
then Mass DE BF , that is, the mass charge to the tank, is
tank is V(tank) C(slurry) ¼ Mass DE. The pre-coat tank is
directly proportional to t(tank).
sized merely to circulate a given mass charge at the highest
The slurry concentration is not so low that the tank volume
concentration that can be circulated while maintaining a uni-
is excessive, nor so high that the slurry is difficult to maintain
form suspension of DE. The body-feed tank is sized based on
as a uniform suspension. Some examples of tank concentra-
both the concentration of the slurry that is feasible and a
tions are provided in Table 14.4, with data and calculated
desired duration for the tank to provide a body feed, which
concentrations given in tabular form.
depends upon C(body feed) to the filters. The relation is, as a
As seen, the pre-coat slurry concentrations are about an
mathematical expression,
order of magnitude higher than the body-feed slurry concen-
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trations, that is, about 120–240 kg=m for the former and
V(tank) BF C(slurry) BF ¼ Mass DE BF 3
10–24 kg=m for the latter. In each case, the slurries should
¼ C(body feed) Q(septum tank) t(tank) (14:9) be agitated to maintain them in suspension; mixers with
rotational speed 40–60 rpm are recommended as high-speed
where mixers may degrade the media particles (McIndoe et al.,
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V(tank) BF is the volume of body-feed tank (m ) 1988, p. 21).
C(slurry) BF is the concentration of diatomite slurry in
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body-feed tank (kg=m ) 14.3.1.2.2 Pipes
Mass DE BF is the mass charge of diatomite to body-feed The main concern in the sizing of slurry pipes is to avoid the
tank (kg) settling of DE due to low velocity. On the other hand, if
C(body feed) is the concentration of body feed to septum velocities are too high, solids may accumulate and compact
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assembly (kg=m ) around bends with consequent clogging (Bell, 1962, p. 1250).