Page 478 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Cake Filtration 433

2
2
0.73 kg=m septum (0.15 lb=ft ), body feed 50 mg=L, and Insufficient body feed causes an exponential increase in the
2
HLR 2.44 m=h (1.0 gpm=ft ). rate of headloss increase. If the body feed is sufficient, then
enough unblocked pores are provided such that the rate of
headloss is proportional to the thickness of the cake, that is,
14.2.2 REMOVAL MECHANISMS
the rate of headloss increase is linear. The particles to be
Straining is the primary removal mechanism in diatomite removed are embedded within the cake and, as noted, are
filtration; the definition of ‘‘straining’’ is expanded (see Gloss- prevented from movement unless the particles to be removed
ary), however, to include a particle being ‘‘embedded’’ within are smaller than the smallest fraction of pore sizes. Viruses
the media. If alum or a polymer are added to coat the media, (10–400 nm), for example, are small enough to move within
then the particles to be removed may also attach to the media, the C-545 cake, but not Giardia cysts (10 mm). Also, bacteria
that is, ‘‘adsorb.’’ (1 mm) are small enough to move within the pore structure of
C-545, but not within the pores of the smallest grades of DE,
14.2.2.1 Straining and Embedding for example, Filter-Cele, which has an average pore size of
Figure 14.9 is a scanning electron photomicrograph (SEM) at about 1.5 mm (Logsdon and Lippy, 1982, p. 655).
2000 showing Giardia cysts embedded in C-545t as depos-
ited on a septum (obtained by Harris Walton, 1988, then at 14.2.2.3 Adsorption
Manville Corp., Denver, Colorado). The cysts are distin- The zeta potentials of the DE particles were measured
guished from the media by their oblong form. The cysts are in distilled water as 47 z 24 mV (Oulman and
embedded in the media not able to progress further. To be Baumann, 1964, p. 920). Adding a trivalent ion, that is,
more specific, the hydraulic shear force on any given cyst Al 3þ or Fe , will cause a coating on the DE particle
3þ
would be resisted by an equal and opposite force of the surface, with consequent charge reversal, that is, a positive
diatomite particles on the cyst. If, however, the hydraulic zeta potential in the range of þ22 z þ30 mV using
shear forces cause movement of the cyst, progress will be about 0.05 g Al =g DE (similar data were obtained by
3þ
limited since some fraction of the pores will be smaller than Schuler and Ghosh, 1990, p. 68). A cationic polymer coat-
the cysts. Embedding might be considered as a subset of ing will result in z þ40 mV. The coating may be by
straining, that is, the particles strained are larger than the mixing the alum or polymer with the pre-coat and the
openings. body feed prior to application. A zeta potential, z þ60 mV,
was achieved with C(Purifloce601) 0.001 g Purifloce601=g
14.2.2.2 The Role of Body Feed C-545, which enabled the adsorption of clay particles in
The idea of body feed is to prevent the particles to be removed accordance with a Langmuir isotherm relation (Baumann
from accumulating within the pores and causing blockage. and Oulman, 1970b, p. 689). In other words, the DE media,
having negative charge, is likely to repel negatively charged
particles. By charge neutralizing the media, however,
negatively charged particles, for example. Giardia cysts,
may be attracted.

14.2.2.4 Comparisons between Filtration Processes
At this point it is useful to review the three main filtration
processes and their respective filtration mechanisms:
(1) depth filtration works by the particles to be removed
penetrating the filter media and ‘‘attaching’’ to the grains of
media; (2) slow sand filtration works by the straining of the
particles to be removed by previously deposited matter that
has been retained on the surface of the media bed and by
depth filtration for those particles that penetrate the media
bedattaching tobiofilms on sand grains; and (3) cake
filtration works by straining=embedment within the media
cake, that is, the particles to be removed are enmeshed
within the matrix of media grains and blocked from move-
ment by those grains.
5 μm
14.2.3 HYDRAULICS
FIGURE 14.9 C-545 diatomite with Giardia cysts interspersed.
(From Walton, H.G., Diatomite filtration: Why it removes Giardia Flow of water through the filter cake and associated pumps,
from water, in Wallis, P. M., and Hammond, B. R. (Eds.), Advances pipes, and tanks that comprise a diatomite filtration system is
in Giardia Research, University of Calgary Press, Calgary, Alberta, hydraulic in nature. The sections that follow review the
Canada, 1988, Figure 8. With permission.) hydraulic principles relevant to diatomite filtration.