Page 470 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Cake Filtration 425

Septum assembly
Pressurized tank DE pre-coat
Body-feed
tank
Pre-coat
tank
X
Outlet tube
P
P
X
Flow meter
X FM X
(a)
Septum assembly
DE cake
Body-feed Pressurized tank
tank

X
Pre-coat
tank
Outlet tube
P
P
X
Flow meter
Raw water
X FM X
(b)
Septum assembly
Body-feed Pressurized tank DE cake
tank

Filtered water
Jets P
Pre-coat
tank
P
P
X
X
Flow meter Drainage of spent DE
X FM X
(c)
FIGURE 14.2 Cycles of diatomite filtering operation: (a) pre-coat, (b) body feed, and (c) cleaning.

for the situation. The run length is determined by the rate of fossilized diatom is hydrous silica, that is, SiO 2 nH 2 O, in
headloss increase and the selected pressure differential limit. composition (Kadey, 1975, p. 605). Figure 14.3a shows a
mixture of diatoms that illustrates the wide variety in shapes
14.1.2 MEDIA and sizes. The scale is 50 mm. Most diatoms are a few
microns to over 100 microns in size, the latter applying to
The media used in cake filtration are manufactured
rod-shaped diatoms. The species of diatoms number 12,000–
products. Selection of a product must be matched to the
16,000 (Kadey, 1975, p. 609). At Celite’s Lompoc mine, the
removal task at hand.
ore comprises seawater species. Figure 14.3b through d shows
14.1.2.1 Kinds of Media three examples. At the Nevada deposits of Eagle-Picher the
species are freshwater.
Three kinds of media are used in cake filtration: (1) diatomite;
(2) volcanic ash, for example, Perlitet; and (3) cellulose. The
14.1.2.2 Sources of Media
most common media is diatomite and is the basis for the
The filter media, that is, diatomite and perlitet, are mined as
discussions in this chapter.
minerals in ‘‘rock’’ form with about 40% moisture and then
14.1.2.1.1 Diatomaceous Earth: Fossil Diatoms processed to form desired grades of filter media. Deposits of
Diatomite is the manufactured form of a product obtained DE are located in many locations over the world. In the
from a sedimentary rock that comprises fossil diatoms. The United States, the largest deposit is at Lompoc, California,