Page 468 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

14 Cake Filtration

As with all unit processes, ‘‘cake’’ filtration has its own backwash water, treatment of industrial process water, indus-
nomenclature and technology. Also, as with other unit pro- trial wastewater treatment, clarification of swimming pool
cesses, it has its own ‘‘subculture,’’ defined largely, in this water, and filtration of aquarium water. Although this chapter
case, by the industry that manufactures the filter media. The is built around the application to potable water treatment, the
latter relates to how the field evolved from the discovery of principles apply to any of the applications mentioned.
the geologic deposits of fossil diatoms in Germany through its
14.1.1.2 Definitions
application to drinking-water treatment. Finally, as with all
unit processes, the design of a system is circumscribed, to a Most of the terms here are requisite to further learning about
large extent, by its past. Understanding these themes, that is, diatomite filtration. The ‘‘glossary’’ expands on the termin-
the past, the technology, and the process principles, gives ology given.
rationale for the design.
14.1.1.2.1 Filter Aids
Filter media have been called, in the industry, a ‘‘filter aid.’’
14.1 DESCRIPTION
The most common medium is diatomaceous earth, called
Figure 14.1 encapsulates the key aspects of the cake filtration ‘‘diatomite’’ by those in the industry. Sometimes it is called
process. As illustrated, a ‘‘cake’’ filter (also called a ‘‘pre-coat’’ ‘‘DE.’’ Another filter aid, less common, is a volcanic ash,
filter) has a media, usually diatomaceous earth, deposited on a manufactured under the name, perlitet. Yet another is cellu-
‘‘septum,’’ comprising a stainless steel fabric. The mesh size of lose, made from wood pulp.
the fabric must be small enough to retain the media. The circu- 14.1.1.2.2 Septum
lation of a specified mass of slurry through the septum transfers
the media to a ‘‘pre-coat’’ deposit on the septum, illustrated as The filter media are retained on a fabric, called a ‘‘septum.’’ The
the portion of the filter cake just to the left of the septum in most commonfabric is stainless steel, but other materials include
Figure 14.1. After the pre-coat forms, the filtration process can carbon steel, titanium, hasteloy, polypropylene, etc. The mesh
begin. The raw water is introduced, along with a ‘‘body feed’’ opening most often cited is 60 mm. Theseptummustbesup-
injected into the raw-water flow to give a specified concentra- ported by a structural grid of several layers to withstand the
tion, usually of the same material as the pre-coat. At this point pressure differential between the two sides, for example,
the filter run begins. The body-feed deposits on the pre-coat, 103–280 kPa (15–40 psi), depending upon the system and to
building up the filter ‘‘cake,’’ illustrated in Figure 14.1 as the minimize flex that could cause cracks in the pre-coat.
portion of the filter cake just to the left of the pre-coat. When the
14.1.1.2.3 Pre-Coat
headloss reaches a specified design limit, or when the cake
An initial deposit of filter media on the septum is called a
thickness reaches a limit as defined by the spacing between
pre-coat. The pre-coat has three primary functions: (1) to
septum leaves, whichever occurs first, then the cake is removed
limit the passage of particles at the start of a filtration cycle
and the cycle is repeated. As also illustrated in Figure 14.1, the
(‘‘immediate clarity’’), (2) protect the septum from ‘‘fouling’’,
suspended matter deposits within the matrix of the cake that has
and (3) aid in cake release at the cycle termination. The rule of
been deposited. Without the body feed, the suspended matter 2 2
thumb for the deposit is 0.5–1.0 kg=m (0.1–0.2 lb=ft ). A mid-
would deposit as a layer, forming its own ‘‘cake’’ on the pre- 2 2
range, that is, 0.75–1.0 kg=m (0.15 lb=ft ) was applied in
coat, resulting in a rapid rate of headloss increase. Not shown in
studies that adopted DE for portable field water treatment
Figure 14.1 is the structural support for the septum that must be
units for military use (Lowe et al., 1944). The higher number,
provided in order to withstand the pressure differential between 2 2
that is, 1.0 kg=m (0.2 lb=ft ), was recommended to alleviate the
the two sides of the filter cake.
passage of cyst-size radioactive beads (Logsdon et al., 1981,
p. 113) and Giardia murus cysts (Logsdon and Lippy, 1982,
14.1.1 CAKE FILTRATION IN-A-NUTSHELL
p. 655). The thickness of the deposit is about 3–5mm (1=8in.).
Cake filtration has its own nomenclature, operating proced- 14.1.1.2.4 Body Feed
ures, and process characteristics. These are described briefly
The filter media are metered into the raw-water flow, which
to provide an orientation for the chapter.
add to the pre-coat deposit, building up the thickness of the
14.1.1.1 Applications filter cake. The media concentration is established by trial
Applications of diatomite filtration have included potable- such that the rate of headloss increase is linear, that is, it
water treatment, tertiary treatment of wastewater, treatment of does not increase exponentially, as would be the case without
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