Page 374 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

12 Rapid Filtration

The common filtration technology since about 1900 in the lower effluent concentration of suspension. The resulting filter
United States has been ‘‘rapid filtration.’’ As we have gained effluent should have low turbidity, for example, 0.10 neph-
more understanding the ‘‘process,’’ as distinct from the ‘‘tech- elometric turbidity units (NTU).
nology,’’ has been called ‘‘depth’’ filtration.
The topics of this chapter include (1) a description of rapid 12.1.1.1.1 Filtration Processes
filtration, (2) a review of the theory, (3) elements of practice, Two kinds of filtration processes are as follows: (1) straining,
(4) a description of operation, (5) the use of pilot plants, and and (2) depth filtration. Straining is the retention of particle
(6) an introduction to proprietary systems. that are too large to pass through the pores of the media.
Depth filtration is the retention of floc particles that ‘‘attach’’
to the media grains or to floc that has previously attached to
12.1 DESCRIPTION OF RAPID FILTRATION
media grains. Such attachment is due to bonding by electro-
As a technology, rapid filtration evolved from a variety of static and surface forces.
proprietary innovations in the 1880s and as an empirical
12.1.1.1.2 Clogging and Headloss
practice from about 1900 through perhaps the 1980s. By the
mid-1980s process theory began to influence design and Straining causes an exponential headloss increase with time,
operation. The ‘‘process,’’ called ‘‘depth filtration,’’ has to which is not desired. With depth filtration, however, the time
do with the removal mechanisms of particulates being ‘‘trans- rate of headloss increase is linear, which is desired. As a filter
ported’’ to the granular media comprising the filter bed and run progresses, the media ‘‘stores’’ the attached floc, which
‘‘attaching’’ to the surfaces, that is, being removed from the encroaches on the void spaces, and is termed ‘‘clogging.’’
fluid flow. A goal of the filter design is that the clogging progresses
The modern design of a depth (or rapid) filter system has through the bed depth, that is, not just within the top layer
two phases: (1) process design, and (2) design of the technol- of media. Media design is important to permit approaching
ogy support system. Process design, in the modern sense, this goal, along with control of floc size. As clogging occurs,
requires knowledge of theory coupled with a pilot plant the headloss across the bed increases.
study. The technology support system requires knowledge of
the practices that have been developed since the first reinforced 12.1.1.1.3 Length-of-Run
concrete rapid filters were put on line after 1901 (according to At some point, the clogging causes a designated ‘‘terminal
Fuller (1928) the first was in 1901 at Little Falls, New Jersey). headloss’’ to be reached. At the same time, the effluent par-
The technology support has to do with hydraulic design, for ticle concentration may reach a designated ‘‘breakthrough’’
example, getting coagulated water to the filter, collecting limit, as measured by turbidity or particle counting. Which-
filtered water, backwashing, collecting the backwash water, ever occurs first determines the ‘‘length-of-run.’’ The break-
and disposal of waste solids (see also Qasim et al., 2006, through limit should occur first, however, which means
pp. 355–422). setting an adequate depth of the filter box.
12.1.1.1.4 Transport
12.1.1 FILTRATION TECHNOLOGY
Removal of floc particles within the filter bed is a two
The filtration process and the supporting components to step sequence: (1) transport and (2) attachment. The transport
make it work is called here, ‘‘filtration technology.’’ The step is the movement of a suspended particle through the
‘‘process,’’ that is, ‘‘depth filtration,’’ occurs within the water to a media grain. Mechanisms include (1) diffusion,
media bed. The supporting components include an under- (2) gravity settling, and (3) interception. The ratio of particles
drain system, pipes, valves, controls, and various instru- striking a media grain to those approaching is the transport
ments. Numerous variations exist which are both generic coefficient, h. The value of h is determined by media size,
and proprietary. the floc particle size, temperature, filtration velocity, etc.
(Yao et al., 1971).
12.1.1.1 In-a-Nutshell
The media bed is where the ‘‘process’’ part of rapid filtration 12.1.1.1.5 Attachment
occurs, that is, it is where the changes occur. Floc particles in Attachment is the bonding between the particle and the media
suspension (following coagulation-mixing and possibly floc- grain, which is affected by the zeta potentials of the media and
culation-settling) are removed within the media, resulting in a the floc particles. Depth filtration is thus transport from the

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