Page 430 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Rapid Filtration 385

construction contract. The time period for completing these that the pressure drop across a steel mesh screen
phases of the work should be no more than six years.’’ with 35 mm pore opening increased exponentially
The City of New York was planning to filter their Croton with volume of suspension passed. Applied to the
water supply, which would have brought a resolution to the surface deposition on a deep bed filter, the Boucher
approximately 100 years of concern on the question of filtra- equation describes the ‘‘blinding’’ of the filter (see
tion of the Croton water. The issue may have been resolved as Ives, 1975b, p. 186).
far as the City of New York was concerned when the discov- Breakthrough: The third and final phase of a filtration cycle
ery was made about 1998 that ultraviolet (UV) light could kill (excluding backwash) is called ‘‘breakthrough.’’ It is
Cryptosporidium oocysts; subsequent reports were that a UV the rising leg of the C(t) z curve and occurs when the
installation was underway. One reason for not moving ahead filter is near exhaustion with respect to its capacity to
on filtration was the very high cost involved. store floc. Actually, this phase is merely the obser-
vation of the ‘‘wave front’’ as it emerges from the
bottom of the filter bed.
GLOSSARY
Breakthrough curve: The C(t) z curve in its entirety may be
Advection: The mass flow of a quantity that occurs with the sometimes called the breakthrough curve. More
bulk velocity. Same as convection. commonly, breakthrough curve refers to the final
Air binding: Gas, that is, oxygen and nitrogen, that displaces phase of the C(t) Z¼Z o curve in the filtration cycle in
volume within the filter bed causing blockage which the curve begins to rise steeply.
of pores and thus higher headloss is called ‘‘air Cake filtration: The slurry, that is, the body feed is the
binding.’’ filtering agent which accumulates and forms a cake.
Air-wash: An adjunct to backwash to help dislodge attached Filtration of small particles may occur in the depth of
floc by impacts between filter grains. Protocols vary the diatomite layer. (Ives, 1975a, p. 1) The mechan-
but an investigation by Amirtharajah (1985) showed ism is primarily straining.
that a ‘‘collapse pulsing’’ procedure at subfluidiza- Chemical conditioning: The first phase of the filtration cycle
tion of the filter bed was most effective. in which the concentration declines with time to
American filter: During the early days of the rapid filtration some acceptable level, that is, C C(limit), which
technology, the prevailing filtration technology in is maintained for the second phase of the filtration
Europe was slow sand. Since rapid filtration was an cycle called the ‘‘steady state’’ phase. The duration
American invention, the filters were called ‘‘Ameri- of the chemical conditioning phase may be say
can filters.’’ 15 min to about 2 h and depends, most likely, on
Attachment: The bonding between a particle and a filter the initial value of the attachment coefficient, a.As
grain (which may be called a ‘‘collector,’’ depending floc particles attach to the media, they will serve as
on the context). The bonding is due to surface forces collectors, presumably with higher a than with the
principally van der Waals attractive forces after bare media.
reduction in electrostatic repulsive forces. Chemical Clogging front: A term used by Adin and Rebhun (1977)
factors affect the amount of reduction of the electro- as a designation for the s(Z)t profile. The wave
static repulsive forces. front may be a measurable surrogate for the clogging
Attachment coefficient: The ratio of particles striking a filter front. Another surrogate that indicates the approxi-
grain to those that attach. mate end of the clogging front is the headloss
Backwash: The reverse flow through a filter, that is, through versus Z profile at a given time; the intersection
the under-drain system, upward through the media, of the curve with the clean-bed headloss versus
with waste floc removed by overflow launders. The Z curve, a straight line, indicates the end of the
bed of granular media is ‘‘fluidized’’ by the back- clogging front.
wash. Coagulant aid: The connotation is that a polymer is used in
Bed expansion: The bed expansion is usually a percentage addition to the metal coagulant. The intent is that
referenced to the ‘‘at rest’’ bed, that is, bed microflocs may bind to sites on a polymer which
expansion ¼ (h h o )=h o . In theory, the expanded may occur partially during coagulation.
bed porosity is also used as a parameter of bed Collector: A granular media surface that exhibits surface
expansion. The relationship is (h h o )=h o ¼ forces that may cause particle attachment. A single
(e e o )=(1 e), in which h o and e o refer to bed grain of granular media, for example, sand or anthra-
depth and porosity before expansion and h and e cite or garnet, is called a ‘‘collector.’’ As the term
refer to bed depth and porosity after expansion. implies, the particles-to-be-removed ‘‘attach’’ to col-
Blinding: A deep bed filter that retains suspended particles by lector surfaces and are removed.
straining may cause an exponential increase in head- Collector: May refer to a porous medium grain that provides
loss, which is then said to be blinded. a surface for particle adhesion. The term may be used
Boucher’s law: The relationship, Dp ¼ k 1 exp(k 2 V), is in the context of a particular model of a porous
Boucher’s law. It was discovered in observing medium. Some choices in geometry include capillary