Page 611 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

566 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

Concentrate flow: The flow that is retained by the mem- Foulant: A material, e.g., bacteria, organic matter, particles,
brane, that is, does not pass through the membrane that deposit on the surface of a membrane which
as permeate flow. causes a reduction in permeate flux.
Concentration polarization: Deposition of a layer of solute Fouling: An effect that causes a reduction in permeate flux.
on the membrane surface caused by the accumulation Gel polarization: See concentration polarization.
of salts due to the rate of advection of salts toward the Hollow fiber (HF): A type of membrane that is a dead-end
membrane surface exceeding the back-diffusion of tube with about 1 mm internal diameter and 2–3 mm
salts. outside diameter. Cheryan (1986, p. 87) mentions
Cross-flow: In a spiral-wound membrane, the feed flow with id 1.1 mm, length 635 mm, 660 fibers=tube,
velocity parallel to the membrane tube is called v(bore) 1.0 m=s. HF membranes been used since
‘‘cross-flow.’’ the early 1990s, mostly in MF. Two types of HF
Dalton: (1) A unit of mass for expressing masses of atoms, membranes are bore feed and shell feed. Bore feed
molecules, or nuclear particles equal to 1=12 of the has been used for UF (Cheryan, 1986, p. 64), with an
atomic mass of the most abundant carbon isotope, interior ‘‘rejecting skin’’ and with bundles of
6C 12 , which is about 1.66043 10 27 kg—called 50–3000 fibers, and they have been manufactured
also mass unit (http:==unabridged.merriam-webster. for RO applications (Hydranautics, 2003, http:==
com). (2) Also synonymous atomic mass unit, www.membranes.com). The shell-feed HF is
which is a mass unit exactly equal to 1=12 the mass through the pressure vessel, with the permeate pass-
of a carbon atom-12 atom (Silberberg, 1996, p. G-2). ing through the bore and out the end of the fiber
The term is used commonly in membrane vernacular where the flow from a single bore is joined with the
to give an idea of the sizes of molecules that may other flows, leaving the pressure vessel. Cleaning is
pass through the membrane, and perhaps an indica- by a hydraulic flush around the fibers to remove
tion of a pseudo-‘‘pore size.’’ There are several retained solids. To be consistent with the nomenclat-
measures of atomic size (Shriver and Atkins, 1999, ure for a spiral-wound membrane element, a bundle
p. 23), for example, atomic radii (metallic radius and is also called here an ‘‘element.’’
covalent radii) and ionic radius. To give an idea of Housing: See pressure vessel.
sizes, the atomic radius of carbon is given as 0.77 Å Hyperfiltration: A membrane with pore sizes mainly <5Å.
(p. 24), while for lead, it’s 1.75 Å. Sometimes, the term ‘‘hyperfiltration’’ is used
Depth filter: A filter in which removal of material occurs instead of ‘‘reverse osmosis’’ (RO) since it is con-
throughout its depth. sistent with the series: MF, UF, and NF.
Dialysis: The transfer of solute molecules across a membrane International Desalination Association (IDA): Inter-
by diffusion from a concentrated solution to a dilute national organization started in 1985 focused on
solution. desalination technology, science, and practice with
Electrodialysis: The transfer of ions across membranes by an headquarters in Topsfield, Massachusetts; the asso-
electric field (adapted from Helfferich, 1962, p. 397). ciation publishes the journal Desalination.
Element: The part of a membrane package as manufactured Intrinsic permeability: A characteristic of the inherent resist-
for the insertion into a pressure vessel (or open tank, ance of a membrane, k,defined in terms of Darcy’s
in the case of a vacuum system). The ‘‘element’’ may law that does not include viscosity (see Equation E.4,
be a single spiral-wound membrane, a ‘‘bundle’’ of K ¼ krg=m). Alternatively, terms in the Poiseuille
hollow fibers, a ceramic tube, etc. equation may be combined to give the same thing.
Equivalent: Molecular weight of an ion divided by its Mass flux: Flow of dissolved and=or suspended solids.
Membrane: (1) A thin soft pliable sheet or layer especially
þ þ
charge. For example, Eq(Na ) ¼ MW(Na )=1 ¼
of animal or vegetable origin. (2) A limiting proto-
þ 2þ
23.5 g=mol=1Eq=mol ¼ 23 g Na =Eq; Eq(Ca ) ¼
MW(Ca )=1 ¼ 40 g=mol=2Eq=mol ¼ 20 Ca g=Eq. plasmic surface or interface (http:==www.merriam-
2þ
2þ
Faraday: A Faraday, F ¼ 96,485 C ¼ 96,485 A-s. webster.com, 2003). (3) As used in this chapter, the
Feed water: The influent flow of water to a membrane reference is, in general, to synthetic membranes, for
element. example, CA, PS, polyamide, etc., manufactured for
Flow balance: A key principle of analysis is that flow-in and the purpose of separating contaminants from water
flow-out of a membrane pressure vessel must be by causing the water to be transported through
accounted for, that is, Q F ¼ Q P þ Q C , in which the membrane by means of an hydraulic gradient,
Q F ¼ feed flow, Q P ¼ permeate flow, and Q C ¼ con- that is, pressure differential, Dp. Contaminants are
3
centrate flow, also called ‘‘reject’’ flow (all in m =s). retained based on kind of membrane and associated
Figure 17.1a is a schematic depiction for a cross- pore size. For NF and RO, pore size may be a
flow membrane. misconception since the water passes through the
Flux: A flow of something, for example, water flux, salt molecular structure of the membrane (actually, the
flux. retaining layer, as distinguished from the support
Flux density: Flux per unit area. layer).