ABSTRACT
Pressure-driven membrane separation processes employ
membranes that act as selective filters to separate colloidal
particles or dissolved species based largely on differences
in size. Regardless of the material of construction, several
general features of membrane structure determine perfor-
mance. These membranes act as screen filters, like sieves,
that retain particles on their surface, vis-a`-vis, depth filters
typically used in dead-end filtration that trap particles
within a tortuous matrix of randomly-oriented fibers or
granules. Consequently, membranes can be assigned a
quantitative rating related to the pore size distribution on
the separation surface. The filtration spectrum is divided
into the processes known as microfiltration (MF), ultrafil-
tration (UF), nanofiltration (NF), and reverse osmosis
(RO) based principally on the size of the solute or particle
retained by the membrane. MF, UF, and NF are membrane-
based separations processes capable of rejecting particles
and dissolved molecules larger than 0.1 mm, 2 nm, and <2 nm, respectively. RO is a membrane-based process in
which an applied transmembrane pressure causes selective
transport of solvent against its osmotic pressure gradient.
The properties of a membrane that affect its retention and
flux characteristics include: chemical composition, pore
size, and the distribution of pore diameters, pore density
(number of pores per unit of membrane surface area),
porosity or void volume (fraction of the membrane not
occupied by the membrane material), and the structural
relationship between the active surface and the support
layers. Membranes are formed as either flat sheets, hollow
fibers, or coated tubes.
