Page 39 - Membranes for Industrial Wastewater Recovery and Re-Use

Membrane technology 19

Table 2.2 Membrane materials by type
Membrane Manufacturing procedure Applications
Ceramic Pressing, sintering of fine powders MF, UF. Aggressive and/or highly
followed by sol-gel coating fouling media
Stretched polymers Stretching of partially crystalline foil MF. Aggressive media. sterile
filtration, medical technology
Track-etched polymers Radiation followed by acid etching MF (polycarbonate (PC) or
polyethylene terephthalate
(PET) materials). Analytlcal and
medical chemistry, sterile filtration
Supported liquid Formation of liquid film in inert Gas separations, carrier-mediated
polymer matrix transport
Integral asymmetric, Phase inversion MF. UP, NF, GT
microporous
Composite asymmetric, Application of thin film to integral NF. RO, PV
microporous asymmetric microporous membrane
to produce TFC
Ion exchange Functionalisation of polymer material ED

liquid separations usually comprise a high-viscosity, hydrophobic liquid
immobilised in a polymer matrix. The supported liquid may contain a carrier, a
component that reacts chemically and reversibly with the desired component in
the liquid mixture and thereby assists its transport through the membrane.
By far the most significant of the polymeric membranes applied to the water
industry are those produced by phase inversion (Table 2.3), a fabrication process
which produces an anisotropic material which may then be used as a substrate
for a TFC membrane, The ultrathin surface layer of a TFC RO or NF membrane is
invariably either a polyamide (usually aromatic) or some co-polymer blend
based on polyamide. The permselectivity and throughput of such membranes is
then critically dependent upon the precise polymers selected and the fabrication
method used. For example, Filmtec (part of the Dow Chemical Company) produce
both a reverse osmosis membrane (the FT-30) and a series of nanofiltration
membrane materials (NF45, NF70 and NF90) which are all based on aromatic
polyamide, yet the permeability and salt rejection capability of these membranes
varies considerably.
The phase inversion process involves dissolving the polymer in a suitable
solvent and then casting it in a film, less than 1 mm thick, and then adding
another liquid to precipitate the polymer. The membrane skin forms at the
interface between the solvent and the second liquid, in which the membrane is
only sparingly soluble, Careful choice of the solvent and non-solvent liquids,
concentration of the polymer, temperature and reaction times can produce the
desired physical membrane characteristics.
Most polymeric materials are resistant to moderate pH swings, the principal
exception being cellulose acetate, but most have limited resistance to organic
solvents. Only PTFE and PVDF can be considered highly stable in this regard. The
main limitation of the more robust polymeric materials, in terms of resistance to