Page 36 - Membranes for Industrial Wastewater Recovery and Re-Use

16 Membranes for Industrial Wastewater Recovery and Re-use

Table 2.1 Dense and porous membranes for water treatment (adapted from Stephenson et
aJ., 2000)
Dense Porous
Membrane separation processes
Reverse osmosis (RO) Ultrafiltration (UF)
Separation achieved by virtue of differing Separation by sieving through mesopores
solubility and diffusion rates of water (solvent) (2-50 nm)a.
and solutes in water.
Electrodialgsis (ED) Microfiltration (MF)
Separation achieved by virtue of differing ionic Separation of suspended solids from water by
size, charge and charge density of solute ions, sieving through macropores ( > 50 nm)a.
using ion-exchange membranes.
Pervaporation (PV) Gas transfer (GT)
Same mechanism as RO but with the (volatile) Gas transferred under a partial pressure gradient
solute partially vaporised in the membrane by into or out of water in molecular form.
partially vacuumating the permeate.
Nanofiltration (NF)
Formerly called leaky reverse osmosis. Separation achieved through combination ofcharge rejection,
solubility-diffusion and sieving through micropores ( < 2 nm).
Membrane materials
Limited to polymeric materials. Both polymeric and inorganic materials available.
a IUPAC (1985).

although they are unable to offer a significant barrier to dissolved gases and
certain low-molecular-weight organic molecules.
Membranes may also be categorised according to the material composition,
which is either organic (polymeric) or inorganic (ceramic or metallic), or on the
basis of their physical structure, i.e. their morphology. The membrane
morphology is dependent on the exact nature of the material and/or the way in
which it is processed. In general, however, membranes employed in pressure-
driven processes tend to be anisotropic: they have symmetry in a single direction,
and hence are often referred to as asymmetric, such that their pore size varies
with membrane depth (Fig. 2.2). This arises out of the requirement for a thin
permselective layer (or skin) to minimise the hydraulic resistance, the porous
support providing minimal resistance and acting purely to provide the necessary
mechanical strength. For integral (i.e. single, non-composite material)
asymmetric polymeric membranes, such as the one shown in Fig. 2.2a, the skin
is normally 2-5 pm in thickness.
Flat sheet reverse osmosis membranes have an additional ultrathin active
layer, less than 0.5 pm in thickness, attached to the anisotropic substrate to
produce a thin-film composite (TFC) membrane (Fig. 2.3). This ultrathin layer
provides the required permselectivity, rejecting all charged species and
permitting only the passage of water and small organic molecules. On the other
hand, ion exchange membranes, which are also dense by definition, are
essentially homogeneous. These membranes comprise a three-dimensional
array of fixed ionogenic sites (i.e. functional groups capable of dissociating to
form charged species) which facilitate transport of either cations or anions