Page 35 - Membranes for Industrial Wastewater Recovery and Re-Use

Mfmhmne technologg 15

0 have reasonable mechanical strength,
0 maintain a high throughput, and
0 be selective for the desired permeate constituent.

These last two parameters are mutually counteractive, since a high degree of
selectivity is normally only achievable using a membrane having small pores
and thus an inherently high hydraulic resistance (or low permeability). The
permeability also increases with increasing density of pores, and the overall
membrane resistance is directly proportional to its thickness (in accordance with
Darcy’s law). Finally, selectivity will be compromised by a broad pore size
distribution. An optimum physical structure for any membrane material is thus:

0 a thin layer of material,
0 a narrow range of pore size, and
0 a high porosity.

Membrane materials can be categorised as either dense or porous, and by the
mechanism by which separation is actually achieved (Table 2.1). Separation by
dense membranes relies to some extent on physicochemical interactions between
the permeating components and the membrane material, and relate to
separation processes having the highest selectivity (Fig. 2.1 ). Porous
membranes, on the other hand, achieve separation mechanically by size
exclusion (i.e. sieving), where the rejected material may be either dissolved or
suspended depending on its size relative to that of the pore. Since some
membranes exhibit properties that can be associated with more than one process
type, the boundaries between the adjacent membrane processes in Fig. 2.1 are
somewhat nebulous. For example, IUPAC (1985) state that the upper and lower
boundary limits for mesopores, as are characteristic of a UF membrane, are 2 and
50 nm. According to Kesting (1989), howcvcr, these boundaries are at 1 and 20
nm. respectively.
Whilst microfiltration membranes are assigned a characteristic pore size, the
exact value of which is dependent on the method of measurement, ultrafiltration
membranes are mostly rated on the basis of the size of the smallest molecule the
membrane can be expected to reject. This is routinely expressed as the molecular
weight cut-off (MWCO) in daltons (i.e. grams per mole). As the precise
relationship between MWCO and pore size is obviously dependent on the
physical and chemical nature of the solute molecule, precise cross-referencing is
impossible. The actual pore size of nanofiltration and reverse osmosis
membranes is of little practical consequence, since there are other mechanisms
more dominant than simple sieving that determine membrane performance. The
purification performance of these membranes can only be rated according to
their actual demonstrated permselectivity, i.e. the extent of the rejection of key
contaminants by the membrane, under some defined set of conditions.
Nanofiltration membranes, which have a charge rejection component, are
generally designed to be selective for multivalent rather than univalent ions.
Reverse osmosis membranes are designed to reject all species other than water,