Page 57 - Membranes for Industrial Wastewater Recovery and Re-Use

Membrane technology 3 7

desirable to suppress CP by promoting turbulence and/or operating at a flux
below that at which CP starts to become significant.
CP effects on specific processes are summarised in Table 2.6. All membrane
processes are subject to CP, but it is only in specific cases where certain CP
phenomena become significant. The importance of elevation of osmotic pressure
depends upon the concentration of the rejected solute in the feedwater, since
osmotic pressure is directly related to ion concentration by the van’t Hoff
equation (Section 2.3.9). Thus for nanofiltration processes, where only part of
the feedwater ion content is affected by CP, the osmotic pressure elevation effects
are commensurately smaller than for RO which rejects ions almost
quantitatively. CP also increases the permeation of the rejected materials
through the membrane because of the increase in the transmembrane
concentration gradient generated. This can affect the permeate water purity,
although the effect is minor. The increase in concentration from the bulk
solution to the membrane surface can also change the selectivity of the
membrane, particularly for nanofiltration, although again the effect is not
generally significant.
Scaling presents a more substantial limitation to operation under conditions
of CP. Both RO and NF are subject to scaling by divalent salts formed through the
CP of ions contributing to hardness. In ultrafiltration, precipitation of sparingly
soluble organic solutes produces a gel layer whose permeability and
permselectivity often differs from that of the membrane on which it sits. This gel,
or dynamic, layer then determines the process performance with respect to both
the hydraulics and the product water quality.
In the case of electrodialysis, the effect of polarisation in the liquid film is to
deplete ions in that region as ions are extracted through the membrane faster
than they arrive at the interface from the bulk solution (Fig. 2.18). Depletion of
the permeating ion has two principal effects:

0 the electrical resistance increases, and
0 the concentration of the permeating ion at the membrane surface of the
depletion side decreases to a level that approaches that of the innate water
dissociation products (hydroxide and hydrogen ions).

Table 2.6 Concentration polarisation effects
Process Osmotic Electrical Scaling Gel layer Selectivity
pressure resistance formation change
elevation elevation

a More marginal effect.
Depletion polarisation.