Page 81 - Membranes for Industrial Wastewater Recovery and Re-Use

Membrane technologg 61

Scalants and scaling indices
Scalants are low-solubility salts whose precipitation onto the membrane is
promoted by the conversion of water into permeate and further encouraged in
pressure-driven dense membrane processes both by concentration polarisation
and the pH shift produced by carbon dioxide permeation. The scale formed can
reduce the membrane permeability and permselectivity. As with colloidal and
particulate fouling, scaling is also a problem in membrane filtration processes.
Any water containing calcium carbonate close to or beyond its thermodynamic
saturation limit, as is the case for many dairy and pharmaceutical effluents, can
produce calcite (the most common crystal form of calcium carbonate) at the
membrane surface.
Scale formation propensity is usually apparent from chemical
thermodynamics, and specifically the solubility product K, (Table 2.14), although
it can never be unequivocally predicted. The solubility product represents the
maximum value of the product of the molar concentrations of the two component
ions of the salt. If the solubility is exceeded then the salt will precipitate. The
general rule of thumb to avoid precipitation is that the ionic product should
not exceed 80% of the solubility product. The appropriate constants for
thermodynamic equilibria appropriate to some of the more common scalants,
such as salts of the divalent alkaline earth elements of magnesium, calcium and
barium, are normally included in CAD packages for designing RO arrays (Section
4.1). The thermodynamic relationships include, in the case of calcium carbonate
formation, data pertaining to hydrolysis. The significance of this is outlined below.
Calcium carbonate is very insoluble in water and readily precipitates to form a
scale on pipework, heat transfer surfaces and membranes. The equilibrium
constant for the dissolution reaction is represented by:
K, = [ca2+][co:-] (2.30)

and so:
(2.31)

When carbon dioxide dissolves in water it forms carbonic acid, which
dissociates producing acid and bicarbonate ions thus:

C02(dissolved) + H20IH+ + HCO, (2.32)

This is the origin of the pH shift in reverse osmosis. Because the membrane allows
free passage of carbon dioxide, the C02/HC03- ratio in the permeate is high and
that of the retentate low. Since, according to Equation (2.32), the acid (i.e. H+)
concentration relates directly to the C02/HC03- ratio, the retentate pH is
correspondingly high whilst that of the permeate is low.
Bicarbonate ions further dissociate to carbonate:

(2.33)