Page 23 - ro membanes

6 1. INTRODUCTION TO SALINE WATER PRETREATMENT
that the rate of convective flow of water is typically higher than that of the diffusion of flow of salts, the salts rejected by the membrane tend to accumulate in the boundary layer with highest salt concentration occurring at the inner surface of the membrane (shown as Cs on Fig. 1.3). Besides salts, for the same reasons, the boundary layer also accumulates particulate solids.
This phenomenon of concentration of salts and solids in the boundary layer has four sig- nificant negative impacts on membrane performance:
1. It increases osmotic pressure at the membrane surface;
2. Increases salt passage through the membranes;
3. Creates hydraulic resistance of water flow through the membranes; and
4. Induces potential for accelerated scale formation and particulate fouling on the mem-
brane surface because of the salt and solids concentration in the boundary layer.
The ratio between the solute (salt) content at the surface of the membrane (Cs) and in the
bulk feed water (Cb) is referred to as concentration polarization factor and is denoted as Beta (b), that is:
b 1⁄4 Cs=Cb (1.1)
The Beta value is always higher than 1, even when new RO elements are installed and the RO system is well designed and operated. The higher the Beta value, the greater the concen- tration difference, and the worst the negative impacts of this difference on membrane perfor- mance. Under best-case scenario the Beta could be in a range of 1.1e1.2, while in the worst case, the Beta value could exceed 2.0.
Eq. (1.2) indicates the impact concentration polarization has on RO-membrane flux (e.g., the freshwater production per unit membrane surface):
J 1⁄4 A1⁄2Fp ðbOp þPp þ0:5PdÞ (1.2)
where J is the membrane permeate flux; A is the membrane water permeability coefficient, which is unique for each type of commercial RO membrane; Fp is the feed pressure applied to the RO membranes; Qp is the osmotic pressure of the saline water; Pp is the permeate pres- sure (also known as permeate backpressure); and Pd is the pressure drop (e.g., the difference in pressures) between the feed and concentrate sides of the RO membranes.
As seen from Eq. (1.2), the RO-membrane freshwater production (flux) decreases as con- centration polarization increases. This formula indicates that in practical terms, the osmotic pressure that will need to be overcome by the RO-system feed pumps to produce the same volume of water will be increased proportionally to the concentration polarization factor.
In full-scale RO-desalination systems, A is constant specific to the RO membranes incorpo- rated in the system; Qp is also constant for a given source water salinity and target permeate salinity; Pp is constant determined by the target product water pressure; and Pd is of a preset value determined by the fouling propensity of the saline source water. Therefore, the only variable parameter the RO plant operator can adjust to maintain the same production flow as concentration polarization increases is the RO-system feed pressure (Fp). Such adjustment, however, is limited in practice by the maximum design delivery pressure of the RO feed