Environmental Engineering Reference
In-Depth Information
Cake formation (
N
F
>
N
Fc
)
Concentration polarization layer
y
Cake layer
Particle
suspension
x
Membrane
Permeate flux
FIGURE 27.10
Same membrane as in Figure 27.9 but igure depicts the process of membrane fouling that is initiated by con-
centration polarization. The cake layer is a dense particle layer adjacent to the membrane surface. This layer can
irreversibly foul the membrane as the polarization layer remains over the membrane surface. (From Chen, J.C.,
Q. Li, and M. Elimelech,
Advances in Colloid and Interface Science
, 107, 83, 2004.)
As the cake or scale layer continues to build, it can become permanently attached to the
membrane surface, causing irreversible fouling that diminishes the lux through the mem-
brane, which cannot be recovered even after the membrane is cleaned. Cake or gel layer
formation occurs at a critical pressure and solute or particles concentrate at the membrane
surface [32]. Concentration polarization initiates gel layer formation; however, if the par-
ticles are very small (generally ions or non-fouling macromolecules), the polarization layer
behaves as if there is an additional or virtual membrane at the physical membrane surface,
as depicted schematically in Figure 27.11. Since fouling and precipitation are generally a
function of concentration, the formation of an area of increased concentration next to the
membrane surface is undesirable. Hence, many methods of reducing concentration polar-
ization have been developed. Current methods of concentration polarization mitigation
can be classiied into three broad categories: (i) mechanical, (ii) chemical, and (iii) electrical.
Mechanical methods of polarization reduction include any method that agitates the luid
surrounding the membrane, including but not limited to physical mixing, vibration, and
low pulsing. Chemical methods include chemical surface modiication of the membrane
or solution to be separated. Electrical methods include applying an electrical, magnetic, or
electromagnetic ield on or near the membrane to increase lux by mitigating concentration
-
+
-
+
+
-
-
-
+
+
+
-
Unimpeded
membrane
After concentration
polarization onset
FIGURE 27.11
Schematic depicting the effect of concentration polarization. After the onset of concentration polarization, the
system at hand behaves as if there were additional membranes added. Consequently, the easiest conceptual
framework for visualizing concentration polarization is to recognize the presence of virtual membranes.
