Air sparging induces shear forces at the membrane surface as rising air bubbles travel near the membrane. The shear forces increase the mass transfer in the concentration polarization layer on membrane surface, which facilitates particle transport away from the membrane surface.
Two mechanisms are predominantly responsible for the high surface shear forces in both immersed flat sheet and hollow fiber membranes:
|•|| Falling film effect – Localized high liquid velocities in a falling film could increase the mass transfer at a membrane surface by up to two orders of magnitude depending on the size of the air bubbles, when compared to non-air-sparged conditions (Ghosh, 1999; Judd, 2001).
|•||Wake effect – Eddies that occur in the wake of a rising air bubble (Bérubé, 2006a, 2006b) can generate a short period of very high shear stress twice as large as that induced by the localized high liquid velocities in the falling film depending on the air bubble size.|
The shear stress in local membrane area can be measured using electrochemical shear probe as described in Cabassud (2003) and Bérubé (2006a, 2006b). Fig. 1 shows an experimentally measured shear stress profile using electrochemical shear probes as a function of bubble position on membrane surface, where the shear stress is proportional to the voltage reading from the sensor. When a rising ellipsoidal bubble is approaching the sensor marked as a white dotted line, initially stable shear signal starts to rise (frame 1, 2). When bubble passes the sensor (frame 3), shear stress starts to rise sharply due to the falling film effect. Right after the bubble passed the sensor (frame 4, 5, 6), shear stress reaches its maximum due to the wake of the rising bubble. The patterns of shear stress profile are variable depending on the bubble shape, but the general trend remains same (Chan, 2007).
In immersed flat sheet membrane, falling film effect appears one of the major mechanisms of anti-fouling since all the bubbles generated must pass through the narrow channels in between two panels. However, in hollow fiber membrane, bubbles tend not to rise along the fiber, but rise through the spaces among fiber bundles. Therefore, it is not certain that how much falling film effect plays a role in hollow fiber membranes. Falling film and wake effects are important mainly for flat sheet membranes, short hollow fiber membranes, and tightly held hollow fiber membranes that do not move laterally.
Fig. 1. Typical shear profile and images of an ellipsoidal bubble rising in tightly held fibers. The translucent vertical streaks seen in the pictures are the eight fibers in the bundles. The white horizontal dashed line across the six images show the position of the shear probe on the test fiber. Gas sparging rate= 0.5 ml/min, Diffuser nozzle size=2mm (Chan, 2007).
It has been known that not only the magnitude of upflow velocity (or shear stress) on membrane surface, but the time derivative of the upflow velocity (or shear stress) is important as well (Yeo, 2006; Chan, 2011). Here time derivative represents how much the water velocity tends to vary over time. As shown in Fig. 2, when 9 hollow fibers were used to filter 1 g/L bentonite, final TMP that indicated the extent of membrane fouling decreased as standard deviation of flow velocity increased. This result suggests that giving a variation in air flow rate while maintaining a constant overall air flow is an effective approach to enhance membrane scouring, which is in-line with the intermittent aeration employed by processes with hollow fiber membranes.
Fig. 2. Effect of standard deviation of upflow velocity on final TMP. Particle image velocimetry was used to monitor flow velocity around hollow fiber membranes (Yeo, 2006)
Specifically for hollow fiber membranes, three additional mechanisms in the following are responsible for the high shear forces on membrane surface.
|•|| Lateral liquid migration facilitates mass transfer between the internal and the external spaces of a hollow fiber membrane bundle (Fig. 3). The concentrated particles in the internal spaces due to the loss of permeate through the membranes are transported outside by the lateral liquid migration (Cui, 2003).
|•||Random fiber displacement (or sway) perhaps plays a major role in hollow fiber membranes. Once a part of hollow fiber is pulled suddenly by random turbulent motion of liquid, rest of the fiber is also pulled. As a consequence, the longer the fiber is, the higher the chances of the fiber being pulled.
|•||Physical contacts among fibers can also generate high shear forces on membrane surface. The magnitude of the surface shear forces resulting from lateral velocity and physical contacts are dependent upon a number of complex parameters relating to the configuration of the submerged membrane system (e.g. fiber length, looseness, density, etc.) and the air sparging practices (Bérubé, 2006b).|
Fig. 3. Mechanism of cake depolarization in hollow fiber membrane (Cui, 2003)
© Seong Hoon Yoon