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Gas-Sparged Ultrafiltration: Recent Trends, Applications and Future Challenges 691
7. PRACTICAL ISSUES AND FUTURE CHALLENGES
The use of gas sparging in membrane processes for controlling concentration polarization
and subsequent fouling in order to enhance permeate flux is now widespread. However, there
are some practical issues and challenges which remain unaddressed and these constrain wider
usage. One major concern is the tendency of foaming in gas sparged processes. Excessive
foaming and bursting of bubbles can damage proteins and in some cases cells during bioseparation.
Foaming and bubble damage are closely related. Clarkson et al. (89–90) studied
protein denaturation during foaming using a bubble column. They found that damage to the
protein is mainly due to surface denaturation at the gas–liquid interface. The degree of
denaturation was found to correlate directly with the interfacial exposure area. Shear stress
does not contribute significantly to the degree of damage and neither does oxidation. When the
gas flowrate is low, the foaming is less and the damage due to it is also less significant.
However, with high gas flowrates, foaming can cause severe problems (90). In a related work,
Clarkson et al. (91) investigated the conditions under which protein damage due to foaming
can be reduced. They concluded that protein damage could be reduced by operating at optimal
ionic strength and pH, and to a lesser extent, by the addition of sugars. There are some other
possible mechanisms of bubble-induced damage to proteins such as high shear rate resulting
from bursting bubbles and aggregation due to the change of protein structure (1).
Gas distribution is another challenge in gas-sparged UF. It has to be ensured that injected
bubbles influence the entire membrane surface area in a module. Most conventional membrane
modules are therefore not suitable for gas-sparged UF and special modules need to be
designed. Gas sparging is also difficult to implement in multi-pass operations and cannot be
used in spiral wound modules.
Gas-sparged UF in MBRs for the treatment of municipal as well as industrial wastewater is
the most promising application of this technique. Gas sparging (with or without backwashing)
has been successful in controlling concentration polarization and cake deposition. However,
there are some important issues that need more attention viz. optimization of the energy need
for bubbling (e.g. intermittent or cyclic, 73, 81), and optimization of bubble size and
frequency for a given application.
8. CONCLUSIONS
Gas sparging has been successfully applied to ultrafiltration to enhance its performance.
The bubbles are responsible for creating a secondary flow which enhances mass transfer and
hence the permeate flux. Bubbles also contribute towards fouling control by eroding the mass
transfer layer, physical scouring action, pressure pulsing and vibration. Gas sparging is found
to be effective even at modest gas flowrates. The flux enhancement is more significant at
lower liquid flowrates, higher solute/particle concentrations and higher operating pressures.
Most current applications of gas sparging are in the area of submerged MBRs for water and
municipal wastewater treatment. With controlled bubbling and better module design, the use
of gas sparging could be extended to food and biotechnology processes. In the case of MBRs,
cake deposition and fouling is minimized by gas sparging. Further improvement in efficiency