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Gas-Sparged Ultrafiltration: Recent Trends, Applications and Future Challenges 683
was observed when particle deposition was more severe (i.e. at high feed concentrations and
low liquid flow rates). This clearly indicated that the mechanism behind such enhancement
was the disruption of the fouling layer.
In a recent study, Cheng and Lin (54) reported the flux behavior of an inclined flat-sheet
membrane module of 100 kDa MWCO using dextran T500 as the test media. It was found
that an increase in cross-flow velocity increased the permeate flux and decreased the time
required to reach the steady state. Under the experimental operating conditions, examined
maximum flux enhancement was observed when the membrane inclination was 180 , i.e.
with a horizontally installed module with the feed below the membrane. At this angle the gas
slugs were the closest to the membrane surface and thereby disrupted the concentration
boundary layer more effectively. The effect of inclination was more pronounced when the
system was operated at a low liquid flow rate.
4.4. Mechanisms of Flux Enhancement
It is clear from the experimental results discussed in the previous sub-sections that flux
enhancement was more significant when the concentration polarization was more severe.
Generally, the gas bubbles disrupted the concentration polarization layer thereby enhancing
the permeate flux. However, the differences in the performance of gas sparging in tubular and
hollow fibre membranes suggest that the exact mechanisms could be somehow different (27).
Two main mechanisms for flux enhancement have been identified:
1. Bubble induced secondary flow – The secondary flow is generated by the buoyancy driven bubble
motion. The wake following bubble promotes local mixing in and near the mass transfer boundary
layer, thereby decreasing concentration polarization (27). This is similar to that observed in gassparged
enhancement of heat transfer (55–56).
2. Physical displacement of the concentration polarization layer – Taylor bubbles in tubular
membranes disrupt the concentration polarization layer by physically eroding them, leading to
high mass transfer rates. With hollow fibre membranes, the thickness of the liquid film between
the membrane wall and a gas slug is usually lower than the calculated boundary layer thickness.
In addition to these two mechanisms, other factors such as pressure pulsing caused by
passing gas slugs, increase in superficial cross-flow velocity and vibration (in the case of
hollow fibre membranes) are also responsible for flux enhancement (1).
5. GAS-SPARGING IN SUBMERGED MEMBRANE SYSTEMS
In aerobic MBRs gas sparging is utilized for biological oxidation and enhancement of
membrane filtration. The advantages of MBRs in municipal wastewater treatment include
excellent quality of treated water, small treatment plant footprint, reduced sludge production
and better process reliability. Submerged MBRs are operated in the suction mode and hence
the need for a pressurized module is eliminated. However control of fouling is challenging
since the biomass concentration in the feed is high. The nature and extent of fouling
are strongly influenced by three factors: biomass characteristics, operating conditions and