Improvements to the performance of membrane systems by applying collapsible-tube-induced pulsatile flow

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Copyright: Wang, Wanxin
The major drawback of crossflow membrane filtration is that permeate flux declines with time as a result of the increase in total membrane resistance. Pulsatile flow is well known to reduce the resistance and enhance permeate flux. This study applied pulsatile flow induced by the oscillation of a collapsible tube to microfiltration and ultrafiltration, to improve filtration performance expressed as permeate flux enhancement and backflushable resistance reduction. Three membranes (ceramic tubular microfiltration, PVDF spiral-wound microfiltration and PS hollow-fibre ultrafiltration) and two media (bentonite suspension and whey solution) were used. In bentonite pulsatile microfiltration with the tubular membrane, up to 300% of permeate flux enhancement and 90% of backflushable resistance reduction were achieved. In bentonite and whey pulsatile microfiltration with the spiral-wound membrane, moderate improvements were gained: for bentonite, the highest increase in permeate flux was 51% and decrease in backflushable resistance was 45%; for whey, the highest permeate flux enhancement and backflushable resistance reduction were 36% and 38% respectively. In ultrafiltration of both media, no significant performance improvement was found. This is thought due in the one case to the relatively minute membrane pore size, and in the other to the large irreversible resistance created by whey solution. Transmural pressure at the collapsible tube downstream end indicates the tube compression and influences the pulsation vigour. Increasing the transmural pressure was an effective way to improve filtration performance. In bentonite microfiltration with the tubular membrane, increasing crossflow velocity was also effective, but increasing transmembrane pressure was not. Analysis of pulsatility parameters showed that the pulsatile flow always resulted in enhanced wall shear, and induced pore backflush always in the tubular membrane and sometimes in the HF membrane. These mechanistic findings helped to understand the filtration performance improvements. The analysis of energy consumption in bentonite microfiltration with the tubular membrane clearly demonstrated the benefit of applying the collapsible-tube-induced pulsatile flow in energy utilisation. The system specific energy could be reduced more than 70 % relative to the equivalent steady microfiltration permeate flux. For a given specific energy, the permeate flux could be increased by a factor of nearly four.
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Wang, Wanxin
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PhD Doctorate
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