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Effect of bubble size and sparging frequency on the power transferred onto membranes for fouling control Jankhah, Sepideh


Fouling control through air sparging in membrane systems is governed by the hydrodynamic conditions in the system and the resulting shear stress induced onto membranes. However, the relationship between hydrodynamic conditions and the extent of fouling control is not well understood. As a result, sparging approaches are designed using a capital and time intensive empirical trial-and-error approach that does not guarantee that optimal conditions are identified. To address this knowledge gap, the present research focused on characterizing the hydrodynamic conditions in a membrane system under different sparging conditions (bubble size and frequency) and on finding a correlation between the induced hydrodynamic conditions and fouling control efficiency. New concepts of zone of influence of bubbles and power transferred were defined to characterise the hydrodynamic conditions in the system. A non-homogenous fouling distribution was observed in the zone of influence of bubbles due to a non-homogenous distribution of velocity and shear stress in this zone. Fouling rates generally decreased with an increase in the area of the zone of influence, the root mean square of shear stress induced onto membranes and the rise velocity of bubbles. However, none of these parameters on their own could accurately describe the effect of the hydrodynamic conditions on fouling rate. On the other hand, power transferred onto fibers, which incorporates the effect of all the three parameters, could more effectively describe the effect of the hydrodynamic conditions on the rate of fouling. Power transfer efficiency into the system, defined as the ratio of power transferred onto membranes to the power input in the system, was used to identify optimal sparging approaches. For all cases investigated, the power transfer efficiency to the system was consistently much higher for pulse bubble than for coarse bubble sparging. The results also indicated that as sparging frequency and the size of the bubbles increased, the width of zone of influence increased, suggesting that the spacing between the spargers could be increased when sparging with larger bubbles or at higher frequencies. Increasing the spacing would not only decrease the number of spargers, but also the volume of the gas required for sparging.

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