Unstable filtration behavior with submerged hollow fiber membranes

Abstract

Freshly wetted, submerged hollow fiber polypropylene membranes have been observed to exhibit unstable behavior characterized by fluctuating suction pressure–time profiles on filtration of MilliQ water. Filtration results with single hollow fibers reveal that the observed suction pressure fluctuation could be caused by an unstable local resistance at some location inside the fiber lumen. The evidence from X-ray microimaging of the fiber lumen suggests that abnormal local flow conditions were induced by the presence of stagnant bubbles firmly attached to the internal wall of the hollow fiber membranes. The formation of these stagnant bubbles is attributed to some ‘dry’ points existing on the internal surface of the hollow fiber membrane. It appears that these ‘dry’ points cause significant and unstable local resistances for the permeate flow inside the fiber lumen. For a fixed average flux the high local resistance results in increased suction pressure in the lumen region downstream of the ‘dry point’ and this shifts an additional flux load to this region. The resultant maldistribution of local fluxes caused by abnormal local flow resistances can markedly affect the filtration behavior of the hollow fiber membrane. Interestingly the effect was not observed for all fibers and appears to require local ‘dry point’ existence within the lumen. The filtration of latex particle suspensions under different conditions showed that those fresh fibers with fluctuating suction pressure–time profiles also exhibited a sharp increase in the suction pressure and had limited run times for filtration. A pre-treatment protocol using pressurized water (PW) applied to the freshly ethanol-wetted fibers was found to result in marked improvement in the stability of the hollow fiber membranes tested.