Fouling behavior of microstructured hollow fibers in cross-flow filtrations: Critical flux determination and direct visual observation of particle deposition

The fouling behavior of microstructured hollow fiber membranes was investigated in cross-flow filtrations of colloidal silica and yeast. In addition to the as-fabricated microstructured fibers, twisted fibers made by twisting the microstructured fibers around their own axes were tested and compared to round fibers. In silica filtrations, the three different fibers showed similar behavior and increasing Reynolds number increased the critical fluxes significantly. In yeast filtrations, the twisted fiber performed similar to the round fiber and better than the structured fiber. Among the three fibers, during yeast filtrations the critical flux for irreversibility was highest for the twisted fiber. The Reynolds number had little effect on the critical fluxes for particle deposition, which was attributed to the strong adsorption of yeast particles on the membrane. On the other hand, the critical fluxes for irreversibility increased with increasing Reynolds number for all three fibers. Direct visual observation of yeast particles on the surface of the three different hollow fibers revealed that for the structured and twisted fibers, the initial deposition rate on the fins is much lower than that in the grooves. This is attributed to the shear-induced migration of the yeast particles from areas of high shear (fins) to those of low shear (grooves). Furthermore, on the fins of the twisted fiber the deposition rate was lower than that on the fins of the structured fiber. This observation, together with the observed high critical fluxes for the twisted fiber led to the conclusion that the twisting induces a secondary flow in the liquid. This secondary flow is effective in depolarizing the buildup of micron-sized yeast particles since the diffusion of these particles is strongly effected by gradients in shear rate. On the other hand, for the silica colloids which are much smaller, shear-induced diffusion is not significant and twisting does not have an improving effect on filtration. (C) 2011 Elsevier B.V. All rights reserved.