Experimental assessment of filtration of biomass with transverse and axial fibres

The orientation of hollow fibres (vertical or transversal) is of interest for the design of submerged hollow fibre modules. The purpose of this paper is to assess the effect of fibre orientation on performance for the filtration of biomass. A crossflow fibre cell was built to hold hollow fibres in well-controlled transverse or axial orientation. Two sizes of polypropylene hollow fibre, 0.65 and 2.7 mm (o.d.) were tested. A microscope and video-camera system was used to monitor particle deposition on the fibres. Filtration experiments with and without bubbling were carried out using yeast as model particles. The experiments showed that the effect of fibre orientation on the filtration process depends on the system conditions (with or without bubbling, fibre diameter, etc.). For filtration without bubbling, the transverse flow resulted in about 50% enhancement in filtration performance for 0.65 mm fibres but the benefits was insignificant for 2.7 mm fibres. The different filtration behaviour between 0.65 and 2.7 mm fibres in the filtration of biomass with transverse flow was attributed to the effect of wall shear around the transverse fibres, fibre movement, vortex formation, and inertial impaction. On the other hand, for filtration with bubbling, around 5-15% lower flux declines occurred with axial orientation for both 0.65 and 2.7 mm fibres, Comparing the results obtained with and without bubbling, the effect of fibre orientation on filtration with the smaller fibre decreased with injection of air. This implies that for filtration with bubbling the process tends to be dominated by the turbulence caused by the two-phase flow. However, axial bubble flow, rather than transverse does promise better performance in the membrane filtration of biomass. (C) 2002 Elsevier Science B.V. All rights reserved.