Estimation of shear stress working on submerged vertically set hollow fibre membrane in MBRs

Abstract

This study is aimed at elucidating the mechanism by which rising air bubbles induce shear stress on the surface of hollow fibre membranes in submerged membrane bioreactors (MBRs). Shear stress working on vertically set hollow fibre membrane surfaces caused by aeration was measured directly using a two-direction load sensor connected to one hollow fibre at the centre of the ‘vegetation’ of the module and was found to be in a downward direction suggesting the existence of strong downward flow at the lower part of the module caused by eddies behind the pod. Velocity profile inside the membrane module was measured using a laser Doppler velocimeter showing slow upward direction flow inside the module which was caused by the influence of the hollow fibre vegetation. Just one hollow fibre was vertically set free in the bubble flow to measure shear stress on the fibre. A computational fluid dynamics (CFD) method was employed to numerically calculate the shear stress suggesting the effectiveness of CFD for the evaluation of shear stress on hollow fibre membrane.