Submerged hollow fibre membrane module - design options and operational considerations

A new membrane module concept, the submerged hollow fibre membrane, has been widely accepted for the wastewater membrane bioreactor. This paper focuses on operational considerations and design aspect of the submerged hollow fibre membrane module. Experiments with both crossflow and 'dead-end' submerged systems have demonstrated that bubbling is effective in enhancing filtration performance, but the effect of gas flow rate is constrained because of the limited effect of bubbling on turbulence in two-phase flow. The experimental results also indicated that module configuration exerts crucial effects on the performance of the system. A better filtration performance can be obtained with vertical axial fibre orientation, small fibre diameters, and a loose fibre bundle. A model describing the filtration behavior of the submerged hollow fibre at steady state shows that when the maximum initial flux is lower than the critical flux, the flux distribution along the fibre can be estimated according to a dimensionless parameter x = 4LR(1)(-3/2)R(m)(-1/2) which includes fibre length and radius and hydraulic resistance. When the average imposed flux is lower than the critical flux but the maximum local initial flux is higher than the critical flux, a steady state can be achieved after an initial deposition over some of the fibre length. The filtration resistance caused by the initial deposition becomes significant when the fibre radius is smaller than 0.2 mm, particularly with long fibre lengths and high ratios of average imposed flux to critical flux. The simulation also suggested that the optimal combination of fibre radius and length goes to small radius and short length.