Flux Increase Occurring When an Ultrafiltration Membrane Is Flipped from a Normal to an Inverted Position-Experiments and Theory

The effects of flipping membranes with hydrophilic/hydrophobic asymmetry are well documented in the literature, but not much is known on the impact of flipping a membrane with dense/porous layer asymmetry. In this work, the pure water flux (PWF) of a commercial polyethersulfone (PES) membrane and a ceramic ultrafiltration (UF) membrane was measured in the normal and inverted positions. Our experimental results showed that the PWF was two orders of magnitude higher when the PES membrane was flipped to the inverted position, while the increase was only two times for the ceramic membrane. The filtration experiments were also carried out using solutions of bovine serum albumin and poly(vinylpyrrolidone). A mathematical model was further developed to explain the PWF increase in the inverted position based on the Bernoulli's rule, considering a straight cylindrical pore of small radius connected to a pore of larger radius in series. It was found by simulation that a PWF increase was indeed possible when the solid ceramic membrane was flipped, maintaining its pore geometry. The flow from a layer with larger pore size to a layer with smaller pore size occurred in the backwashing of the fouled membrane and in forward and pressure-retarded osmosis when the membrane was used with its active layer facing the draw solution (AL-DS). Therefore, this work is of practical significance for the cases where the direction of the water flow is in the inverted position of the membrane.

Automated summary

The impact of flipping a membrane with dense/porous layer asymmetry on water flux was investigated in this study. The experimental results showed that when a membrane with hydrophilic/hydrophobic asymmetry was flipped, the water flux increased by two orders of magnitude, while for a ceramic membrane, the increase was only two times. A mathematical model based on Bernoulli's rule was developed to explain the increase in water flux in the inverted position. This work is of practical significance for cases where the direction of water flow is in the inverted position of the membrane.