Enhanced double-skinned FO membranes with inner dense layer for wastewater treatment and macromolecule recycle using Sucrose as draw solute

Internal concentration polarization (ICP) that occurs in the membrane sublayer is considered a serious problem restricting the performance of forward osmosis (FO) processes. Aiming at reducing ICP and fouling propensity, novel nanofiltration (NF) hollow fiber membranes with two apparently dense skins have been designed from cellulose acetate (CA) for FO applications by manipulating different phase inversion rates and degrees of annealing at the inner and outer layers. For the CA hollow fibers precipitated rapidly and then annealed at the lumen side, the surface pores within the inner skin layer show a very narrow size distribution with a mean radius of 0.34nm. Being also relatively dense, the outer skin layer may keep the feed solutes from entering the sublayer and avoid their accumulation within the sublayer if the feed solutes are macromolecules or multi-valence ions with relatively larger sizes. Thus, the double-skinned FO membrane would have improved performance by suppressing ICP at a cost of additional external concentration polarization (ECP) at the outer surface. In the FO process, the CA hollow fiber membrane with inner selective layer generates a water flux of 17.1LMH (Lm ^-2h ^-1) with 2.0M MgCl ₂ draw solution running at the lumen side of the fibers and DI water feed at the shell side. When using 1.0M Sucrose (26.7bar osmotic pressure) as the draw solution at the shell side and DI water feed at the lumen side, a water flux of 12.9LMH is obtained with a negligible reverse Sucrose flux. This FO performance is comparable to that created by 1.0M MgCl ₂ draw solution although 1.0M MgCl ₂ has a much higher osmotic pressure of 93.7bar. With wastewater feed containing 200-2000mgL ^-1 mixed metal ions at the lumen side and 0.5M Sucrose draw solution at the shell side, water fluxes in the range of 9.9-6.5LMH with minimal reverse Sucrose fluxes are observed. These results have revealed great potential of the newly developed double-skinned CA hollow fiber membranes as well as using Sucrose as the draw solute for wastewater reclamation and macromolecule recycle.