(287b) An Advanced Design of Membrane Structure for Wastewater Treatment Through Forward Osmosis

Jincai Su¹, Rui Chin Ong¹, Bradley J. Helmer², Jos S. de Wit², Tai-Shung Chung^1,*

¹Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576

²Eastman Chemical Company, P.O. Box 1972, Kingsport, Tennessee 37662

*Corresponding author, Email: chencts@nus.edu.sg, Fax: (65)-6779 1936

Key words: Forward osmosis; Hollow fiber; Double-skin; Wastewater treatment

  Abstracts

Forward osmosis (FO), utilizing the natural phenomenon of osmosis, is an emerging membrane process driven by the osmotic pressure gradient created across a membrane by two flowing streams of varying concentrations. Considered as an energy-effective technology, FO has attracted much attention in water-related R&D activities. The 1st key component of the FO process is the membrane material which should be semipermeable, i.e., allowing water to permeate through while blocking all the solutes in the draw and feed solutions. To maximize the water permeation flux, FO membranes are designed with asymmetric structure, i.e., an ultra thin dense layer on the top of a highly porous sublayer. With this structure, either running the draw solution or the feed along the porous sublayer would result in severe internal concentration polarization (ICP), which would greatly reduce the effective osmotic pressure gradient across the membrane. Nonetheless, running the draw solution along the porous sublayer would be more practical since the feed may contain foulants which would be easily stuck in porous matrix. Once the membrane sublayer is fouled, it is very difficult to clean. To date, how to effectively reduce the effect of ICP is still a big challenge.

We have designed and fabricated very interesting hollow fiber FO membranes with two apparently dense skins by manipulating the phase inversion rates during the spinning and the degrees of subsequent thermal annealing at the inner and outer layers. The surface pores within the inner skin layer show a very narrow size distribution with a mean radius of 0.34 nm. Being also relatively dense, the outer skin layer is able to keep certain solutes such as macromolecules or multi-valence ions from entering the sublayer and avoid their accumulation within the sublayer. The double-skinned hollow fiber membranes have been used for Sucrose concentration and treatment of wastewater containing mixed metal ions through a forward osmosis-nanofiltration (FO-NF) hybrid system (Fig. 1). The FO system draws water from the wastewater while the NF system is used to regenerate the diluted draw solution. The experimental results have revealed great potential of the newly developed double-skinned CA hollow fiber membranes for wastewater water reclamation and macromolecule recycle.

Fig. 1. The FO–NF hybrid system for wastewater reclamation