To overcome the problems described above, Toshiba focused on the forward osmosis membrane method enabling concentration treatment at low temperature without application of pressure (Figure 1). The forward osmosis membrane method utilizes the phenomenon that water spontaneously diffuses across a forward osmosis membrane that separates aqueous solutions with different solute concentrations (diffusing from a low solute concentration solution to a high solute concentration solution) to keep the concentrations of both solutions equal. The energy consumption of the conventional reverse osmosis membrane method is approximately 4 kWh/m3, and this can be reduced to one-fourth by using the forward osmosis membrane method (1 kWh/m3). Toshiba independently developed draw solutes for the forward osmosis membrane method: dissolving of this material into a solution with a high solute concentration enhances spontaneous water transport, thereby achieving highly effective concentration and separation of water in an energy-saving manner. The draw solutes can be separated from water and used repeatedly by desorption of pre-absorbed carbon dioxide in the aqueous solution containing the draw solutes.
Toshiba tested this draw solutes using an aqueous solution containing salt (sodium chloride) as a model, and found that water moved to the aqueous solution containing the draw solutes, resulting in an increase in the salt concentration to 19% (*5), which was 2.4 times higher than the salt concentration achievable by the conventional reverse osmosis membrane method (8%: Figure 2 (a)).
When the aqueous solution containing the draw solutes, which had been diluted after movement of water molecules, was heated at 70 °C, desorption of carbon dioxide occurred and this led to the separation into an draw solutes layer and a water layer (Figure 2 (b)), demonstrating that this material can be regenerated and well separated from water at less than 100 °C. Exhaust heat can be used to separate the draw solutes from water. After desorption, the carbon dioxide generated can also be recycled to use in the preparation of the aqueous solution containing the draw solutes.
The forward osmosis membrane method developed achieves a higher concentration rate than the conventional reverse osmosis membrane method, as well as good separation from water. It can be used for a system that concentrates valuable materials to high concentration under normal temperature and pressure conditions and for a system that highly concentrates effluents for reuse of water.