Toward a combined system of forward osmosis and reverse osmosis for seawater desalination

Forward osmosis (FO) is an osmotic process that uses a semi-permeable membrane to effect separation of water from dissolved solutes by an osmotic pressure gradient. Unlike reverse osmosis (RO), FO does not require high pressure for separation, allowing low energy consumption to produce water. However, the internal concentration polarization in FO is an important factor affecting the performance of FO processes. This paper was intended to investigate the characteristics of FO and RO processes. A simple film theory model was applied to consider concentration polarization in FO and RO processes. This model allows the estimation of internal and external concentration polarization effects in FO process. A laboratory-scale FO device was used to find the model parameters for further calculations. The calculated flux was compared with experimental flux under a variety of operating conditions. It was found that the combination of FO and RO may result in a higher flux than FO-only process under some operating conditions. Further research will be required to investigate the effect of membrane materials on energy efficiency of FO and RO hybrid system.

In this work, the characteristics of FO and RO processes were investigated using a simple film theory model and laboratory scale tests. The following conclusions can be drawn from this work: (1) The simple film theory mode can match the FO experimental data well. The permeate flux of RO membrane in FO mode was less than 3 L/m2-hr under 3 M of draw solutions due to internal concentration polarization. (2) As the salt concentration of feed water increases, the permeate flux decreases even at high draw solution concentrations. Increasing crossflow velocity may result in a higher flux by decreasing concentration polarization. (3) Combination of FO and RO allows an increase in permeate flux under some operating conditions. Further work will be required to optimize the design and operation conditions of FO+RO systems.