Reverse Draw Solute Permeation in Forward Osmosis: Modeling and Experiments WILLIAM A. PHILLIP, JUI SHAN YONG, AND MENACHEM ELIMELECH* Department of Chemical Engineering, Environmental Engineering Program, P.O. Box 208286, Yale University, New Haven, Connecticut 06520-8286 Received March 20, 2010. Revised manuscript received May 19, 2010. Accepted May 21, 2010. Osmotically driven membrane processes are an emerging set of technologies that show promise in water and wastewater treatment, desalination, and power generation. The effective operation of these systems requires that the reverse flux of draw solute from the draw solution into the feed solution be minimized. A model was developed that describes the reverse permeation of draw solution across an asymmetric membrane in forward osmosis operation. Experiments were carried out to validate the model predictions with a highly soluble salt (NaCl) as a draw solution and a cellulose acetate membrane designed for forward osmosis. Using independently determined membrane transport coefficients, strong agreement between the model predictions and experimental results was observed. Further analysis shows that the reverse flux selectivity, the ratio of the forward water flux to the reverse solute flux, is a key parameter in the design of osmotically driven membrane processes. The model predictions and experiments demonstrate that this parameter is independent of the draw solution concentration and the structure of the membrane support layer. The value of the reverse flux selectivity is determined solely by the selectivity of the membrane active layer. Introduction Forward osmosis (FO) and pressure retarded osmosis (PRO) are two emerging technologies that fall under the classifica- tion of osmotically driven membrane processes (1, 2). These technologies take advantage of the osmotic pressure differ- ence that is generated when a semipermeable membrane separates two solutions of differing concentrations. By using the osmotic pressure difference to drive the permeation of water across the semipermeable membrane, osmotically driven membrane processes may be capable of addressing several of the shortcomings of hydraulically driven membrane processes, such as reverse osmosis (RO). Unlike RO, FO does not require a high applied hydraulic pressure, thereby decreasing capital and energy costs (3). Furthermore, recent investigations have demonstrated a lower fouling propensity with FO (1, 4-6), implying lower operating costs. Several studies have taken advantage of these benefits and demonstrated the use of osmotically driven membrane processes to desalinate seawater and brackish water (1, 7-9), treat wastewater (4, 10), and reclaim waste- water using an osmotic membrane bioreactor (11). A significant portion of the efforts to improve FO and PRO operations has focused on tailoring the membrane structure to decrease the effects of internal concentration polarization (ICP) (12-19) or on developing new draw solutions (20) that are capable of generating large osmotic pressures, but are still relatively easy to separate from water. Further developments in these areas are still needed for successful commercialization of these technologies. However, one area of research that has received limited attention, but could be a significant impediment to the viability of osmoti- cally driven membrane processes, is the reverse permeation of draw solute from the draw solution into the feed solution (11, 21). An ideal semipermeable membrane would prevent any dissolved draw solute from permeating into the feed solution. However, no membrane is a perfect barrier, and a small amount of dissolved solute will be transported across the membrane. If an expensive draw solute is used, the cost of replenishing the draw solute lost to the feed solution could make the process less economical. Alternatively, if the draw solute was detrimental to the aquatic environment, an additional treatment step of the feed solution concentrate would be required prior to discharge. Therefore, a thorough understanding of the phenomenon of reverse solute per- meation is critical to the effective development of osmotically driven membrane technologies. The objectives of this paper are (i) to formulate a model that describes the reverse permeation of a single draw solute across an asymmetric membrane in a forward osmosis operation, (ii) to validate the model through laboratory experiments with a well characterized forward osmosis membrane and draw solution, and (iii) to use the model to gain insights into the processes that govern draw solute permeation in forward osmosis. In our model, the draw solute reverse flux is described in terms of experimentally accessible quantities and common transport parameters. The implica- tions of our results for the future development of forward osmosis are further evaluated and discussed. Theory A schematic of an asymmetric membrane operating in FO mode (i.e., with the selective layer facing the feed solution) is shown in Figure 1. For the draw solute to leak into the feed solution, it must first diffuse through the support layer, where its diffusion is opposed by the convective flow of solvent, until it reaches the interface between the support layer and the active layer. Once there, the draw solute partitions into the active layer before diffusing across it. After diffusing across the active layer, the draw solute partitions into the feed solution, which has a negligible concentration of draw solute. This process can be described by considering the mass transfer through the support layer and then the active layer in series. Draw Solute Mass Balance in the Support Layer. For the support layer, a steady-state mass balance can be written on a differential volume where J s S is the total flux of draw solute, c is the solute concentration, D S is the solute diffusion coefficient in the support layer, and J w is the superficial fluid velocity, which is equivalent to the solvent permeate flux. This mass balance is, in principle, the same as that used to describe the * Corresponding author phone: (203)432-2789; e-mail: menachem. elimelech@yale.edu. dJ s S dz )-D S d 2 c dz 2 + J w dc dz ) 0 (1) Environ. Sci. Technol. 2010, 44, 5170–5176 5170 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 13, 2010 10.1021/es100901n  2010 American Chemical Society Published on Web 06/07/2010