Desalination 197 (2006) 1–8 0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserved *Corresponding author. Internal concentration polarization in forward osmosis: role of membrane orientation Gordon T. Gray, Jeffrey R. McCutcheon, Menachem Elimelech* Department of Chemical Engineering, Environmental Engineering Program, Yale University, P.O. Box 208286, New Haven, Connecticut 06520-8286, USA Tel. +1 (203) 432-2789; Fax +1 (203) 432-2881; email: menachem.elimelech@yale.edu Received 27 December 2005; accepted 10 February 2006 Abstract The mechanisms governing internal concentration polarization (ICP) were studied using well-controlled forward osmosis experiments. The relationship between osmotic pressure and water flux was observed across a range of solute concentrations and molecular weights. The effect of membrane orientation on ICP was also studied. Two regimes of ICP — dilutive and concentrative — were described and characterized, and their governing equations were tested. Resistances to solute diffusion within the membrane porous support layer were calculated under each regime and found to be consistent across a wide variety of experimental parameters. Keywords: Forward osmosis; Direct osmosis; Internal concentration polarization; Osmosis; Desalination; Draw solution 1. Introduction While pressure-driven membrane processes such as reverse osmosis (RO) have dominated for several decades, new processes are now emerging that are driven by forward osmosis (FO). Several nascent applications based on FO could be of strategic importance in the coming decades. Fore- most among these is a novel process for saline water desalination, which may outperform reverse osmosis (RO) both economically and environ- mentally [1,2]. Additionally, pressure-retarded osmosis (PRO), a derivative process of FO, has been proposed as a clean and renewable source of energy [3]. Forward osmosis is similar to RO in that water transports across a semi-permeable membrane that obstructs transport of the solute. But instead of the hydraulic pressure difference by which RO is driven, FO uses an osmotic pressure gradient to drive water transport through the membrane. The “draw” solution, which is on the permeate side of the membrane, has a significantly higher osmotic