Patent ID: 7566402

Claim:
A method for providing an elevated pressure by forward osmosis in a system using pressure retarded osmosis through at least one semi-permeable membrane, which is built up of several layers, where at least one part of the osmotic pressure is maintained in the system, comprising contacting a first water feed stream containing salt with a non-porous material as a diffusion skin of said at least one semi-permeable membrane; where at the same time a second water feed stream containing less salt is contacted with a porous material in a porous layer of said at least one semi-permeable membrane, where the porous layer has, when water wetted, a porosity φ, thickness x (m), and tortuosity τ related to one another as given by the expression x·τ=φ·S Equation (1) where S has a value of 0.0015 meter or lower, the porosity φ has an average value of more than 50%, the tortuosity τ is less than 2.5, a water permeability of the porous layer is greater than 1×10 −11 m/s/Pa, and a salt permeability of the porous layer is less than 3×10 −8 m/s, and whereby water from the second water feed stream is driven through the semi-permeable membrane by forward osmosis and creates an osmotic hydraulic elevated pressure on a permeate side of the membrane defined by the diffusion skin.