Delamination and low water permeability are two issues limiting the applications of dual-layer hollow fiber membranes in the pressure retarded osmosis (PRO) process. In this work, we first developed a universal co-casting method that is able to co-cast highly viscous dope solutions to form homogeneous dual-layer flat sheet membranes. By employing this method prior to the tedious dual-layer hollow fiber spinning process, both time and material consumptions are significantly saved. The addition of polyvinylpyrrolidone (PVP) is found to eliminate delamination at the sacrifice of water flux. A new post-treatment method that involves flowing ammonium persulfate (APS) solution and DI water counter-currently is potentially to remove the PVP molecules entrapped in the substrate while keeps the integrity of the interface. As the APS concentration increases, the water flux in the PRO process is increased while the salt leakage is slightly decreased. With the optimized APS concentration of 5 wt%, the post-treated membrane shows a maximum power density of 5.10 W/m(2) at a hydraulic pressure of 15.0 bar when 1 M NaCl and 10 mM NaCl were used as the draw and feed solutions, respectively. To the extent of our knowledge, this is the best phase inversion dual layer hollow fiber membrane with an outer selective layer for osmotic power generation. (C) 2014 Elsevier B.V. All rights reserved.