Continuous and discontinuous pressure assisted osmosis (PAO)

Forward osmosis (FO) processes, due to internal concentration polarisation, are limited with regards to flux. Improved flux performance will allow FO to compete with fluxes achieved by hydraulically driven membrane processes. Pressure assisted osmosis (PAO) is proposed to enhance FO performance, by adding hydraulic pressure (0.1-0.8 bar) on the feed side. An FO mass transport model (active layer to feed side orientation) incorporating pressure was developed to describe the fluxes in PAO. Continuous and discontinuous PAO operations on laboratory scale were proposed and evaluated using draw solutions equivalent to 24 bar osmotic pressure. The fluxes increased with increasing hydraulic feed pressures for all PAO experiments, including activated sludge feeds, owing to the increased driving force and membrane deformation. Discontinuous PAO was found to have an adverse effect on the salt fluxes, due to the subsequent pressure release. This study emphasizes the benefits of PAO for use in innovative membrane systems, while illustrating the importance of developing more rigid membranes and better support designs.