Pressure assisted osmosis using nanofiltration membranes (PAO-NF): Towards higher efficiency osmotic processes

Abstract

The pressure assisted osmosis (PAO) process has been recently considered as a new strategy to enhance water flux and to limit reverse salt transport (RST), hereby overcoming two critical limitations of forward osmosis (FO) operation through the use of additional pressure on the feed side. With the aim to achieve higher (economically sustainable) fluxes, the use of commercially available nanofiltration (NF) membranes under PAO operating conditions is considered for the first time in this study. When operated under PAO mode, the tested commercial NF membranes clearly outperform the state of the art FO membrane with fluxes up to 36Lm−2h−1 and RST below 0.01gL−1, demonstrating the relative potential of PAO-NF. For flat sheet NF membrane osmotic contribution was minimal due to intense internal concentration polarisation. However, the tested hollow fiber NF membrane allowed for low internal concentration polarisation and exhibited significant flux even in FO operation. However, transmission of salts from feed to draw solutions (forward salt transport, FST) also occurred and was observed to be linearly proportional with the NF top layer rejection. Therefore, it is an important parameter to consider in the membrane selection in addition to the trade-off between large flux and low RST. The PAO-NF concept also proved to be more efficient in terms of water flux than direct NF operation, owing to a small but significant contribution of osmotic pressure. A modelling study demonstrated that further flux enhancement could be obtained through modification of NF membranes support layer to improve the osmotic contribution. These encouraging initial results warrant the need for a more detailed assessment of the long-term operation and energy requirement of the PAO-NF concept for a range of industrial applications.