Analysis of enhancing water flux and reducing reverse solute flux in pressure assisted forward osmosis process

Pressure assisted osmosis (PAO) has been recently suggested as a way to overcome the current limitations of forward osmosis (FO), since water flux can be increased by additional hydraulic driving force. To validate its feasibility more fundamentally, the effect of hydraulic pressure combined with osmotically driven FO process was evaluated experimentally, and compared with theoretical modeling. Four different FO and NF membranes were selected and their performance characteristics were determined by both RO- and FO-based methods, since PAO is a simultaneous osmotic- and pressure-driven membrane process. The degree of enhancing water flux and reducing reverse solute flux (RSF) in the PAO process clearly differed according to the membrane type and their performance parameters. Modeling PAO performance, using the values of A, B, and S determined from both RO- and FO-based methods, often failed to exactly match experimental observations, particularly for thin film composite (TFC) FO membranes, suggesting that PAO membrane performance parameters are apparently pressure-dependent. Discernible compaction in the support layer of TFC FO membranes was identified and confirmed by FO tests at different operating modes and SEM analysis, partially explaining large variations in S values under pressurized PAO operation and thus resulting large deviation from theoretical predictions.