Impact of hydraulic pressure on membrane deformation and trace organic contaminants rejection in pressure assisted osmosis (PAO)

• Novel commercial TFC FO membranes allowed for high rejection of TrOCs.
• Novel commercial FO membranes have improved steric hindrance to TrOCs transport.
• Hydraulic pressure impacts active and support layer of high permeability TFC FO membrane.
• Water flux was improved with TFC FO membrane in FO and PAO operation.
• Hydraulic pressure decreased TrOCs rejection due to membrane deformation.

This study provides for the first time an extensive comparison of trace organic contaminants (TrOCs) rejection by commercial cellulose tri-acetate (CTA) and thin film composite (TFC) forward osmosis (FO) membranes from HTI and Porifera operated under pressure assisted osmosis (PAO) conditions. Commercial TFC membranes allowed for higher water permeabilities, higher selectivities and higher water fluxes in FO and PAO operation, compared to the HTI CTA benchmark. As for HTI CTA, TFC membranes suffered from deformation due to the hydraulic pressure applied in the PAO process. However, not only deformation by stretching of the active layer, but also compaction of the support layer was observed, reducing internal concentration polarisation (ICP) and allowing for flux enhancement. In FO operation, the TFC membranes demonstrated a high rejection (>80% for HTI TFC and >90% for Porifera) of the whole range of tested TrOCs due to steric hindrance. It was also noticed that, being more negatively charged, the TFC membranes allowed for very high rejection of negatively charged compounds, but lower rejection of positively charged molecules, as a consequence of electrostatic interactions. In PAO operation, a general decrease of TrOCs rejection was observed. This could possibly be a consequence of decreasing selectivity (due to membrane deformation), increased TrOCs external concentration polarisation and/or lower reverse salt diffusion (less hindrance of forward TrOCs diffusion).