Towards direct potable reuse with forward osmosis: Technical assessment of long-term process performance at the pilot scale

In this study we evaluated the performance of forward osmosis (FO) at the pilot scale to achieve simultaneous seawater desalination and wastewater reclamation. The investigation was performed with a commercial spiral wound FO membrane element for approximately 1300 h of continuous operation, processing 900,000 L of wastewater effluent and producing 10,000 L of purified water through a hybrid FO-RO process. Water and solute fluxes were monitored during the study. Reversible and irreversible membrane fouling was observed; however, water flux was maintained at a relatively constant rate of 5.7±0.2 L m−2 h−1 with MBR permeate feed and seawater draw solution. Subsequent increase of total suspended solids (TSS) concentration in the FO feed (secondary treated effluent with 5 to 16 mg L−1 TSS) resulted in incremental flux decline; however, the membrane typically achieved stable water flux after the initial exposure to foulants. Additional analysis focused on bi-directional transport of inorganic species and a detailed evaluation of dissolved organic matter permeation through the membranes in the hybrid process. Evaluation of sample fluorescence revealed that the FO membrane and the hybrid process provide a strong barrier to protein-like fluorophores associated with wastewater effluent. Results also demonstrated the robust nature of dual barrier membrane systems to achieve greater than 99.9% removal of orthophosphate and dissolved organic carbon and more than 96% rejection of nitrate. Upon completion of the study a comprehensive membrane autopsy was performed on the FO and RO membranes. Organic fouling on the FO membrane was observed to have low adhesion to the membrane surface, but did result in some modification to membrane active layer properties.