Performance and implications of forward osmosis-membrane distillation hybrid system for simultaneous treatment of different real produced water streams

This study investigates the simultaneous treatment of different real produced water streams using forward osmosis-membrane distillation (FO-MD) hybrid system. The water–oil separator outlet (WO) was used simultaneously as FO draw solution (DS) and MD feed solution (FS). The FO process generated stable average fluxes of 13.6 L/m²/h and 15.8 L/m²/h with desalter effluent (DE) and wash water (WW) as FO feed streams, while MD produced 13.2 L/m²/h and 11 L/m²/h, respectively. Monovalent ions induced internal concentration polarization and CaSiO₃ colloidal scaling on the support layer reduced the FO flux. Further FO flux reduction was caused by CaSO₄ and NaCl crystals on the active layer. Moreover, CaSO₄ created partial pore covering while oil and grease depicted pore clogging of the MD membrane, which decreased the MD flux. Volatile fatty acids and organic nano pollutants reached MD permeate and FO FS. Humic acid scaling was fully recovered by ethylenediaminetetraacetic acid by masking calcium ions and reducing the pH. No membrane wetting was observed in the system. No bacteria were found in real streams as analyzed with Epifluorescence microscopy, eliminating the potential of microbial fouling. The hybrid system showed > 93 % removal of oil, inorganics and organics, making the permeate quality excellent for re-injection or industrial reuse. The FS streams were concentrated by 77–84 % resulting in smaller disposal volumes. No utilization of chemicals and water makes this hybrid concept practical and sustainable. The findings of this study can serve as design criteria for future onsite pilot plant applications.