Dual-stage forward osmosis/pressure retarded osmosis process for hypersaline solutions and fracking wastewater treatment

• Dual-stage FO/PRO is proposed for hypersaline water treatment and power generation.
• Two designs were suggested: PRO–FO and FO–PRO systems.
• PRO–FO system generates higher power than the FO–PRO system.
• Increasing draw solution flow rates increased the permeate flow rate and TDS.
• Treated hypersaline water is suitable for RO treatment or discharge to sea.

Hypersaline solution with high TDS is not suitable for direct treatment by the conventional membrane and thermal processes. The current study proposes a dual-stage FO/PRO process for hypersaline solution treatment and power generation. The treatment process reduces the concentration of saline wastewater and hence renders it suitable for disposal directly to sea or treatment by the conventional membrane and thermal processes. The draw and feed solutions in the FO process were the hypersaline solutions and wastewater effluent, respectively. Five concentrations were evaluated for the process treatment with different concentrations ranging from 53 g/L to 157 g/L. The performance of FO membrane was estimated using pre-developed computer software. The results showed that a higher power can be generated from the PRO-FO system than from the FO-PRO system without compromising the concentration of hypersaline solution after dilution. The study also showed that although increasing the flow rate of draw solution resulted in an increase in the permeate flow rate, it caused a reduction in the dilution of draw solution. On the other hand, the study showed a negligible improvement in the performance of FO membrane upon increasing the feed solution flow rate. Finally, the simulation results showed that the concentration of diluted draw solution was suitable for the conventional membrane and thermal treatments or discharge to seawater after the dual-stage FO membrane treatment.