High recovery rate NF–FO–RO hybrid system for inland brackish water treatment

• NF–FO–BWRO system is proposed for inland brackish water desalination.
• The system recovery rate achieved > 90% for a range of feed salinities.
• 80% of total power consumption was due to the BWRO membrane.
• NF and FO contributed to the rest 20% power consumption.
• BWRO system required about 3 times membrane area as FO and NF systems.

Brackish water desalination is a common method for fresh water supply in arid areas. Concentrated brine is the major waste stream generated from the desalination process. The current study proposes a multi-stage Nanofiltration (NF)–Forward Osmosis (FO)–Brackish Water Reverse Osmosis (BWRO) system to increase the recovery rate of brackish water. The simulation results showed that the NF–FO–BWRO system was able to achieve > 90% recovery rate for a number of feed salinities varied from 1 to 2.4 g/L. High permeability NF membrane was used in the first stage to produce the first permeate flow at relatively low power consumption. Concentrated brine from the NF was fed to an FO membrane for power for additional fresh water extraction before disposal. 0.25–0.5 M NaCl was used as the draw solution in the FO membrane. The results showed that system recovery rate increased with increasing the concentration of the draw solution. NF process was responsible of 75% of the total recovery rate while BWRO process contribution was up to 20%. Almost 80% of the total power consumption for desalination was due to the BWRO process. NF and FO processes accounted for the rest 20% power consumption. The BWRO system also required 2 to 3 times more membranes than FO and NF processes. NF–FO–BWRO is flexible and can generate different proportions of permeate flows through controlling the recovery rates of each component.