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

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 N 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. © 2014 Elsevier B.V. All rights reserved.

The NF–FO–BWRO system can increase the recovery rate of brackish water to N90% using a multi-stage membrane treatment. The system was able to produce different water qualities which are applicable for humans' use and agricultural activities. The NF process was responsible of producing 75% of the product water. An additional 15% recovery rate was achieved from the NF concentrated brine by the subsequent FO and BWRO processes. Power consumption was mainly due to the last stage of the BWRO process. NF and FO power consumption was about 20% of the total power consumption. Using NF in the upstream filtration process reduced the propensity of membrane scaling fouling. LSI of the NF concentrated brine was 1.5 for 2.4 g/L feed salinity but decreased to less than 1 for 1 g/L and 1.4 g/L feed salinities. NaCl draw solution, 0.25 M to 0.5 M, was used in the FO process to recover fresh water from the NF concentrated brine. Higher NaCl concentration can be used to increase the system recovery rate. Results also showed that NF and FO processes require less membrane elements than the BWRO process. BWRO requires 3 and 2 times, respectively, more membranes than NF and FO processes. In general, the NF–FO–BWRO system is a very flexible design and the produced water quality and recovery rate can be controlled through changing the recovery rates of membranes and/or the concentration of the draw solution. Pilot and bench scale experiments are required in future studies. Acknowledgment The authors would like to acknowledge the Carnegie Trust Grant No. 31823 for support to this research study.