Brackish water desalination by a hybrid forward osmosis–nanofiltration system using divalent draw solute

In the last decade osmotically driven membrane processes have attracted growing interest in wastewater treatment, desalination and power generation. A hybrid forward osmosis–nanofiltration (FO–NF) system designed for brackish water desalination was systematically investigated in this study. The hybrid FO–NF process was also compared with a stand-alone reverse osmosis (RO) process in brackish water desalination. It is found that the hybrid FO–NF process has many advantages over the stand-alone RO process in brackish water desalination, such as lower hydraulic pressure, less flux decline caused by membrane fouling and higher flux recovery after cleaning. Other benefits like no pre-treatment requirement, higher permeate quality and no need for chemical cleaning in brackish water desalination can also be achieved by the hybrid FO–NF system. Therefore, the proposed FO–NF system using a divalent draw solute (e.g. Na2SO4 or MgSO4) can be a useful alternative to stand-alone RO processes in brackish water desalination.

► A FO–NF system using divalent draw solute is studied for brackish water desalination.
► The hybrid FO–NF system is compared with RO in brackish water desalination.
► Hybrid FO–NF has many advantages over RO in brackish water desalination.
► FO–NF using divalent draw solutes has great potential in brackish water desalination.