Apart at a high recovery for draw solution reconcentration.

Apart
from the RO process, FO can be coupled with NF (i.e., FO-NF) process for
desalination. In these system, the pressure-driven processes utilize membranes
with a more porous rejection layer than that of the RO skin. Therefore, using
DS with solutes of much larger size is necessary. For example, Tan and Ng
evaluated the performance of a hybrid FO-NF system as an alternative to the
conventional RO (standalone) desalination process 17223. By using Na2SO4
as the draw solute (rejection by NF was 97.9%), an FO process integrated with
a double-pass NF process was able to provide product water that met the
requirement of drinking water quality (single-pass NF was not sufficient due to
the high DS concentration). A similar study by Zhao et al. 17324 proposed a
FO-NF system using a divalent draw solute (e.g. Na2SO4 or
MgSO4) for brackish water desalination. Results highlighted the
competitive advantage of using a lower pressure NF operation as compared to the
RO system (< 10 bar for NF against 30 bar for RO) 17324. Besides, it was found that inorganic salts and micro-organic molecules such as EDTA sodium salts 174,17525,26, sodium lignin sulfonate (NaLS) 155 and poly (aspartic acid sodium salt) (PAspNa, Mw ~1313 g/mol) 17627 were deemed as good candidates for draw solutes to be used in the FO-NF system. Fig. 9 represents a schematic flow diagram of FO pretreatment for conventional RO/ NF or thermal (MED or MSF) desalination process 171. Feedwater foulants and scalants are excluded from the draw solution, enabling the conventional desalination process to operate at a high recovery for draw solution reconcentration. In addition, a larger draw solute size normally leads to a higher rejection by FO as reverse solute diffusion can be impeded to a greater extent 17627. However, larger draw solutes tend to have smaller diffusion coefficients, which increase the internal concentration polarization (ICP) effect in the membrane support layer, resulting in a lower FO water flux 171. Hence, selecting a draw solute with an appropriate size is essential to balance its back diffusion and mass transfer coefficient.