UF has attracted considerable attention for drinking water production during the past decade because of its relatively low energy cost compared to RO and NF, and the decreasing cost of membranes [81]. UF is widely employed to concentrate solutions of macromolecules, however, the applications for separation or concentration of ionic species in solution are lacking due to big pore sizes of the membranes [82].
Recently, the use of UF to recover PSA polyelectrolytes after FO was proposed [28]. PSA polyelectrolytes have various molecular weights (Mws) and expanded polymer structure, which enable them to be regenerated by a low pressure-driven UF process. A low gas pressure of 10 bar was used in the UF cell. Results showed that a high rejection rate (>99%) could be achieved for the PSA (Mw of 1800), and that no PSA leaked to product water during UF, as the quality of product water was observed to be comparable to that of DI water. However, the water flux decreased in FO after their recovery by UF and reuse. Chung's group supposed that this decrease might be due to experimental errors [28]. However, there is no evidence given about the quality of product water. In other words, the decrease may also be ascribed to the loss of PSA during UF [18], and the product water might be contaminated by the PSA leakage.
The use of UF for recycling MNPs coated with polyacrylic acid (PAA) (∼5 nm) after FO was also investigated, as the pore size (1–1.5 nm) of UF membranes could be small enough to intercept them [39]. The UF process was operated under a pressure of at least 5 bar. Results showed that the MNPs remained active even after five cycles of UF recovery without increasing their sizes or reducing their osmolality. However, the smaller MNPs were lost by leakage through the UF membrane. To further improve the recovery efficiency of DSs, to design MNPs with narrow particle distributions is needed. Overall, the UF recovery requires lower energy consumption compared to RO and NF, however, the effective use of UF recovery is not suitable for DSs with low Mws, which can produce higher osmotic pressure for water production from brackish water, even seawater [35]