**6. PEG-assisted membranes in wastewater treatment and desalination**

As emerging membrane technologies, FO and MD have received increased attentions for wastewater treatment and desalination [14, 57–60]. FO takes some


**Table 1.** *PEG-assisted FO and MD membranes used in wastewater treatment and desalination.*

*Water Quality - Science, Assessments and Policy*

**5.1 PEG used as pore-forming additive**

membranes for MD [41, 42].

**5. PEG associated with membrane distillation**

In the early 1990s, PEG was used as pore-forming additive to fabricate microporous polyvinylidene fluoride (PVDF) membrane for MD of wastewater discharged from the taurine production [33]. The effects of a series of PEG 400, 1000, 1540, 2000, and 6000 on the pore structure and permeate performance of poly(vinylidene fluoride-co-hexafluoropropylene) (F2.6) flat-sheet membranes were investigated by evaluating average pore radius, porosity, and morphology, and the membranes reached better higher distilled flux than the PVDF membrane during the DCMD test [34]. Dayanandan et al. studied the influence of the various additions of PEG (0–4 wt%) in the coagulation bath composition during the preparation of PVDF membranes, and they found that the bath-based MD membrane with 4 wt% PEG had relative superior overall performance than other membranes based on various evaluations of elongation-at-break, tensile strength, liquid entry pressure, hydrophobicity, porosity, and water flux [35]. Combined effects of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and the concentrations of the additive PEG 10000 were studied by using a statistical approach, and the optimized membrane achieved salt rejection of 99.5% in the DCMD of 0.1 M NaCl solution at 65°C [36]. Two pore-forming additives PEG and LiCl both at 4 wt% were used in the fabrication of hydrophobic flat sheet and hollow fiber PVDF and PVDF-co-chlorotrifluoroethylene (PVDF-CTFE) membranes for membrane distillation [37, 38]. The effect of mass ratio of PEG and LiCl was further investigated for making PVDF-CTFE membranes for MD [39]. Recently, more effective additives such as organic acids, LiCl, MgCl2, and LiCl/H2O mixtures along with PEG were investigated in the fabrication of flatsheet hydrophobic PVDF-CTFE membranes used for MD [38]. Hou et al. prepared hydrophobic PVDF flat-sheet membranes for DCMD by using various non-solvent additives such as acetone, phosphoric acid, glycerine, LiCl, and PEG 400 [40]. They found that the membrane fabricated with 5 wt% acetone and 3 wt% phosphoric acid exhibited the highest water flux among the various fabricated membranes and showed great performance stability in the 240 h desalination of synthetic seawater. There exist more novel additives, such as calcium carbonate nanoparticles and TamiSolve® NxG along with PEG and LiCl, applied to the fabrication of hydrophobic

During the fabrication of microporous PVDF hollow fiber membranes, two

non-solvent additives PEG 400/1500 and LiCl were added to the feed N,Ndimethylacetamide (DMAc) solution containing PVDF powder [43–46]. The experiment factors such as dope extrusion rate, take-up speed, air-gap concentration of polymer and additives, and bore liquid temperature were investigated in the membrane fabrication through a dry-jet wet phase inversion process, and the obtained membranes were suitable for DCMD than VMD. Recently, Zhang et al. further discovered the regulatory role of coagulation bath temperature during PVDF membrane fabrication when PEG 400 and triethyl phosphate were used [47]. In the fabrication process of PVDF hollow fiber membranes via complex thermally integrated phase separation at 80°C for MD, triethylphosphate (TEP) and PEG 200 were used as weak solvent and weak bore fluid, respectively [48]. PEG 6000 was used to fabricate hollow fiber MD membranes by Garcia-Payo et al. [49, 50]. They also dissolved PEG 10000 in DMAc to prepare PVDF-HFP hollow fiber membranes, and the optimized membrane had high permeate flux and salt rejection [51, 52]. Similarly, the concentration effects of PEG 600 (0–20 wt%) on the performance of PVDF-HFP membranes for MD were also investigated, and the suitable range of 5–20 wt% was found for fabricating hollow fiber MD membranes with acceptable

**62**

advantages of low membrane fouling, feasibility to treat high salinity water, possibility to run at low voltage electricity, and applications in osmotic dilution at low energy demand. When thermal volatile draw solute is used, FO can easily be integrated with low-grade heat, such as waste heat in a power plant and thermal heat gathered by highly efficient solar collector, for draw solute recovery [57, 58]. MD as a thermal membrane separation process has the nature to utilize low-grade heat easily, and it also possesses distinctive advantages of low membrane fouling and low operation energy demand of heating the feed and cooling the permeate when integrating with low-grade heat source and high quality of product water when using VMD and AGMD in the desalination of highly saline water [59]. The utilization of PEG, its derivatives, and copolymers in FO and MD membranes can improve the overall membrane performance, thus enhancing the abilities of these membranes in various applications. **Table 1** summarizes the performance of PEG-assisted FO and MD membranes used in wastewater treatment and desalination.
