## 3 Technical Challenges and Recent Efforts to Address Them
### Membrane Wetting
The fluorinated silica nanoparticles located on the outer surface of the membrane are clearly visible. The composite membranes demonstrated a stable D-CH\({}_{4}\) recovery flux over 300 h along with a very high CH\({}_{4}\) flux as compared to that of a commercial PP degassing membrane. The surface modification of PVDF-HFP (Poly(vinylidene fluoride-\(\alpha\)-hexafluorpropylene)) hollow fiber membrane with a mixture solution comprising perfluoropolyether (Fluorolink S10) and tetraethoxysilane (TEOS) was also reported [62]. The PVDF-HFP surface was firstly treated by an alkaline solution for dehydrofluorination, after which the hydroxyl groups (-OH) newly formed on the membrane surface react with TEOS and Fluorolink S10. After the surface modification, the WCA of the membrane increased from 95.5\({}^{\circ}\) to 127.8\({}^{\circ}\) with a slight decrease in pore size. In a following study, the hydrophobic modification parameters were further optimized [66]. The CH\({}_{4}\) flux of the resulting PVDF membrane was found to be two times greater than that of the commercial PP membrane. In addition, PVDF  
Figure 3: Membrane wetting due to (**a**) a pressure above the critical entry pressure of a membrane and (**b**) the capillary condensation. membranes displayed a stable performance for 8 days under a test with a real anaerobic effluent owing to its excellent resistance to pore wetting. On the other hand, the hydrophobic modification via chemical grafting and spray-coating on a surface of polymeric membranes has been reported as viable chemical approaches to enhance the wetting resistance. For instance, [PERSON] et al. reported the chemical hydrophobic modification of polymeric hollow fiber membranes, which have imide groups [67]. The ring-opening of imide groups in poly(amideimide) followed by the reaction with octadecylamine could create hydrophobic side chains, which inhibit the liquid entrance to membrane pores. The similar strategy was used to poly(etherimide), allowing the increases in both WCA and LEPw. For the case of PVDF membranes, [PERSON] et al. modified the membrane by a dip-coating of amorphous perfluoropolymer (Hyflon AD) on the surface [68]. Hyflon AD/PVDF composite membranes exhibited a significantly enhanced wetting resistance, along with an increased WCA. Meanwhile, polydimethylsiloxane (PDMS) has been employed as a coating material because of its hydrophobic property as well as high gas permeability. [PERSON] et al. prepared two types of PDMS-coated PVDF membranes, namely lumen-modified and bulk-modified membranes and evaluated the  
Figure 4: Surface SEM images of (**a**) Matrimid® hollow fiber substrates and (**b**) Matrimid®fluorinated silica composite hollow fiber membranes. Reproduced with permission from ref [46; 59; 63]. Copyright 2017 Elsevier. (**c**) Cross-sectional SEM images of polydimethylsiloxane (PDMS)-coated polyvinylidene fluoride (PVDF) membranes and (**d**) Si elemental mapping of PDMS-coated PVDF membranes. Reproduced with permission from ref [46; 59; 63]. Copyright 2019 Elsevier. FESEM images of the surfaces of (**e**) PVDF and (**f**) PVDF-fluorinated TiO\({}_{2}\) composite hollow fiber membranes. Reproduced with permission from ref [46; 59; 63]. Copyright 2018 Elsevier. D-CH4 recovery performance of them [69]. Lumen modified PDMS-PVDF (Figure 4c,d) membranes exhibited two times higher CH4 recovery flux than that of a commercial PP degassing membrane. According to the mass transfer analysis, the membrane resistance was only about 15% of total mass transfer resistance due to the highly porous nature of the PVDF substrate. Similarly, [PERSON] et al. modified a hydrophobic polytetrafluoroethylene (PTFE) membrane with silica nanoparticle and PDMS by a spray deposition [70]. The PDMS/silica-coated PTFE membrane exhibited a strong pore wetting resistance together with a high WCA of 158.4\({}^{\circ}\). Furthermore, the PDMS/silica skin layer on the surface did not increase the mass transfer resistance significantly, owing to the high gas permeability of PDMS material.