## 3 Technical Challenges and Recent Efforts to Address Them
### Membrane Fouling  
Membrane fouling, which is the deposition of unwanted particles, colloids, and various macromolecules on the membrane surface or inside the pores of a membrane [71], creates an additional mass transfer barrier on the membrane surface, thus lowering the CH4 permeation flux. For instance, [PERSON] et al. observed severe membrane fouling and consequent CH4 recovery flux decline of 40% after 175-h operation of a PDMS membrane module [38]. [PERSON] et al. analyzed the fouling layers deposited on hydrophobically modified PVDF membrane surfaces after the operation of MC with two types of anaerobic effluents, namely AnMBR and UASB [72]. The overall mass transfer coefficient gradually decreased with time due to membrane fouling, with UASB effluent causing more severe fouling than AnMBR effluent (Figure 5a). The fouling layer analysis revealed that the soluble microbial products such as proteins and polysaccharides form the cake layer on the membrane surface (Figure 5b), which is also typically found in the operation of MBRs. When the fouling progresses, the cake layer becomes thickened and eventually blocks the pore mouths, resulting in a gradual decrease in the overall mass transfer. Although the fouling is an important issue that must be resolved for the practical application of MC in D-CH4 recovery, only a limited number of studies have been conducted to mitigate membrane fouling involving organic-, inorganic-, and biofouling. Nevertheless, the most common approach utilized so far is developing anti-fouling membranes. Herein, we report several recent progresses on the anti-fouling modifications of membranes that can mitigate the fouling effects during the long-term operation of G-L MC. Introducing hydrophilicity on the membrane surface can reduce membrane fouling by decreasing an adhesion force of hydrophobic organic particles on the membrane surface. To this end, developing composite membranes coated with a thin hydrophilic layer on top  
Figure 5: (**a**) FESEM and energy dispersive X-ray spectroscopy (EDS) images of the cross-sectional morphology of the fouled membrane with N, F, and O elemental mappings. (**b**) Mass transfer analyses showing overall mass transfer coefficient (K\({}_{\text{OV}}\)) with respect to the operation time of the membrane contactor for the recovery of dissolved CH4. Reproduced with permission from ref [72]. Copyright 2019 Elsevier. of a hydrophobic membrane substrate was suggested by [PERSON] et al. [13]. In their review, the possible candidates proposed for the hydrophilic layer were poly(2-hydroxyethyl methacrylate) and poly(ether block amide) [73; 74]. Later, the same group successfully realized this idea by creating an anti-fouling layer on PDMS-PVDF composite membrane [40]. Prior to the surface modification, the WCA and surface streaming potential, which can be utilized as essential implications for anticipating biofouling propensity [75; 76], were 120.4\({}^{\circ}\) and -12.8 mV respectively for the PDMS-PVDF membrane. However, after the coating with a PDMS-based amphiphilic copolymer comprising polyethylene glycol (PEG)-like segments, those values were determined as 14\({}^{\circ}\) and \(-\)4.1 mV [40]. Interestingly, the surface-modified membrane showed better performance during MC operation in terms of both fouling resistance and D-CH\({}_{4}\) recovery efficiency probably due to a lower steric hindrance from the short-chain PDMS-based copolymer compared to that of the original PDMS. Consequently, the post-treatment using the amphiphilic copolymer tremendously improved the anti-fouling property of the PDMS-PVDF membrane as evidenced by the successive 7-day operation while maintaining a stable CH\({}_{4}\) recovery flux. A similar observation was also reported by [PERSON] et al. when the membrane surface was modified with a short-chain PEG [77]. Introducing various patterns on the surface of membranes is also an effective method to prevent membrane fouling by creating turbulent hydrodynamics that imposes a high shear stress near the liquid-membrane surface [13].