## 3 Technical Challenges and Recent Efforts to Address Them
### Membrane Fouling
Although this strategy has not yet been implemented in MC for D-CH\({}_{4}\) recovery, herein, we want to introduce some recent examples from relevant membrane technology. The primary advantage of patterning on the membrane surface is that the fouling resistance is created without introducing additional layers on top of the membrane surface [78]. [PERSON] et al. showed that surface patterning using nanoimprint lithography (NIL) on flat-sheet ultrafiltration (UF), microfiltration (MF), and thin-film composite membranes could induce antifouling characteristics on the membranes [79]. By comparing both patterned and unpatterned membranes, it was confirmed that the presence of the surface patterning increased shear stress at the membrane surface, thus mitigating the accumulation of colloids and protein-like macromolecules. Furthermore, [PERSON] et al. found that fabricating a line-shape nanostructure on the surface of the membrane not only minimized the water flux decline but also reduced the deposition of foulants in membrane distillation (MD) [80]. Comprehensive analysis revealed that the patterned membrane exhibited a much lower adhesion force between the membrane surface and protein foulants as compared to the pristine membrane. [PERSON] et al. also suggested that mitigating fouling propensity can be achieved by introducing patterned morphology on the membrane surface [81]. The patterned morphology such as prism and pyramid on a cross-flow MF system enhanced hydraulic resistance on the membrane surface, inducing a local turbulence that inhibits biofouling on the membrane surfaces. Apart from all these efforts to fabricate anti-fouling membranes, the low fouling could also be achieved by designing novel MC systems. As shown in Figure 6, [PERSON] et al. proposed a novel approach to recover D-CH\({}_{4}\) from a model anaerobic effluent by using an omniphobic membrane [82]. Unlike conventional membrane contactor systems, a non-polar (highly methane soluble), non-volatile organic solvent was utilized instead of sweeping gas or vacuum to create the driving force for MC operation. The difference of methane solubility in between the feed solution and the solvent drives the transport D-CH\({}_{4}\) from the feed, achieving over 90% of methane recovery at a temperature range of 15-35 \({}^{\circ}\)C. In the later work, AnMBR effluent was used as the feed solution to evaluate D-CH\({}_{4}\) recovery performance of the solvent-based membrane contactor (SMC) under realistic conditions [83]. The solvent-derived transfer of methane via the omniphobic membrane contactor showed over 90% methane recovery during the 24-h operation. More importantly, the absence of water flux across the membrane could prevent any accumulation of organic foulants on the membrane surface. Once a membrane is fouled, despite all the efforts for anti-fouling, the cleaning should be conducted to restore the original membrane performance. Although the most primitive and effective method of removing membrane fouling is chemical cleaning, it should be considered as the last option, since the cleaning chemicals can alter the membrane surface properties. However, physical cleaning such as back washing or flushing with water can be recommended. The physical cleaning is known to be most effective when the foulants are weakly bound to the membrane surface. To this end, the aforementioned hydrophilic anti-fouling coating on the membrane surface is once again a very effective strategy in controlling membrane fouling. Indeed, [PERSON] et al. demonstrated that the membrane coated with a hydrophilic anti-fouling layer could be readily cleaned by a flushing with tap water, after which the D-CH\({}_{4}\) flux was returned back to the original flux of fresh membrane [40]. It is worth noting that all membranes have a certain lifetime after which a replacement with a new one is inevitable. Owing to the growing concern on the generation of solid waste from used membranes, biodegradable membranes have also been developed in recent years.