## 2 Membrane Contactors for the Recovery of D-CH4 from Anaerobic Effluents
### Influesive Parameters of Hollow Fiber MCs for D-CH\({}_{4}\) Recovery
showed that when the G/L ratio is below a certain value, the gas phase mass transfer can play the major role in overall mass transfer [45]. The same observation was confirmed by a theoretical investigation by [PERSON] et al., and they further revealed that the maximum outlet gas purity can be obtained with a lower G/L ratio reached by adjusting L rather than G [43]. Apart from the G/L ratio, it was found that the membrane inner diameter, which can change the linear velocity at a fixed G/L ratio, gives a direct impact on the CH\({}_{4}\) transport when the liquid is fed to the lumen side [46]. When the membrane inner diameter becomes smaller, the D-CH\({}_{4}\) recovery flux can be increased due to an increase of liquid velocity. However, a smaller membrane diameter not only causes a higher pumping energy requirement but also increases the liquid pressure, which in turn causes a larger pressure drop along the membrane. Therefore, the MC designer should wisely select the membrane diameters and recommend the operating G and L.  
The membrane mass transfer coefficient is closely related to membrane morphology (i.e., porous, dense, or thin film composite) and the type of fluid inside the pores (i.e., gas or liquid). Even though porous membranes can minimize the mass transfer resistance, dense or composite membranes can also be utilized in MC applications with an aim of restricting membrane wetting and/or fouling. The mass transfer correlations for all three membrane morphologies are summarized in Table 2. However, it should be noted that these correlations may be modified based on the flow geometry and the module design [47; 48]. The gas transport mechanism in a porous MC membrane is either Knudsen diffusion or viscous diffusion. Such classification is based on the dimensionless Knudsen number (Kn), which is the ratio between the mean pore radius (r\({}_{\text{P}}\)) of the membrane and the mean free path (\(\lambda\)) of the gas molecule. When Kn \(<1\) (i.e., \(\lambda>r_{\text{P}}\)), the gas transport mechanism is Knudsen diffusion or viscous if otherwise [51]. Meanwhile, in a dense membrane, the mass transport mechanism is governed by the solution-diffusion model, such that the permeability coefficient becomes dominant in the mass transfer coefficient calculation. The mass transfer properties of both dense and porous membranes must be considered together for composite membranes. In particular, [PERSON] et al. and [PERSON] et al. investigated the applicability of dense and porous membranes for D-CH\({}_{4}\) recovery from  
Figure 2: (**a**) Vacuum mode and (**b**) Gas stripping mode for D-CH\({}_{4}\) recovery and energy production. Reproduced with permission from ref [43]. Copyright 2020 Elsevier. anaerobic reactor effluents. They ended up with the same conclusions: (1) the performance of a microporous membrane in terms of CH4 flux and recovery is better than that of a dense membrane, (2) the pore wetting limits the operation of a microporous membrane, and (3) the membrane mass transfer coefficient dominates the overall mass transfer in a dense membrane whereas the liquid phase mass transfer coefficient is dominant in microporous membranes [39; 52]. Later, [PERSON] et al. also compared the performance of polydimethylsiloxane (PDMS) and microporous polypropylene (PP) membranes in D-CH4 recovery [39]. The rates of recovery were recorded as 92.6% and 98.9% for the nonporous and microporous membranes, respectively. In a subsequent study, [PERSON] et al. demonstrated the limitation of the microporous membranes due to membrane fouling and wetting during a prolonged operation and thus recommended intermittent cleaning [37]. Accordingly, several studies have proposed the membrane post treatments or surface modifications to whittle down the wetting and fouling [40]. A key in the surface modification is to maintain the porosity of the membrane to keep a high CH4 flux while improving its wetting and fouling resistances. The details are discussed in the next section.