## 2 Membrane Contactors for the Recovery of D-CH4 from Anaerobic Effluents
### Influesive Parameters of Hollow Fiber MCs for D-CH\({}_{4}\) Recovery  
In the operation of a hollow fiber MC, many parameters such as module configuration, operational parameters, and membrane properties should be optimized. First of all, different flow geometries (i.e., liquid in lumen or liquid in shell) are known to give a significant impact on the mass transfer in MC. However, in D-CH\({}_{4}\) recovery, the residual pollutants such as suspended solids and organics in the feed solution should also be considered together during the selection of the flow configuration. For example, [PERSON] et al., [PERSON] et al., and [PERSON] et al. supplied the liquid to the shell side for the high organic content anaerobic effluents such as UASB and vice versa for low organic-containing effluents such as AnMBR [13; 37; 38; 39]. Even though the lumen side operational mode offers the better CH\({}_{4}\) recovery efficiency, a higher risk of membrane clogging limits such operation mode. To avoid the membrane clogging issue, [PERSON] et al. adopted the lumen side operation with a prefiltered UASB effluent by a filter cloth, thus achieving a higher recovery efficiency [40]. Similarly, [PERSON] et al. filtered an AnMBR effluent with a ultrafiltration membrane prior to feeding to a commercial hollow fiber MC module [41]. The conditions in the gas phase (Figure 2) are known to affect the purity of the recovered gas stream. In the studies of [PERSON] et al. and [PERSON] et al., the vacuum mode (Figure 2a) promoted a greater driving force across membrane Contactors, thus enhancing the overall mass transfer coefficient [11; 38; 39] while acquiring the product with a high CH\({}_{4}\) concentration. However, when the MC comprises a microporous structure, a gradual decline of overall mass transfer coefficient might be anticipated due to the infiltration of water during prolonged MC operation under vacuum conditions. On the contrary, air stripping mode (Figure 2b) has been proposed to save the energy for MC operation as well as to mitigate membrane wetting. The idea is based on the fact that the biomethane recovered is burnt in the presence of air in the power generation process. Hence, if biomethane is consumed to produce electricity without a long-distance transport, this strategy would be highly beneficial. Indeed, [PERSON] et al. employed a combined mild vacuum and air-stripping to maximize net energy recovery [42]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Characteristics and Indicators** & **Aeration** & **Gas Stripping** & **Membrane Contactors** \\ \hline \multirow{3}{*}{Pros} & Simple & Scalable & Modular, \\  & High mass transfer area & Easy to operate & High mass transfer area \\  & & & Independent fluid flow \\  & & & Low footprint \\ \hline \multirow{4}{*}{Cons} & Scalability issue & Low mass transfer area & \\  & Low CH\({}_{4}\) concentration in a & Flooding/foaming & Lack of optimized membranes \\  & gas outlet & High gas/liquid flow ratio & Fouling issues \\  & Difficult to control the & quality of product gas & required & Membrane lifespan \\ \hline Energy requirement & High & High & Low \\ \hline Capital cost & & Similar & \\ \hline Operating cost & High & High & Low \\ \hline CH\({}_{4}\) mass transfer coefficient & & Low & High \\ per unit volume & & & \\ \hline Overall CH\({}_{4}\) mass transfer & & & \\ coefficient & High & High & Low \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Advantages and disadvantages of membrane contactor (MC) over conventional separation devices [18; 19; 23; 24; 27; 31; 32; 33; 34; 35; 36]. The most common findings in D-CH\({}_{4}\) recovery by hollow fiber MCs are a gradual increase in overall mass transfer coefficient with an increase in liquid flowrate (L) and a negligible impact of gas flowrate (G) on the mass transfer. This is due to the dominant effect of liquid phase resistance on the overall mass transfer [44]. Nevertheless, [PERSON] et al.