## 2 Membrane Contactors for the Recovery of D-CH4 from Anaerobic Effluents  
### Comparison of MC with Conventional Gas-Liquid Separation Devices  
Several gas-liquid contacting processes including aeration, gas stripping, and membrane technology have been proposed for the removal of D-CH4 from effluents. The aeration (i.e., bubble columns) and gas stripping (i.e., spray tower, packed tower, tray tower, etc.) devices exploit the concept of direct contact mass transfer between liquid and gas phases. In bubble columns, the stripping gas is bubbled through the liquid-filled column, where the size and formation rate of bubbles govern the mass transfer. A high D-CH4 recovery efficiency ranging from 90 to 100% was reported by [PERSON] et al. in bubble columns; however, foaming and difficulty in handling the liquids with solids were the major drawbacks [14; 15]. Meanwhile, in spray towers, the nozzles at the top of the chamber generate liquid droplets from the feed, which are then directly contacted with air. The droplet size, falling height, velocity, and the type of nozzle used determine the mass transfer rate of the process [16; 17]. [PERSON] et al. reported over 60% of D-CH4 recovery efficiency in their study with a spray tower after optimizing operation parameters [16; 17]. The back mixing of recovered gas with the liquid was found to be a major issue lowering the mass transfer in the spray tower [18]. Furthermore, a high pressure is required for the formation of fine liquid droplets, leading to a high energy penalty [19]. On the contrary to spray towers, a packed tower is filled with packing materials that facilitate the mass transfer by increasing the effective mass transfer area. The liquid is supplied through spray nozzles from the top of the column, ensuring a uniform distribution over the cross-section. Flooding, channeling, and wall flow are the major drawbacks in this type of mass transfer devices, limiting the operation ranges of liquid and gas flows [20; 21]. [PERSON] et al. reported over 90% of D-CH4 removal efficiency in a packed tower fed with groundwater [22]. As compared to the aforementioned conventional G-L separation devices, the MC technology has recently attracted much attention due to its alluring features such as high mass transfer area, safe operation (no foaming, flooding, channeling, and wall flow), independent control of gas and liquid flows, and compact design [23; 24]. In an MC device, the membrane acts as a physical barrier, providing a nondispersive contact between the gas and liquid phases for mass transfer [25]. Here, the mass transfer area is well-defined at the G-L interface, and it is maintained stably compared with conventional mass transfer devices. In general, the separation effectiveness of conventional mass transfer devices is determined by the height of transfer units (HTU). As the k\({}_{x/y,a}\) (i.e., k\({}_{x/y}\): overall mass transfer coefficient based on liquid or gas phase, a: mass transfer area per unit volume) increases, the HTU deceases, resulting in the increase of separation effectiveness [26]. Typically, for MC devices, the mass transfer area available per unit device volume ranges between 500 and 1000 ft\({}^{-1}\), whereas those for conventional tray or packed columns are in the range of 10-100 ft\({}^{-1}\). Hence, a significant improvement in separation efficiency can be achieved even though the overall mass transfer coefficient is almost the same or slightly lower than those of conventional G-L separation devices [23; 27]. Indeed, in the CO2 absorption process, a representative application of MC, the gas absorption flux per unit device volume of MC was reported to be 2.7 times higher than that of the packed tower [28; 29; 30]. In addition, an economic analysis conducted by [PERSON] et al. revealed that by switching the CO2 capturing technology from a conventional process to a membrane absorption, 30-40% of installation and operational cost could be saved [30]. Thus, these examples imply that MC is a promising technology in both economic and energy viewpoints.