## 1 Introduction  
Waste-to-energy conversion technologies have been extensively studied and developed in recent decades as a sustainable waste management solution while producing additional energy. With this regard, the anaerobic digestion of wastewater has received much attention owing to its compelling advantages such as potential energy recovery in the form of biogas and low operation cost [1; 2; 3; 4]. Of all benefits, methane (CH4) can be recognized as the valuable energy source component in the biogas, having a heating value of 55 MJ kg\({}^{-1}\) that can be convertible to thermal and electrical energy [5]. Nevertheless, the dissolution of the produced CH4 in the liquid effluent drives the anaerobic digestion of wastewater to be less appealing than expected [6; 7]. The theoretical estimation based on the Henry's Law revealed that around 45% of the produced CH4 can be dissolved in the liquid effluent, even at 30 \({}^{\circ}\)C, and the portion in liquid further increases with decreasing temperature [7; 8; 9]. The supersaturation of biogas in the liquid, which is often observed when a mixing is not applied in the reactor, worsens this situation, causing a huge economic loss as well as a safety concern. It was reported that the supersaturation values in up flow anaerobic sludge blanket (UASB) reactors can be ranged in 1.3-6.9. However, in anaerobic membrane bioreactors (AnMBR) that are operated under proper mixing, the CH4 concentrations in the liquid are approximately at the saturation [8]. Apart from theses energy and economic losses, releasing of gaseous CH4 to the environment upon discharging the liquid effluent creates critical environment complications such as global warming. Noticeably, around 9% of global environmental CH4 emissions arises from the wastewater treatments and their related activities, which should be seriously taken into account while discharging anaerobically treated liquid effluents [6]. Moreover, the explosive nature of CH4 (lower and upper explosive limits are 4.6 and 15.8 vol % in air, respectively) necessitates the safe handling of anaerobically treated wastewater effluent in enclosed environments [10]. In general, the anaerobically treated wastewater effluent contains 10-25 mg L\({}^{-1}\) of dissolved CH4 (D-CH4), whereas to avoid explosions in closed environments, the D-CH4 concentration should be lower than 0.14 mg L\({}^{-1}\)[8; 11]. Hence, downstream processing of anaerobically treated wastewater effluent to recover the D-CH4 is of utmost importance for phasing out the potential environmental and safety hazards while exploiting the economic gains in turn, thereby accomplishing a sustainable anaerobic wastewater treatment process. In this respect, harvesting CH4 from both gas and liquid phases is necessary to make the process more attractive and efficient [7; 9; 12]. For instance, [PERSON] et al. demonstrated the potential feasibility of self-sustaining anaerobic processes based on the recovery of the produced CH4 from both the headspace (gas phase) of the reactor and the D-CH4 in effluents [8]. Previously, we also proposed a conceptual system of energy self-sufficient anaerobic wastewater process enabled by employing a membrane contactor (MC), as shown in Figure 1[13]. The rationale behind Figure 1 is to maximize the recovery of produced CH4 in the gaseous form at the head space as well as dissolved form in the liquid effluent (i.e., recovery of D-CH4 using an MC) followed by the conversion of CH4 to electricity using a micro turbine. The techno-economic feasibility of MC for CH4 recovery from anaerobic liquid effluents is yet to be finely investigated, since to date, no sufficient studies have been conducted at an industrial scale. Herein, we critically investigate the recent advances in gas-liquid (G-L) MCs for D-CH4 recovery from anaerobic effluents. The present report covers general operational principles of G-L MC, technical challenges, and recent progress with focus on the efforts to address the technical challenges.