## 5 Future Directions  
Even though the G-L MC system is a promising technology to recover D-CH4 from anaerobic effluents and thus produce useful energy from wastes, technical challenges have limited its large-scale practical applications. To capitalize the potential benefits of MC systems in practical applications, the following aspects should be investigated and improved in future studies. * The development of anti-wetting and anti-fouling membranes is an essential component of the utilization and application of membrane contactors. Membrane contactors without fouling and wetting resistances require frequent membrane cleaning and membrane replacements, which eventually lower the economic feasibility of the process. Thus far, many studies have been carried out by using commercially available MC membranes such as PP and PDMS membranes, and the membrane fouling was not the major focus in most of studies. Noticeably, the biofouling that occurred in the D-CH4 recovery process would be hard to tackle, as microorganisms in anaerobic effluents can even fouf the surface of the hydrophilic layer at a prolonged operation [13]. In addition, the anti-wetting property of the membrane should be addressed properly, considering the realistic conditions, including the fouling effect. For example, the surface property of the membrane can be altered by the deposition of a fouling layer, although a hydrophobic membrane surface was successfully created. Accordingly, it is recommended to develop high-performance membranes possessing both anti-wetting and anti-fouling properties together. * Switching lab-scale to large-scale testing is needed to acquire more reliable data. Both membrane and operation parameters should be optimized together to maximize the net energy production in large-scale MC operation. Especially, the membrane module should be effectively designed in such a way that both liquid and gas streams are uniformly distributed inside the module, since ineffective flow stream often limits the overall performance of large-scale hollow fiber module. It is worth noting that a success in a lab-scale test using a few pieces of lab-made hollow fibers cannot guarantee the success in a large-scale practical operation. The operation parameters that are effective in a large-scale system would also be different from those found in a lab-scale system. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Membrane**} & \multirow{2}{*}{
\begin{tabular}{c} **R\({}_{\text{M}}\)****a** \\ **(s m\({}^{-1}\))** \\ \end{tabular} } & \multicolumn{2}{c}{**E\({}_{\text{Net}}\)****b**} & \multicolumn{2}{c}{**Long-Term Operation**} & \multirow{2}{*}{**Reference**} \\ \cline{2-2} \cline{6-6}  & **(s m\({}^{-1}\))** & & **(MJ m\({}^{-3}\))** & & **AnMBR** & **UASB** \\ \hline Modified PVDF & & & & & \\ (PHM-PVDF) & 4518.0 & 0.112 & Stable flux for 8 days & Wetting after 3 days & [46] \\ \hline PDMS-PVDF & & & & & \\ LM-5.0 & 6225.0 & 0.135 & Stable flux for 8 days & 25\% flux drop within 10 days & [69] \\ \hline PDMS-co-P1 & 6528.9 & 0.115 & - & Stable flux for 5 days, then & [40] \\ \hline Commercial PP & 9512.6 & 0.102 & 10\% flux drop within 8 days & 40\% flux drop within 3 days & [40] \\ \hline Commercial PDMS & 12668 & 0.077 & - & 25\% flux drop within 10 days & [40] \\ \hline \hline \end{tabular}  
* The membrane resistance was obtained from the Wilson plot method. \({}^{\text{b}}\) The E\({}_{\text{NET}}\) was calculated at the liquid velocity of 0.06 m s\({}^{-1}\).
* PDMS-PVDF membranes were utilized for the post-modification (fouling-resistant coating) with PDMS copolymers (polydimethylsiloxane-co-3-(2-(2-hydroxyethyl)ethoxy)propylmethylsiloxane) and termed as PDMS-co-P1.  
\end{table} 
Table 3: Comparison of the membranes tested. More rigorous economic analysis is required based on data from large-scale operations under realistic conditions to provide more convincing evidence to potential users.