Design and applications of vapor-gap membranes for water and resource recovery

Wetting-resistant, microporous membranes (i.e., vapor-gap membranes) have found promising applications in desalination and resource recovery from wastewater. When immersed in liquids, the vapor-gap membrane captures air bubbles in its pores, and a selective transport of volatile compounds is driven by the chemical potential gradient of the compound across the membrane. Here, we discuss two important attributes of the separation processes using vapor-gap membranes: productivity and selectivity of target compounds.

In the first part, we discuss the water productivity in the trade-off relation with membrane wetting resistance. Using microporous membranes that possess different surface wettabilities, we observed a decreasing trend of water vapor flux as the wetting resistance increases. An electrochemical impedance spectroscopy, combined with fluorescence microscopy, revealed that the membrane with a lower wetting resistance exhibits a larger area of liquid-vapor interface inside the membrane pores, allowing for higher evaporative water. This finding provides a guideline to design vapor-gap membranes for water desalination to ensure a maximized water flux for given feed water properties that require a certain wetting resistance.

In the second part, we present a novel application of vapor-gap membranes for selective recovery of dissolved methane from anaerobically treated wastewater as a renewable energy source. In this process, termed solvent-based membrane contactor, an omniphobic vapor-gap membrane is placed between a methane-rich anaerobic effluent and an organic draw solvent that has an order of magnitude higher methane solubility than water. Driven by the solubility difference, we demonstrate over 90% recovery with minimal membrane fouling and negligible influence of other dissolved gas species on methane recovery. An energetics analysis also shows the possibility of the process to contribute towards net energy generation, by harnessing the methane gas.