Wetting indicators, modes, and trade-offs in membrane distillation

In this study, we propose and demonstrate a simple solute mass balance to determine vapor flux and liquid flux in membrane distillation for several membrane materials under baseline and wetting conditions. We observe that distillate salinity can increase due to an experimental artefact, but that liquid flux can be used to conclusively diagnose wetting, quantify wetting rates, and indicate the relative number of wetted pores. We identify two wetting modes: “constant wetting”, characterized by increasing distillate salinity and constant liquid flux, and “increasing wetting”, characterized by increasing distillate salinity and increasing liquid flux. Constant wetting indicates that isolated pores remain wetted throughout operation, while increasing wetting indicates that more pores wet over time. Constant wetting may be tolerable at low liquid flux, requiring no intervention and resulting in no trade-off between wetting resistance and water production. In contrast, increasing wetting requires intervention. Reducing membrane pore size or increasing thickness can increase wetting resistance and delay liquid flux – but can also reduce vapor flux, resulting in trade-offs between wetting resistance and vapor flux. These results provide new understanding of wetting modes and trade-offs in membrane distillation and can guide membrane design and operation, for conventional and challenging applications. [Display omitted] •Solute mass balance to determine vapor and liquid flux from net flux in MD.•Analysis of distillate salinity and liquid flux enables diagnosis of wetting in MD.•Two wetting modes exist in MD: constant wetting mode and increasing wetting mode.•Constant wetting may be tolerable, but increasing wetting requires intervention.•Liquid/vapor flux tradeoffs occur due to membrane pore size, thickness.