Hydrophobicity recovery in membrane distillation through water flushing: Effect of uneven wetting distribution phenomenon along hollow fiber membrane

Hollow fiber membranes are extensively utilized in membrane distillation (MD) due to their expansive filtration area and high flux. However, the elongated length and slender diameter of these fibers often result in an uneven distribution of fluid characteristics, leading to heterogeneous wetting phenomenon along the membrane and impeding the complete restoration of hydrophobicity during membrane cleaning process. This study elucidated the impacts of uneven wetting kinetics along the hollow fiber membrane on the hydrophobicity recovery through water flushing using the ultrasonic time-domain reflectometry (UTDR) technology. The results revealed that an increase in temperature and flow rate mitigated the uneven wetting depth in MD process, while the decrease in surfactant concentration aggravated this effect. Computational fluid dynamics (CFD) model evidenced that this phenomenon was caused by the uneven distribution of temperature, velocity and surfactant concentration fields along the membrane. Furthermore, we observed that membrane wetting can be completely recovered using a lower temperature and a lower flow rate of cleaning solutions. Based on force-balance theory, the critical cleaning depth at the duration of in-situ water flushing was determined. This work provides a reliable strategy for membrane cleaning to effectively restore the hydrophobicity of hollow fiber membranes in MD.