Crossflow-induced membrane vibration and its effects on fouling and scaling in direct contact membrane distillation

Abstract

Membrane distillation (MD) is driven by vapor pressures rather than hydraulic pressures. Consequently, certain regions of the membrane may be in an unrestrained state, which potentially lead to membrane vibrations due to the turbulence caused by crossflow. However, this crossflow-induced membrane vibration has been largely overlooked in the previous studies. Here, we made the first attempt to investigate the crossflow-induced membrane vibration through the observation using optical coherence tomography (OCT), and explore the effect of this vibration on membrane fouling and scaling. The experimental results revealed that the vibration exhibited a self-excited vibration mode, with amplitudes reaching tens of micrometers. Moreover, the amplitude could be readily adjusted through changing crossflow velocity, transmembrane pressure, and the mesh size of spacer. For humic acid (HA) fouling, the membrane vibration could effectively reduce the thickness of HA cake layer, thereby stabilizing the specific flux at a constant level of 0.85, in contrast to the continuous decline of the flux under non-vibration condition. For gypsum scaling, the membrane vibration allowed the MD under low crossflow velocity (5 cm/s) to achieve comparable scaling occurrence concentration and scaling layer thickness as those under high crossflow velocity (10 cm/s). Our findings emphasized the potential of crossflow-induced membrane vibration as an effective strategy to mitigate membrane fouling and scaling in MD and other non-pressure driven membrane systems.