Direct visual observation of particle deposition in the different zones of MD flow field: Mechanisms of deposition and release

Membrane fouling induced by foulant deposition is an urgent issue for the actual application of membrane distillation (MD), thereby a bench-scale MD system coupled with the direct observation through membrane (DOTM) technique was established to visually observe particle deposition and release in MD system. Drag force generated by gravity and vapor pressure as well as the attractive interfacial interaction was the prime culprit for initial particle deposition, namely dragging deposition. Vapor pressure, as the driving force of MD, posed certain impacts on the particle deposition at lower cross-flow velocity (CFV), while the flow shear induced by cross-flow was the crucial factor for particle deposition in MD system. The effect of MD driving force on particle deposition was remarkably weakened at high CFV of 0.168 m/s. The regions near the side wall or corner were severely covered by glass particles likely owing to the diffused distribution of slow flow in these boundary zones. Five different patterns including dragging deposition, dead zone deposition, trapping deposition, boundary retention deposition, and aggregation deposition were proposed for the particle deposition mechanisms in MD system. Membrane flushing or introduction of small air bubbles during membrane washing might be feasible for the efficient removal of deposited particles from MD membranes.