Membrane scaling in Vacuum Membrane Distillation - Part 1: In-situ observation of crystal growth and membrane wetting

This paper presents an experimental approach to investigating scale formation in Vacuum Membrane Distillation using in-situ optical crystal growth tracking and synchronized identification of membrane wetting. An incident light microscopy setup was used to monitor crystal growth on the membrane in the feed channel. An image processing strategy called Digitally Simulated Dark Field Illumination helped overcome the extremely low contrast between translucent crystals and the membrane surface. Further image processing and statistical evaluation enabled us to quantitatively track the population of crystals on the membrane. Simultaneously, membrane wetting was monitored using tracer-based Laser Induced Fluorescence. This allowed us to locate membrane leakages. The combination of these two experimental techniques gave insight into the coupling mechanisms of scaling and membrane wetting in Vacuum Membrane Distillation, where classical methods for wetting detection could not be applied. The methodology was tested with different scalants under process conditions to rate its performance and identify limitations.