In-situ monitoring and quantification of fouling development in membrane distillation by means of optical coherence tomography

Abstract

Fouling formation in membrane distillation limits process stability and could cause besides wetting a rapid flux decrease making the process inefficient. Process performance is conventionally assessed by flux monitoring. Optical Coherence Tomography (OCT) enables in-situ visualization and quantification of fouling layers dominated by scaling in a fully operated direct contact membrane distillation system (DCMD). This study presents an innovative methodology for 3D dataset analysis allowing for the detection and quantification of predominant scaling on the membrane surface. Defined fouling parameters permit a correlation between process performance indicators, especially flux decrease and quantified fouling layer coverage. Additionally, structural information about the fouling layer was derived from the fouling parameters calculated from OCT C-scans across the feed channel within the flat sheet membrane unit. The method was successfully applied in DCMD of hot spring water, showing a critical fouling ratio between RS = 50–60 µm3/µm2, resulting in an overall flux decrease of 80%. The consideration of covered membrane area in RC proved the correlation of increasing fouling layer coverage (up to 90%) inducing a flux decline (by 80%) verifying the hypothesis, that the share of covered membrane area is the limiting step for MD performance. The presented method was able to link scalable fouling parameters with macroscopic process parameters. Hence making performance monitoring possible while enabling an adapted process control.