In-situ observation and quantification of membrane deformation in reverse osmosis, using optical coherence tomography

Deformation of the membrane can seriously hamper the performance of a reverse osmosis process. Currently, the most used approach to obtain detailed information on membrane deformation is by conducting membrane autopsy. However, ex-situ analysis can only observe irreversible deformation. In this study, optical coherence tomography (OCT) was introduced as method for in-situ 3D visualization and quantification of membrane and feed channel deformation during operation with varying transmembrane pressure (0–55 bar). In-situ monitoring of membrane surface geometry in a lab-scale flow-cell revealed the formation of specific patterns with ridges and valleys. The deformation became more pronounced with increasing applied pressure, due to membrane intrusion into the permeate spacer. The asymmetric structure of the permeate spacer causes a clear distinction in spatial distribution of ridges and valleys, depending on which side faces the membrane. Interestingly, membrane surface displacement increased linearly with TMP, with slope depending on spacer orientation (A vs B) and type (BW vs SW). Under pressure, we found the following displacements: Ridge – BW-A 1.7 ± 0.1 μm/bar, BW-B 1.9 ± 0.1 μm/bar, SW-A&B 1.5 ± 0.3 μm/bar; Valley – BW-A 3.2 ± 0.1 μm/bar, BW-B 2.7 ± 0.1 μm/bar, SW-A 1.7 ± 0.2 μm/bar, SW-B 1.8 ± 0.3 μm/bar. Measuring the membrane position under pressure revealed that the displacement involves the whole membrane-permeate spacer structure, i.e., the permeate spacer is also deformed. By alternating high and low feed pressure, we could distinguish between reversible and irreversible deformation. In-situ measurement of membrane deformation can contribute to understanding of pressure related performance and can serve as input for advanced modeling.