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In-situ monitoring techniques of reverse osmosis (RO) desalination systems, particularly those with chemical sensing capabilities, can provide the means for better understanding important scaling mechanisms as well as early scaling detection. In this work, both calcium sulfate and calcium carbonate scaling on RO membranes were detected concurrently in real time using Raman spectroscopy to provide a unique chemical fingerprint. Two different sampling methodologies (manual and automated) were employed, and their performance was evaluated by comparing the Raman detection times to concurrent values of flux decline. The manual sampling strategy resulted in the detection of calcium sulfate and calcium carbonate at mean permeate flux declines of 13 ± 10 % and 22 ± 3 %, respectively. The automated sampling strategy provided better performance, with detection of calcium sulfate and calcium carbonate at mean flux declines of 8 ± 5 % and 4 ± 3 %, respectively. The increasedsensitivity and decreased variability of the automated sampling strategy provided valuable preliminary insights for the selection of optimized sampling strategies. The ability to identify the chemical composition of different scaling crystals including their polymorphs is an important step toward better understanding of the crystallization pathways of multi-component feed streams used in seawater and brackish water RO desalination. 

Direct monitoring techniques of fouling in membrane-based filtration processes can be implemented as part of an effort to reduce the negative effects of membrane fouling. In particular, monitoring techniques with chemical characterization capability are crucial for the formulation of effective fouling prevention and mitigation strategies. In the present work, Raman spectroscopy was applied as an in-situ monitoring technique for calcium carbonate scaling on commercial reverse osmosis membranes. The bench-scale Raman monitoring system allowed for a qualitative chemical assay of the scaled membrane surface at sequential downstream and upstream axial positions. The time evolution of the downstream and upstream calcium carbonate Raman signal was evaluated with respect to computed values of local concentration at the membrane surface, revealing a statistically significant dependence (p < 0.001). The real-time Raman data were bolstered by results of post-mortem analysis (scanning electron microscopy, gravimetric measurements, and laser interferometry). The employed technique was capable of detecting crystals with characteristic lengths <50 μm. Preliminary evidence of polymorph detection was also presented with recommendations for improvements in the technique.