Reverse osmosis (RO) desalination is the leading technology for industrial and municipal water production in water-stressed regions. While developing chemical-free scaling control strategies helps mitigate the environmental impact of brine discharge, it also increases the risk of membrane scaling due to high salt concentrations. Establishing methods for early detection and localization of scaling is essential, as well as understanding the impact on key operational parameters. This study evaluated optical coherence tomography (OCT) for real-time monitoring of growth- and deposition-driven gypsum fouling in RO systems. Membrane fouling simulators were operated under constant flux conditions using unsaturated and supersaturated synthetic water solutions. Real-time monitoring of operational parameters revealed that growth and deposition fouling had a greater impact on transmembrane pressure than pressure drop increase. OCT imaging visualized scaling progression, with optical and SEM imaging confirming distinct morphologies: sharp, translucent crystals in growth-driven scaling and a white, amorphous fouling layer in deposition. Data processing further provided quantitative assessment of area coverage and fouling volume, with membrane autopsy indicating higher porosity in the deposition case. Crystal detection from OCT imaging evidenced sensitivity for early-stage scaling detection. In the growth case, a strong correlation was observed between initial crystal formation and regions of maximum saturation index, as revealed by CFD with multicomponent solute transport simulations. The variation in induction time across detection methods highlights the importance of sensitivity of monitoring techniques, positioning OCT as a valuable tool for early scaling detection, before conventional indicators point out to significant scaling.

In this study, a biofouling index based on the relative pressure drop is presented to quantitatively evaluate the amount of fouling in spacer-filled membrane filtration channels. The biofouling index was defined as the inverse of the time to reach a relative pressure drop of 100% and can be interpreted as a fouling rate or cleaning frequency. The index was applied to evaluate biofilm growth in membrane fouling simulators with reverse osmosis membranes and commercial feed spacers operated with different feed water nutrient concentrations and crossflow velocities. Biofilm accumulation on the membrane and feed spacer was characterized in situ using optical coherence tomography. We showed that the biofouling index is directly related to the volume of biofouling independent of the applied crossflow velocity and a suitable tool for improved quantitative comparison of the biofouling rate. Furthermore our results suggest that the pressure drop is better described as function of the velocity at the perimeter of a spacer cell instead of the average velocity in the channel. Although the biofouling index is developed for biofouling, the index may be applied to quantitatively assess mitigation strategies in spacer filled channels for a wider range of fouling types.