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.

Gradients in dissolved gas concentrations are expected to affect the performance of large reactors for anaerobic gas (CO, H2, CO2) fermentation. To study how these gradients, and the dissolved gas concentration level itself, influence the productivity of the desired product ethanol and the product spectrum of C. autoethanogenum, we coupled a CFD model of an industrial-scale gas fermentor to a metabolic kinetic model for a wide range of metabolic regimes. Our model results, together with literature experimental data and a model with constant
dissolved gas concentrations, indicate high ethanol specificity at low dissolved CO concentrations, with acetate reduction to ethanol at very low dissolved CO concentrations and combined ethanol and acetate production at higher CO concentrations. The gradient was predicted to increase both the biomass-specific ethanol production rate and the electron-to-ethanol yield by ~25%. This might be due to intensified ferredoxin and NAD+ redox cycles, with the rate of the Rnf complex – a critical enzyme for energy conservation – as key driver towards
ethanol production, all at the expense of a reduced flux to acetate. We present improved mechanistic understanding of the gas fermentation process, and novel leads for optimization and fundamental research, by coupling observations from various down-scaled lab experiments to expected microbial lifelines in an industrial-scale reactor.