In-situ and real-time fouling monitoring in RO desalination: Bridging smart sensing with operational control

Reverse osmosis (RO) is central to seawater desalination and water reuse, yet membrane fouling continues to impose operational and economic burdens through increased energy consumption, more frequent cleaning, and reduced module lifetime. Although fouling monitoring is most effective when linked to practical mitigation measures, earlier reviews often treat sensing, diagnosis, and mitigation as separate topics, limiting their direct impact on plant operation. This review critically examines in-situ, real-time fouling detection at the RO module level and develops a framework for translating early fouling signals into actionable operational responses. The review first summarizes fouling mechanisms and dynamics, then discusses the main sensing families used for RO fouling detection. To enable consistent assessment across heterogeneous technologies, the review introduces fouling detection metrics (FDMs), including limit of detection (LOD), alarm lead time (ALT), false-alarm rate (FAR), and stability under temperature and salinity variations. These metrics are linked to a decision-oriented framework that supports four operator responses: Monitor, Mitigate, Prepare cleaning-in-place (CIP), and Trigger CIP, linking detection directly to operational action. The review also examines the role of signal processing and machine learning in fouling diagnosis, forecasting, sensor-data fusion, uncertainty reporting, and operational decision support. Challenges in standardization and multi-fouling identification, along with a perspective on readiness for plant use, are also highlighted. By combining sensing, FDM reporting, and action thresholds, the review provides a practical path from early detection to better-timed mitigation and cleaning decisions, with the potential to reduce unnecessary cleaning, lower energy consumption and chemical use, and extend membrane life.