Process-aware early warning of fouling-related instability in flow-reversal reverse osmosis

Flow-reversal reverse osmosis (FRRO) enables high-recovery desalination but exhibits complex dynamics that can lead to rapid performance deterioration near stability limits. In this study, a process-aware early-warning framework is developed using physics-based feature engineering, a physically interpretable drift indicator, continuous risk assessment, and rule-based decision logic. Analysis of pilot-scale FRRO data from the OCWD GWRS shows progression across stable, marginal, and unstable operating regimes. Under the studied conditions, increasing recovery from approximately 91% to 92% was accompanied by declining downstream permeability, increased hydraulic stress, and delayed upstream permeability recovery following block rotation. Using temporally separated calibration based exclusively on data preceding the 92% recovery increase, the nominal framework generated its first persistent warning 13.8 h before the first sustained Stage 3 feed-pressure excursion at or above the 21-bar (305 psi) operational high-pressure reference. Varying the operational reference from 20 to 22 bar yielded warning lead times of 10.65–14.02 h, while benchmark performance depended strongly on the persistence criterion and its interaction with the nominal 60-min FRRO block residence period; at shorter persistence settings, Stage 3 feed pressure and permeability generated earlier alarms than the multivariable warning. Direct post-event measurements following ROP flushing and restart indicated partial rather than complete physical recovery. The framework should therefore be interpreted as a process-aware system-level indicator that integrates multiple physical signals and FRRO operating history, rather than as a method that necessarily provides earlier alarms than individual process variables. Because the analysis contains one dominant degradation episode, additional independent events and full-scale systems are required to establish generalizability.