Early detection and control of CaCO₃ scaling in closed-circuit reverse osmosis under antiscalant-free conditions

Recently, discontinuous operation modes, such as closed-circuit reverse osmosis (CCRO), have gained importance. CCRO allegedly offers various benefits, such as increased scaling resistance. However, there is limited understanding of how CCRO operation affects membrane performance parameters. Beyond that, the alleged scaling resistance is poorly understood and, in some instances, even subject to contradictory findings. In addition, scaling detection in CCRO remains largely unexplored. To investigate these open questions, we operated a semi-technical plant with a spiral-wound element fed with a real water matrix inducing CaCO 3 -scaling and organic fouling without antiscalant dosing. Under non-scaling conditions rejection increases within a closed circuit, correlating with rising concentrate pH. Monitoring concentrate pH proves effective for detecting CaCO 3 scaling, as it is first indicated by the flattening of the pH curve and later by a decreasing pH during closed-circuit operation. This method prevented membrane scaling in long-term experiments (>54 days). Process inherent flushing partially removes organic fouling. Moreover, flushing triggers the dissolving of CaCO 3 deposits, likely due to the relatively short duration under high saturation. This leads to the formation of calcite precursors like amorphous CaCO 3 , as supported by water chemistry simulations. This research extends, thus, current knowledge on anti-scaling mechanisms in discontinuous operation and opens up a new direction for antiscalant-free desalination. • Concentrate-pH indicates incipient CaCO 3 -scaling, the earliest and most reliable. • Early detection enables long-term antiscalant-free operation. • Inherent flushing removes fouling and scaling layers. • Short time at high saturation promotes formation of mineral precursors. • Higher solubility of precursors enables dissolution by inherent flushing.