Incipient CaCO3-scaling detection for scaling mitigation

The formation of scale deposits on the membrane surface due to the precipitation of sparingly soluble salts is a major obstacle limiting the recovery of inland membrane desalination. A simple approach to mitigating scaling is its early detection, allowing for timely countermeasures. Early scaling detection is mainly limited to laboratory studies with flat sheet membranes and synthetic feedwaters using sophisticated detection tools. This impedes transferability to an industrial scale, where spiral-wound elements are commonly used. The present study advances the previous research using pilot and laboratory-scale spiral-wound elements and a real water matrix. It demonstrates that monitoring changes in concentrate quality is an effective and practical way to detect incipient CaCO 3 scaling. It was found that the concentrate-pH is a reliable indicator that recognizes incipient CaCO 3 formation well before a scaling-induced drop in permeability. By monitoring concentrate pH in real time, countermeasures such as forward flushing can be triggered in a timely manner to reliably prevent CaCO₃ scaling. This allowed stable operation near the onset of scaling (calcite saturation index of 1.7 to 2.0) even without dosing an antiscalant for over two months. Forward flushing not only removed the concentration polarization layer but also led to the dissolution of CaCO 3 deposits that had already formed. This shows for the first time that incipient CaCO 3 scale deposits are reversible if intervention is carried out in time. Moreover, water chemistry models and simulations were conducted to gain a deeper understanding of the underlying mechanisms, particularly regarding the late response of the permeability and conductivity. • Early CaCO 3 -scaling detection via concentrate-pH before permeability drop • Concentrate-pH enables operation at scaling threshold without antiscalants • Conductivity and turbidity are slower or less reliable scaling indicators • Water chemistry simulations unraveled the delayed conductivity drop • Permeability drop is masked by concentration polarization