Antiscalant-free strategy for nanofiltration: Prolonged induction period of gypsum crystallization at ultralow pressure

Gypsum scaling is a major obstacle to the broad application of nanofiltration (NF). Although high-pressure operation combined with antiscalants is typically employed to improve the techno-economic feasibility of NF, it frequently results in increased energy and chemical consumption. In this study, an antiscalant-free approach, ultralow-pressure NF, was applied to regulate the crystallization process and maintain a stable flux. The gypsum scaling behavior under different pressures as well as the applicability of ultralow-pressure NF were investigated. The results revealed that the normalized flux remained above 0.95 at low pressure but declined rapidly at high pressure. Minimal mineral deposition and a loosely structured scaling layer were observed at low pressure, whereas high pressure led to extensive crystal formation, covering the membrane surface and blocking pores seriously. These differences in scaling behavior were attributed to the variations in the induction period under different pressures. Numeric simulation results demonstrated that system pressure regulated the induction period by affecting concentration polarization and altering the migration trajectories of gypsum crystals and precursors. A simple flushing process was found to effectively reset the induction period, thereby impeding gypsum scaling. As lowering the pressure significantly prolonged the induction period before flux decay, the flushing intervals could be extended. An economic assessment confirmed the feasibility of ultralow-pressure NF. This study proposes an energy-efficient and antiscalant-free strategy for achieving sustainable separation in the context of low-carbon water treatment.