Chemical precipitation-tubular membrane filtration for silica removal from reverse osmosis brine: full-scale performance and comparative analysis

The high concentration of silica in reverse osmosis (RO) concentrate is prone to cause membrane fouling and scaling in subsequent crystallization processes, significantly restricting the efficiency and stability of brine treatment. This has become a core technical bottleneck in the advanced treatment of high-salinity wastewater. To address this issue, this study specifically developed and systematically evaluated the chemical precipitation-tubular membrane filtration (CP-TMF) process to construct an efficient silica removal system for RO concentrate. Using sodium aluminate (NaAlO₂), magnesium chloride (MgCl₂), and magnesium oxide (MgO) as precipitants, the study systematically explored the regulatory mechanisms of reaction pH, reagent dosage, temperature, and contact time on precipitation effects. The optimal reaction conditions for NaAlO₂ were found to be pH 8–10, dosage ratio of 2:1 (wt/wt%), 30 °C, and a reaction time of 30 min, achieving a silica removal rate of 95%. For MgCl₂ and MgO, the removal rates reached 93% and 91%, respectively, under conditions of pH 11 and 12, dosage ratios of 5:1 and 7:1 (wt/wt%), and a reaction time of 60 min, meeting the demands of different water quality scenarios. Long-term monitoring over six months confirmed that both the NaAlO₂ and MgCl₂ systems maintained stable silica removal efficiencies above 93%, solving the problem of performance decay in traditional processes during long-term operation. The CP-TMF process, through the synergistic action of precipitation and membrane filtration, significantly reduced the fouling load on subsequent RO membranes, enhancing the stability of system operation and laying a reliable foundation for the large-scale application of the process. Analysis of silica removal efficacy and applicability indicates that this coupling system is compatible with multiple components in RO concentrate, offering advantages of effective silica removal and simplified operation, demonstrating potential for engineering applications. This study clarified the mechanisms of different precipitants for silica removal, perfected the theoretical framework for silica control in high-salinity wastewater, and provided an implementable technical solution, offering key theoretical support and practical guidance for the large-scale application of silica control in high-salinity wastewater.