Novel nanofiltration-reverse osmosis-high pressure nanofiltration membrane brine concentration (NF-RO-HPNF MBC) system for producing high purity high concentration sodium chloride brine from seawater

A pilot-scale study of a nanofiltration (NF) - reverse osmosis (RO) - membrane brine concentration (MBC) system was carried out using novel NF membranes for the NF and MBC systems. Two commercially available NF membranes with high magnesium rejection ability were tested for the removal of divalent ions from seawater. These membranes exhibited 81–87 % rejection of Ca and 95–97 % rejection of Mg at an 85 % recovery rate, resulting in a 4.3–5.3 times higher concentration of Ca and 5.4–6.7 times higher concentration of Mg in the NF reject than in the feed. Accordingly, the NF permeate had a high NaCl/TDS index of 96.7–97.8 %, facilitating the downstream production of high-purity NaCl salt. The potential of using NF membranes for brine concentration was investigated, and it was found that if a NF membrane could be operated at 75 bar, the NaCl solution could be concentrated to over 230 g/L. A pilot-scale trial was carried out on a 3-stage high pressure NF (HPNF) system, which achieved a concentration of 220 g/L from 78 g/L SWRO brine and 80 days of continuous operation. At a higher operating pressure, the system successfully concentrated the brine to 251 g/L as designed. The specific power consumption of the industrial 3-stage HPNF MBC system was estimated to be 111.0–123.3 kWh/t NaCl while concentrating from 75 g/L to 225 g/L and producing diluted steam at 43 g/L. This demonstrates competitiveness with conventional concentration methods such as thermal evaporation and electrodialysis.