A diafiltration-nanofiltration-reverse osmosis (DiaNF-RO) process for ion fractionation towards resource recovery in seawater desalination: Comparison with a RO-DiaNF process and effect of NF membrane performance

The diafiltration-nanofiltration-reverse osmosis (DiaNF-RO) is an innovative process that aims to achieve ion fractionation of divalent/monovalent ions, i.e., Mg/Na (SF1_Mg-Na), an important step towards resource recovery, while producing clean water in seawater desalination at a reasonable specific energy consumption (SEC). Firstly, the novelty of DiaNF-RO process is compared with the RO-DiaNF process where ion fractionation is performed at the pretreatment stage (i.e., seawater) in the former and post-treatment stage (i.e., SWRO brine) in the latter. DiaNF-RO outperforms RO-DiaNF in many aspects, including higher Mg recovery, higher SF1_Mg-Na, lower scaling risk, and more efficient cost utilization, which emphasizes the benefits of initiating ion fractionation at low salinity in brine management. Secondly, the process practicality is reiterated by showing how desirable performance is achievable at low number of stages with some modifications. One possible adjustment is to improve NF membrane performance in the priority order of higher water permeability, lower Na rejection, and higher Mg rejection, which enables the 2DiaNF-RO to achieve the desirable SF1_Mg-Na of 13.7 at 5.71 kWh/m³. Another possible adjustment is to integrate the 2nd RO stage for more product water recovery to achieve a competitive net SEC (4.01 kWh/m³). These results further solidify the industrial implementation possibility of the DiaNF-RO process.