Fracsis: Ion fractionation and metathesis by a NF-ED integrated system to improve water recovery

Abstract

Desalination of brackish and wastewater is a challenging task in case the water contains sparingly bivalent ions. To avoid membrane scaling and obtain a high water recovery, an integrated membrane process is proposed in this investigation: ion fractionation and metathesis by a nanofiltration (NF) – electrodialysis (ED) integrated system (namely “Fracsis”). In the first step, this system makes use of the advantage of NF which can fractionate the bivalent ions from the monovalent ions. Results show that the tested NF membrane has a satisfying selectivity for this application, with a retention of Na+ 38–49%, Cl− 20–38%, and Ca2+ >98%, SO42− >85%. Afterwards, both NF permeate (monovalent stream) and retentate (divalent stream) were sent to the ED for metathesis. By ED metathesis, salts with high solubility (CaCl2 and Na2SO4) were synthesized in two separate streams and hence a high water recovery was achieved. The water recovery rate may reach 98.5% in case a continuous mode is used. Finally, the operational cost was calculated on this bench scale Fracsis system. Results indicate that although the energy consumption is higher than that of a RO system, Fracsis has its advantages in a higher water recovery, lower cost on chemical consumption and sludge treatment expenditures. To a broader context, this technology may possibly be used for chemical purification and synthesis in various industries in case mixed inorganic/organic salt streams are concerned.