Energy-efficient hybrid FCDI-NF desalination process with tunable salt rejection and high water recovery

Abstract

Seawater and brackish water constitute ~ 97% of the water on Earth. Therefore, future water shortages could be alleviated if we develop more efficient and cost-effective desalination technologies. Reverse osmosis (RO) has been established as the best available technology for commercial seawater (SW) desalination during the last two decades. Because the standard SWRO membrane element is designed to achieve a very high salt (NaCl) rejection (> 99%) with a low water recovery (~ 15%), current SWRO desalination plant design is based on the staging of arrays of RO membrane elements to achieve an overall water recovery of ~ 50% with an energy consumption of 3–4 kWh per m3 of water treated. Moreover, SWRO desalination plants generate substantial amounts of brines that must be disposed of. In addition, SWRO desalination plants require post-treatment including remineralization. Here, we report an energy-efficient hybrid desalination process for high salinity brackish water. This new desalination process couples flow capacitive deionization (FCDI) with nanofiltration (NF). Our experiments and energy calculations using a 10000 ppm NaCl solution as model brackish water show that the energy consumption of the new FCDI-NF unit (0.460 kWh_total m−3) is lower by 16–20% than the best reported energy consumption (0.571 kWh m−3) and the 1-stage practical minimum energy consumption (0.550 kWh m−3) of a brackish water reverse osmosis (BWRO) unit treating the same feed at 70% water recovery, while final total dissolved solids (TDS) are in the drinking water range.