Can capacitive deionization outperform reverse osmosis for brackish water desalination?

Abstract

Capacitive deionization has received significant attention as an energy-efficient technology for brackish water desalination. The capacitive deionization suffers from lower water recovery and high electrode discharge time, that hampers its economics. The low recovery ratio increases the brine disposal cost and is also environmentally polluting. In this paper, the feasibility of capacitive deionization as a brackish water desalination technology is studied. The capacitive deionization performance is first optimized and is later compared with the state-of-the-art reverse osmosis. Both modeling and experimental studies are carried out to optimize the performance of capacitive deionization. A new cost correlation as a function of the capacitive deionization system size is proposed. By optimally designing the system, the treated water cost for capacitive deionization is reduced by 36%. A feasibility map for capacitive deionization is generated and compared to reverse osmosis as a function of brackish water concentration and different brine disposal options (see the graphical abstract). Compared to reverse osmosis, capacitive deionization economics are sensitive to the feed concentrations but becomes superior for salt concentrations less than (a) 70±15 mM (surface brine discharge), (b) 65±13 mM (deep well injection), and (c) 43±10 mM (evaporation pond). For zero liquid discharge, the CDI is not economical compared to RO regardless of salt concentrations. Based on the feasibility map generated in this paper, the optimal technology for desalination can be selected as a function of brackish water concentration and brine disposal.

Keywords

• Capacitive deionization
• Reverse osmosis
• Techno-economic analysis
• Brine disposal
• Feasibility map