Hybrid system of reverse osmosis -membrane capacitive deionization-reverse electrodialysis for energy efficient high-salinity seawater desalination

Abstract:

High-salinity seawater, above 35000 mg L^-1 TDS, desalination with a conventional sweater reverse osmosis (SWRO) process (50% recovery) often demands an additional desalination process to meet produced water qualities. Although treating the permeate of SWRO with a brackish water reverse osmosis (BWRO) process, so called two-pass RO, fulfils the required water qualities, the energy requirements and the difficulties in optimization and flexible operation of these consecutive RO processes remain challenges.

This study introduces a novel concept for a hybrid system of reverse electrodialysis (RED), membrane capacitive deionization (MCDI), and RO processes to treat high-salinity seawater with enhanced energy efficiency. Based on process modeling and the specific energy consumption (SEC) analysis, the MCDI process exhibited better energy efficiency than the BWRO and the produced water quality met the regulations (500 mg L^-1 TDS) by the World Health Organization (WHO). For example, with 30% and 80% water recovery of the first and second processes, respectively, the SEC of BWRO was more than twice as high as that of MCDI (0.16 and 0.38 kWh m^-3 for MCDI and BWRO, respectively). In addition, the RED unit harvested the energy from the salinity gradient between RO brine (a high concentration solution) and MCDI brine (a relatively low concentration solution). Various operating conditions for the RED unit were investigated and the results showed that substantial energy can be generated from the RO–MCDI–RED hybrid system that would reduce the overall SEC for high-salinity seawater desalination as well as the environmental and ecological impact induced by the discharge brine. Furthermore, the RED process generated more power integrating with the RO-MCDI system compared to the conventional two-pass RO system (2.8 and 2.5 W m^-2 for RO-MCDI and two-pass RO, respectively) mainly because the larger concentration difference between RO and MCDI brines resulted in the higher salinity gradient in RED.

In summary, this study highlights the superior potential of RO-MCDI-RED system as a substitute for the conventional two-pass RO system. With those notable advances, an RO-MCDI-RED hybrid system could be a next-generation seawater desalination system.