An energy-saving “nanofiltration/electrodialysis with polarity reversal (NF/EDR)” integrated membrane process for seawater desalination. Part III. Optimization of the energy consumption in a demonstration operation

Abstract

For the proposed energy-saving “nanofiltration/electrodialysis with polarity reversal (NF/EDR)” integrated membrane process for seawater desalination, studies on the NF process and the optimization of the ED process have been previously conducted. Based on the obtained results, a multi-stage, multi-passage ED process is presented in this manuscript to realize the optimization of energy consumption. In addition, the integration between NF and EDR processes at a pilot scale was accomplished, in which actual seawater was the feed water and >1200 h of demonstrative operation was implemented. During the NF process, 15 NF90-4040 elements with high desalination capacity were applied. During the EDR process, the membrane stack consisted of three stages with six passages were utilized, in which 250 cell pairs were included with unequal pairs of membranes for each stage. From the first to the third stage, there were 100, 80, and 70 membrane pairs, respectively. In addition, the energy-saving process was achieved by thinning the thickness of the electrode chamber to 2 mm and filling the electrode chambers with mixed ion exchange resin at an appropriate volume ratio. Furthermore, the voltage and the current for each stage can be independently controlled. The results show that the total desalination rate of the integrated process can be 99.26%, the system energy consumption is <2.15 kWh/(m3 produced water), which is significantly lower than that of SWRO, suggesting that the proposed integrated membrane process is an efficient and economically feasible seawater desalination system.