Optimization of a nanofiltration and membrane capacitive deionization (NF-MCDI) hybrid system: Experimental and modeling studies

Typical reverse osmosis (RO) systems achieve a high level of performance in removing salt from feed water; however, they are relatively energy-intensive even for brackish water applications, due to the high water pressure required to overcome their high membrane resistance. Thus, we propose an energy-efficient hybrid system in which nanofiltration (NF) is sequentially coupled with membrane capacitive deionization (MCDI). The performance of the NF-MCDI hybrid system was demonstrated through experiments and modeling under various operating conditions. First, we experimentally evaluated the impacts of feed concentration, NF recovery rate, and MCDI flow rate on removal rate and energy consumption of the individual NF and MCDI processes. Then, a response surface methodology (RSM) analysis demonstrated the significant dependence of salt removal and energy consumption on the feed concentration, NF recovery, and MCDI flow rate within the NF-MCDI system. Although NF alone is insufficient to remove a large amount of salt from salty water, the proposed system configuration using MCDI was found to improve salt removal by up to 95%. Furthermore, it was found that the optimized NF-MCDI system (at <= 10 g/L feed concentration) can outperform conventional RO systems in terms of energy efficiency while meeting drinking water standards (<= 0.5 g/L).