## 3 Results and Discussion
This result indicates that a CB[6] thin-film nanocomposite (TFN) membrane could achieve high salt rejection almost without paying the price for the increased SEC, unlike a process approach. To qualitatively weigh up the SEC change stemming from the process and material approaches to improve salt rejection, we roughly estimated the ideal SEC (SEC\({}_{i}\)) obtainable under ideal operating conditions (i.e., no leakage of energy recovery devices (ERD), perfect pressure transfer efficiency of ERD, and perfect overall pump efficiency) without consideration of the salt concentration based on the following relationship [38]:  
\[\text{SEC}_{i}=\bigg{(}\frac{1\ -\ R_{w}}{R_{w}}\bigg{)}\Delta P+\Big{(}P_{f}-P_{o} \Big{)} \tag{3}\]  
where \(R_{w}\) is the desalinated water recovery, \(\Delta P\) is the pressure difference across the membrane module, \(P_{f}\) is the feed pressure, and \(P_{o}\) is the permeate pressure. The unit of SEC\({}_{i}\) can be converted from Pa to kWh/m\({}^{3}\) with a conversion factor (3.6 \(\times\) 10\({}^{6}\) Pa \(\approx\) 1.0 kWh/m\({}^{3}\)). If \(R_{w}\) is constant while \(\Delta P\) and \(P_{o}\) are negligible for simplifications, the SEC\({}_{i}\) has a nearly linear correlation with \(P_{f}\). In this regard, a process approach such as increasing applied pressure should accept an additional concomitant SEC\({}_{i}\). In contrast, a material approach does not increase the SEC\({}_{i}\), since it does not require a change of applied pressure. Furthermore, the correlation between water permeability and SEC also supports that a material approach is more beneficial than a process approach. Specifically, the SEC soars as water permeability drops (Figure 3) according to the correlation between water permeability and SEC obtainable from the data for BWRO in the previous study [39]. Given that a process approach entailed a significant loss of water permeability (5.54 to 3.31 LMH/bar; about a 40% reduction), it is not a surprise that a process approach may be accompanied by a substantial increase in the SEC. On the other hand, the SEC change by a material approach is likely to be negligible, as evidenced by the CB[6] TFN membrane exhibiting only a marginal reduction in water permeability (4.30 to 4.01 LMH/bar; about a 7% reduction). Although a quantitative assessment could not be made with regard to the SEC due to the lack of information required to model at different pressures with variable salt rejection, those qualitative comparisons showed which approach we need to pursue to improve the selectivity of RO membranes without a substantial increase in the SEC (i.e., a  
Figure 2: (**a**) Water flux and salt rejection and (**b**) water permeability and salt permeability coefficients of the TFC-D1 membrane at different applied pressures. material approach is necessary to make a fundamental improvement in the selectivity of RO membranes). Lastly, it is worth exploring which materials have the potential to increase the selectivity of the commonly used RO membranes (i.e., polyamide RO membranes) in the field and what conditions need to be satisfied to make high-performance RO membranes with high selectivity. As for the specific way to a valid material approach, fabricating thin-film nanocomposite (TFN) RO membranes using porous nanomaterials could be the most appropriate as of now in that the approach can make the existing manufacturing lines to produce conventional RO membranes readily available, facilitating the commercialization of newly developed membranes. When it comes to the potential nanomaterials, it would be desirable to possess pores with low tortuosity and adequate size falling between hydrated ions (Na\({}^{+}\): 7.2 A; Cl\({}^{-}\): 6.6 A) and water molecules (about 2.75 A) to effectively inhibit the salt passage while allowing water passage, thereby increasing the selectivity without significant loss of water permeability.