## 1 Introduction  
The world has sought a sustainable and environmentally friendly society to overcome the crisis caused by climate change threatening food and water security as well as biodiversity [1, 2, 3, 4]. For instance, many countries have been struggling to reduce carbon emissions to about 50% of 1990 levels by 2030 and reach carbon neutrality by 2050, since the European Green Deal was announced [5, 6]. To achieve carbon neutrality, it is necessary for humankind to reduce the amount of carbon burned for energy generation in all areas. It is also the case for the industries related to water supply and water infrastructure. Many people have also made continuous efforts to reduce the carbon footprint in membrane-based water and wastewater treatment in various ways. For example, researchers have tried to improve the energy efficiency required for wastewater treatment using membrane bioreactors via the development of antifouling membranes or cleaning methods [7; 8; 9] while producing green energy such as biomethane generated from an anaerobic digestion process [10; 11; 12].  
An attempt to enhance the energy efficiency in membrane desalination also has a long history. For example, the specific energy consumption (SEC) of a reverse osmosis (RO) process has continuously decreased (from 12 kWh m\({}^{-3}\) in the 1970s to about 2 kWh m\({}^{-3}\) in 2006) over the three decades thanks to the advances in membrane materials as well as module and process designs [13; 14; 15; 16; 17]. Consequently, new light was shed on the thought of having to improve the selectivity rather than permeability [18], leading to calls for greater focus on enhancing salt rejection of the membrane to guarantee higher retention of inadequate solutes (e.g., boron [19; 20]), thereby improving treated water quality while reducing the costs imposed by pre- and post-treatment processes [21; 22]. As if to reflect the new trend, research work to meet the demand for improved salt rejection has recently been published [2] in contrast with the existing mainstream to overcome the permeability-selectivity trade-off by enhancing water permeability while maintaining salt rejection [23; 24; 25]. However, it is still hard to find research papers empirically dealing with increasing the salt rejection of RO membranes. In particular, it is rare to find research papers in the literature addressing the SEC change caused by the trade-off between water permeability and salt rejection possibly occurring while increasing the selectivity of RO membranes.  
If improving salt rejection causes the SEC to jump, we will no longer be able to claim that it would take precedence over enhancing water permeability. Note that improving water permeability also brings merits such as a small footprint, saving the membrane area, and cutting down on the cost of chemical cleaning agents and membrane replacement [26]. Accordingly, one should remember that the gains from increasing salt rejection could surpass the benefits resulting from higher water permeability on the condition that it would not be too costly to ameliorate salt rejection (i.e., without a jump in the SEC). With that in mind, this paper examined whether the SEC of an RO process can be significantly influenced by an increase in salt rejection at the expense of a decrease in water permeability. Furthermore, a qualitative assessment of the SEC was carried out to weigh up a better approach to increase salt rejection between material and process perspectives.