## 1 Introduction  
Despite being called the Blue Planet because two thirds of it is covered by water, 97% of the water of the Earth is seawater (Figure 0(a)) and only 3% is freshwater, in the form of glaciers (2.5%) and other water bodies of groundwater (aquifers), superficial water (lakes and rivers), atmosphere (water vapor), and even organisms [1, 2]. A combination of the exponentially rising population, the advance of drought and desertification as consequences of global warming and climate change, and the scarcity and unequal global distribution of water resources and other associated problems such as pollution is endangering, aggravating, and accelerating the loss of water available for human consumption, hydric stress,and the water crisis. This water stress is based on freshwater availability and affects different levels (ecology, biology, economy, social, etc.) depending on the geographic location. Around 4 billion people live in areas that suffer from severe physical water scarcity for at least one month per year [3]; these regions are mostly located between the tropics and the Northern Hemisphere (Figure 1b, [4]). It is estimated that more than 1.8 billion people will live in regions without water, and two-thirds of the global population will live under hydric stress. Unfortunately, the scarcity of water and water quality deterioration go hand in hand with famine, low development of agriculture, and low social development. Figure 1: (a) Distribution of the total water on Earth in percentages (Source: [5]). (b) Water risk Index Map (Sources: [6; 7]). (c) Desalination process. According to the United Nations World Water Development Report [8], global water demand is expected to continue increasing at a similar rate until 2050, accounting for an increase of 20 to 30% above the current level of water use, mainly due to rising demand in the industrial, agricultural, and domestic sectors. Water demand is increasing, while supplies are shrinking (due to superficial and aquifer contamination, glacier melting, droughs, increasing urbanization, etc.). To cover this demand, the water industry must make better and more efficient use of available water resources, by means of water purification, water desalination, and waste water treatment and in industrial activities that require the use of water. Due to the huge proportion of seawater (Figure 1a) and the fact that most countries have easy access to it (Figure 1b), one of the most feasible ways to obtain freshwater is through desalination. This water treatment is based on liquid-solid separation techniques such as evaporation and reverse osmosis (RO), but these techniques are not completely efficient (between 60-98%) due to the generation of a waste product known as brine (Figure 1c). A desalination plant operating using RO commonly recovers 40%, i.e., 0.40 m\({}^{3}\) of purified water is produced from each 1 m\({}^{3}\) of seawater, and 0.60 m\({}^{3}\) is transformed into brine to be discharged back into the ocean. The state of desalinization and brine discharges has been recently and thoroughly reviewed [9]. It is estimated that more than 15 thousand operational desalination plants produce 95 million m\({}^{3}\) of desalinated water per day, and more than 140 million m\({}^{3}\) of brine per day. Most of these plants are installed in the Middle East and North Africa (coinciding with areas under hydric stress, Figure 1b); although half of them are destined for industrial purposes, more than 62% of desalinated water is destined for human consumption. RO is by far the dominant treatment technology (84% of the operational plants). The downsides of desalination are the intake and pretreatment strategies, use of land, elevated power costs, membrane separation saturations, chemical addition and biofouling, ecological and environmental problems associated with the brine returning into the sea, and social rejection [10; 11; 12; 13].