## 1 Introduction  
Water scarcity is becoming one of the most challenging issues worldwide, especially with the rapid increase of the population. Nowadays, about 25% of the world's population does not have access to a satisfactory quality and quantity of potable water, and around 70 countries had medium to extremely high water stress levels by 2019 [1, 2]. Increasing water demand, either domestic or industrial, along with the raising of voices to solve water issues, are pushing the growth of water desalination technologies forward in technology and penetration. Desalination of brackish or sea water now represents a consolidated system to resolve the water scarcity emergency. The main drawbacks of this solution, however, remains the high energy consumption, and also the high amount of greenhouse gases (GHG) produced in the case of traditional water desalination plants powered by conventional fuels. Reverse osmosis (RO) is a membrane separation process and is considered one of the physical/chemical processes. Dissolved organic and inorganic substances such as viruses or ions can be removed from water by the reverse osmosis procedure. The RO process remains one of the most widespread water desalination technologies deployed, highlighted by its low energy demand compared to other commercial technologies. RO technology became a competitive water desalination technology due to several of its properties, like low energy consumption and operation flexibility. However, pretreatment processing of the brackish or sea water is necessary, like filtration and anti-scalant addition [3]. According to the International Desalination Association (IDA), the total number of desalination plants worldwide is more than 20,000 (as of June 2018) including conventional desalination plants. Those produce more than 100 million cubic meters of desalinated water per day to meet the demands of more than 300 million people in around 150 countries worldwide [4]. Figure 1 illustrates the distribution of the installed water desalination plants globally [5]. It is noticeable that the interest in using water desalination plants and the realization of their necessity are increasing. According to [PERSON]. [PERSON] et al. [6], the total water demand in the Middle East and North Africa (MENA) will increase from 9000 million cubic meters in 2010 up to 13,300 million cubic meter in 2030, because of the growing population and depletion of surface and groundwater resources. Consequently, the desalination capacity in the MENA region is expected to grow from 21 million m\({}^{3}\)/day in 2007 to nearly 110 million m\({}^{3}\)/day by 2030 (of which 70% will be in Saudi Arabia, the United Arab Emirates, Kuwait, Algeria, and Libya) [6]. Figure 2 illustrates the return flow ratio; this is a scale to measure the overall water risk depending on its quality. Return flow ratio measures the percentage of available, previously used water that is discharged upstream as wastewater. Higher values indicate higher dependence on treatment plants and potentially lower water quality in areas that lack enough treatment infrastructure and policies. Arid areas with low water use are shown in grey and scored as low stress when calculating aggregated scores [7]. This clearly indicates the urgency to adapt advanced technologies to diversify the sources of clean water and to utilize non-conventional water resources, which will include desalination resources as a major share. Figure 1: (**a**) Global installed desalination capacity 1960–2020, (**b**) operational capacity by desalination technology. Reproduced with permission from [PERSON] et al., Science of the Total Environment); published by ScienceDirect, 2019 [5]. Water desalination plants powered from renewable energy resources are growing, especially in arid regions with substantial solar energy potential such as the MEAN region. Many of the existing renewable-energy-powered desalination systems, nowadays, are implemented in small capacities from a few m\({}^{3}\)/day up to 10 m\({}^{3}\)/day.