# The Synergy of Renewable Energy and Desalination:
## 2 Overview of Desalination Technologies
### Membrane-Based Methods  
Reverse osmosis (RO) is a widely used water purification technology that employs a semipermeable membrane to remove impurities from water. While the trapped concentrated brine is either released or processed for possible reuse, the water molecules that flow across the semipermeable membrane are captured as freshwater. To meet certain water quality requirements, the freshwater generated by an RO system is subjected to further treatment. The process involves applying pressure to overcome osmotic pressure, forcing water through the membrane while leaving contaminants behind [37; 38]. RO is effective in removing a variety of substances, including salts, bacteria, viruses, and organic compounds, making it a popular choice for desalination and wastewater treatment. The technology is particularly valuable in desalination, where it can convert seawater into potable water. In desalination applications, RO systems provide several advantages. These include lower energy and capital costs, less physical space needed, greater recovery ratios, and less vulnerability to corrosion and scale, because the membrane can withstand microbial contamination and operates at room temperature. Despite its effectiveness, RO has some limitations, such as the need for pre-treatment to prevent membrane fouling and the generation of concentrated waste streams. Additionally, RO systems can be energy-intensive due to the pressure required for operation. RO is the most popular desalination technology at the moment for seawater desalination because of its greater recovery rate, lower energy usage, and cheaper water production costs. Recent advancements focus on improving membrane materials and configurations to enhance efficiency and reduce costs. With modern RO methods, seawater requires from 3 to 8 kWh/m\({}^{3}\) at discharge pressure of 55 to 82 bar, while brackish water from big to medium-sized plants requires 1.5 to 2.5 kWh/m\({}^{3}\)[39]. Nanofiltration (NF) is a membrane filtration process that operates between microfiltration and reverse osmosis, effectively removing divalent ions and larger organic molecules while allowing monovalent ions to pass through. The NF membranes have pore sizes typically ranging from 1 to 10 nanometers, making them suitable for applications that require selective separation without the energy-intensive processes associated with RO. NF is particularly effective in desalination and softening hard water by removing calcium \begin{table}
\begin{tabular}{c c c c c} \hline  & **MSF** & **MED** & **VC** & **Solar Still** \\ \hline Cost & High & Moderate to High & Moderate & Low \\ \hline Energy consumption & High & Moderate & Low to Moderate & Very Low \\ \hline Technology Maturity Level & Mature & Mature & Mature & Emerging \\ \hline Water Quality & High & High & Moderate to High & Moderate \\ \hline Environmental Impact & Moderate & Moderate & Low & Very Low \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of thermal-based desalination technologies. and magnesium ions, making it valuable in both municipal water treatment and industrial processes [40; 41]. It is also employed in wastewater treatment to recover valuable resources and reduce environmental impact. One of the advantages of NF over RO is its lower operational pressure requirement, which can lead to reduced energy consumption. However, NF membranes are still susceptible to fouling, necessitating effective cleaning protocols and pre-treatment methods. Overall, nanofiltration presents a versatile solution for diverse separation challenges while contributing to sustainable water management practices. Forward osmosis (FO) is an emerging water treatment technology that utilizes the natural osmotic pressure gradient between two solutions to draw water through a semipermeable membrane. The FO system has a low osmotic pressure feed solution on one side and a high osmotic pressure draw solution (DS) on the other.