Potable water can be made from sea water, through a process known as desalination. It is preferable to make potable water from fresh water reserves rather than from sea water. This is because removing the large amount of sodium chloride (35 grams in every kilogram of sea water) requires a lot of energy.
Sea water is heated until it boils. The salt remains in the liquid, and the steam is pure water. The steam is cooled and condensed to make potable water.
Distillation requires a lot of energy to boil the water, and also to cool the steam down to condense it. The waste water is very salty and can be difficult to dispose of in a sustainable way which does not harm marine ecosystems.
Water is put under high pressure and passed through a membrane which has tiny pores (holes) in it. The pores allow water molecules through, but prevent most ions and molecules from passing through. Reverse osmosis requires expensive membranes and also produces a large volume of waste water, so its efficiency is often quite low.
Which method of water purification would be best suited to a country in the Middle East which has a large amount of fossil fuel reserves?
Distillation would be the most appropriate method of desalination because the country would be able to use fossil fuels to provide the energy needed at a relatively low cost.