Introduction
Reverse osmosis is a process by which a solvent (in this case water) passes through a semipermeable membrane in the direction opposite to that for natural osmosis (hence the name ‘reverse’) when subjected to an amount of pressure, leaving behind any solutes such as ions, molecules, and larger particles. The semi-permeable membrane only allows water molecules and molecules which are smaller than water to pass through. Hence, anything which are larger than water molecules, like dissolved minerals and bacteria, will not be able to pass through the semi-permeable membrane. Reverse osmosis can be used in water desalination, recycling, and wastewater treatment.

Explanation
To understand reverse osmosis, we need to first understand what osmosis is. Osmosis is the tendency of molecules of a solvent to pass through a semipermeable membrane from a less concentrated solution into a more concentrated one. This process occurs without energy input.

For osmosis, water flows from the less concentrated solution to the more concentrated solution across the semipermeable membrane. Osmotic pressure is built up so that osmosis is stopped and there is no net flow of water. The opposite is observed for reverse osmosis. When an external pressure is applied to seawater, it forces water molecules to flow from the more concentrated solution to the other side which is less concentrated across semipermeable membrane to obtain freshwater.

To stop osmosis, meaning to stop the net flow of water molecules, pressure is applied on the the solution which is more concentrated to overcome the pressure generated by the movement of solvent molecules from the other side. The minimum amount of pressure needed is called osmotic pressure.

When the pressure applied is greater than the osmotic pressure, the flow of solvent will be reversed. This results in the flow of water molecules from the higher concentrated solution to the less concentrated solution. The act of applying pressure to reverse the flow is called ‘reverse osmosis’. Hence, pure water can be obtained since ions like Na+, Ca2+, F– and organic molecules like glucose, urea will be unable to pass through the semi-permeable membrane and retained at the other side.

One of the main use of reverse osmosis is in water desalination which is to remove minerals from seawater. A simplified explanation of how reverse osmosis works in water desalination is as follows: we have saltwater solution on one side of a tank and pure water on the other side, separated by a semipermeable membrane. Then, pressure applied to the saltwater side of the tank will be sufficient to counteract the osmotic pressure from the pure water side, and to push the saltwater through the filter. Water molecules from the saltwater will pass through the membrane to the pure water side, leaving the salt on the other side. Moreover, chemicals (such as fluoride) and other impurities are also filtered out in the process.

However, the water obtained is still not 100% suitable for drinking after reverse osmosis. Impurities like volatile organic chemicals (VOCs) are generally smaller in size than water. Hence, it will also pass through the semi-permeable membrane together with water molecules. Secondly, reverse osmosis removes alkaline minerals like magnesium, sodium and calcium which then produces acidic water. This will be harmful to our body. Hence, additional process is required, like in case of NEWater, ultraviolet disinfection is then required to remove any remaining impurities and microorganisms and addition of chemicals to restore the pH of water.

References
http://www.allaboutwater.org/reverse-osmosis.html
https://www.pub.gov.sg/watersupply/fournationaltaps/newater
https://en.wikipedia.org/wiki/Osmosis
https://en.wikipedia.org/wiki/Reverse_osmosis
http://science.howstuffworks.com/reverse-osmosis.htm/printable
http://www.chem1.com/acad/webtext/solut/solut-4.html