As the name suggests, Reverse Osmosis (RO) is the exact opposite process osmosis. Osmosis is the process in which the solvent from a region of lower solute concentration crosses over a semi-permeable membrane to a region of higher concentration. For instance, if a raisin is kept in water for some time, it swells up by gaining water. Here the sugar and other solvent content are much higher inside the raisin as compared to the ambient water surrounding it. The skin or the cover of the raisin allows water to cross through it but not the other molecules like sugar etc. So its cover is a semi-permeable membrane. Due to the concentration gradient, the natural process of osmosis takes place wherein water molecules pass through the raisin cover into the raisin. Here the driving force is the concentration gradient. If this process of osmosis could be reversed using any other driving force, then this would be called Reverse Osmosis. In fact, a pressure called osmotic pressure must be applied to restrict osmosis across a semi-permeable membrane when there is a concentration gradient across the membrane. From there emanates the idea of Reverse Osmosis through the application of pressure. In practice, Reverse Osmosis is the process in which pressure is used to force the solvent from a higher solute concentration region to a lower concentration region through a semi-permeable membrane.
Nowadays, Reverse Osmosis process is extensively used for purification of water. The process of osmosis has been first scientifically noted in 1748 but the reverse of it that is Reverse Osmosis was first obtained in 1950 wherein salt from seawater was removed to obtain pure water. But it was at the laboratory scale. It was in 1977 when Reverse Osmosis was first used to real life applications. Presently, Reverse Osmosis process is used both at the municipal level and as a home appliance.
In Reverse Osmosis process, a polymer-based semi-permeable membrane is used for filtering the salts present in the water. A pressure higher than the osmotic pressure is applied to the impure water containing excess salts and minerals to make the water molecules cross the semi-permeable polymer membrane. The membrane being semi-permeable does not allow the impurities, including salts and minerals, to pass. Therefore in a Reverse Osmosis purification unit, pure water is obtained on the other side of the semi-permeable membrane. As the pure water is dispelled from the input raw water to be treated, the concentration of solutes goes on increasing. This, in turn, raises the osmotic pressure. To sustain further purification, the applied pressure has to be increased but the semi-permeable membrane in the Reverse Osmosis unit has a finite sustenance against increasing pressure. So, in order to maintain the semi-permeable membrane or the Reverse Osmosis membrane, the commercial Reverse Osmosis units discards the leftover water through a waste water outlet. This dispelled thus becomes hard water is usually unsuitable for daily usage but can be used in applications like the toilet flush.
To summarize, a basic commercial Reverse Osmosis unit consists of a motor pump to apply pressure, a waste water outlet, a Reverse Osmosis (semi-permeable) membrane, and a water storage unit. In modern-day Reverse Osmosis units, Ultraviolet Light treatment and pre Reverse Osmosis carbon and fabric filters are also included. The Ultraviolet Light kills the microorganisms whereas the carbon and fabric filters restrict silts and other large impurities from reaching the Reverse Osmosis to enhance its durability.