Put a raisin in water for some time, strange it may seem but the raisin will swell up by absorbing water. The high sugar and other solute content inside the raisin draw the water inside. The skin or the cover of the raisin is a semi-permeable membrane which allows water to cross through it but not the larger molecules like sugar etc. This process of solvent permeating a semi-permeable membrane from a rarer medium to a denser medium is called Osmosis. In other words, 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.
In Reverse Osmosis Water Purification System, a polymer-based semi-permeable membrane is used for filtering the salts present in the water. This, additionally, filters all the microbes present in water. A pressure higher than the osmotic pressure is applied to the impure water containing excess minerals and impurities 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 Reverse Osmosis water treatment 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.
Reverse Osmosis water treatment 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. Finally, modern day Reverse Osmosis water treatment units, like Nasaka’s, includes innovative technology like OrpH+ to add immunity boosting properties to the purified water.