When two solutions with different concentrations of a solute are mixed together, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions. This is achieved by diffusion, in which solutes will move from areas of higher concentration to areas of lower concentrations until the concentration in all the different areas of the resulting mixture are the same, a state called equilibrium.
Instead of mixing the two solutions together, they can be put in two compartments where they are separated from each other by a semipermeable membrane. The semipermeable membrane doesn't allow the solutes to move from one compartment to the other, but allows the solvent to move. Equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration. Equilibrium is, instead, achieved by the movement of the solvent from areas of low solvent concentration to areas of high solvent concentration. When the solvent moves away from low concentration areas, it causes these areas to become more concentrated. On the other side, when water pores into areas of high concentration, there concentration will decrease. This process is termed osmosis.
In reverse osmosis, in a similar setup as that in osmosis, pressure is applied to the compartment with high concentration. In this case, there are two forces to consider regarding the movement of water: the force of solute concentration difference between the two compartments and the force caused by the externally applied pressure. In the same manner, the solvent cannot move from areas of high pressure to areas of low pressure, because the membrane is not permeable to them. Only water can move in this way. When the effect of the externally applied pressure is greater than that of the concentration difference, net water movement will be from areas of high solute concentration to low solute concentration, and reverse osmosis occurs.
Reverse osmosis in use
In July 2002, Singapore announced that a process named NEWater would be a significant part of its future water plans. It involves using reverse osmosis to treat domestic wastewater before discharging the NEWater back into the reservoirs.