Reverse Osmosis
In cases where the water is very hard or salty, then it is unprofitable to use professional conventional filters. In this case, the solution is provided by Reverse Osmosis. In short, osmosis is the phenomenon of passage of solvent molecules (eg water) through a semi-permeable membrane, from the solvent to the solution. Or from a solution of lower concentration (diluter) to one of higher concentration (thicker). A semipermeable membrane (or osmosis membrane) is a membrane that allows solvent molecules (eg water) to pass through its pores and does not allow solute molecules to pass through it. All home reverse osmosis water filters have such membranes.

What is reverse osmosis?
Suppose we have in a tube, where it has a membrane in the middle and to the right of the membrane there is a solution. In the solution, the solute is sugar and has concentration C and the solvent is water.

On the left side we have pure solvent (water) with pure solute concentration C=0.

Remember that membranes only allow solvent molecules to pass through it, so only pure water can pass through the membrane. At first, water flows faster from left to right than from right to left.

After a while though, the concentrations will be the same, meaning water going from left to right will run at the same speed as water going from right to left.

The above process is called osmosis.

Let us now see what reverse osmosis is.

If on the right side we apply an external pressure greater than the osmotic pressure of the solution (i.e. Pex > Π), then the phenomenon is reversed and solvent molecules will leave the solution towards the pure solvent (or from the denser to the thinner solution, i.e. from right to left). This is reverse osmosis.

In essence, we force the dirty water to pass through the reverse osmosis membrane. However, since only pure water can penetrate the membrane, the rest of the water is excess. The excess water leaves through the drain and we drink only the clean water that has penetrated the membrane.