Forward osmosis, or just osmosis, occurs when a solution with a lower solute concentration passes through a semipermeable membrane to a region of higher solute concentration. In other words, let’s say we have a solution of salt water, where the water is the solvent and the salt is the solute. On one side, the water contains less salt, so the solute concentration is low. On the other, the water contains more salt, so the solute concentration is high. The semipermeable membrane is a thin film of material which, here, allows water to pass through but not salt. Therefore, the water on the side with less solute passes through to the side with more. In actuality, water is travelling across both sides: left to right and right to left. The overall, or net, flow occurs from lowest solute concentration to highest solute concentration. Eventually, the two sides reach equilibrium and will have the same concentration of salt. The osmotic pressure is the pressure exerted by the net flow of water through the membrane and is the pressure that would need to be applied to push the water back to the left side.
Osmosis requires no external force and occurs naturally in many instances, such as water absorption through plant roots, dehydration after consuming salt, and the strengthening of blood cell walls. Water passes through the membranes of plant cells to keep their cell walls rigid. As cell walls become rigid and more water comes through, that water passes through more and more cells until the water reaches the cells that need it. Similarly in animals, after consuming salt, the red blood cells will excrete water to balance out the salt concentration in the animal’s body. The kidneys regulate this to make sure the red blood cells do not become dehydrated (or overhydrated when absorbing water). Like plant cells, blood cells use water to keep their membranes rigid and strong.
Reverse osmosis (RO) occurs when an external pressure is applied to the solution of highest solute concentration, i.e., the saltiest side. This forces water molecules to pass through the membrane, leaving behind salt. Do remember that just because salt is used in the example, that is not the only thing that is usually filtered out. Chlorine, lead, detergents, fluoride, insecticides, and many more impurities can be removed using RO filtration. Some impurities typically remain, as it is very difficult to remove all of them or even all of a single one. A good filter will reduce the concentration to 10% or less. There are some that are able to reduce it to 0%. RO is commonly used in the desalination of saltwater, as well as electrolysis.