A semi-permeable membrane, like a membrane composed of cell walls or like the arrangement of cells in the bladder, is selective against objects going through it. Generally, this membrane is very easy to traverse by water because of its small molecular size block other contaminants from trying to get through it. As experiments, water is loaded on both sides of the membrane, where water on one side has different concentrations of dissolved minerals, since water has the property of moving from a low concentration solution to a higher concentrated solution, the water will move (diffuse) through the membrane from the side low concentration to the higher concentration side. Thus, the osmotic pressure will counter the spreading process and will form an equilibrium.
The best reverse osmosis system process moves water from high contaminant concentrations (as raw water) to water reservoirs that have very low concentrations of contaminants. By using high-pressure water on the raw water side, it can create a reverse process of the natural process of osmosis. By still using a semi-permeable membrane it will only allow the water molecules through it and dispose of various dissolved contaminants. The specific process that occurs is called exclusion ions, in which a number of ions on the membrane surface as a barrier allows the molecules of water to pass through as they release other substances.
The semi-permeable membrane in the early experiments of osmosis originated from the swine bladder. Prior to 1960, these types of membranes were considered highly inefficient, expensive, and unreliable for the use of osmosis applications outside the laboratory. Modern synthetic materials, capable of solving this problem, make membranes more effective at removing contaminants and making them stronger to withstand greater water pressures as operating efficiencies.
Despite its ability to purify raw water, a Reverse Osmosis system must be periodically cleaned to prevent the formation of the crust on the membrane surface. The Reverse Osmosis system requires carbon as the initial filter to reduce the chlorine content that will damage the Reverse Osmosis membrane and also requires sediment filters to filter dissolved materials from raw water so that they do not penetrate the membrane.