Osmosis is a natural process that occurs when two volumes of water are separated by a semi-permeable membrane. A semi-permeable membrane has holes that are small enough to trap contaminants, but allow water to flow through. Water will flow through the semi-permeable membrane from the side of the low solute concentration (fewer contaminants) to the side of the high solute concentration (more contaminants) in order to restore equilibrium between the two sides. This flow of water may be stopped or even reversed if external pressure is applied to the higher solute concentration. This process is used in water purification, manufacturing plants, and chemical laboratories.
Reverse osmosis occurs when pressure is applied to a volume of water. So, for example, when pressure is applied to a volume of saltwater, the water (the solvent) will pass through the semi-permeable membrane into the other container, leaving behind the salt. This is one way that saltwater can be made potable.
Reverse osmosis (RO) is only one stage of a typical RO system. Sediment and carbon filtration systems are normally included with an RO system, with each stage of filtration contributing to the purification process.
The first stage of filtration is the sediment filter, which reduces suspended particles such as dirt, dust, and rust.
The second stage of filtration is the carbon filter. This filter reduces volatile organic chemicals (VOCs), chlorine, and other compounds that might impact the taste or odor of the tap water. Chlorine should be removed from the water before the water enters the membrane. This will help preserve the life of the membrane.
The center and third stage of a reverse osmosis system is the semi-permeable membrane. It is responsible for rejecting up to 98% of the total dissolved solids in the water. This is where the purification takes place.