Reverse osmosis (RO) is a water purification technology that employs a semi-permeable membrane to remove ions, unwanted molecules, and larger particles from drinking water. In reverse osmosis, pressure is applied to overcome osmotic pressure, a colligative property driven by chemical potential differences of the solvent, a thermodynamic parameter. This process can remove many types of dissolved and suspended chemical species as well as biological ones (primarily bacteria) from water, and is used in both industrial processes and the production of potable water.
The mechanism of reverse osmosis relies on a high-pressure pump to increase the pressure on the salt side of the RO and force the water across the semi-permeable RO membrane, leaving almost all (around 95% to 99%) of dissolved salts behind in the reject stream. The amount of pressure required depends on the salt concentration of the feed water. The higher the concentration, the more pressure is needed to overcome the osmotic pressure. The desalinated water that is demineralized or deionized, is called permeate (or product) water. The water stream that carries the concentrated contaminants that did not pass through the RO membrane is called the reject (or concentrate) stream.
As the feed water enters the RO membrane under pressure (enough pressure to overcome osmotic pressure), the water molecules pass through the semi-permeable membrane and the salts and other contaminants are not allowed to pass and are discharged through the reject stream. The membranes used in reverse osmosis systems are made of various materials, including thin-film composite membranes, which are characterized by permeability and selectivity. These membranes are engineered to allow only water to pass through while preventing the passage of solutes (such as salt ions). This selective process is critical for the effective production of clean water.
Reverse osmosis is used in various applications beyond just drinking water purification. It is instrumental in the dialysis processes used in medicine, as a vital part of both kidney dialysis and water purification for medical uses. It's also employed extensively in the food and beverage industry, for example in the concentration of fruit juices and the dairy sector for the production of whey protein concentrates. Moreover, RO has significant industrial applications, such as in the production of ultrapure water for semiconductor manufacturing and power plants. The technology's versatility and efficiency make it a critical component in many sectors, highlighting the ongoing importance of advanced filtration methods.