Osmosis is the natural movement of water from an area of high water concentration (low salt concentration) through a salt barrier to an area of low water concentration (high salt concentration). Flow is driven by the difference in osmotic potential of the two solutions, quantified as osmotic pressure. Applying an external pressure to reverse the natural flow of water through the barrier is reverse osmosis.
The process of osmosis through a semipermeable membrane was first observed in 1748 by Jean-Antoine Nollet. Reverse Osmosis was known in the 1950s, but was not practically demonstrated until the early 1960s with the discovery of asymmetric membranes at University of California at Los Angeles by Sidney Loeb and Srinivasa Sourirajan. These membranes are characterized by a thin “skin” layer supported atop a highly porous and much thicker substrate. This basic structure remains as the basis of reverse osmosis membranes today.
The reverse osmosis process is this: a salt solution is pressurized against RO membranes. Salts and impurities are retained on the pressurized side of the membrane and purified water flows through the membranes. The salty reject stream is called brine and the purified water or the product of the RO process is called permeate. RO requires flow across the membranes or cross flow to keep the membrane surfaces clear of concentrated impurities, allowing continuous and almost constant flow of permeate. This is distinct from conventional filtration processes in which impurities embed in the filter or build up as a cake that must be flushed or removed periodically to restore productivity.
The minimum pressure required to separate pure water from impure water can be considered a barrier. The height of this barrier depends upon the osmotic pressure or potential of the water, which in turn depends upon its salinity and composition. Saltier water has a higher osmotic potential and requires more pressure to desalinate. To drive permeate through membranes at reasonable fluxes, the osmotic potential must not only be met but overcome. This overpressure depends partly upon the permeability of the membranes. In addition, the salt concentration immediately adjacent to the membrane surface is elevated, which increases the osmotic barrier above that in the bulk of the salt water – a phenomenon known at concentration polarization. Concentration polarization is reduced with cross flow. Other non-idealities in RO systems that elevate energy requirements include resistance to flow through the membranes, mechanical and volumetric efficiency losses in pumps, and viscous friction losses in flowing water. Also, membrane surfaces can be blocked with contaminants including organic and inorganic foulants or salts that precipitate out of solution.
Today, RO is used in a variety of applications. The most common use is for industrial water treatment. Industrial RO applications include supply water purification, wastewater recycling and wastewater concentration. Nearly 70% of factories in the U.S. require water treatment, and for many, RO is the treatment process of choice. RO is also used in agriculture and for municipal water supply.
Read More: Reverse Osmosis Systems and Configurations