A Summary of Reverse Osmosis
Reverse osmosis (RO) is a water purification technology that uses a semipermeable membrane to remove ions, molecules, and larger particles from water. The RO process has been studied as early as the 1850s, though its significant value was not realized until 1931 when the first process for RO-based water desalination was patented. Despite its potential for water purification, initial RO desalination technologies were not able to process water quickly enough to be practical. The first commercially viable RO desalination membrane was eventually developed in 1962 and led to the construction of the first pilot-scale commercial RO desalination plant based on its technology in Coalinga, California in 1965. Since then, RO desalination has become the world’s most used water purification technology with uses in thousands of processes and applications. As clean water becomes increasingly valuable in the future, the market for water purification technologies is expected to grow significantly and RO is sure to play a significant role in satisfying this demand.
To understand the process of reverse osmosis it helps to first be familiar with standard forward osmosis, commonly referred to as just “osmosis.” Osmosis is a naturally occurring, spontaneous process that takes place when two solutions of different concentrations are separated by a semipermeable membrane, which allows for passage of the solvent (e.g. water), while acting as a barrier for the passage of any dissolved or suspended solutes or contaminants (e.g. salt). In osmosis, water from the less concentrated solution passes through the membrane into the more concentrated solution in order to make the concentrations of both solutions more similar, as seen in the figure on the right. Solvent from the less concentrated solution will always migrate into the more concentrated solution until the concentrations on either side of the membrane are identical. Osmosis is an integral process driving life on earth and can be observed in a wide variety of biological processes in daily life. A common example of osmosis in nature is the ability of plants to absorb water through their roots from the soil.
Understanding Reverse Osmosis
In RO, the forward osmosis process is reversed by applying sufficient pressure to the more concentrated solution to reverse the natural flow of the solvent. Instead of bringing the concentrations of the solutions closer together, reverse osmosis drives the concentrations on either side of the membrane further apart, as seen in the figure on the right. Using seawater as an example, when freshwater and seawater are separated by a semipermeable membrane, the natural forward osmotic process would see freshwater pass through the membrane into the seawater. However, pressure can be applied to the seawater side to drive a reverse osmosis process where pure water is extracted from the saline seawater solution. This is the fundamental principle behind the operation of RO desalination plants and is widely used around the world today.
Standard osmosis sees the natural flow of pure water into salt water in order to make the concentration of salt water on both sides of the membrane more similar.
In RO, the pressure is applied to the saltwater side of the membrane to reverse the flow of standard osmosis. This concentrates salt on one side of the membrane and produces pure water on the other side.
In a continuous process, the stream of pure water is collected and referred to as the permeate. The concentrated waste stream is referred to as the retentate, which is typically rejected and disposed of but may also be recycled or harvested as a product itself.
An Introduction to
Reverse Osmosis at Work
RO processes are operated by applying a high pressure pump to increase the pressure on the concentrated side of the RO membrane. This drives the water across the RO membrane, removing 95-99% of dissolved salts and rejecting them into a waste stream known as the retentate. The retentate is then drained off or can be fed back into the feed water supply in some circumstances to be recycled through the RO system to save water. The purified water is produced on the other side of the reverse osmosis membrane and is known as permeate. The minimum amount of pressure required depends on the osmotic pressure, or the pressure applied by the forward osmotic process that must be overcome to drive reverse osmosis. This pressure depends on the salt concentration of the feed water; the more concentrated the feed water, the stronger the osmotic pressure, and thus the more pressure is required to overcome it and to drive the RO process. Today, RO is the most energy-efficient process for seawater desalination from small to industrial scales and remains the benchmark to beat for any potential innovators. An example configuration for a typical industrial RO desalination process can be seen in the figure below.