Reverse osmosis, often abbreviated to RO, is a process that is used to purify water with the help of a semipermeable membrane. It involves the removal of unwanted ions and molecules from water by applying enough pressure to overcome the osmotic pressure of the solvent. It is important to note that reverse osmosis can also be used to remove certain microorganisms (such as bacteria) from water. Potable water can be produced via reverse osmosis on an industrial scale as well as a domestic scale.
Comparison Between Osmosis and Reverse Osmosis
In traditional osmosis processes, the solvent moves through a selectively permeable membrane (or semipermeable membrane, SPM) from a region where the solute concentration is low to a region where the solute concentration is high. However, in reverse osmosis, the solvent moves through the SPM form a region where the solute concentration is high to a region where the solute concentration is low.
The primary driving force behind the movement of solvent molecules in osmosis processes is the reduction in the free energy present in the system. This movement of the solvent molecules generates osmotic pressure, which can be overcome by applying external pressure on the region where the solute concentration is high. If the external pressure is greater than the osmotic pressure, then the flow of solvent molecules is reversed and the solvent begins to flow from a region of high solute concentration to a region of low solute concentration.
Purification of Drinking Water via Reverse Osmosis
Many household water purification systems employ the reverse osmosis process as one of the steps. Typically, the purification of water (at a relatively small scale) involves the following steps:
- The trapping of particulate matter such as calcium carbonate and rust with the help of a sediment filter.
- The trapping of finer particles with a secondary sediment filter featuring smaller pores.
- Filtration of chlorine and some organic compounds via an activated carbon filter.
- Removal of other contaminants via reverse osmosis. This step usually involves a thin film of a composite membrane.
- A secondary filter made up of activated carbon to filter any remaining contaminants.
- The sterilization of the water with the help of an ultraviolet lamp (this step kills any lingering microbes in the water).
Thus, the contaminated water can be purified and converted into potable water at a domestic scale with the help of an RO water purifier.
What are the Disadvantages of Reverse Osmosis?
At a domestic scale, water purifiers that are based on reverse osmosis typically waste a lot of water. In fact, domestic RO setups usually retain only 5-15% of the total water that enters the system. The remainder of the water is flushed out as waste. Since this water discharge is rich in contaminants, it cannot be used for other domestic activities such as bathing and laundry. Therefore, this water is usually funnelled into the drainage system, where it adds to the load on the septic system. Another key disadvantage of reverse osmosis is that it removes some desirable minerals from the water along with the contaminants.