Reverse Osmosis

Understanding Reverse Osmosis

Reverse Osmosis, commonly referred to as RO, is a process where you demineralize or deionize water by pushing it under pressure through a semi-permeable Reverse Osmosis Membrane.

Osmosis

To understand the purpose and process of Reverse Osmosis you must first understand the natural process of Osmosis.

Osmosis is a naturally occurring phenomenon and one of the most important processes in nature. It is a process where a weaker salt solution will tend to migrate to a stronger salt solution. Examples of osmosis are when plant roots absorb water from the soil and our kidneys absorb water from our blood.

Below is a diagram showing how osmosis works. A solution that is less concentrated will have a natural tendency to move into a solution with a higher concentration. For example, if you have a container of water with a low salt concentration and another container of water with a high salt concentration and they are separated by a semi-permeable membrane, then the water with the lower salt concentration will start to migrate towards the container of water with the higher salt concentration.

Semi-permeable membrane.

A semipermeable membrane is a membrane that allows some atoms or molecules to pass through but not others. A simple example is a screen door. It allows air molecules to pass through but not pests or anything larger than the holes in the screen door. Another example is Gore-tex clothing fabric which contains a very thin plastic film in which billions of tiny pores have been cut. The pores are large enough to let water vapor through, but small enough to prevent liquid water from passing through.

Reverse Osmosis is the process of Osmosis in reverse. Whereas Osmosis occurs naturally with no energy required, to reverse the process of osmosis you need to apply energy to the saltier solution. A reverse osmosis membrane is a semi-permeable membrane that allows the passage of water molecules but not most dissolved salts, organics, bacteria and pyrogens. However, you need to 'push' water through the reverse osmosis membrane by applying pressure greater than the natural osmotic pressure to desalinate (demineralize or deionize) the water in the process, allowing pure water through while retaining the majority of the contaminants.

Below is a diagram outlining the Reverse Osmosis process. When pressure is applied to a concentrated solution, water molecules are forced through a semi-permeable membrane and contaminants are not allowed to enter.

Read also: understanding sea water osmosis

How does Reverse Osmosis work?

Reverse Osmosis works by using a high-pressure pump to increase the pressure on the salt side of RO and force the water across a semi-permeable RO membrane, leaving almost all (about 95% to 99%) of the salt dissolved in the rejected small stream. The amount of pressure required depends on the salt concentration of the feed water. The more concentrated the feed water, the more pressure is needed to overcome the osmotic pressure.

The demineralized or deionized desalinated water, called permeate (or product) water. The water stream carrying concentrated contaminants that do not pass through the RO membrane is called reject (or concentrate) stream.