Desalination of salty waler

Reverse Osmosis System

December 06, 2018


Reverse Osmosis is a technology that is used to remove a large majority of contaminants from water by pushing the water under pressure through a semipermeable membrane.

How does Reverse Osmosis work?

When seawater or just a salty water is separated with fresh water by a semipermeable membrane, the salty water attracts the fresh water and the water flows from fresh water to salt water because of the presence of salts. The quantity of salts in water exerts certain natural pressure called as Osmotic pressure which is directly proportional to the number of salts present in salty water.



It can be seen that the fresh water level is going down while the salty water level is going up and the equilibrium maintained as salty water high and fresh water level low. This is the natural phenomena called as Osmosis system which seems to be of no practical use in water purification systems.

When a force is applied from the salty water end certainly higher than the natural osmotic pressure the water flow through semi-permeable membrane in the reversed direction and the fresh water level increased. It means that we are applying force against its natural pressure to make the water flow direction reverse in order to get fresh water. That is why we call it Reverse Osmosis system. The system is used to turn brackish or sea water to get potable water on the large scale.

Reverse osmosis system is simply a filtration through a molecular sieve which is called a semi-permeable membrane.

RO is the most common use is in desalination plants, which converts seawater into drinkable water. The desalination Project near San Diego, the biggest plant in the Western Hemisphere, produces roughly 52 million gallons of fresh water per day.

RO technology supplied by Dow based on breakthrough polyamide membranes developed by John Cadotte in the '70s. There are several membrane manufacturers of membranes such as FilmTec, Osmonics, Hydranautics.

This article covers the following topics:


How does Reverse Osmosis work?

Reverse Osmosis works by using a high-pressure pump to increase the pressure on the salt side of the RO and force the water across the semipermeable 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 more concentrated the feed water, the more pressure is required 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.

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