Reverse Osmosis

The process through which solvent is pushed from region of high concentration solute to region of low concentration solute using semipermeable membrane is called as Reverse osmosis(RO) process. When solution is pushed through semipermeable membrane, the pure solvent passes to the other side whereas solute is trap on one side.

The membranes that are normally used in this process is made of dense polymer barrier layer. In this process high pressure is exerted from the region of high concentration side of the membrane. The pressure exerted is more than the normal osmotic pressure.

Reverse Osmosis (RO) is considered as one of the most suitable process for desalination. To achieve desalination, the feed water is forced with high pressure through the semi permeable membrane that results in the separation of salts from the water. The concentration of salts in the feed water, pressure applied and the salt permeation constant of the membranes are some of the key factors that decides the quality of the water. To further enhance the quality of the water, the water obtained during the first phase can be made to pass through the membrane second time.



Technical Process of Desalination
In the RO process, using water-permeable membrane, dissolved salt and water is separated from a pressurized saline solution. The pressure difference created between the pressurized feedwater and the product water enables the permeate (the liquid flowing through the membrane) to flow through the membrane. In this process no change of heat or phase takes place. It is only during the initial pressurization of the feed water the maximum energy is required.

To pressurized the feed water it is pumped into a closed container against the membrane. The passing of the product water through the membrane results in the concentration of brine solution and the feed water. The concentrated feedwater-brine solution is withdrawn from the container in order to reduce the concentration of remaining dissolved salts. This is essential because this enables to prevent the the continuous concentration of dissolved salts in the feed water.


Desalination through Reverse Osmosis
Major Components of Reverse Osmosis System
Following processes are followed in reverse osmosis system:

Pretreatment
The feed water before introduction to membrane is pretreated for adjusting the pH, removing suspended solids, and to control scaling which is the caused by constituents such as calcium sulphate.

Pressurization
The pressure of the pretreated feed water is raised by the pump to make it suitable for the membrane and the salinity of the feedwater.

Separation
The dissolved salt is obstructed by the permeable membranes whereas desalinated water is allowed to pass through. Thus the membrane separates the product feed water into freshwater product stream and a concentrated brine reject stream. However, small amount of salt passes through the membrane and remains in the product water no membrane is as perfect as to filtrate all the salts present in the water. The popular membrane that are used for reverse osmosis process (RO) are spiral wound and hollow fine fiber membranes. These membranes are mainly used for brackish water and seawater desalination.


Spiral Bound Semipermeable Membrane for Reverse Osmosis

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Stabilization
The pH adjustment and degasification of the product water is necessary before it is made fit for consumption. The pH value of the water is raised from the level of 5 to 7 by passing the water through an aeration column.

Applications of Reverse Osmosis Process Reverse Osmosis technology is applied for various purposes apart from desalination of sea and brackish water. Some other applications are: It is used for desalination of ground water. In industries where the concentration of a wet process stream is needed. The popular industries in which they are used are: Electronic parts manufacturing Specialty foods Pharmaceuticals For purifying water to be used in green house irrigation

Maintenance of the Reverse Osmosis System
If the reverse osmosis system is carefully planned and designed they can work for the long time provided that day to day monitoring of the system is done and a systematic program of preventive maintenance is followed. For preventive maintenance following steps need to be taken

• Chemical feed inspection and adjustment
• Pump adjustment
• Instrument calibration
• Leak detection and repair
• Structural repair of the system on a planned schedule.

Fouling of the membrane is one of the most common problem of the reverse osmosis system which is caused by the clogging of salts over membrane. This fouling can be corrected cleaning, backwashing and replacing cartridge filter elements thrice in a year.

Advantages
The Reverse Osmosis (RO) system and process has following advantages:

• The system is simple which is easy to operate and install.
• Cost of installation is low.
• Space to production capacity ratio of the plant is quite high which range between 25 000 to 60 000 l/day/m2.
• The use of chemicals in whole process is quite low.
• The impact on environment is low.
• It is suitable both for organic and inorganic contaminants
• For processing brackish water energy use range from 1 to 3 kWh per 10001 of product water.