INTRODUCTION AND WORKING PRINCIPAL OF REVERSE OSMOSIS
Today we discussed about the reverse osmosis system in water purification system . now these days Water is an important resource for Industrial development. With increased automation and technological development, more stringent quality of water is specified and required. This has led to new process technology in water treatment. Membrane technology has developed considerably and is being used for both domestic and industrial water treatment. Membrane processes are being used in conjunction with Ion Exchange process and hence some basic understanding of this process is essential for any one servicing or operating an Ion Exchange plant.
The benefit of membrane processes are:-
1) Reduction in cost achieved by savings on chemicals, operation and maintenance requirements.
2) Reduction in waste treatment.
3) Reduction in man power requirements in monitoring regeneration etc.
Basic working principle of Reverse Osmosis :
The flow of solvent from dilute solution to a concentrated solution which is separated by a semi permeable membrane is known as OSMOSIS. Imagine a container which is installed with a semi preamble membrane and containing solution of unequal concentration. The dilute solution flows through the semiperamble membrane due to the difference in pressure in two solution. The flow stops when equilibrium established. Osmosis pressure is that pressure which establishes equilibrium between two solutions and stops the solvent flow through the membrane. Reverse Osmosis acts exactly in the opposite direction. Pressure is applied on the concentration solution to force water through membrane to dilute solution to produce pure water. The pressure applied to overcome the osmotic pressure is known as Reverse Osmosis. The pure water produced is known as Pure water or permeate and the concentrated water solution is known as reject or brine water.Reverse osmosis is a pressure driven membrane separation process that is capable of separating dissolved solutes from a solvent, usually water. The solute may be organic or inorganic in nature and range in size from 1-10 angstroms or less. The ability of reverse osmosis membrane to reject organic substances depends upon the molecular weight, geometry of the solute, and other factors. A well-designed RO system is capable of removing 90-99% of the most of dissolved organic and inorganic compounds. Reverse Osmosis membranes are constructed from cellulose acetate, polyamides or other polymers. The present generation of high rejection – high flow membranes are manufactured by depositing thin films of rejecting materials over bases selected for their superior support and flow characteristics. Most current reverse osmosis applications are related to water treatment for commercial, industrial, municipal, agricultural and military facilities.
Reverse osmosis is being used in wastewater treatment/recalculation, metal
recovery and custom industrial separations due to energy saving operation
versus competitive processes such as distillation.
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Process: When a salt solution is separated from demineralised water by semipermeable membrane, the higher osmotic pressure of the salt solution causes demineralised water to flow into the salt solution compartment (see figure above). Water will continue to flow and rise in the solution compartment until the increase in water heights equals the osmotic pressure of the salt solution. If the pressure is exerted on the salt solution compartment, water can be made to flow in the reverse direction. This is the process of reverse osmosis.
The osmosis pressure is a function of the specific solute and its concentration in
water and in practical terms, it is the minimum pressure required to produce
the first drop of pure water from a solution of solute at s specific concentration.
The membranes used in the reverse osmosis process have pores in the order of 5-angstrom units. The exact mechanism of separation is still not clearly understood and different theories have been put forward. However, it is well established that the higher the ionic charge (valance) of an ion, the better its rejection by the membrane. Thus, monovalent salts like sodium chloride will pass through the membrane at a rate higher than the multivalent salts like calcium sulfate. The molecular weight cut off lies in the range of 100-1000 MW for the reverse osmosis membranes.
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