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Ships are always surrounded by literally unlimited quantity of water, except perhaps when undergoing drydock repairs. Since there are human beings on board on all types of ships, we certainly need drinking water on board. We have already studied about the machine known as the fresh water generator which is used to produce pure water out of the salty sea water. Basically the process of the FWG machine is simply to boil of the water and collect the vapour just as you would do you in your kitchen if you didn't have access to fresh water.
But there are other methods for purifying water and machines which work on different principles than evaporation and condensation. One such type of machine is the reverse osmosis water generation system which is used on board marine vessels. It is important and interesting for a marine engineer to know about the working and theory of these machines. Let us start by learning the meaning of osmosis before we delve into the details of the technique.
Definition of Osmosis
When two solutions of different concentrations are separated by a partition of a Semi-Permeable Membrane, the solvent from solution of less concentration would flow into the solution of higher concentration due to the natural process called Osmosis.
Semi-Permeable Membrane is a porous sheet barrier, which blocks the flow of salts and other solutes but allows only water to flow through.
Given below is a picture of one such generator used on ships. We will take up the detailed construction and working of one such machine in a later article, but for the moment let us concentrate on the fundamentals first, as given in the subsequent sections.
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Osmosis, Osmotic Pressure and Reverse Osmosis
This natural process called Osmosis takes place due to existence of Osmotic Pressure between two solutions separated by the Semi-permeable Membrane. The flow rate of the solvent will depend on the Osmotic Pressure, which varies with the difference in the concentration of the two solutions. The flow can be stopped or, even reversed by application of pressure on the solution in the high concentration side. This is called Reverse Osmosis and is used to produce fresh water from seawater or brackish water.
Now consider a vessel with Semi-Permeable Membrane partition containing salt-water solution A of high concentration & solution B of low concentration or pure water at same levels. In normal conditions, the water will flow from solution B to solution A through Semi-Permeable Membrane until there is difference in levels on two sides equivalent to the Osmotic Pressure when the flow will stop.
Take a look at the adjacent diagram which pictorially depicts this scenario and the simple sketch shows two solutions in a container separated only by a semi-permeable membrane between them which performs the required task
When the level on two sides was same, flow could be stopped by application of pressure on solution A equal to the Osmotic Pressure and, the flow could be reversed by application of pressure more than the Osmotic Pressure on the solution A. This is the process of Reverse Osmosis. Higher the difference in concentration of solutions A & B, more will be the pressure required to reverse the flow.
This is the principle used for producing fresh water fro salt / brackish water. It is applied to water purification and desalination, waste water treatment, and many other chemical and biochemical laboratory and industrial processes.
Image of R.O. Generator from Website of FCI Water Makers