After reading this article you will learn about chemical and reverse osmosis methods for water purification.
Chemical Methods for Water Purification:
Complete water purification by chemical reagents which kill pathogenic bacteria or microorganism, are called sterlization.
(1) Aeration:
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The process of aeration consists of bubbling compressed air through raw water or impure water. Aeration is useful for the removal of CO2 and bad odour.
(2) Ozonisation:
In this process the water is treated with ozone (O3). The complete procedure is performed in a long tower made of iron, and divided into several compartments by means of perforated celluloid partitions. The perforated partition breaks up the gas and water stream into minute bubbles, as a result of which intimate contract between gas and water is effected.
Ozone is produced by passing a high voltage current through air using either plate or cylindrical electrodes of stainless still or aluminum.
Conversion of Saline Water to Drinking Water:
Sea water contains about 35000 ppm of dissolved solids the process of liberalization of saline water or brackish is called desalting or desalinize. No single process of treatment can be safe for making such water safe.
Vapour compression method, ion exchange method, solar distillation, freezing osmotic process and ultrasonics are the methods which may be employed for purification of such water. The process of demineralization of saline water or brakish water is called desalting or desalination.
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Ion exchange method is more promising when the concentration of dissolved material is below 4000 to 5000 ppm. Several plants for applying this method have been constructed recently in U.S.A.
The freezing method is carried out by freezing he highly saline sea water so as to form a slurry of ice crystals and brine. The ice crystals are separated, rinsed and melted while the concentrated solution of salt (brine) is left behind.
Reverse Osmosis Method for Water Purification:
It is a another promising method of desalting in which water is squeezed out of the waste instead of taking the waste out of water. When a solution is separated by a semi-permeable membrane from a pure solution sample.
There is a tendency for the water to move through the membrane to the solution side due to the almost pressure than osmotic pressure, the process known as osmosis. Hence ordinary osmosis cannot be used to dieseline water because the movement is in wrong direction.
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Osmosis can, however, be prevented by applying pressure on the solution side that is just equal to the osmotic pressure. It the applied pressure exceed the osmotic pressure, water moves out the solution to the pure water side of the membrane.
This process known as reverse osmosis. Accomplishes the objective of desalination, the extraction of pure water from salt water. Schematic representation of set up used for the reverse osmosis is shown in the figure.
The salt water is fed into the top of the cell, the bottom part of which consists of a semipermeable membrane made of polystyrene, cellophane, polyvinyl chloride or ethyl cellulose. None of these is very satisfactory since they all go to pieces quite rapidly. Thus search for an appropriate material for semipermeable membrane is still going on.
Normally fresh water tends to move through semipermeable membrane towards the salty side, but by putting a sufficiently large pressure on the inflowing saline water the normal osmotic flow can be reversed and fresh water literally squeezed through the membrane so as to come and from the hormone pipe.