Although in the process of recarbonation due to the formation of bicarbonates of calcium and magnesium some hardness is imparted to water, but recarbonation is necessary to avoid the above noted troubles. Recarbonation of water is carried out in a recarbonation plant. The carbon dioxide gas produced in a coke burner is passed through a chamber containing lime stone over which water trickles.
The gas then passes through excelsior or moisture trap for its partial drying. It is then passed through a dryer containing steel chips or turnings, to remove remaining water and active oxygen. Then with the help of a compressor the carbon dioxide gas is diffused at the bottom of a carbonation chamber which contains the effluent from the sedimentation tank. The minimum reaction time allowed in the carbonation chamber is about 20 minutes. The recarbonated water is passed through filters to ensure complete clarification.
The filters may be of ordinary type rapid sand filters or pressure filters. This occurs when causticity caused by calcium hydroxide or sodium hydroxide of 20 to 50 p. The sludge can either be discharged directly into river or stream or municipal sewers or it can be used for raising the level of low lying areas.
In the absence of recarbonation a thick layer of calcium carbonate will be deposited in the filtering media and distribution system. However, for public water supply water of zero hardness is not required. As such this disadvantage is not that serious. In this process no chemical are added to water as in the case of lime-soda process, but instead of this hard water is passed through a bed of ion-exchange material or ion exchanger commonly known as zeolite, which has a property of interchanging base or ion.
Thus when hard water passes through zeolite bed, calcium and magnesium are removed from water as these are substituted by sodium by ion exchange phenomenon. As such this process is also known as base-exchange or ion-exchange process. Zeolites are complex compounds of aluminium, silica and soda, which occur in nature and are therefore available in natural form.
However, these may also be prepared synthetically. The naturally available zeolite is green in colour and it is therefore known as green sand or glauconite. The exchange value of green sand is to gm of hardness per m 3 of zeolite. The most common artificially prepared or synthetic zeolite is Permutit. It is manufactured from feldspar, kaolin clay and soda. These chemicals are mixed in the required proportion and the mixture is fused in a furnace. After attaining a certain degree of fusing it is allowed to cool.
The material thus formed is then crushed to form particles of diameter varying from 0. Permutit is white in colour and it has the appearance of coarse sand with uniform hard lustrous grains. The exchange value of Permutit is to gm of hardness per m 3 of zeolite, which is much higher than that of glauconite or green sand.
However, glauconites or green sands are more rugged than synthetic zeolites. An increase in SiO 2 content of a synthetic zeolite increases its resistance to aggressive attack but it decreases its exchange value. When hard water is passed through a bed of permutit the following chemical reactions take place:. The above equations indicate that both calcium and magnesium present in water are replaced by sodium and thus hard water is softened.
The sodium salts that are formed in these reactions are soluble in water and no hardness is imparted to water by these salts. By zeolite process the hardness of water is reduced almost to zero. Since for public water supply water of zero hardness is not required, the usual practice is to soften only a portion of water to zero hardness and then to mix it with unsoftened water so that the resulting hardness is about 50 to 90 p. Due to continuous use of zeolite the sodium present in it is exhausted.
At this stage zeolite needs to be regenerated to make it again effective for removal of hardness of water. Zeolite can be regenerated by passing a solution of salt through it.
The following chemical reactions take place during regeneration of zeolite. The above equations indicate that sodium of salt solution replaces calcium and magnesium of the exhausted zeolite. The regeneration of a zeolite bed may be carried out either at a fixed interval of time or after a certain quantity of water has been softened, or when the effluent obtained has reached a predetermined level of hardness.
In the first two cases with the help of suitable controls it is possible to have automatic regeneration of zeolite bed. In the third case the regeneration of zeolite bed may be carried out as and when its need is indicated by the results of laboratory tests or field soap test. The first step in the regeneration of zeolite bed is to backwash the bed in the manner similar to a rapid sand filter, to loosen the particles and remove any material that might have been deposited on the bed.
The proper amount of 5 to 10 per cent of salt solution is then introduced into the bed and it is allowed to stand in contact with the entire bed for sufficient time. These make it more difficult for the water to form a lather with soap. Temporary hardness is caused by dissolved calcium hydrogencarbonate which is removed by boiling. Permanent hardness is caused by dissolved calcium sulfate which is not removed by boiling.
How does sodium carbonate remove water hardness? Sodium carbonate is soluble but calcium carbonate and magnesium carbonate are insoluble. The water is softened because it no longer contains dissolved calcium ions and magnesium ions.
It will form lather more easily with soap. What is the normal hardness of water? What are the two types of hardness in water? There are two types of water hardness, temporary and permanent. Permanent hardness is due to calcium and magnesium nitrates, sulphates, and chlorides etc. This type of hardness cannot be eliminated by boiling. What are the advantage of hard water? Advantages of hard water It is more difficult to form a lather with soap.
Some people prefer the taste. Scum may form in a reaction with soap, wasting the soap. Calcium ions in the water are good for children's teeth and bones. What is Clark's method?
Clark's method is a process for the large-scale removal of temporary hardness from water. What are the types of hardness? There are two types of hardness — temporary hardness and permanent hardness. This hardness can be removed by boiling the water. When we boil the water, the reaction takes place in the following manner:. CO 2 escapes the surroundings. Let us discuss the methods of removing the temporary hardness of water:. We remove the precipitates by filtration.
Clark Method. In this method, we add some amount of lime Slaked lime to the water. The lime reacts with Mg HCO 3 2 :. This is how we can get rid of these precipitates and remove the temporary hardness of the water. Now, let us study the permanent hardness of the water. You might have seen a hard white coating on electric kettles, showers, or any bath asset or utensils in which we boil the water.
This white coating is due to the permanent hardness of the water. So, what is the permanent hardness of the water?
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