Removal of Permanent Hardness from Water | Water Engineering

Permanent hardness of water can be removed by the following methods:

Method # 1. Lime-Soda Process:

In this method lime [Ca(OH)₂] and sodium carbonate [Na₂CO₃] (or soda ash) are used to remove permanent hardness from water.

The chemical reactions involved in this process are as follows:

The compounds calcium carbonate CaCO₃ and magnesium hydroxide Mg(OH)₂ are insoluble in water and these can therefore be removed in the sedimentation tanks. The other compounds formed during the chemical reactions are soluble in water and these do not impart the property of hardness to water.

Equation (i) indicates the chemical reaction between lime and carbon dioxide present in water.

Equations (ii) and (iii) indicate the removal of temporary hardness by the action of lime on the bicarbonates of calcium and magnesium. Equation (iv) indicates the reaction between magnesium carbonate and lime. It presents excess lime treatment over Eq. (iii) to convert magnesium carbonate into magnesium hydroxide.

Equation (v) indicates the chemical reaction between lime and magnesium sulphate. The reaction produces calcium sulphate and hence, there is no softening of water as such.

Equation (vi) indicates the chemical reaction between sodium carbonate (or soda ash) and calcium sulphate. Thus calcium sulphate already present in water and also that formed by chemical reaction indicated by Eq. (v), is removed by this chemical reaction.

Equation (vii) indicates the chemical reaction between lime and magnesium chloride. The reaction produces calcium chloride and hence, there is no softening of water as such.

Equation (viii) indicates the chemical reaction between sodium carbonate (or soda ash) and calcium chloride already present in water and that formed by chemical reaction indicated by equation (vii), is removed by this chemical reaction.

Equation (ix) indicates the chemical reaction between sodium carbonate (or soda ash) and magnesium chloride.

A summary of the chemicals required for the removal of the different types of hardness is indicated in the following table:

Lime used for water softening may be either quick lime CaO, or hydrated lime Ca(OH)₂. Quick lime is preferred for large plants because it is less bulky and cheaper. It can be fed as dry feed or added as made-up slurry. Sodium carbonate (or soda ash) can also be added to water as dry feed or as a solution.

The amount of water softening chemical i.e., lime and sodium carbonate (or soda ash) required to complete these reactions depends on the following:

(i) Amount of free carbon dioxide present;

(ii) Amount of half-bound carbon dioxide, which is a portion of that in the bicarbonate alkalinity;

(iii) Non-carbonate hardness;

(iv) Total magnesium; and

(v) Contents of commercially available lime and sodium carbonate (or soda ash).

The amount of lime required for softening of water is determined by the amounts of free and half-bound carbon dioxide and of magnesium that are present.

For determining the amount of half-bound carbon dioxide in the bicarbonate alkalinity the following reaction is considered-

It therefore, indicates that the half-bound CO₂ amounts to 44% of bicarbonate alkalinity. Also the ratio of molecular weights of CO₂ and CaO is 44: 56 or 1: 1.27. Thus for 1 p.p.m of CO₂ present, 1.27 p.p.m of lime CaO will be required. Since 1 p.p.m is equal to 1 kg per million litre (or 1 mg/l), 1.27 kg of lime CaO will be required per million liters of water for 1 p.p.m of CO₂.

If instead of quick lime CaO, hydrated lime Ca(OH)₂ is used, then the chemical reaction is as following:

The molecular weight of Ca (OH)₂ is [40 + (2 × 16) + (2 × 1)] = 74, and the ratio of molecular weights of CO₂ and Ca(OH)₂ is 44: 74 or 1: 1.68. Thus for 1 p.p.m of CO₂ present, 1.68 p.p.m of hydrated lime Ca(OH)₂ will be required, or 1.68 kg of hydrated lime Ca(OH)₂ will be required per million litres of water for 1 p.p.m of CO₂.

The molecular weight of magnesium is 24 and hence the ratio of molecular weights of magnesium and CaO is 24: 56 or 1: 2.33. Thus for 1 p.p.m of magnesium present, 2.33 p.p.m of lime CaO will be required, or 2.33 kg of lime CaO will be required per million litres of water for 1 p.p.m of magnesium.

Further molecular weight of CaCO₃ (non-carbonate hardness) is 100 and molecular weight of sodium carbonate (or soda ash) Na₂CO₃ is [(2 × 23) + 12 + (3 × 16)] = 106. Hence amount of sodium carbonate (or soda ash) Na₂CO₃ required to remove I p.p.m of non-carbonate hardness from 1 million litres of water = 106/100 = 1.06 kg.

Lime-Soda Water Softening Plant:

Lime-soda water softening plant consists of the following units:

(i) Feeding and Mixing Devices:

These devices are similar to those used for coagulation.

(ii) Settling Tank or Settling Basin:

This tank is similar to coagulation- sedimentation tank. However, in this case a longer detention time, varying from 2 to 4 hours, is provided to obtain greater clarification. Mechanical devices for continuous removal of the sludge are also ordinarily provided.

(iii) Recarbonation Plant:

Most of the calcium carbonate CaCO₃ and magnesium hydroxide Mg (OH)₂ which are formed in this process get deposited in the sedimentation tank. However, the effluent from the sedimentation tank may contain some quantity of calcium carbonate and magnesium hydroxide as finely divided particles, which should be removed; otherwise these may cause troubles by getting deposited in sand filters and also cause incrustation in pipes of the distribution system.

This is usually accomplished by recarbonation process in which carbon dioxide CO₂ gas is diffused through the effluent so that the insoluble calcium carbonate and magnesium hydroxide combine with carbon dioxide to again form the soluble bicarbonates of calcium and magnesium as indicated by the following equations-

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.

(iv) Filters:

The recarbonated water is passed through filters to ensure complete clarification. The filters may be of ordinary type rapid sand filters or pressure filters.

The various advantages and disadvantages of lime-soda process are as follows:

Advantages of Lime-Soda Process:

(i) The process is economical.

(ii) The pH value of water treated by this process is increased which results in decrease in corrosion of the distribution system.

(iii) The process is suitable for turbid, chalybeate (i.e., impregnated with iron) and acidic waters for which zeolite process cannot be used.

(iv) When this process is adopted, less quantity of coagulant will be required for coagulation.

(v) In this process there is removal of iron and manganese also to some extent.

(vi) There is reduction in total mineral content of water.

(vii) There is likelihood of killing of pathogenic bacteria in this process. This occurs when causticity caused by calcium hydroxide or sodium hydroxide of 20 to 50 p.p.m is retained in the treated water for a period of about 4 to 5 hours.

(viii) The whole process is easy and simple and it can be accommodated in the existing filter plant of any water supply scheme.

(ix) The process is better for excessively hard waters, particularly those high in magnesium hardness, and for water high in sodium.

Disadvantages of Lime-Soda Process:

(i) A large quantity of sludge (i.e., insoluble precipitates of calcium carbonate and magnesium hydroxide) is formed in this process which needs to be disposed-off by some suitable method. 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.

(ii) The process requires skilled supervision for its successful working.

(iii) In this process recarbonation is required. In the absence of recarbonation a thick layer of calcium carbonate will be deposited in the filtering media and distribution system.

(iv) Calcium carbonate is slightly soluble in water to the extent of about 30 mg/l, and hence by lime-soda process water of zero hardness cannot be produced. However, for public water supply water of zero hardness is not required. As such this disadvantage is not that serious.

Method # 2. Zeolite Process:

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 7000 to 9000 gm of hardness per m³ 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 0.25 mm to 0.50 mm.

Permutit is white in colour and it has the appearance of coarse sand with uniform hard lustrous grains. Its chemical formula is 2SiO₂ Al₂O₃Na₂O. The exchange value of Permutit is 35000 to 40000 gm of hardness per m³ 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 SiO₂ 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.p.m.

Regeneration of Zeolite:

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. The salt solution is then flushed out and the zeolite bed is put back into service, but the effluent from it should be allowed to go to waste until it shows a hardness of less than 1 p.p.m.

The zeolite bed is considered to be satisfactorily regenerated if the first 10 per cent of the water softened between successive regenerations shows a chloride content of less than 10 p.p.m. higher than that in the unsoftened water and there is no increase in the chloride content of the water during the remainder of a run between successive regenerations.

Zeolite Water Softener:

The zeolite process is carried out in zeolite water softeners which resemble rapid sand filters of either pressure type or gravity type. The thickness of zeolite layer varies from 75 cm to 190 cm.

The smaller thickness of zeolite bed is most common because smaller the thickness of zeolite bed, greater will be the capacity of the softener. The flow of water through the bed may be upward or downward, but the downward flow is more common as there is less danger of loss of material and the filtering action of the bed is better.

The usual rate of flow of water through zeolite bed is about 250 litres per minute per square metre area of bed. Further, these softeners are usually equipped with automatic regeneration control to regenerate either at a fixed interval of time or after a certain quantity of water have been softened.

The various advantages and disadvantages of zeolite process are as follows:

Advantages of Zeolite Process:

(i) In this process sludge is not formed and hence there is no problem of sludge disposal.

(ii) The zeolite unit is compact in design and hence it requires small space.

(iii) It can be easily operated and does not require any skilled supervision.

(iv) There is no problem of deposition of layer of calcium carbonate in the distribution system.

(v) By this process it is possible to reduce hardness of water to zero. Hence this process is quite useful for softening of water to be used for boilers and certain textile industries.

(vi) The water of any desired hardness can be prepared by adding softened water of zero hardness to unsoftened or raw water.

(vii) The process is almost completely automatic and highly skilled labour is not required for its operation.

(viii) The process proves to be economical where salt is cheaply available.

(ix) The process is independent of change in quality of raw water.

(x) In this process since no chemicals are added to water there is no danger of excess chemicals being present in the effluent.

(xi) The chemicals involved are easy to handle.

(xii) The first cost and operating cost of the process are comparatively low.

(xiii) By the use of pressure type softener repumping of water is not necessary.

Disadvantages of Zeolite Process:

(i) The zeolite process cannot be adopted for highly turbid water, because the suspended particles get deposited around zeolite particles, and hence cause obstruction to the working of zeolite.

(ii) This process is unsuitable for water containing iron and manganese. This is due to the fact that iron zeolite or manganese zeolite formed during the reaction cannot be reconverted into sodium zeolite. The zeolite will thus be wasted in such a case.

(iii) The process is unsuitable for acidic waters which irreversibly substitute hydrogen for sodium in the zeolite. Moreover acidic water may aggressively attack zeolite by dissolving alumina or silica from it.

(iv) Zeolite water softeners should be operated carefully to avoid injury or damage to the equipment, bed of zeolite and quality of water.

(v) When ion-exchange capacity of zeolite has been exhausted it has to be regenerated. After regeneration when zeolite bed is put back into service certain amount of effluent is wasted.

(vi) There is likelihood of growth of bacteria on the bed of zeolite. It should therefore be flushed annually with chlorinated water.

Method # 3. Demineralisation Process (or Deionization Process):

This process is similar to zeolite process with the difference that in demineralisation process the metallic ions viz., calcium, magnesium, etc., are exchanged for hydrogen ions. In this process hard water is passed through a bed of ion-exchange material or ion exchanger such as resin or carbonaceous material which is also called a hydrogen exchanger.

The chemical composition of such an exchange is expressed as H₂R, where H represents hydrogen ions and R represents the organic part of the exchanger.

The chemical reactions involved in the cation exchange process are as follows:

The above equations indicate that calcium, magnesium and sodium present in water are replaced by hydrogen and thus hard water is softened. Further during these reactions various acids viz., carbonic acid, sulphuric acid and hydrochloric acid are formed which results in an increase in the acidity of treated water which is not desirable.

The resulting increased acidity in treated water can be removed by:

(i) Diluting treated water with raw water,

(ii) Neutralizing treated water with alkaline substance, or

(iii) Absorbing excess acids by De-Acidite (“D”) a proprietary substance regenerated with sodium carbonate or caustic soda.

The following reactions take place in this exchange process:

Due to continuous use of hydrogen exchanger its hydrogen content is exhausted. At this stage it needs to be regenerated to make it again effective for removal of hardness of water. Hydrogen exchanger is regenerated by passing through it a solution of sulphuric acid or hydrochloric acid of suitable strength.

The following chemical reactions take place during regeneration of hydrogen exchanger:

The effluent obtained in the demineralization process is free from minerals and it has a quality almost equal to that of distilled water. However, the process is too costly and hence it is not used for large public water supplies and its use is limited to treatment of water on a small scale for industrial purposes where water free from minerals is required.

The water to be treated by this process should have a turbidity of less than 5 to 10 p.p.m. Moreover, the equipment used in this process should be capable of resisting acids and alkalies, since the entire process involves the use of acids and alkalies.