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Softening of Hard Water






As has already been mentioned, Softening of Hard Water that are possessed of temporary hardness may be softened by boiling. This process is, however, very expensive and impracticable upon a large scale. A more convenient method is that of Clark, patented in 1841, according to which the excess of carbon dioxide is precipitated by addition of the requisite quantity of slaked lime. The soluble alkaline earth bi- carbonates are thereby converted into their insoluble normal carbonates and separate out. The reactions involved may be represented as follows:

CaH2(CO3)2 + Ca(OH)2 = 2CaCO3 + 2H2O
MgH2(CO3)2 + Ca(OH)2 = MgCO3 + CaCO3 + 2H2O.

In neither case is precipitation perfectly complete, for solubility is a relative term, and although the normal carbonates are relatively insoluble when compared with their bicarbonates, they are not absolutely insoluble. About 1 grain of calcium carbonate per gallon of water remains in solution in the former reaction, whilst in the latter case some 28 grains of magnesium carbonate may remain behind, although this may be considerably reduced by addition of excess of lime which precipitates magnesium hydroxide, which is less soluble than either carbonate. Water treated in this manner retains, therefore, about 2 degrees of hardness due to dissolved calcium carbonate, and may contain several degrees of hardness due to dissolved magnesium carbonate, in addition to any permanent hardness that was originally present. Although the treatment, even in the case of waters possessing no permanent hardness, does not entirely remove their hardness, it greatly improves them. Furthermore, during sedimentation the calcium carbonate carries down with it any oxides of heavy metals, such as iron and manganese, and also considerable quantities of organic material and living organisms. This partial sterilisation of the water is a very valuable side feature of the process.

Permanent hardness is frequently much more difficult to combat. Distillation will effect its removal completely, but only in exceptional cases can such drastic treatment be applied, as, for example, in preparing drinking water from sea water on steamers during long voyages, and in the preparation of pure water from tap water for special scientific purposes for which water of a high grade of purity is required. In softening water for domestic purposes it is usual to remove the temporary hardness only, since small amounts of sulphates of calcium and magnesium have not usually an adverse influence on the general health of the public. But for many manufactures, particularly for steam raising purposes, it is desirable to remove the permanent hardness also.

This Softening of Hard Water may be done by the addition of caustic soda or sodium carbonate. The latter reagent converts the dissolved calcium and magnesium salts into relatively insoluble carbonates and thus effects their removal by precipitation. Thus:

CaSO4 + Na2CO3 = CaCO3 + Na2SO4
MgSO4 + Na2CO3 = MgCO3 + Na2SO4.

Caustic soda will convert the magnesium salts into the still less soluble hydroxide and thus soften the water more effectively. If sufficient free carbon dioxide is present, dissolved in the water, caustic soda, or a mixture of sodium carbonate and milk of lime, may with advantage be added instead of sodium carbonate, as it is transformed into this latter salt by the carbon dioxide, and then reacts according to the above equations. This "fixes" the dissolved carbon dioxide and prevents it from converting into bicarbonates the normal carbonates precipitated when sodium carbonate is first used, and thereby converting a permanently hard water into one possessing temporary hardness.

In the Stanhope or Gaillet and Huet process, lime water and sodium carbonate are employed as softening agents. The former is prepared as a saturated solution by passage of a certain proportion of the incoming hard water through milk of lime in a saturator. The water enters at the bottom and as the saturated solution rises, the suspended solid particles gradually sink and the clear liquid is discharged at the top. It now mixes with hard water to which soda has been added, and the resultant mixture passes slowly up a clarifying tower, down which the precipitated solids gradually settle, and is freed from the last traces of suspended matter by filtration through wood-wool filters. This mechanical removal of the gradually separated calcium and magnesium compounds is common to most precipitation methods of softening.

The various commercial lime-soda processes for Softening of Hard Water are based on the foregoing principles, and differ essentially only in their mechanical details. Their underlying principle is more commonly applied than any other for the purification of water for boiler-feed purposes.

As already explained, it is desirable to precipitate the calcium compounds in the form of carbonate and the magnesium compounds as hydroxide. Commonly a mixture of dissolved sodium carbonate and suspended slaked lime is introduced in the correct proportion by an automatic measuring device. If the hardness is represented as the equivalent number of parts of calcium carbonate per 100,000 parts of water, a simple calculation leads to the following formula for the necessary amounts of sodium carbonate and lime in pounds per 100,000 gallons of water.

Weight of sodium carbonate (Na2CO3) = 10.6 P,
Weight of lime (CaO) = 5.6 (T + M),

where P represents the permanent hardness, T represents the temporary hardness, and M represents hardness due to magnesium compounds.

For this purpose the determination of the magnesium hardness is easily effected by treating a measured volume of the carefully neutralised water with a definite quantity of sodium hydroxide or calcium hydroxide solution of known concentration. After allowing a sufficient period for the precipitation of the magnesium hydroxide, the residual excess of the precipitant is estimated, and hence the amount of magnesium which has been removed. As an alternative, the magnesium hardness can be directly estimated by titration after the removal of the calcium compounds by addition of sodium oxalate to the neutralised water.

During continuous running of a boiler water-softening plant it is not necessary to make frequently repeated measurements of the magnesium present, the only information required for the constant adjustment of the. amount and relative proportions of the softening agents being the total hardness, the temporary hardness, and the alkalinity of the water towards phenolphthalein.2 The softening process reaches its maximum efficiency when the whole of the calcium has been converted into calcium carbonate and the magnesium into hydroxide without any unnecessary excess of the agents. The solubility of calcium carbonate, which is the more soluble of these two products, therefore represents the approximate limit of efficient softening. At this point the total hardness is entirely "temporary" in character. Also, dissolved calcium carbonate, like sodium carbonate, on titration with mineral acid with phenolphthalein as indicator gives a neutral reaction as soon as conversion into bicarbonate is complete, whilst methyl orange indicates neutralisation only after decomposition of the bicarbonate. If, therefore, the acid used is of equivalent concentration to the soap solution, the following simple relationship will approximately hold at ideal softening:

M0 = T,
M0 = 2P

where M0 represents the volume of acid required to neutralise the carbonate in the presence of methyl orange, and therefore represents also the temporary hardness,
T represents the total hardness, and P represents the amount of acid required for neutralisation using phenolphthalein.
All these quantities refer, of course, to equal volumes of the water under examination.

If T is greater than M0, the water still contains some permanent hardness and a greater proportion of sodium carbonate is needed for the softening operation. If P is less than M0/2, the water still contains bicarbonate and a greater proportion of calcium hydroxide is required. If M0 is greater than T, too much "softener " has been applied; and if M0 is less than 2P, an excessive quantity of lime has been used.

If the hardness is represented as parts of calcium per 100,000 parts of water, the limit without excess of softening agent is represented approximately by the values

M0 = T = 2, and P = 1.

In all such precipitation methods of Softening of Hard Water the actual separation of the " insoluble " products takes an appreciable time, and if the water is used or tested before precipitation is complete, misleading results may easily be obtained. For satisfactory separation a period of three hours is advisable during which the treated water circulates slowly through a large filtering chamber or chambers charged with pine wood shavings or similar material. This works more efficiently after it has become coated with the precipitated compounds.


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