CHAPTER XXVIII

HARD WATERS-METHODS OF SOFTENING

Outline

Kinds of Hard Waters

Effects of Hard Waters

Treatment for Hardness

(a) In the Home

(b) For Commercial Enterprises

Effects of Coagulants upon City Supplies
Hardness as Related to Soap

Method of Estimating Hardness

1. Kinds of Hardness. Any water containing mineral matter in solution which will cause a precipitate with a soap solution is said to be hard. The soap solution must be perfectly clear: when added, the formation even of a slight cloudy appearance shows that the water is distinctly hard. More often hardness is caused by the presence of calcium or magnesium salts, but occasionally by iron. When the hardness is such that boiling will remove it, it is said to be temporary; otherwise it is permanent hardness. This does not mean, however, that there is no possible way of removing it. Both kinds of hardness may be present and often are. Temporary hardness is caused by the presence in the water of the acid carbonate of calcium or magnesium, often called the bicarbonate, Cal, (CO). When boiled, decomposition takes place, with the formation of the normal carbonate. This being insoluble in water precipitates out, thus,

CaCO3.H2CO3 → CaCO3 + CO2 + H2O.

It is the compound, CaCO3, which collects upon the inside of teakettles as a rough deposit, also in the hot water coils

in furnaces and water heaters, and in all steam boilers which use hard water. It is one of the greatest annoyances with which the engineer has to deal as well as one of great expense. All sorts of "water softeners" have been put upon the market under various names, many of them worthless. Fig. 62 shows two actual cases of sections of pipe taken from two manufacturing plants, which show that the "boiler scale" as it is called had nearly closed the pipes. It is said that a layer 1/4 inch in thickness lessens the heating effect by half, or in other

words it is the equivalent of an iron pipe 5 inches in thickness. In the cases shown in Fig. 62 the scale was about 3/4 inch thick. Under such conditions, in gas heaters or furnace coils, in the home, the water in the tank is warmed very slowly, while the iron of the pipe is being rather rapidly burned away. Finally, it becomes so thin that under the water pressure it bursts, and must be renewed.

Permanent hardness is caused usually by the presence of sulphates of calcium or magnesium, although the chloride, especially of magnesium, may be the cause. As these

compounds are not decomposed at the temperature of boiling water, they remain in solution.

2. Removal of Temporary Hardness. Since temporary hardness is caused by the presence of an acid salt, it would seem that naturally an alkali would remove it. For laundry and bath purposes at home, dilute ammonia water is thus often employed. The reaction is shown by the equation,

CaCO3.H2CO2+2NH,HO → → CaCO3 + (NH4)2CO3.

As the calcium carbonate is not soluble in water it settles to the bottom and leaves the water soft. The ammonium carbonate, unless present in large quantities has little effect upon the soap, hence is not objectionable. If much water is to be treated, this method is too expensive. Therefore, steam laundries, many of which use from 50,000 to 100,000 gallons of water per day, and other similar establishments, must employ a cheaper alkali. The cheapest known is lime water, and this is used. Daily analysis of the water is made so as to know exactly the amount of hardness present. This being known, into large settling tanks, holding sufficient for a day's run, milk of lime is added sufficient to combine with the acid carbonate and convert it into the normal salt. This reaction takes place,

CaCO3.H2CO3 + Ca(HO), → 2CaCO3 + 2H2O.

The precipitated carbonate settles to the bottom and is then drawn off to the sewers. Lime water cannot be employed in the home because the quantity of water used is relatively small and any excess of the reagent would leave the water as bad or worse than before. Many cities, with river water more or less muddy, use milk of lime as already mentioned on p. 45 in removing the turbidity. Not only does it accomplish this, but to

prevent any excess, even minute, of the alum or whatever is employed as a coagulant, a very slight excess of lime is used. This reacts, as shown above, with the precipitation at least of a portion of the temporary hardness. In such treatment of water, however, for turbidity, the permanent hardness is always increased as the equation will show,

Al2(SO4)3 + 3Ca(HO), →
→ 3CaSO + Al1⁄2(HO) 6.

The aluminum hydroxide is the gelatinous coagulum which drags down the mud, but the calcium sulphate is soluble to a degree and in this way adds to the permanent hardness as stated.

3. Removal of Permanent Hardness. For removal of this in large plants the water is treated with the requisite amount of sodium carbonate in solution. This reaction then takes place,

CaSO4 + Na2CO3 → CaCO3 + Na2SO4.

The sodium carbonate is added at the same time as the milk of lime previously mentioned. The precipitate, it will be noticed, is the same as in the other case; both will settle together and be removed at the same time. Sodium sulphate in this case remains in the water, but in small quantities causes no appreciably bad results. In large amounts any sodium salt would cause the precipitation of the soap when added, from ionic reasons. This was found true when hydrogen chloride was passed into a solution of common salt to produce pure sodium chloride. (See p. 309.)

The Permutit System. This is sometimes called the zeolite process, and is protected by rigid patents. The water containing the calcium or magnesium compounds is made to flow through cylinders containing an artificial sodium compound known as zeolite. In the process the

magnesium and calcium in the hard water are removed by interchange with the sodium in the zeolite. It is said the water is rendered perfectly soft. In the case of very hard waters, however, considerable amounts of sodium salts are introduced into the softened water. As this is objectionable, the water must be further treated with lime as described already.

4. Effects of Hardness upon Soap.-Without the softening of the water used in large plants, the cost of the soap item would be enormous. No work can be done by soap as long as the water is hard, for it reacts with the compounds of calcium present thus,

CaSO4 + 2NaC1,H,COO → Na2SO, + Ca(C12H5COO) 2.

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The equation shows by the interaction that a calcium stearate is formed, a species of soap, which is insoluble in water, and floats. It forms the disagreeable, greasyfeeling scum upon such waters and adhering to the sides of the bath tub. In laundry work it sticks to the clothing and under the hot iron it melts like wax and is thus spread out upon the garment as a dark gray or dirty looking spot. So with hard water, aside from the expense, high quality work in a laundry is impossible. When the soap has combined with all the calcium salts present in the water then it can begin to form emulsions for the removal of the foreign matter from the clothing, but not before. In all cities using hard water the soap bill is a constant tax upon the people. In Glasgow, Scotland, a few years ago, a change was made from the hard water supply which had been used for years to one much softer. In the first year the saving in soap bills was estimated at $200,000. In the home, therefore, if the water is appreciably hard, small quantities of sal soda with very little ammonia water should be used both as a means of economy and for better results. Borax, as a water softener, when delicate