The available water of many villages and small towns is that of the chalk beds, but it is invariably very hard, and should be softened. We have received so many inquiries respecting a simple means of carrying out Clarke's water-softening process, that the following description of a set of apparatus devised for this purpose by Messrs. Law and Chatterton, MM.I.C.E., may interest many besides those who contemplate the construction of small waterworks supplied by the chalk springs.
The apparatus, as made in various sizes by Messrs. Bowes, Scott, and Read, of Broadway-chambers, Westminster, we illustrate by the accompanying engravings.
The disadvantages attending the use of hard water either for drinking purposes, steam generation, lavatory purposes, and for many manufacturing purposes, are well known, but as there are several methods of softening waters which are hard in different degrees by different substances, we may be pardoned if we here reproduce, for the convenience of some of our readers, a few passages from the sixth report of the River Pollution Commission, 1874, pages 21 and 201-16, which give some very valuable information on the relative merits of hard and soft waters in domestic and trade uses. "Some of the mineral substances which occur in solution in potable waters communicate to the latter the quality of hardness. Hard water decomposes soap, and cannot be efficiently used for washing. The chief hardening ingredients are salts of lime and magnesia. In the decomposition of soap these salts form curdy and insoluble compounds containing the fatty acids of the soap and the lime and magnesia of the salts. So long as this decomposition goes on the soap is useless as a detergent, and it is only after all the lime and magnesia salts have been decomposed at the expense of the soap, that the latter begins to exert a useful effect.
As soon as this is the case, however, the slightest further addition of soap produces a lather when the water is agitated, but this lather is again destroyed by the addition of a further quantity of hard water. Thus the addition of hard water to a solution of soap, or the converse of this operation, causes the production of the insoluble curdy matter before mentioned. These facts render intelligible the process of washing the skin with soap and hard water. The skin is first wetted with the water and then soap is applied; the latter decomposes the hardening salts contained in the small quantity of water with which the skin is covered, and there is then formed a strong solution of soap which penetrates into the pores, and now the lather and impurities which it has imbibed require to be removed from the skin by wiping the lather off with a towel or by rinsing it away with water. In the former case the pores of the skin are left filled with soap solution; in the latter they become clogged with the greasy, curdy matter which results from the action of the hard water upon the soap solution which had previously gained possession of the pores of the cuticle.
As the latter process of removing the lather is the one universally adopted, the operation of washing with soap and hard water is analogous to that used by the dyer and calico printer for fixing pigments in calico, woolen, or silk tissues. The pores of the skin are filled with insoluble greasy and curdy salts of the fatty acids contained in the soap, and it is only because the insoluble pigment produced is white, or nearly so, that so repulsive an operation is tolerated. To those, however, who have been accustomed to wash in soft water, the abnormal condition of skin thus induced is for a long time extremely unpleasant.
Of the hardening salts present in potable water, carbonate of lime is the one most generally met with, and to obtain a numerical expression for this quality of hardness a sample of water containing 1 lb. of carbonate of lime, or its equivalent of other hardening salts, in 100,000 lb.--10,000 gallons--is said to have 1° of hardness. Each degree of hardness indicates the destruction and waste of 12 lb. of the best hard soap by 10,000 gallons of water when used for washing. Hard water frequently becomes softer after it has been boiled for some time. When this is the case, a portion at least of the original hardening effect is due to the bicarbonate of lime and magnesia. These salts are decomposed by boiling into free carbonic acid, which escapes as gas, leaving carbonates of lime and magnesia; the latter being nearly insoluble in water, ceases to exert more than a very slight hardening effect, and produces a precipitate. As the hardness resulting from the carbonates of lime and magnesia is thus removable by boiling the water, it is designated temporary hardness, while the hardening effect which is due chiefly to the sulphates of lime and magnesia, and cannot be got rid of by boiling, is termed permanent hardness.
The total hardness of water is therefore commonly made up of temporary and permanent hardness. A constant supply of hot water is now almost a necessity in every household, but great difficulties are thrown in the way of its attainment by the supply of hard water to towns forming thick calcareous crusts in the heating apparatus.
Waters with much temporary hardness are most objectionable in this respect, and the evil is so great where the heating is effected in a coil of pipe, as practically to prevent, in towns with hard water, the use of this most convenient method of heating water. The property of being softened by boiling which temporarily hard water possesses is not of much domestic use, for water is, as a rule, either not raised to a sufficiently high temperature or not kept at it for a long enough time. Seeing then the disadvantages attendant on the use of hard water, it remains to be considered how best to soften it. Four processes are known to the arts. They are: Distillation, carbonate of soda, boiling, lime. Of these processes the first and second are the most effective, but owing to their expense are not applicable on a large scale. The third and fourth processes are efficient only with certain classes of water, rendered hard by the presence of the bicarbonate of lime, magnesia, or iron. The fourth is, however, a very cheap process, and is easily applicable to the vast volumes of water supplied to large cities, provided the hardening ingredients are of the character described.
By evaporation, water is completely separated from all fixed saline matters, and consequently from all hardening matters. Distilled water, however, has a vapid and unpleasant taste, due partly to deficient aeration and partly to the presence of traces of volatile organic matter; and though filtration through animal charcoal will remove this, and the aeration can begin chemically, the process is too expensive, except in certain cases, as on board ship, or at military or naval stations where no potable water exists.
The hardness of water, as already explained, being principally due to the presence in solution of bicarbonates and sulphates of lime and magnesia, can be reduced by addition of carbonate of soda, which decomposes these salts slowly in cold water but quickly in hot, forming insoluble compounds of lime and magnesia, which are slowly precipitated as a fine mud, leaving the water charged, however, with a solution of bicarbonate and sulphate of soda. This process, on account of expense, is only applicable on a small scale to the water for laundry purposes, as the water acquires an unpleasant taste from the presence of the soda salts. For laundry purposes it is, however, valuable, as it effects a great saving of soap.
That portion of the hardness of water due to the presence of bicarbonate of lime, magnesia, or iron, is corrected by boiling the water for half an hour. During ebullition the bicarbonates, which are soluble, become carbonates, which are insoluble, giving off their carbonic acid as gas, rendering--by the precipitate produced, but not allowed in a boiler time to settle--the water muddy, but incapable of decomposing soap. To raise the temperature of 1,000 gallons of water to the boiling point and to maintain it for half an hour requires the consumption of about 2½ cwt. of coal, or by the wasteful appliances found in households, probably three times that amount. Softened by boiling, then, 1,000 gallons of water would cost about 7s. 6d., while the cost of softening the same amount by soap is 9s., at £2 6s. 6d. per cwt.