The economy which carbonate of soda exhibits in comparison with soap as a softening material is far surpassed by the use of lime. Lime costs about 8d. per cwt., and this weight of lime will soften the same volume of water as would require the use of 20¼ cwt. of soap. From the above it is evident--so soon as it is conceded that there is an advantage in using soft water--that the lime process is by far the most economical. Besides the chemical action affecting the hardness, it has another most important mechanical action, in consequence of the weight of each particle composing the precipitate produced by it. These particles during subsidence become attached to the almost microscopical organic impurities present in all river water, and drag them down to the bottom of the settling tank, whereby the water is rendered, after some eight hours, clear as crystal. The average cost of the water supplied by the leading metropolitan water companies is £10 10s. 9¾d. per million gallons. The charge made by the companies to consumers is about 6d. per 1,000 gallons, or £25 per million gallons.

It has been found that water can on a large scale be softened from 14° hardness to 5° at a cost of 20s. per million gallons--that is, 10 per cent. on the cost of the water to the companies, or 4 per cent. as the price charged to consumers. This estimate does not take into account the value of the precipitated chalk, which has a market price, and is used for many purposes, being, in fact, whiting of the purest quality. The operations necessary in Clarke's process are four in number: (1) The preparation of milk of lime; (2) the preparation of a saturated solution of lime; (3) the mixture of this solution with the water to be softened; (4) the classification of the softened water by the separation of the precipitated substances Messrs. Law and Chatterton effect these processes by simple mechanical means which are so far automatic that they only require the presence of a person, without technical knowledge, once in each twenty-four hours. No filtering medium whatever is required, which is a great advantage for the following reasons: (1) Filtering materials require periodical cleaning and renewal, which not only occasion much trouble and mess, but are also frequently inefficiently performed. (2) Experience has shown that the filtering material, whether cloth, charcoal, or other substance, is extremely liable to become mouldy or musty, which makes the wafer both unwholesome and unpalatable.

This system is especially adapted for small water supplies and for use in country houses, there being no operation to perform requiring either technical, chemical, or mechanical knowledge, nor producing dust or dirt.

The Softening Of Water By Lime 392 6a

Fig. 1.--LAW AND CHATTERTON'S
WATER-SOFTENING APPARTUS.

The following is a description of this apparatus as fitted at the Hoo, Luton, Bedfordshire, for the supply of Mr. Gerard Leigh's house, grounds, and home farm. The mixing of the lime and the subsequent stirring of the water is effected by water power obtained from a turbine. The whole of the apparatus and tanks occupy a space 60 ft. square, 3,600 ft. area, and soften a daily supply of 50,000 gallons.

The Softening Of Water By Lime 392 6b

Fig. 2

A pump driven from the turbine forces the water to a reservoir in the park and on to the house, an ingenious automatic arrangement worked by the overflow from the cistern throwing the pump out of gear when the tank is full. A, B, and C. Figs. 1 to 6 herewith, are three tanks in which the water remains to be softened, each capable of holding one day's supply. D and E are two smaller tanks in which the lime water is prepared; X is the automatic valve apparatus by which the connections between the several tanks are effected in the order and at the times required; H and H show the positions in which two pumps should be placed, the former for pumping unsoftened water into the tanks, the latter to pump the softened water into the supply cistern. J is the pipe from the well or other source of supply--in case the supply is at a higher level, one pump can be dispensed with. The operation consists in adding to the water to be softened a certain quantity of lime water, depending upon the degree of hardness, and in then allowing the mixture to rest in a state of perfect quiescence until the whole of the lime has been deposited and the water has become perfectly clear. The tank, A, has been filled with unsoftened water.

Tank B contains the water and lime in process of clarification by subsidence after mechanical agitation by the screw. Tank C contains the softened water--and the precipitate--in process of removal for consumption. The mode of working is as follows: The milk of lime, prepared by slaking new lime in a "Michele mixer"--not shown. One of the tanks, D, having been filled with softened water, run by gravity from one of the tanks, A, B, or C, the requisite amount of milk of lime is allowed to flow into it from the lining machine, and the whole having been thoroughly mixed by the patent agitator, G, is left in a quiescent state for some hours, when the superabundant lime falls to the bottom, and the tank contains a perfectly clear and saturated solution of lime. The requisite quantity of lime water is then suffered to flow by gravity into whichever of the three tanks is empty. In the mean while, the softened water is being withdrawn by pumping or gravitation, as the case may be, from the tank C, until, upon the water being lowered to within a certain distance of the bottom, an automatic arrangement shifts the valve, X, so that the supply then commences from B, the unsoftened water flows into C, and the water is in process of clarification in A, and thus the operation proceeds continuously.