WATER: ITS PURIFICATION, STORAGE, AND DISTRII;UTION.
Quality of the quality of water for town and city supplies, the following conditions are prerequisite: It should be transpar ent, inodorons, cool, and soft; it should contain but little dissolved solid matter, and should be free from organic substances. Chlorine and nitro gen compounds of the alkalies should be tolerated only in extremely small quantities, inasmuch as their presence is an almost positive indication of contamination by organic impurities (sewage). While the wholesomeness of water for drinking- purposes may not be materially affected by the pres ence of salts of lime and magnesia, it inay be rendered thereby quite unsuit able for many domestic and industrial uses.
'lard ana' Soft containing notable quantities of the salts of the alkaline earths (lime, magnesia) is distinguished as " hard " water; when these salts are absent, or present only in very minute quantities, the water is termed "soft." In consequence of the presence of these com pounds, hard water, when treated with a solution of soap, will decompose it, and there will be formed insoluble lime and magnesia soaps, which are precipitated. As no soap remains in solution, the water will not form suds until a quantity of soap has been added in excess of that required to pre cipitate all the lime and magnesia present. Consequently, such water is unsuited for washing. Furthermore, hard water, when evaporated by boil ing, has the undesirable property of leaving behind it solid matter in the form of a residuum which attaches itself to the surfaces of the vessel, form ing the deposit known as " kettle-stone," " boiler-scale," etc.
Conzrrsion of I faro' into Soil 11'a/cr.—Where no other water is available, it generally becomes necessary to render hard water soft before it is fit to be used. In cases where the alkaline-earth bases are present in the form of carbonates, simple boiling will be sufficient. As soon as the carbonic acid gas is driven off by heat the lime and magnesia carbonates are precip itated, and the water is rendered soft. The same object will be accom plished by the addition of milk of lime in proper quantity. To determine how much of the precipitant must be employed to effect complete precip itation without leaving an excess of free lime in the water, the amount of hardness in each case must be previously ascertained. For convenience,
the relative hardness of water is expressed in degrees, one degree corre sponding to one part of lime in one hundred thousand parts of water.
It should be remembered, however, that boiling suffices to render hard water soft where the hardness is caused by the presence of the alkaline earth carbonates. The chloride of calcium and the sulphate of lime, and the corresponding magnesia salts, remain in solution in the boiled water; neither will the employment of lime-water serve to precipitate these salts. To effect the complete separation of lime and magnesia under such condi tions, it is necessary to add a suitable quantity of the carbonate of soda, bv which all the lime and magnesia present is precipitated as carbonates, and an equivalent quantity of the corresponding salts of soda is formed, Vol.. V.-22 which remains in solution. The quantity of soda to be added will depend upon .the degree of hardness. Water of ten to fifteen degrees of hardness may still be called soft, and is available for all purposes.
Consumption of Thzter Citics.—In planning water-works, a careful estimate of the quantity of water that will be required is of the first import ance, for by this the size and capacity of the works will be governed. Sta tistics exhibit wide variation in the consumption of water per capita in various countries, depending-, apparently, upon climatic conditions, the available supply, and the habits of the people. Thus, the daily quantity per head for the population in Rio Janeiro is two gallons; in Edinburgh, eleven gallons; in Munich, seventeen and five-tenths gallons; in Paris, twenty gallons; in London, twenty-oue gallons; in New York, one hundred and twenty-five g-allons; in Rome, two hundred and seven gallons; etc. A daily supply of seventeen and five-tenths gallons per head may therefore be taken as a fair average. It may be remarked incidentally that the require ment is higher during the daytnne than at night, and in suininer than in winter. Of the total amount, it may be estimated that forty-three per cent. is used for drinking and domestic service, eighteen per cent. for manufac turing purposes, twenty-five per cent. for watering the streets, g-ardens, etc., four per cent. for public fountains, and ten per cent. for baths. This estimate is based on the conditions which obtain in Vienna.