WATER-SUPPLY of from 2 1 to 3 gallons per square foot of area per hour. The filter must periodically be cleaned by scraping off the top surface of the sand, which becomes choked with the matter removed from the- water ; and after a time a fresh layer of sand has to be provided. Filtration, though in itself a purely mechanical operation, has been found to reduce the organic impurities in the water, which must be due either to their oxidation from exposure in thin layers to the air in the process of filtering or to the actual removal of very minute organisms floating in the water, or probably to both causes combined. Filters of spongy iron mixed with gravel were set up at the Antwerp waterworks in 1881, and arrested a quantity of matter which had passed through an upper layer of sand, and proved very efficient agents for purification. Their large cost, however, their becoming rapidly clogged and caked on the top, necessitating the removal of the upper layer of sand to break up the hard top iron crust, and the rather frequent renewal of the iron required, as compared with sand, were a bar to the extension of the system, which had become inadequate for its work. Eventually, it was found that the purification could be effected more economically and rapidly, and quite as effectually, by scattering cast-iron borings in the water as it flowed through horizontal revolving cylinders, furnished with projecting plates fastened at intervals round the inside, which, in revolving, scooped up the iron at the bottom, which had fallen through the water, and dis charged it on reaching the top. The iron is first converted into ferrous oxide, FeO, which dissolves, at least partly, as bicarbonate, and then by further oxidation into ferric oxide, Fe 2 O 3 , which readily precipitates, and is easily removed by filtration through sand, a small portion only of the oxide remaining in solution. Iron appears to purify water from organic matter, partly by its fatal influence on the growth and even life of micro-organisms, and partly by dragging down the floating organisms in the process of precipitation. Softening Water. Many waters drawn from springs, wells, and rivers fed by springs contain inorganic salts in solution, and, though innocuous and pleasant for drinking, are not good for general domestic and manufacturing purposes, owing to their curdling soap and encrusting kettles, boilers, and pipes. This quality, known as hardness, is mainly due to salts of lime, which are found in large quantities in waters drawn from the chalk. Most of the lime in solution is in the form of bicarbonate, CaO.2CO.,, having been produced from the very slightly soluble carbonate of lime, CaO.CCX, of which chalk and other limestones are mainly composed, by combination with free carbonic acid, CO 2 . It is only therefore neces sary to remove half of the carbonic acid from the bicar bonate to reconvert it into the insoluble carbonate. This can be done either by boiling, which drives off the excess of carbonic acid, depositing the carbonate of lime which forms the troublesome incrustation in pipes and boilers where chalk water is used, or by adding caustic lime, CaO, to the water, which, combining with the excess of carbonic acid, reduces the bicarbonate, forming carbonate of lime, which is precipitated. This reaction is indicated by the formula CaO + CaO.2C0 2 = 2CaO.CO 2 , and is the basis of Dr Clark s process for softening water. To indicate the degree of hardness of various waters, Dr Clark s scale of 1 degree of hardness for each grain of chalk in 1 gallon of water is employed. Some waters have 22, or even more degrees of hardness; and all waters are termed hard which contain more than 5; but by the softening process waters of 22 of hardness can be reduced to about 5. The remaining or permanent hardness consists of sulphate of lime and other soluble salts. The conversion of the bicarbonate in the process of softening is rapid ; but the difficulty of dealing with the fine precipitate, which requires time to settle, has hindered the general adoption of the process, though it has been applied successfully at various works deriving their supply from chalk wells. The precipitation of the carbonate of lime in the softening process has been observed to remove to a great extent the micro-organisms in the water, confirming the view expressed above, that precipitation sweeps down with it the minute germs, as in this case the chemical action could not influence them. The waters obtained from mountainous districts are very soft, and therefore very valuable for manufacturing districts ; but they have more action upon lead, and are more liable to absorb organic impurities than water highly charged with inorganic salts. STORAGE. Quantity of Daily Supply. The water-supply required is estimated in gallons per head of population, with additions in manufacturing districts for trade purposes. The consumption varies greatly in different towns, ranging from about 12 to 50 gallons per head per day ; and it depends more upon the fittings and other sources of waste than upon the habits of the population, though the con sumption per head is greater in the wealthier quarters. An ample supply, for domestic and general requirements, is from 20 to 25 gallons per head daily. The actual rate of consumption varies with the time of day, and also with the period of the year, being greatest between 7 and 10 A.M., and in June, July, and August, and least from 9 P.M. to 5 A.M., and in January, February, and March. Where the quality of the supply drawn from different sources varies, as in London (the water from the deep wells in the chalk being far superior to that derived from the Lea), and where filtration has to be largely adopted, it is unfortunate that the best supply cannot be devoted to drinking and cooking, and the inferior in quality and unfiltered waters used for cleaning, for gardens, stables, water-closets, flushing sewers, watering the streets, and extinguishing fires. Besides, however, the cost of a duplicate system of mains and pipes, with the addition, in London, of difficulties between independent companies, the carelessness of persons in drawing from the two supplies has been considered a bar to this separation. It is possible, however, that, when the population becomes still more dense, and pure water more difficult to obtain, these objec tions may be overruled, and the purest supply devoted to special uses. Service-Reservoir. To provide a sufficient reserve for sudden demands, such as for a fire, and to ensure an adequate supply to every house, a service-reservoir has to be constructed, into which the water from the source of supply is led. The reservoir consists generally of a brick or concrete tank, rendered inside with cement, sunk in the ground, and roofed over with brick arches resting on the side walls and intermediate pillars, over which a covering of earth is spread. By this means light and heat are excluded, which, together vith a depth of at least 15 feet of water, prevents the growth of minute aquatic plants, of which the germs are found -in some well waters, particularly from the New Red Sandstone, and maintains the water at a tolerably even temperature. The reservoir should have a capacity of not less than twenty- four hours supply, and should be at a sufficient elevation to command the whole of the district it serves, and if pos sible afford a good pressure on the fire-hydrants. Where a town stands at very different levels, separate reservoirs at different elevations for supplying the high-level and low-level districts are advisable, to equalize the pressure.
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