Natural Process of Purification from Organic Matter.—Although, by means of sand and other filters, or of the liming process, organic contamination of water may be much reduced, there still remains enough to render the water unsafe for use. Is water' then, once corrupted with organic matter, hopelessly and permanently so? This question can be answered in the negative. Filthy water has a tendency topurify itself, and this in two ways. In the first place, in any shallow stream of polluted water, such as the ken nels of a street, there may be observed long brushes of a sort of slimy vegetation adhering to every projection of the bottom. All this matter has been disengaged from the water, which thus flows away so much the purer. The second and most effective part of the natural purification consists in the actual decomposition of the impurities. The nitrogen of the decaying matter, then, goes to form nitric acid, which, unitiug with bases, forms salts of the class called nitratek of which salpeter is one. Thus, what was in a state of putrefactive change, offensive to the senses, breeding loathsome insects, and causing dangerous disorders, is changed in course of time into a stable and harmless product. This process is constantly going on in rivers and other waters containing organic matter. In the case of streams passing through populous districts, the con tamination goes on at a rate far beyond the power of natural purification; but we can easily conceive how a river, very much contaminated with organic impurities at one part of its course, may, after flowing a long way through an uninhabited tract, be almost restored to its natural state. The process is one of oxidation, and takes place at the expense of the free oxygen, of which, iu healthy, normal water, there ought to be 29 per cent of the entire volume of gases held in solution.
The oxidation is much favored and hastened when the water percolates or filters very slowly through porous beds of earth. If the filtration has. been sufficiently pro longed to convert all the decaying matter into carbonic acid or nitrates, the water will be pure, as far as the organic taint and the presence of animalcules are concerned, and will, in fact, be neither disagreeable nor unwholesome, the amount of the dissolved carbonates or nitrates being unimportant.
Dr. Smith has proved by direct experiment that decomposing organic matter passed through a filtering-bed is changed into nitric acid. "A jar, open at both ends, such as is used with an air-pump, was filled with sand, and some putrid yeast, which con tained no nitric acid, was mixed with pure water, and poured on the sand, and allowed to filter through. The production of nitric acid was abundant." It Is not improb able that other earthy matters, such as loam and clay, may have a still more decided influence in hastening the formation of the nitrates; and perhaps by imitat ing more closely the slow mode of filtration by which nature converts surface-water into spring-water, it may yet be practicable to make the most contaminated waters fit for use.
Conveyance, Storage, and Distribution.
Into the engineering operations connected with the conveyance of water from its source to the town to supplied, we need not enter, beyond noticing that when the source is below the level of the houses, steam or other power is necessary to lift or propel the water to the necessary height; while in the more general and more desir able case of the source being higher than the place where the supply is to be deliv ered, the water is made to flow by its own gravitation, either in a channel or culvert with a contiguous descent, as in the ancient aqueduct (q.v.). or in the simpler and more economical modern plau of a line of cast-iron pipes following the inequalities of the surface. In many cases, both principles are employed, the water flowing for the most part in a gently-sloping conduit, tunneled through hills where necessary, and being carried through valleys in tubes decouding and ascending—an inverted siphon, as tt is called. The Croton aqueduct, which supplies New York, is carried across the Manhattan valley, upward of 100 ft. deep, in this way. The Glasgow supply from loch
liatrine flows mainly in it sloping channel carried through tunnels and over bridge's; but there are four miles of iron pipings across valleys.
The extent of the storage in reservoirs depends on the nature of the supply. If water is derived from perennial springs, whose minimum flow equals the maximum demand, the storage may be the least possible. If a river is the source, the reservoirs should be large enough to hold such a stock as will carry the consumers over the periods when the river is polluted by rains; they should also be large, on the principle of allowing time for purification by subsidence, especially if artificial filtration be not etu ployed. In places where the supply is obtained from surface drainage, or from a small stream, the practice is to build reservoirs capable of containing a five or six months' supply, it being necessary to provide against the greatest droughts that ever happen in any season.
The reservoirs should be deep, so as to prevent vegetation, and the distributing or service reservoirs should be roofed.
In distributing water over a town, two different methods have been adopted, known respectively as the intermittent and the constant systems of supply. On the intermittent system, water is laid on once a day, or once in two or three days, as the case may be, and fills a tank attached to every separate house, and from this tank the water is drawn off as required. The feeding-ptpe of such a tank or cistern is provided with a ball-cock which ingeniously shuts off or admits the supply, as the cistern may be full or empty, On the constant system, no tank is absolutely needed, but the house-pipes are kept con stantly charged through their unbroken connection with the distributing reservoir, which must therefore be higher than the highest house to be served. The intermittent supply was &ail lately employed everywhere in .the metropolis; but it is universally admitted that the other system is vastly superior in every respect. The disadvantages of the intermittent practice have been strongly set forth in all the recent official reports on sanitary improvement; the expense of the erection and repair of cisterns, the trouble requisite to keep them clean, the contamination of the water by the neighborhood of sources of pollution, the frequent waste of water that occurs, the difficulties imposed on the poorer class of tenements where cisterns are not provided—are a few of the objections urged against this mode of supply. In a letter in the Times, January 3, 1860, Dr. II. Jeaffreson thus describes the condition, in regard to water-supply, of the centers of typhus infection in Lambeth, Southwark, Bethnalgreen, etc.: "Those houses the best supplied lave each a butt, holdino. about 80 gallons, into which water flows from' a stand-pipe for from ten minutes to half ° an hour each day, and is supposed to supply the wants of 20 persons for cooking, the washing of their persons, house, and linen. and for the rinsing down of the water-closets at such times as it may suit the caprice of any one of the inmates. At other places, a larger butt, 'but in relation to the number of persons pro portionally smaller, supplies a whole court of ten or more three-roomed houses, which have no back yards, and a population of 1N people—members of 30 different Indies. Oa Sundays, even this supply is absent, the water of the day before is gone, and in *any houses, that for the Sunda): cooking has to be begged from neighbors who may have provided themselves with a target butt, who are more provident or more dirty. More than nine-tenths of these water-butts :lave no covers; and fully half are so placed RA to catch the drippings from the foul eaves of the houses, and are lined internally with scum and slimy vegetation." One important advantage, arising from the constant system, is the ease with which water can be had in time of, fires. The water being supplied at high-pressure, all that Is necessary is to affix a hose to the water-plug in the street, when a jet corresponding in height to the pressure is obtained, which can be immediate directed against the fire.