Page:The New International Encyclopædia 1st ed. v. 20.djvu/430

WATEB-WOBKS. Quantity, while not affecting so vitally the health of:i comniuuity as quality, since only a small fraction of the total water supply is de- voted to liygienic uses, is nevertheless of great importance" from the standpoint of health, com- fort, aesthetics, and industry. In a purely resi- dential town, with lawn sprinkling for grounds of moderate size, an average daily consumption of 35 to 50 gallons per capita is not only ample for legitimate use, but makes provision for what is termed unavoidable waste. With a moderate amount of manufacturing the consumption ought not to go above 75 gallons; while for all but ex- ceptional conditions 100 gallons per capita should be considered excessive rather than mere- ly ample. For well-managed works, where popu- lar sentiment permits the use of meters, figures no higher than these are reported in America, and still lower ones abroad, but it is no uncom- mon thing in the United States to hear of cities using (and wasting) all the way from 100 to 200 gallons per capita, while some cities go up to 250 gallons or over, and very likely would go higher still if the capacity of their works were not overtaxed by the excessive demands upon them. Under present conditions few American engineers deem it safe to plan for a water sup- ply of less than 100 gallons per capita, with a provision for some increase in the near future. The tendency of nearly all American cities, for years past, has been to show rapid increases in per capita water consumption.

Pressure is that quality which is imparted to mobile liquids, like water, through differences of elevation, or head, and is due to the force of gravity; or it may be produced artificially by the use of pumping machinery. It is es- sential in water-works systeins that the pres- sure be ample to give a full and constant supply for all domestic and industrial pur- poses, and particularly for protection against fire. This may be effected by choosing a source of supply far enough above the general level of the community for the difference in level to ' overcome the pipe friction and cause the water to rise to the upper stories of buildings for ordi- nary service, and be sufficient to throw several fire streams over the tops of the highest build- ings. Where natural pressure cannot be had pumping is necessary.

Source and Mode of Supply. Modes of supply are classed as gravity or pumping, and the latter mode is sub-classified as pumping direct to the distributing mains and consumers, to standpipc, to tank, or to reservoir. In some cities a combination of two or all these modes is in use. Where there are marked varia- tions in the levels of a city it is customary to divide the works into high and low and perhaps intermediate services. This saves strain on the street and house pipes in the lower levels, and where the water is pumped it may greatly re- duce plumbing expenses. In addition, lower pres- sures result in less leakage at defective joints and pumping fixtures. Several ])umps in one atation may rai-<e water to different levels, or the water may be repumped for the higher districts. Whatever the source and mode of supply, an nmple quantity of water should always be avail- able near (he centre of population and fire risk. To nceomplish this end, storage reservoirs, or in the ca.se of pumping plants, duplicate pumps. may be required; and several independent lines of main pipes may also be essential.

The choice between the various classes of sup- ply, such as springs, wells, streams, and lakes, will be governed by the relative availability of each possible source to meet the three essentials of quality, quantity, and pressure. For large cities surface supplies are most commonly the chief reliance, and in such cases, for all cities not on the larger rivers and lakes, impounding reser- voirs are generally employed. Even relatively small supplies from wells are liable to yield less, after a few years, than their original rated ca- pacity. In point of qualit.v, underground waters, except when drawn from wells in populous areas, are generally free from dangerous organic matter, but they may have, or give rise to, bad tastes and odors, and their water may be hard, or may contain iron in such large quantities as to make it unfit for use. Tastes and odors are liable to develop in water from underground sources when exposed to the light. This is due to the life processes of miscroscopic organic growths, mostly of vegetable origin. Water con- taining iron compounds, on exposure to the air, ma.v become fairl,v red with a precipitate resembling iron rust. Surface waters are generally soft, unless from limestone drainage areas. They sometimes, on storage, foster odor and taste producing organisms. Their chief danger is from sewage pollution. Surface supplies are relatively warm, and underground waters cool. Surface supplies are also liable to be high in color and turbidity. Swampy or peaty gathering grounds, heavily wooded areas, and improperly cleaned sites for storage reservoirs give rise to color, while readily eroded clay or silt lands contribute turbidity. The Ohio, Mississippi, and many Southern and Western rivers are notable for their turbidity. Swamps may sometimes be drained, or cut out of the collecting area. Soils rich in organic matter may be removed from reservoir sites. See W.ter Supply. Pumping Works vary in character with the source and mode of supply, the motive power, the cost of fuel, if fuel is used, and the working- pressure on the pumps. The latter is determined by the elevation to which the water is lifted and the frictional resistance in the pipes through which the water is forced. The chief bearing of the source of supply on the pumping plant is the depth of the water below {lie surface of the earth. If the water is drawn from a depth of more than say 20 to 25 feet as a maxiinum the pumps must be lowered correspondingly. This suggests, or may render necessary, ji vertical rather than a horizontal pump, to economize in the matter of excavations and foundntioiis. (See PuMPixo Machinery for illustration.) If the pump must be located at a considerable depth below the surface two sets of pumps are used: one in an excavation made to receive it, known as the pump-pit, and the other and main pump at the surface. If the w.itor be very deep seated some form of dee]>-well pump, or else the air lift, is required, in order that the motive power may be kept at the surface. In addition to the depth of the source of supply, the necessity of purify- ing the water is a factor in determining the character of pumping plants. (See Watkr Puri- fication.) Thus, it may he necessary to lift the water from a river to a settling basin, or from