Page:The New International Encyclopædia 1st ed. v. 20.djvu/418

* WATER PUBIFICATION. 3-i8 WATER PURIFICATION. Aeratio>' is acconiplished by forcing air into the water requiring treatment, by discharging water over weirs, or by means of fountains. In a few American cities stored water of surface origin is treated with compressed air delivered into the reservoir throngli perforated pipes. It is claimed that this practice has been very ef- fective in preventing organic growths that give rise to bacl tastes and odors. Aeration is also used to precipitate iron before filtration, as men- tioned farther on. As a rule, aeration is merely supplementary to other processes, making good a deficiency of oxygen. Where water is high in organic matter any amount of aeration which is economically applicable is insufficient for its re- moval. The process has little or no effect upon the bacterial contents, and is directly opposed to removal of the latter by sedimentation. FlLTR.Tlox plants are divided into slow sand and mechanical or rapid, according to the rate of filtration and the methods of cleaning the filters. The filtering material in either case is generally sand. Slow sand filfcrs are beds of sand, supported on gravel, and provided with underdrains, the whole being inclosed in a watei"- tight basin or basins. The water is admitted on the top of the bed, percolates through the pores of the filtering material, is collected by the underdrains. and then passed to a clear water reservoir or to the consumer. Each bed has an area of one acre or less. The rates of filtration range from 2,000,000 to 5.000.000 gallons per acre per day, according to the character of the water and the fineness of the sand. The filtering sand is commonly two to four feet deep and rests on one to two feet of gravel. The latter is placed in layers, increasing in size toward the bottom until stones as large as hens' eggs are found. The underdrains are four inches and upward in diameter, of either clay, tile, or vitrified pipe. Automatic devices are employed to regulate the rate of filtration. Mien the head reaches a maximum fixed by theory and experience to suit the local conditions, ranging from three to six feet, one or more beds are thrown out of use for cleaning. The water is drained down to a point somewhat below the level of the top of the bed, and a thin layer of fouled sand is removed by broad shovels. This is washed immediately or else allowed to accumulate for washing at in- tervals. The washed sand is not replaced until the thickness of the sand bed has been greatly reduced, oftentimes to as little as 12 inches. This is made possible by the fact that the bulk of the work is done in a very thin upper layer of the bed, sometimes not more than one-eighth of an inch deep. The sand is washed ( 1 ) by playing hose upon it and allowing the sand and water to fiow together through a long flume; (2) by means of running water in a revolving cylinder, provided with arms, or a screw: and, (3) what is more common in .America, by .i scries of hoppers and ejectors. In the latter process the sand is thrown into a wooden or steel hopper, in the bottom of which is placed an ejector. The jet of water carries the sand up with it to the top of another ejector, where Ihc dirty water is drawn oft'. The process is continued until the sand is clean. Slow sand filtration removes prac- tically all the bacteria and suspended organic mat- ter, a considerable amount of organic matter in Bolution, more or less turbidity, and a relatively small amount of color. Where the water has been polluted with sewage the chief object is to reduce the bacteria to a minimum. An average removal of 98 to 99.5 per cent, of the original number is common enough. This is generally considered satisfactory provided the actual num- ber remaining does not exceed say 100 to 200 per cubic centimeter as an average. It is be- ^^^^^^^mmmmsmmmii -f ^ I Concrete Bcft^m--' Main Collector TTPIC.U. SECTION OF SLOW SAXD FILTER BED. lieved that the pathogenic germs succumb more rapidly than the harmless water bacteria, which comprise the great majority of those present, so if the total bacterial reduction be sufficiently high the w'ater is considered perfectly safe. If the water to be treated is very turbid it wall clog slow sand filters so rapidly that keeping them clean becomes an economic and even physical impossibility. In such cases slow sand filtra- tion is supplemented by sedimentation. If the two processes combined are inadequate, without too great an outlay for settling basins and filter beds of large area, then coagulation may be em- ployed. In stich cases it is the practice, at least in America, to substitute mechanical filtration, which almost invariably includes both coagula- tion and filtration, and frequently sedimentation as well. In very cold climates it is essential to the best sanitary and economic results that th-e filter beds be covered as a protection against frost and ice. Masonrj' vaulting is employed for this purpose. Mech.xic.l FiLTB.iTTOX is, first of all, a straining process, in which the natural capabili- ties of the filtering medium are aided by coagu- lation. The gelatinous coagulating material, combined with the more or less sticky organic matters in the water, and with the finely divided clay and silt, form a layer on the surface of the filter and for a greater or less extent on the sides of each sand grain. Thus the effectiveness of the strainer is greatly increased by the reduction in the interstitial passages and by its adhesive qualities. After a comparatively short period, ranging from say two days to twelve hours, the filter liecomes clogged. The impurities penetrate the whole mass, so all the filtering material must be washed. This is done by the simjile mechani- cal process of reversing the fiow of '.vatcr through the filter, so it passes upward from the bottom. The dirty water is wasted at the top. Prior to or in connection with the reversed flow tlio sand is loosened by means of power-driven revolving rakes, gradually lowered into the filtering ma- terial. In place of the rakes, compressed air, admitted from the bottom, is used in some me- chanical filters. The filter sand is supported on a false bottom, in which are placed the ))ipcs for collecting the filtered and admitting the wash water. Strainers of perforated metal plates or wire netting give the water access to and permit it to fiow from these pipes. The coagulant is-