FILTRATION 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 eaNe is generally sand. .Slot' sum/ filters are beds of sand. supported on gravel, and provided with underdrains, the whole being inclosed in a water 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 Led has an area of one acre or less. The rates of filtration range from 2,000,000 to 5.000,000 gallon-s 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 bens' 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. When 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 flow 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 a series
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 the dirty water is drawn on'. The process is continued until the sand is clean. Slow sand filtration removes prac tieally all the bacteria and suspended organic unit ter, a considerable amount of organic matter in solution, 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 OS 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 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 water is eansidered perfectly safe. If the water to be treated is very turbid it will 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 he em ployed. In such 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 the filter beds be covered as a protection against frost and ice. Masonry vaulting is employed for this purpose.