From Hugh S. Canamines 'Investigation of the Potomac Watershed,' it appears that the District of Columbia obtains its supply of water from the Potomac River at Great Palls 150;4 feet above tide-water. The water is conducted through a circular conduit nine feet in diameter moss of the 15 miles distance by gravity to the Delmarlia, Georgetown and McMillan Park res ervoirs, from the latter of which the water is pumped up 21 feet to the slow sand filtration plant, one of the best in the country, comp-is m 29 filters, each having an area of an acre. altogether having a daily capacity of upwards of 100.000,000 gallons. In the Georgetown reser voir the water is sometimes treated with sulphate of aluminum, used as a coagulant to assist sedimentation before slow sand filtra tion and in some cases it is used where no fil tration is employed.
Plants for supplying such preparation of alum as a coagulant have recent y been installed at Trenton, N. J., Springfield, Mass., Columbus, Ohio, and at Omaha. Neb. Slow sand filters were used by the Chelsea Water Company of London in 1829 to remove turbidity from water. As now constructed their daily capacity is from 2.004000 to 3.000,000, and in some exceptional cases as high as from 6,000.000 to 8,000,000 gal lons per acre. When they become clogged, as they necessarily do, then the coagulated deposit is removed. Then the superficial layers of sand from one-half to one and one-half inches in depth are scraped off, then washed and dried and replaced. There are several means for sand washing. including the Nichols separator, the Blaisdell filter sand-washing machine and surface raking, which increases the efficiency of the process of slow sand filtration. In all such plants at least 12 inches of sand must he main tained and from two to three feet of sand is much more efficient. Many cities and villages are using the slow sand filtration process for the filtration of water, including New Orleans, Fittsburgfi, Superior. Zurich, Yokohama and Osaka. where in 1905. the bacteria were reduced from 200 to 2S per cubic centimeter, while at Lawrence. Mau, the bacteria in the Merri mac River water were reduced from 12.700 to 70 per cubic centimeter as a result of the installa tion of the slow sand filtration plant there in 11193. The chlorine disinfection of that eater was reported on by Clark and Gage in 1909, showing the reduction of bacteria at different temperatures of the water. Such plants arc at Albany, N. Y., and at Wilmington, Del., and many are in use in Europe. Some of them are equipped with automatic controllers. The rate of filtration is slow and the filter must be cleaned and that is done by scraping off the superficial layers of sand and then washing, drying and replacing them. Allen Hazen says
that sand filtration alone, without preliminary treatment, is able to remove nearly all of the objectionable bacteria, as well as other organ isms, from many waters, at the same time puri fying them in other ways.* The bacterial con tent of the filtered water is very low, but not entirely free from organisms. It was on the re port of James P. Kirkwood of his investigations in Europe in 1866, that Slow Sand Filtration plants were subsequently constructed at Pough keepsie, Lowell, Columbus and Toledo.
4. Rapid sand filters, by some also denomi nated mechanical filters, require less area than do the former type, but they are more elaborate and somewhat complex in construction. Typical plants comprise pumping stations, preliminary settling basins, coagulation basins for the treat ment of the waters by chemicals, chemical rooms and mixers, filter tanks with connecting pipes, cleaning apparatus, controlling mechan ism, covered reservoirs for the filtered water, a drainage system and other equipment to meet existing conditions, as to locality, characteristics of raw water and amount of filtered water de sired. Filters of this type will clarify 125,000. 000 gallons of water a day per acre of its tur bidity and of 90 per cent to 99 per cent of its bacteria. Strictly modern rapid sand filter plants under skilful management with the proper use of • 'dal chemicals are nearly 100 per cent dFicient in the elimination of all bacteria from ordinary running surface waters. Some waters, however, carry in solution an abnormal amount of foreign substances and those may partially neutralize the germicidal agents of the usual dosage and in such cases the bacterial content may not be entirely negli gible. In such cases the foreign matter may partially consume the disinfectant, so that some of the bacteria may escape and appear in the effluent. Such effluents may then be treated with chlorine, which will destroy the remaining bacteria, unless they be of a class immune to such treatment.
Rapid sand filters of the improved type are in use in Little Falls, N. J., Cincinnati, Colum bus, Cleveland, Youngstown and Toledo in Ohio, Louisville. Ky, Saint Louis, New Orleans, Harrisburg. Minneapolis, Baltimore and else where. The Cincinnati, Louisville, New Orleans and some others also have preliminary settling basins, where the suspended colloidal matter in the water settles. The water is then drawn off into the coagulation basins and therein coagu lation is effected by the introduction of com mon alum, or aluminum sulphate alone or with caustic lime or ferrous sulphate (copperas) or other chemical coagulant. When brine is used in some cases caustic alkali is also needed.