Joseph W. Ellms, author of 'Water Puri fication states that 'such electrolytes as acid basis and salts will coagulate colloidal suspen sions and so will hydrochloric acid, caustic soda, caustic lime or ordinary salt solutions.' in that manner all organic matter including bacteria and inorganic matter suspended in the water are entangled and deposited on the layer of sand at the bottom of the basin. The bac teria and colloidal matter so deposited form a coating, or film over the surface of sand im pervious to bacteria and other suspended mat ter but not so to water, which flows rapidly through it. Many devices serve to facilitate the operation of that process of the purification of the raw river waters, which are the source of supplies for such cities.
Rapid sand filtration operates rapidly, owing in part to the reverse flow of water through the sand, which flow washes away the accumulation of coagulated matter and bacteria into the drainage pipes. Several hundred cities are using this process tor the purification of their waters, including many foreign cities. The bacteria in the Polar River water at Bethman galia, India, were reduced from 4,350 to 13 per cubic centimeter.
The construction and operation of the rapid sand filtration plant at Baltimore will suffice to illustrate that process of purification of water supplies.
Baltimore formerly obtained its water supply from Gunpowder River, collected into Loch Raven and from Jones Falls collected into Loch Roland. Both sources were polluted by patho genic Inicteria, including B. typhati and B. tali communes in great quantities.
Baltimore still obtains its principal water supply from the new impounding reservoir at Loch Raven, which has been enlarged by a new dam 48 feet above the bedrock. The water is drawn through a tunnel 12 feet in diameter into Lake Montebello and from that lake it is pumped through a venturi meter, an rerator gate house, head house with tower 80 feet high containing chemical storage bins, a mixing basin, coagulating basins and thence to the filters. The design involves the handling of wash waters in the settling res ervoirs, a drainage system, effluent pipe details, a head house, a pumping station, a baffle mixing chamber, two coagulating basins, covered fil tered water reservoirs and other equipments.
A nine-foot conduit, recently built, connects the Montebello reservoirs with the distributing systetn at Lake Clifton. Altogether the Bal timore new mechanical filtration plant, also known as the rapid sand filter, at Lake Mon tel has 32 units, each with a capacity of 4,000,000 gallons daily.
5. The disinfection of water supplies by various chemicals. In 1774 the Swedish chem ist, Karl Wilhelm Scheele, made an analysis of manganese dioxide and from that he was led to the discovery of chlorine. Hydrochloric acid, consisting of chlorine and hydrogen, was isolated by J. Priestly in 1772. Chlorine de pends upon the oxidation of that acid, whose salts are known as chlorides. In 1800 chlorine was used as a disinfectant in France and in England. In 1854 it was used in London to deodorize sewage. Another chlorine disinfect ant WA,. Flan de Javelle made by Percy. near Paris in 1792, also known as Labarraque solu tion. chloros and chlorozone, according to Joseph Race The germicidal nature of chlo rine. however, Was not understood until after the discovery of living organisms in water. Experiments were made with chlorine in France, England, Germany and in the United States. In 1890 a plant was erected for its manufacture at Bradford, England, and in 1893 one was erected at Brewster, N. Y., and 'elec trozone' was applied to the sewage then pol luting the Croton water supply. Dr. A. C. Houston of London is said to be the first to apply chlorine to the purification of water. The history of disinfectants now very widely and generally used is given to show the slow prog ress made in the evolution of such agencies for the purification of water supplies. Had chlorine been in general use within half a century after its discovery, the mortality in London in 1854, due to cho ra, and in Germany also due to cholera, might have avoided..
The chemicals now in use as such disin fectants are liquefied chlorine gas, calcium hypochlorite or bleaching powder, sodium hypochlorite,r sulphate in minute quanti ties, ammonia, sulphate of iron, caustic and hydrated lime, carbonate of soda, chloramine and possibly others or compounds of some of these. Most of these germicides are of recent discovery and their application to water supplies has necessitated the renova tion of most of the water plants of this and other countries. Since the proposal of Webster in 1889 to use electrolysed sea-water as a dis infectant, several processes have been utilized for the transmission of a current of electricity through some of the foregoing and other sub stances to produce coagulants and disinfectants.