The processes in general use for the purifi cation of water include; (1) Sedimentation with or without chemicals; (2) preliminary treatment for slow sand filters; (3) slow -.ind filtration; (4) rapid sand filtration; (5) the infection of water supplies by various chemi cals; (6) the application of (a) ozone, or (b) ultra-violet rays of light, and (7) by sterilization. Two or more of these processes may be used in succession in the same plant, or they may form parts of a single system though operating independently of each other.
In the application of these processes many subsidiary details are involved and the com pleted waterworks of a city are often its most costly, complex and elaborate public works.
A brief description of each of these several processes for the purification of water may show something of their value and adaptability to the services which they are required to per form in rendering water suitable for domestic and potable purposes.
1. Sedimentation is the clarification of tur bid or of other waters holding particles of for eirn matter in suspension. This may be aided at Cincinnati and Covington ; Dr. A. C Houston reported to the London Metropolitan Board that 'thirty days' storage of river-water is tamount to sterilization' as to pathogenic crobes. William P. Mason, however, states that 'bacteria sink but slowly in water.' If that be so, a long period of subsidence is sary to make sedimentation an efficient process for their removal from water supplies.
2. The preliminary treatment of water for slow sand filters may be affected by the use of sulphate of lime to coagulate the clay particles in suspension as in the Potomac, or by the use of concrete sedimentation basins with roughing filters of coarse stone, called baffles, to strain out the coarser materials in some waters coagulated by the introduction of chemicals and removed therefrom before the water passes into the sedi mentation basins, as in the filtration plant at Pittsburuh. or still by the use of such prelimi by the introduction of such chemicals as tend to promote coagulation of the particles in sus pension in the water. Sedimentation is the process used in the removal of such large par ticles as are found in the waters of the Hudson above Poughkeepsie, in those of the Potomac above Washington, in the Ohio above Cincinnati and Louisville and in the Mississippi above New Orleans. in all of which cities sedimentation
reservoirs and coagulation basins are in use. The process removes most of the colloidal mate rial in suspension. Percy Frankland found that sedimentation removed 82 per cent of the bac teria in the Grand Junction Company's reservoir and about 87 per cent of the bacteria from the water that had passed through to storage res ervoirs of the West Middlesex Company. Offi cial reports show that from 87 per cent to 97 per cent of the bacteria were removed from the waters after 32 days subsidence in the reservoirs nary filters without coagulation, as those of the Torresdale type in use at Philadelphia, where since their installation it has been found necessary to install a sedimentation basin for the use of coagulants on account of the tarp quantity of turbid waters to be treated, or still by the use of such prefilters as those described by William F. Johnson. in the Pucch-Chabal system extensively used in Europe. That sys tem consists of a series of decreasing in size roughing filters, a subsiding basin and a coagu lating basin.
3. The slow sand filtration process is in use in Washington, D. C. A large area of sand is required and many water-tight basins of sand are necessary, where a large volume of water is to be clarified. The Belmont and Queen Lane filters of Philadelphia are of this type and comprise many shallow sand beds underlain with layers of gravel. Such filters have masonry or concrete walls and are ered over where necessary to prevent freezing. Philadelphia has five slow sand filtration as the Upper and Lower Rox Belmont, Torresdale and Queen Lane = with an aggregate daily capacity of 405, gallons, the largest in the world. Some of these have settling basins, sedimentation basins, covered preliminary filters, filtered water reservoirs, the Torresdale plant of 240,0001300 gallons daily capacity having 120 covered me chanical preliminary filters. Philadelphia ob tains its supply from the Sehuylkill and Dela ware rivers and the latter is becoming so pol luted, that other sources may be required. The arrangements there for treating water with chemicals were elaborate and it was necessary to use chlorine, for the Tonna'le filters did sot remove all the bacteria.