Garbage reduction aims to recover grease and fertilizing material from animal and vegetable household and market wastes, while at the same time affording a sanitary means of final dis posal. The process requires, for its greatest suc cess, a rigid exclusion of all other wastes from those named, both to reduce the bulk of inert and unprofitable refuse and to prevent damage to the plant. The first step is the extraction of grease. This is effected by melting with steam heat, com bined with pressure, or by means of such solvents as naphtha and benzine. Sulphuric acid. has been more than enough steam to run the plant. The paper and like salable material is baled ready for shipment.
The first permanent English furnaces, built at Manchester, England, were designed by Alfred Fryer. Since then many other styles have been tried and found unsatisfactory. In most of the plants, and for the greater part of the garbage now being treated, steam is used. Where a sol vent is employed, naphtha is more often chosen than benzine, but the general process is much the same in either case.
In the steam plants the grease is extracted in rendering tanks, after which the residue, or tankage, is pressed and then dried to free it from moisture. Where naphtha is employed, the drying generally takes place before the solvent is applied. The rendering tanks, or digesters, are cylindrical, some 5 feet in diameter and 15 feet high, with tightly fitting covers for the charging holes and a horizontal valve at the foot of the conical bottom, to empty the charge. Pipe connections are provided for admitting steam or chemicals, as the case may be, and pipes or other channels for leading away the various liquids after the garbage has been treated for a sufficient length of time, generally a number of hours. The tankage is sometimes pressed by steam in the ren dering tank and sometimes in presses of either the cheese-cake or roller type. The driers are generally steam - jacketed horizontal cylinders, fitted with revolving stirring arms, mounted on a longitudinal axis. Grinding-mills are some times provided for such of the tailings from the screens as are of value, chiefly bones. The dried and screened tankage is generally sold to fer tilizer manufacturers, but at a few plants phos phates and other rich fertilizers are mixed with it, so as to produce a finished or commercial fer tilizer. The grease and water are separated by
gravity, in tanks, the grease rising to the top and being skimmed off. In some cases the grease is refined at the reduction works, but usually there is little attempt to do much refining. It may be shipped to buyers in tank cars or in barrels. Where naphtha is used it is recovered by distil lation. The water from the digesters is some times discharged directly into a sewer, stream, or lake. In other cases it is evaporated to 'stick,' and mixed with the dried tankage, increasing the value of the latter, and at the same time not polluting any body of water. In the best plants plants require an extensive equipment of boilers, engines, pumps, tanks, driers, and other ap paratus, the capital charges on which, together with the expenses for fuel and other supplies, and for labor, make up a large total. On the other hand, there is a considerable revenue from the sale of grease and tankage. All the reduction plants in the United States are owned and oper ated by private companies, which do not wish to reveal the details of their business to their rivals nor to the municipalities with whom they have or from whom they hope to secure con tracts. It is, therefore, impossible to give re liable figures as to the cost of constructing and operating reduction plants, or the amount and value of the grease and fertilizer recovered and sold. Obviously, these figures vary widely with the character and amount of garbage handled and with local conditions governing the cost of con struction, of fuel, and of labor. Information collected under the direction of the lath Col. George E. Waring, Jr., in 1895, showed that 3000 tons of summer garbage, from different cities and treated by various methods, gave the follow ing average composition: The selling value of the products from a ton of such garbage was given as follows: all objectionable vapors are condensed and the gases are purified by scrubbing, or else conveyed to and burned in the boiler furnaces. Reduction Based on the total weight of tankage, the value of the 400 pounds would be about one-third cent per pound. Winter garbage would contain a larger percentage of grease and less moisture. The price of both grease and tankage is liable to wide fluctuations.