Advantages.—The cells of the material are more completely broken down so that a larger amount of grease can be more readily recovered; all material is enclosed during the process so that the gases are more readily deodorized with less volume to be deodorized; in the modern plants the mechanical condition of the by-products is better.
Disadvantages.—Increased fixed cost of building and equipment; increased operating cost; increased maintenance cost.
Osborn believes that by-products produced by either method have the same relative market value. In plants that have been operated by both methods, the experience has been that the additional amount of grease recovered by the cooking method has more than offset the increased costs and at the same time the odors were eliminated to a larger extent.
In establishing a reduction plant, Rudolph Herring says that the great fear is creating a nuisance. He further asserts that, owing to unpleasant odors apt to arise at the works, it is necessary to have good ventilation and also a subsequent treatment of some of the vapors and liquids which result from the process. These contingencies make it advisable, he thinks, to locate the plant in a neighborhood where the possibility of occasional unpleasant odors will not materially injure value of adjoining property.
The Chicago Waste Commission gives this suggestion as a solution of the odor problem: “In addition to the steam and electrical power that can be furnished from a destructor plant to operate a reduction plant, the exhausting of all gases carrying odors from the reduction works and passing them through the destructor would prove one of the greatest advantages from a sanitary and economical standpoint to be derived from a combined method of disposal of all municipal wastes.”
Osborn says: “Economical results may be obtained by utilization of heat in the disposal of garbage mixed with other refuse, by burning, but to prove satisfactory the maximum sanitary results must be obtained at a minimum cost, and when the quantity is such that it will warrant utilization the reduction method will continue to show more economical results, and with proper attention given to details and sanitary features the work can be conducted without nuisance.”
Reduction is a method which can be adopted only by large cities. It seems to be usually agreed that cities with less than 100,000 population and producing less than 75 tons of garbage daily will find the reduction process will not pay as a business venture. One writer says in no place of less than 150,000 population can these kind of plants be operated successfully. Venable places the minimum population at 100,000. He says that as approximately 80 per cent. to 90 per cent. of kitchen garbage is water and only 10 per cent. to 20 per cent. is composed of grease and other substances it takes a large amount of garbage to make reduction plants profitable.
The cost of a reduction plant will range from $1,500 to $3,000 per ton daily capacity, according to published reports.
The gross cost of garbage destruction by the reduction method varies from $1.50 to $2.50 per ton of raw garbage. In only a few instances does the sale of the by-products meet or exceed expenses. In a majority of cases, the process is carried on by private companies, the most being subsidized by cities to amounts varying from 50 cents to $2.50 per ton. A few companies pay the city for all garbage delivered to the plant.
The by-products of the reduction method are grease and tankage. It is generally agreed that ordinary garbage contains from 2 per cent. to 3 per cent. by weight of grease and must yield from 200 to 400 pounds of tankage per ton.