TABLE 13. DISTRIBUTION OF WASTE-FOR-DISPOSAL Information concerning the method of storage, collection and transport, processing, utilization, or final disposal of commercial solid wastes is either inadequate or grouped in such a manner with other municipal practices as to be impossible to identify as a separate portion of the total solid wastes stream. It must be assumed that the major categories of these wastes are paper, plastic, metals, and glass, and that a lesser portion is related to those wastes more commonly called food wastes or organic waste materials. To some extent on-site compaction or incineration of refuse is practiced by commercial establishments, but the magnitude and impact upon the environment is unknown. Limited information has been published on how the reclamation or recycling of paper products is achieved, and its order of magnitude is unknown. The final disposal of these commercial solid wastes, however, doubtless utilizes the same practices as used for other solid wastes in each community. Incineration. Combustion is one of the oldest methods of dealing with the organic fraction of urban solid wastes. In its systematized application to municipal refuse it has advanced through the years from simple open piles or pits to highly mechanized and carefully engineered systems. The best incinerators are capable of reducing the volume of organic matter by as much as 90 percent. The combustion products include fly ash and residue, which may be sterile, as well as gases and particulates, which are discharged to the atmosphere. At its best, in the most favorable geographical situations and using properly designed and operated plants and air cleaning equipment, air pollution control can be achieved. At its worst the process creates objectionable air pollution from fly ash, and leaves a residue of as much as 40 percent of the original volume of solid wastes. Modern design and intelligent operation is the answer to environmental control. Current incineration installations are tabulated by states in Figure 5. This practice may be expected to grow in the future with many technological improvements and more enlightened engineering. The modern incinerator is a high-cost facility. Land requirements for incinerators include that for disposal of the residue, bulky and oversized wastes, and any unburnable wastes. Some 142 central incinerators out of a possible 300 plants in the country as a whole have been covered thus far in the National Survey of Community Solid Waste Practices. Ninety-six percent of the incinerators are publicly owned and operated. They are generally located in areas zoned for industry and on the average process 188 tons per day. It is estimated from the survey data that approximately 8% of collected solid wastes in the United States is processed by central incineration. FIG. 5. DISTRIBUTION BY STATES OF THE INSTALLED RATED CAPACITY Further analysis of the data indicates significant differences in plant size, effectiveness of waste reduction and operating costs, all as related to the age of the incinerators, with the plants divided into three groups: one consisting of plants with a construction year centering around 1962, a second centering around 1958, and a third centering around 1945 with some plants built as early as the 1920's. The more modern group reported operating costs averaging $3.27 per ton of waste processed and accomplished an average weight reduction of 79% By contrast the oldest group achieved only a 65% weight reduction with an average operating cost of $5. 37 per ton processed. Seventy percent of the 142 incinerators are in the oldest groups while only 15% are in each of the other two age brackets. More than a half-dozen municipal incinerators in the Los Angeles area were closed down for economic reasons when stringent regulations were enacted for control of air pollution. Compliance would have been more costly than disposal to the large sanitary landfills of the Los Angeles County Sanitation Districts. New York City reports that the installation of air pollution control equipment on their existing incinerators is expected to cost $500,000 to $700, 000 for a furnace of 250-ton-per-day capacity. Such an estimate includes modification and provisions for electrostatic precipitators or waterspray scrubbers. New York City has some 27 furnaces which will require these modifications to meet the air pollution control standards of the area. In addition, the city has 19 furnaces which are too antiquated to warrant modifications to meet air pollution control standards. New York City and Chicago are working with the design and construction of new incinerator facilities to include waterwall furnaces and electrostatic precipitators. The total plant will cost from $10,000 to $12,000 per ton of rated capacity. Steam generation will recover heat value and power from the solid wastes. Air pollution controls should meet regional standards. Present-day incinerators may be considered under two general classifications: continuous-feed system, where solid wastes are fed into the furnace in a steady flow; and batch or intermittent-feed system, where solid wastes are fed into the furnace at regulated intervals. The continuous-feed furnace operates continuously, maintains its air seal continuously, and requires a minimum of manual stoking. Solid wastes move through the incineration process in the following fashion: trucks deliver the material to the tipping area; wastes move from the receiving pit by means of a mechanical conveyor to the charging hopper which feeds them continuously to the combustion grates. These grates are motor driven and terminate at the residue hopper, which makes possible continuous removal of ash. This type of incinerator operates continuously at full capacity as long as its charging hopper is kept full. Problems may arise with continuous feeding of the heterogeneous materials in solid wastes. Batch charge incinerators control these problems by operating in what approximates continuous fashion. Charges of solid wastes are fed to the incinerator at close intervals. Solid wastes move to the incinerator furnace in the following fashion: trucks deliver the solid wastes to the storage pit in the tipping area; material is lifted from the receiving pit by a clamshell bucket which drops a large quantity or batch into a charging hopper that leads to a charging chute located over the combustion cell. High capacity loading buckets operated from bridge cranes are common in incineration plants in the U. S. and Europe. Mechanical stokers may be horizontal traveling grates similar to those used in coal-burning furnaces, or may be specifically designed roller or reciprocating grates adapted to the complete combustion or solid wastes. The grates are set on a relatively steep slope inside the furnace so that mixing and turnover will expose all of the material to the flames. Developments in furnace design and waste heat recovery have been notable in France and Germany. In the United States progress is being made through research to apply techniques and processes from the aero-space industry to the design of incinerators. A project sponsored by the Solid Wastes Program of HEW employs a combination of different unit operations such as salvage, grinding, pressurized fluidized combustion beds, electrostatic precipitators, and gas turbine engines. Recovery and recycling of waste materials and of thermal energy is thus made possible. |