pm1 B REFUSE REDUCTION PROCESSES 101 tions are made by actually sampling the flue gas in a scientific manner, filtering the dust from the sampling stream, drying and weighing the dust, and comparing the dust weight with the weight or volume of gas flowing. Correction is made to a constant excess air content of the stack gases, so that the comparison with a standard or results from other plants would be on the same basis and thus meaningful. For this purpose the flue gas is analyzed for the volumetric proportions of the principal gases. The example dust loading may be expressed in several equivalent ways: Lb per ton of refuse charged = 3.0 lb Lb per 1,000 lb actual flue gas corrected to 50% excess air = 0.270 Grains per cu ft of actual flue gas at 50% excess air, 68 F, 29.92 in. Hg = 0.139 Milligrams per cubic meter at 0 deg C, 760 mm Hg and 7.0 percent COn = 211 U.S. dust emissions standards rake from 0.85 to 0.20 lb per 1,000 lb of Rue gas at 50 percent excess air. The standard applicable throughout West Germany is 150 mg per standard cubic meter, which is equivalent to 0.192 lb per 1,000 lb of flue gas at 50 percent excess air, or 0.099 grains per cubic foot. To meet the West German standard, the example incinerator would have to have a dust emission of 2.13 lb per ton of refuse. The more restrictive new U.S. and European standards can be met by the use of electrostatic precipitators, gas scrubbers, arid bag filters of high cfficicncy. Such equipment has been in industrial use for years. Gas scrubbers have been applied to several large incinerators. It is expected that electrostatic precipitators will soon be installed on incinerators in this country. European Incinerator Art In Europe under conditions of high fuels costs, lower labor costs, and a high technological level of construction and plant operation, as well as the desire to conserve land area, the incinerator art has flourished since 1962. The objective of reducing refuse to minimum volume has been combined with the desires for heat economy and low air pollutipn. The combination is mutually assisting. As a member of the U.S. Study Team of June-July, 1967, led by Mr. Leo Weaver, Chief of the Solid Wastes Program, Public Health Service, it was my privilege to see several of these plants. Descrip- 102 KAISER Proceedings tions and technical information are also available in several excellent papers published by the American Society of Mechanical Engineers in the proceed- ings of the 1964 and. 1966 National Incinerator Conferences. These new-type refuse reduction plants consist of refuse receiving pits, cranes with grapples to elevate the refuse to hoppers, stoker-fired boilers, electrostatic precipitators to trap the flue dust, and chimneys 260 to 390 feet high. Because of the water-tubed furnaces, the refuse can be burned with 1.6 times the stoichiometric air, instead of 3 times as in U.S. practice; the weight and volume of flue gas to be cleaned is reduced considerably. The cooling of the gases to 500 to 600 F in the boiler-superheater-economizer contracts the gas `volume without the addition of spray water. The electrostatic pre- cipitators, although large, are half the volume that would be required without the boiler.a The precipitators are guaranteed at 98 to 99 percent collection efficiency, with test results exceeding guarantees. Finally, the gases are dispersed from high chimneys. The steam generated is used for the production of electric power and for district heating, in conjunction with the local electric utility. For district heating, high-pressure hot water can also be produced for circulation through mains. U.S. refuse is lower in moisture and ash, higher in calorific value, and hence capable of generating more steam per ton of refuse. American Incinerator Art The U.S. incinerator art is on the threshold of a rapid evolution to meet rising requirements for capacity to consume refuse, better plant appearance, low emission of odor and air pollutants, minimum putrescibles in the residue, and less effluent water. The possibilities for steam and power generation from refuse are being restudied. The disposal of incinerator residue, salvage of metals, and utilization of residue are also under investigation. The plants will be more highly engineered, and will require better control and operating personnel to match. Close engineering ties are maintained with European progress. The burning of oversized burnable waste with or without prior shredding is being developed. Trees, furniture, pallets, mattresses, truck and auto tires, and demolition lumber reduce to even less final residue volume than does the equivalent weight of normal refuse. A major stimulation is the Solid Wastes Program of the Public Health Service. Through research and demonstration grants, conferences, educa- Panel B REFUSE REDUCTION PROCESSES 103 tional and field efforts, and allied activities, new advances and trained personnel are resulting. AS public officials and the general public become aware of the long-range implications and opportunities of waste management programs, larger capital investments will become available for incineration plants and allied facilities. The regional approach to waste disposal will lead to larger and better in- cinerators. Engineers look forward to the opportunity to design plants which are in the long-range interest of the public, rather than to satisfy minimum first cost. The total annual cost of refuse incineration will thereby not cscced about $6 per inhabitant served. Destructive Distillation and Gasification of Refuse Esperimentation here and abroad indicates that the organic matter in municipal refuse can be converted to gaseous, liquid and solid products by heating to 1,300 to 1,500 F out of contact with air. After the distillation of the moisture, the organic matter is converted to roughly equal weight per- centages of water vapor, gases, liquids and char. In descending order of volumes, the fixed gases are mainly CO,, CO, CH, plus higher hydrocarbons, hydrogen, and nitrogen. The liquids range from alcohols to tars. The char is primarily carbon and ash.D Refuse can also be gasified in a deep bed gas producer supplied by air at less than half the stoichiometric combustion requirement. Pilot-scale work is in progress to determine yields and costs. It is too early for predictions of the outcome. However, as a method of reducing waste, the residue would require the same landfill space as the residue from incineration. ACKNOWLEDGMENT This paprr is a result of investigations conducted at New York University under rrsrarch grant support of the Solid Wastes Program of the National Center for Urban and Industrial Health of the U.S. Public Health Service, Grant NOS. SWOOO27, SW00035 and SWOOO43. The Leonard S. Wegman engineering firm of New York City kindlv provided incinerator illustrations used in the presentation of the paper. The American Design and Development Corporation of Whitman, Mass., supplied slag samples for density determinations. REFERENCES 1. Crrstle, R. W., and D. A. Kemnitz. Atmospheric emissions from open burning. Paper 67-135. Presented by Air Pollution Control Association, Clevrland, June 16, 1967. 2. Kaiser, E. R., and J. Tolciss. Incineration of automobile bodies and bulky waste materials. 111 American Public Works Association Yearbook. Chicago, American Public Works Association, 1960. p. 178-192. 104 KAISER Proceedings 3. Wiley, J. S., and 0. W. Kochtitzky. Composting developments in the United States, Comport Science, 6(2) :5-9, Summer 1965. 4. Wiley, J. S. A discussion of cornposting of refuse with sewage sludge. In Amer- ican Public Works Association Yearbook. Chicago, American Public Works Asso- ciation, 1966. p. 198-201. 5. Kaiser, E. R. Combustion and heat calculations for incinerators. In Proceedings. National Incinerator Conference, American Society of Mechanical Engineers, 1964, New York. p. 81-89. 6. Requardt, G. J., and W. M. Harrington, Jr. Utilization of incinerator ash as landfill cover material. In American Public Works Association Yearbook. Chicago. American Public Works Association, 1962. p. 216-225. 7. Bump, R. L. The use of electrostatic precipitators for incinerator gas cleaning in Europe. In Proceedings, National Incinerator Conference, American Society of Mechanical Engineers, 1966. New York. p. 161-166. 8. Kaiser, E. R. Prospects for reducing particulate emissions from large incinerators. Combusrion, 38(2) :27-29, Aug. 1966. 9. Kaiser, E. R., and S. B. Friedman. The pyrolysis of refuse components. Paper to be presented at 60th Meeting, American Institute of Chemical Engineers. New York, Nov. 26-30, 1967. RECYCLlNG AND UTILIZATION C. I. Harding * MOST RECYCLING and utilization schemes involve some type of Salvage and composting. A working definition of refuse composting is "the aerobic, thermophilic degradation of putrescible material in refuse by micro-organ- isms." There is no clear definition at this time of when a material becomes "compost " nor is there any general agreement upon the composition of the material which is referred to as compost. Operationally, the stabilized refuse or compost should not go anaerobic during storage either in bags or in bulk. I\`ith this crude criterion for what constitutes refuse compost we can examine the bases for the various composting systems available today. Anaerobic decomposition of waste materials has been practiced to produce soil additi\.es in Asia for centuries. Aerobic cornposting has been practiced in Europe since the 1920's and 1930's but the European practices are not directly applicable to refuse composting in the United States because of the difference of refuse composition in the two areas.1 Studies by Wiley 2 and Schultze " showed that the majority of putrescible material in U.S. refuse can be stabilized in five to seven days with aerated bin processes. This work and subsequent commercial developments served as a basis for the selecting of five to six days as the average decomposition time for the ground refuse in U.S. mechanical composting processes. Windrow systems require a much longer composting period. From two weeks to three months are required for adequate stabilization of refuse in a windrow operation. The temperature achieved during composting should exceed 140' F for a minimum of four days to insure adequate stabilization. The refuse shouId be ground to a particle size less than one inch, the moisture content of the ground refuse should be increased to about 55 percent (based on total weight) and the carbon-to-nitrogen ratio should be adjusted to approxi- mately 40 for most rapid stabilization. Mixed refuse has a very high paper content. The carbon-to-nitrogen ratio of this material can be expected to exceed 70 most of the time. This requires the addition of either sewage solids or nitrogen solutions to adjust the carbon-to-nitrogen ratio prior to digestion. Mixed refuse has a wide variation ;n chemical and physical composition. Data on composition are found in the book entitIed Municipal Refuse * Dr. Harding is Assistant Professor of Environmental Engineering at the Uni- versity of Florida in Gainesville, Florida. 105 106 HARDING Proceedings Disposal prepared jointly at APWA and APHA.4 Recently contracts have been let by the Public Health Service for development of current data on refuse composition and quantities. The composition data presented in Table I is of primary interest to designers and operators of compost plants. TABLE I COMPOSITION OF MIXED REFUSE RECEIVED AT TWO MECHANICAL COMPOSTINC PLANTS (TARLE ENTRIES ARE WEIGHT PERCENTAGE) Component Newsprint Corrigated cardboard Ferrous metal, total Ferrous metal, cans Ferrous metal, tramp Rags Noncompostable (tailings) Compostable Metrowaste plant ' Houston, Texas 1.7 0.5 7.1 1.8 0.2 2.1 86.6 IDC plant ' St. Petersburg, Florida Not separated Not separated 10 - - Not separated 10 80 U.S. COMPOSTING SYSTEMS All composting operations can be broken into three basic steps: refuse preparation; stabilization; and product upgrading. The preparation includes receiving, sorting and salvaging operations, grinding, and the addition of moisture and nitrogen. Stability or aerobic digestion can be accomplished either in windrows in the open or in mechanical plants. Product upgrading consists of grinding, enrichment, granulation, shipment, and marketing. The details of refuse preparation, product upgrading and the composting systems available will be discussed separately. Refuse Preparation Some degree of hand and mechanical sorting of the incoming refuse is required in any of the cornposting operations in use in the United States. This sorting is required to remove noncompostable material, bulky items, and items which may have some salvage value. Most U.S. systems use hand picking from a slowly moving belt and magnetic separation of ferrous metals. Some systems include inertial separation in an attempt to further separate noncompostable items from the organic matter. Grinding is required for efficient cornposting. This can be accomplished in either hammermills, chainmills, a rasp type grinder, or with wet pulping followed by screw-press dewatering. This latter method of grinding would be successful with only one of the four types of cornposting systems in use in the U.S. today. The power required to operate the grinders varies from panel B RECYCLING AND UTlLlZATlON 107 :; to about 30 hp. per ton per hour grinder capacity. In most plants now l,c*ing constructed, grinders are sized large enough to permit all grinding to be accomplished on a one-shift operating basis. Thus the capacity of the plant could be tripled by simply adding additional digester capacity and operating the pre-and post-treatment units on a three-shift basis. Figure 1 shows the inertial separation phase planned for the Gainesville Compost Plant. The primary grinder is a Centriblast unit which does impart a certain trajectory to the materials leaving the unit. A secondary, inertial separation is imparted by the jet slinger located on the Centriblast exit. The material leaving the Centriblast will then pass through magnetic separation. Two stages of grinding are usually used. The first stage or coarse grinding reduces particle size to about 2 to 3 inches. The second stage grinding usually produces particle size of approximately 0.25 to 1 inch. After grind- ing, the material is moistened with either sewage sludge, water or dilute ammonium nitrate solution, then conveyed to the digestion phase. Product Upgrading The upgrading operations which follow digestion consist of some or all of the following: curing, grinding, screening, pelletizing or granulating, drying, magnetic separation, and bagging. Storage of refuse which has been stabilized to compost by high temperature for 5 to 7 days results in a slow curing or maturing process. This has the net result of producing a darker color material with a shorter fiber length, both changes make the material esthetically more desirable. Curing can be omitted in some plants providing the carbon-to-nitrogen ratio is adjusted to insure that a minimum of 1.5 to 2 percent nitrogen will be in the material when it is used for agricultural purposes. Most plants cure from 10 days to 2 months. When properly stabilized by high-temperature cornposting the material can be piled 15 to 20 feet high and left without turning for up to six months without going anaerobic. During this curing the temperature in the pile will remain near 140" F. The material removed from this type of pile will be very dark brown in color and should serve as an excellent soil conditioner or fertilizer filler. Granulation can be accomplished by use of a short granulator followed by a dryer. The best example of an operating system of this type is found in the Altoona, Pennsylvania, plant where an attractive granular product is produced. The moisture content of the material as shipped in granular form averages about 10 percent versus the 40 to 50 percent moisture which is found in the run-of-the-plant compost produced in most other systems. 263-399 O-67-8 E HEAD PULLEY t-SECONDARY FIGURE 1 Section through the grinders and ballistic separator at the Gainesville, Florida, Metrowastc plant. p.dneI B RECYCLING AND UTILIZATION 109 Windtow Composting The new TVA-PHS Demonstration Compost Plant at Johnson City, Ten- uessee, is of the windrow type. Refuse is brought into the plant, hand sorted, ground in either a Williams hammermill or a Dorr Oliver rasping machine, then is moistened and conveyed to the outdoor decomposition area w,bere it is placed in windrows. The windrows are turned 5 to 10 times with a Cobey-Windrow turner during about 5 weeks of cornposting. After composting, the material is cured for 2 to 4 weeks. Windrow composting of this type has been practiced successfully in many locations. This process requires a moderately large area since the windrows are outside and the material is retained on-site in discrete windrows from one to two months. Calculations contained in Appendix A indicate that about 30 acres will be required for a windrow plant to serve a city of 100,000 population. This type of compost operation should be best suited for smaller cities with adequate land available and around which there exists a strong market for the compost produced. Mechanical Composting Systems Three mechanical systems have proved successful in composting U.S. refuse. They are: the Fairfield system; the Intemationl Disposal Corpora- tion (IDC) system (formerly known as the Naturizer system) ; and the Mctrowaste system. The land required for these plants is much less than that required for windrow plants of comparable capacity. A 5-acre site should serve a city of 100,000 population. The Fairfield System A pilot plant which receives approximately 25 tons of segregated refuse from the city of Altoona, Pennsylvania, has been operating using this type of digestion equipment for several years. A schematic diagram of the process is shown in Figure 2. A Williams hammermill is used as a primary grinder with no prior hand sorting since trash and rubbish are supposedly collected separately. The secondary grinding is done in a wet pulper or hydro pulper. In this unit, sewage solids can be added as the moistening agent and the filtrate from the screw press which follows the hydro pulper can be re- turned to the sewage plant. A bar screen is located between the hydro pulper and the screw press to remove film plastics, tin cans, and other non- compostable items. The wet pulp at 55 percent moisture is fed into a circular digester. This digester is the only one of the three mechanical digesters mentioned in this paper which is a continuous process unit. Air is blown through the perforated bottom to keep the mixture aerobic. Differ- ing amounts of air are fed to various sections of the digester to provide any GRINDING PROCESS METAL WPSHER AND METAL WASHER AND MAGNETIC MATERIAL TO SALVAGE OR LANDFILL, WET PULPING CGNTROL mNEL TO STORAGE FAIRFIELD-HARDY DIGESTER GAGGING FICWRE 2 Typical design for Fairfield Hardy Digester installation and related equipment. pmef B RECYCLING AND UTILlZATION 111 desired temperature profile. The augers which operate on a revolving arm, continuously mix the material and immediately integrate the wet pulp into the composting mixture. Only this digester arrangement is suited for the acceptance of ground refuse from the hydro pulper. After a nominal 5-day detention time in the digester the material is removed and cured' in windrows for about three weeks. The cured material is moistened with a starch suspension, granulated, and dried to provide an excellent quality granular product. For much larger installations it is anticipated that a picking belt kosal problem; that, in fact, their job is to sell more and more containers. llol~fully, they will come up with something that will be degradable but as of now I don't think there is any indication that the industry contem- l~latcs changes that will significantly reduce the refuse disposal problem. ANONYMOUS: Why are not private utilities, that is, electric and gas and particularly electric, regulated as closely as other industrial entities on waste disposal? MR. MICHAELS: I don't know that this is so, necessarily. Certainly, recent legislation in New York City and legislation in other major com- munitics which set limits on air pollution emissions, indicates, considerable control of public utilities; I don't know whether anybody else in the Panel or in the audience has any comments to make on this . . . I'm inclined to feel the premise is not a correct one. Any comments at all?