Solid Waste Disposal And Recycling

16.1 INTRODUCTION

Safe disposal of solid wastes is a serious problem. With our culture, which generates ever larger amounts of disposable materials and an increasing population density, we can no longer simply "throw things away." If we discard them on land, they must be buried for aesthetic, safety, and health reasons. Even this is not enough, because toxic materials can be dissolved and enter the groundwater. Consequently, disposal site construction that minimizes leaching and includes elaborate leachate recovery systems is required; sites must be chosen carefully to ensure that the inevitable accidental breaches of the system used to seal the completed landfill will have minimal impact. Remote sites increase transportation requirements, but of course, sites must not be in our backyards (NIMBY). Ocean disposal has potentially serious effects on ocean life and thus on a vital food supply, as well as having safety and aesthetic consequences when refuse materials wash up on beaches that are heavily used for recreation (see Chapter 1 for other comments on this).

Some waste materials can be disposed of by burning. Unfortunately, combustion of many substances can generate toxic products that are released to the atmosphere and widely dispersed. Special incinerators and scrubbers may be necessary, and even these may not satisfy the concerns of those who live downwind.

Recycling will reduce the amount of material to be disposed of. It has its own problems of collection, sorting, and cost. Composting of degradable organic materials also reduces disposal while producing a useful product, while anaerobic digestion is a possible source of methane fuel. A mix of these processes will be needed in future solid waste handling techniques.

Municipal wastes, that is, those not including industrial wastes or hazardous materials, vary widely in composition. Sometimes they include demolition debris, industrial wastes, or water treatment sludge, for example. Most of the total municipal solid waste generated in the United States is disposed of in landfills; according to 1998 EPA estimates, of the total 220 million tons of waste produced, 55% went to landfills, 17% was incinerated, and 28% recycled. Waste generation is projected to continue to increase, while recycling is expected to take up a significantly larger fraction. Disposal of hazardous wastes is a separate problem.

Estimates of municipal waste production and recycling in the United States in 1998 are given in Tables 16-1 and 16-2 and recent trends are shown in Figures 16-1 and 16-2.

TABLE 16-1

Waste Production and Recycling of Different Types of Material in the United States in 1998

TABLE 16-1

Waste Production and Recycling of Different Types of Material in the United States in 1998

Material

Weight generated

Percent recycled

(lb x106)

Paper / paperboard

84.1

41.6

Plastics

22.4

5.4

Glass

12.5

25.5

Metals:

Iron/steel

12.4

35.1

Aluminum

3.1

27.9

Other nonferrous

1.4

67.4

Rubber and leather

6.9

12.5

Textiles

8.6

12.8

Wood

11.9

6.0

Yard wastes

27.7

45.3 (composting)

Food wastes

22.1

2.6

Miscellaneous materials

3.9

23.1

Miscellaneous inorganic wastes

3.3

Negligible

Source: EPA Environmental Fact Sheet: Municipal Solid Waste Generation, Recycling and Disposal in the United States: Facts and Figures for 1998. http://www.epa.gov/osw.

Source: EPA Environmental Fact Sheet: Municipal Solid Waste Generation, Recycling and Disposal in the United States: Facts and Figures for 1998. http://www.epa.gov/osw.

TABLE 16-2

Examples of Industries Using Recycled Materials as Feedstocks in 1989

Number of Consumption (1989) Recycled material Type of industry Virgin materials manufacturers (tons xlO6) Benefits

Paper

Paper and paperboard mills

Wood pulp, other plant libers

700

85

Less costly feedstock Reduced water consumption Lower capital requirement

Glass

Container manufacturers

Sand, limestone, soda ash

16

11

Energy savings Cleaner furnace firing Potential furnace life extension

Tin cans

Steel mills and detinning plants

Iron ore, lime, coke

50-150

80

Reduced capital requirements Reduced raw material requirements Higher quality input (if purchased from detinners)

Aluminum cans

Primary and secondary aluminum producers

Refined bauxite, carbon

> 60

8

Avoided capital cost Reduced energy consumption

Plastic

Plastic resin, film, and fiber manufacturers and processors

Products of petroleum and natural gas distillation and refinement

>14,000

30

Cost savings Availability

Source: L. F. Diaz, G. M. Savage, L. L. Eggerth and C. G. Golueke, Composting and Recycling Municiple Solid Waste, Lewis Publishers, Boca Raton, FL, 1993.

Firenze 1970 1980 1990

1960 1970 1980 1990 1998

Year

FIGURE 16-1 Total municipal waste production in the United States. From data in EPA Environmental Fact Sheet: Municipal Solid Waste Generation, Recycling and Disposal in the United Stated: Facts and Figures for 1998. http://www.epa.gov/osw.

1960 1970 1980 1990 1998

Year

FIGURE 16-1 Total municipal waste production in the United States. From data in EPA Environmental Fact Sheet: Municipal Solid Waste Generation, Recycling and Disposal in the United Stated: Facts and Figures for 1998. http://www.epa.gov/osw.

Containers and packaging, including cans and bottles, made up 72.4 million tons of waste in the United States in 1998, of which 40% overall was recycled; over half the steel and about 44% of the aluminum packaging (mostly cans) and over half the paper and paperboard packaging was recycled, but only 29% of the glass packaging materials. Nondurable goods provided 60.3 million tons of waste, with paper and paper products contributing 40 million tons in this category. Durable goods (appliances, etc.,) amounted to 34.4 million tons.

16.2 LANDFILLS

Originally, landfills were simply places where waste could be dumped where it did not disturb too many people. As problems associated with this form of

1960 1970 1980 1990 1998

Year

FIGURE 16-2 Municipal waste production in the United States. From data in EPA Environmental Fact Sheet: Municipal Solid Waste Generation, Recycling and Disposal in the United Stated: Facts and Figures for 1998. http://www.epa.gov/osw.

disposal became to be recognized, more elaborate arrangements became necessary. Many old landfills that did not meet modern standards, or that were full, have been closed, and it is not a simple matter to open new ones. In the United States, operating landfills went from about 8000 in 1988 to about 2300 in 1998.

A modern landfill is sealed underneath by a flexible membrane liner such as various forms of polyethylene, rubber, or polyvinyl chloride) that resist degradation, and/or a layer of impermeable clay. Some of the properties of clays for this purpose were discussed in Section 12.3.1. A system for leachate collection also is necessary. The waste is placed in compacted, earth-covered layers, and, when the landfill is full, closed by a water-impermeable top layer. Figure 16-3 is a schematic illustration of a landfill with a double liner and double leachate collection system. Not all landfills will have these redundant features.

Viking Graben

Flexible membrane liners Porous layers Leachate collection system

FIGURE 16-3 Schematic model of a modern landfill.

Flexible membrane liners Porous layers Leachate collection system

FIGURE 16-3 Schematic model of a modern landfill.

The leachate collection system typically consists of plastic pipes leading to a sump with a pumping system to remove the liquid. The pipes are embedded in a porous layer that may be an appropriate soil or an artificial material. A filter layer screens out fine particles of earth that could clog the drainage system. Gases, chiefly methane, are released as the organic materials in the waste decay. These gases must be controlled to avoid the buildup of potentially explosive concentrations. In some cases, the methane can be collected for use as a fuel.

Although a typical landfill is a pit, some are constructed as mounds above the surface. These above-grade deposits can be large (a popular name is Mount Trashmore); the largest is the Fresh Kills landfill site on Staten Island, New York, which is expected to top out as a mound more than 150 m high, along with several smaller mounds, by 2005. This type of landfill avoids excavation and makes leachate collection easier, but more earth must be brought into cover the trash layers, and the mound is more subject to erosion. When completed, the earth-covered mound can be used as a recreational area, as has been done at Virginia Beach and other locations, but any use must avoid penetration of the cap.

Although biodegradable materials do decompose in landfills, the process is not rapid. Conditions are anaerobic and not conducive to rapid biological action. Even after decades, newspapers have been recovered that are readable, and food items identifiable. In some sense, a landfill can be regarded not as a disposal site but as a storage site.

On a volume basis, plastics are more important than their low weight percentage implies. They have low densities, often do not crush well, and have a tendency to work their way to the surface. Discarded automobile tires have a similar tendency, which is one reason many landfills will not accept them.

Although not nominally considered to contain hazardous materials, toxic substances from household chemicals, batteries, paint solvents, and so on inevitably make up some fraction of household wastes. However, EPA regulations prohibit disposal of hazardous wastes in an ordinary municipal landfill. Hazardous wastes, which include flammable, volatile, toxic, and pathological wastes, include wastes from many industrial operations. Radioactive wastes are a special class of hazardous material, discussed in Chapter 14. Hazardous wastes are often disposed of in landfill facilities specifically intended for the purpose, and with extensive documentation. In the past, there was considerable mixing of hazardous and municipal waste and few records of landfill composition or even location were kept; landfill construction was much more casual. Accidental breaching of these disused landfills has had serious effects, as illustrated by the notorious Love Canal1 site near Niagara Falls in New York State.

Love Canal was a disposal site used by a number of firms for dumping of chemical wastes over a long period beginning in the 1930s. Sealed in the 1950s, the site was later used for school and housing development. Such use breached the clay cover that had sealed the site and allowed entry of water. Leaching of chemicals such as benzene and chlorinated hydrocarbons into basements and other sites of exposure caused considerable health risks requiring abandonment of the housing and the expenditure of millions of dollars in cleanup programs. This was the beginning of the program to identify potentially hazardous waste sites in the United States and the setup of the "Superfund" program to monitor them and clean them up. Many such sites exist. They are by no means confined to the United States; a situation very similar to Love Canal occurred in the Netherlands, where a new village (Lekkerkirk) was built on a waste dump in 1970 but had to be abandoned in the 1980s with large costs for cleanup. There are thousands of identified abandoned and potentially hazardous waste sites worldwide, and probably many more that have not been identified.

16.3 COMPOSTING

Biodegradable organic materials have beneficial effects when applied to agricultural land, in part as fertilizer because of their N, P, and K content (although this is typically relatively low), but mostly in terms of improving soil quality

1Love Canal was an uncompleted navigation and electrical generation canal intended to bypass Niagara Falls. It was started in the late 1800s but abandoned with comparatively little excavated. The "ditch" left behind was later used as a site for waste disposal.

by increasing its organic content (see Section 12.3.1). Raw organic waste, such as food processing residues, garden wastes, or sewage sludge (if sterilized) can be applied directly, but composting provides important advantages. (The problem of heavy metal contamination of sewage sludge was referred to in Section 11.5.2; the same considerations apply if this sludge is used in composting.) Composting consists of breakdown of the organic material through microbial activity to derivatives of the lignins, proteins, and celluloses that resist further reaction. It produces a material with the characteristics of humus. Pathogens are destroyed during the process through the action of the heat that is generated. Temperatures of 55-60°C for a day or two will kill most pathogens of concern, and such conditions are produced in normal composting behavior. Obviously, adequate mixing is essential, to avoid cold spots and to ensure that all pathogens spend adequate time in the heated regions. Composting of wastes that contain potentially large concentrations of human pathogens, (e.g., sewage sludge) must be done with careful control of conditions.

Composting involves the interaction of the organic substrate with the organisms in the presence of water and oxygen to produce heat, carbon dioxide, and the decomposed organic materials. Conditions such as substrate composition, aeration, and moisture content affect the process and need to be well controlled to give a good quality product and to ensure operation of a large-scale plant. As with recycling, sorting is necessary to avoid contamination with nondegradable materials. Size reduction, particularly of waste wood, branches in yard waste, and so on, is necessary. The C/N ratio of the substrate strongly affects the rate, and nitrogen-rich materials may have to be added to act as fertilizer. The consistency of the material must allow air to circulate. Therefore the mix must have reasonable particle size and cannot be excessively wet. Materials such as sewage sludge need to be mixed with coarser materials, for example. The process is not rapid. Two weeks and usually more is required between the start of the process and its completion, so that a large-scale composting plant will contain a large volume of material.

One process is static; the organic waste is placed in an appropriate pile over a pipe that passes air into it, or applies suction to draw air through it. Odors can be trapped by a layer of finished compost, which has good adsorption properties. A second process involves the construction of windrows (elongated piles) of the waste, and mechanically turning and mixing them on a regular basis. These piles may be 6 or 7 ft high, 10 to 13 ft wide (narrower with manual turning), and as long as necessary. Shelter is needed for both these systems to keep out excessive moisture from rain, and for temperature protection in cold climates. Odor control is necessary for the windrow method.

An alternate process, mechanical composting, involves the use of rotating drums, or tanks equipped with mixing devices along with aeration, to provide optimal aeration and mixing. These afford more rapid reactions in the initial steps of the process when the greatest generation of odor is likely, but still require considerable time either in the reactor or in a windrow stage for complete reaction to a usable material. Control of possible leachate from the wastes waiting to be processed, and from the composting materials, is necessary.

As indicated, odor is one of the main concerns raised in objections to composting sites. Although the process is aerobic, some anaerobic decomposition can occur locally in the composting mass, generating typical anaerobic decay products such as ammonia, hydrogen sulfide, and organic sulfides such as mercaptans. Aerobic products such as low molecular weight fatty acids (e.g., acetic and butyric acids), aldehydes, ketones, esters, terpenes, and others with objectionable odors also may be released. A properly designed composting facility must have provision for capturing these materials, usually by passing the exhaust air through absorbers or scrubbers of some kind to either trap or chemically (sometimes biologically) destroy the obnoxious materials.

16.4 ANAEROBIC DIGESTION OF BIOLOGICAL WASTES

It was pointed out in Section 11.5.2 that the volume of sewage sludge that must be disposed of can be minimized by anaerobic decomposition to produce methane, a useful by-product. This process can be extended to other organic waste materials, including animal and human wastes, domestic wastes and crop residues. The wastes are made into a slurry and digested by anaerobic bacteria over a period of weeks to produce a gas (called biogas) that is approximately 60% methane. Most of the remainder is carbon dioxide; undesirable components such as H2S are usually quite low. The process can be quite simple and has applications as an energy source in undeveloped areas, since the biogas can be used for heating, cooking, or running engines.

The digestion process uses a number of complementary bacterial species that convert protein, carbohydrate, and lipid material to simpler compounds— amino acids, simple sugars, fatty acids—and finally into methane as the bacteria use these compounds to grow. Except for some chemicals that might be found in industrial wastes, composition of the waste is not critical. Operating conditions, such as temperature and pH, must be kept within certain limits for optimum gas production. The final output from such digesters is a sludge containing half to two-thirds of the original organic content. This can be used as a fertilizer, although there may be concern about the complete destruction of pathogenic bacteria.

16.5 INCINERATION

Combustible wastes have long been disposed of by burning. As is the case with burial, this solution is not simple when carried out on a large scale. Three major problems must be dealt with in burning of waste: smoke, soot, and ash released to the atmosphere; highly toxic compounds produced through incomplete combustion and also released to the atmosphere; and toxic residues, chiefly heavy metals, that contaminate the ash and may enter groundwater from ash disposal. On the other hand, the volume of solid waste that must be disposed of in landfills is markedly reduced, and the energy released in the combustion process may be recovered, either to produce steam for heating or to generate electricity.

In all incineration processes, several possible modes exist for release of undesirable materials in the exhaust. These include material that escapes combustion (e.g., if the residence time is too short, or if the waste, is poorly mixed and does not experience sufficiently high temperature), material that is only partially oxidized to other, perhaps more hazardous compounds, fragments from thermal decomposition (pyrolysis) of larger molecules (e.g., benzene rings, small chlorinated molecules), and new molecules that may form from partially decomposed material in temperature regions that are high enough to promote reaction but not high enough for decomposition to be complete. Dioxins are an example; their formation was discussed in Section 8.10.3. The presence of chlorine- or bromine-containing materials can lead to the formation of hazardous chlorinated or brominated organics, including halogenated dibenzofurans (Section 8.10.3) and biphenyls (Section 8.7), as well as HCl, while sulfur compounds will produce SO2, and the combustion process itself will generate nitrogen oxides. Such incomplete combustion can release hydrocarbons, particularly polycyclic compounds such as benzo-(a)-pyrene, as well as carbon monoxide. Other toxic elements present at trace concentrations in the waste, such as Se, As, and heavy metals, can also escape as fine particulates; fly ash is generally higher in heavy metal concentration than the coarse ash left in the incinerator, but even the latter may have enough heavy metal content to require special disposal.

Various designs of incinerators are available for dealing with municipal wastes. They must allow thorough mixing of the waste with air and maintain a temperature high enough (750-1000°C), along with a residence time of the combustible material in the hot zone long enough (at least 3-4 s) to permit complete degradation of all organic molecules to carbon dioxide. This may involve a two-stage combustion process. Exhaust gases must be scrubbed to remove HCl and other components, and particulate materials must be removed by electrostatic precipitators and filters.

For incineration of municipal waste, two approaches are in use. In mass burning, the waste is burned with only minimal preseparation (e.g., of oversize noncombustibles). Shredding or shearing of large combustibles is necessary to provide reasonable handling. Such incinerators may be used solely for waste disposal, or the heat generated may also be used as a source of energy. The heat value of municipal solid waste ranges from about 7,000-15,000 kj/kg, depending on the composition (cf. anthracite coal, 30,000-37,000 kj/kg). Plastic waste has the greatest heating value per unit weight, while paper and other combustble components are considerably lower. Combustion is a way of recovering some of the energy value of plastic and other wastes if recycling is not practical.2 Although incineration of large amounts of plastics generates more emissions of chlorine and of lead and cadmium from compounds that are still used as stabilizers, studies have shown that properly operating modern plants can maintain emissions well within safe limits.

Complete combustion will preclude release of smoke containing soot, but fine ash particles and gaseous pollutants must be removed by precipitation and/or scrubbing of the stack gases as already mentioned. The waste normally contains considerable water, and under some conditions this will lead to a visible steam plume as the vapor condenses. A plume will form if, upon mixing of the warm stack gas with the ambient air, the temperature and relative humidity of the mix reach a value in which the saturation level of the air is exceeded. This is often interpreted by neighbors as harmful, although in itself it is not unless it hides particulate smoke.

Ash, both the fine fly ash recovered from the gas stream and the coarser material left in the incinerator, must be disposed of, usually in landfills. If the heavy metal content is low enough, however, the ash may be used as a fill or aggregate. Some material, especially ferrous metal, can be recovered from the coarse ash before disposal. If scrubbers are used to treat the stack gases, solid or sludge residues from these will also be placed in landfills.

While incineration as a municipal solid waste disposal method can be safe and effective, the technology is not easy to control because of the variable nature of the material to be burned. Even with careful selection, one can never be sure that a householder has not put some highly hazardous material into the garbage, and thus explosions when the waste enters the incinerator are possible. Care in maintaining the monitoring systems and in controlling the operating parameters is essential if release of highly hazardous organic byproducts is to be avoided.

Municipal incinerators have been criticized not only because of their potential for release of materials to the atmosphere, but also because of their effect on recycling. The cost of construction, particularly if the facility is also used to generate energy, makes it necessary from an economic point of view to operate

2B. Piasecki, D. Rainey and K. Fletcher, Am. Sci., 86, 364 (1998).

an incinerator as continuously as possible. Consequently, removal of combustible materials from the waste stream for recycling may be discouraged. Also, efficient plants need to be large and consequently need large amounts of waste; transportation becomes a factor.

The second approach to incineration of municipal waste is the use of refuse-derived fuels for energy generation. In this approach, the waste is first processed for the recovery of recyclables and to assure a more uniform material that can be handled and burned in a more controlled fashion. The refuse-derived fuel is co-burned with another fuel, usually coal. Typically, in these co-burning systems, 30% of the fuel, producing 15% of the energy, is refuse derived. This is the limit for regulations applying to coal-burning generators. If more refuse is used, more stringent rules requiring more monitoring of potential toxic releases go into effect.

Various special purpose waste incinerators are in use—for example, to burn sewage sludge or hospital wastes. Hazardous wastes that are not highly combustible and must be burned at an appropriately high temperature for safety reasons are treated in incinerators with auxiliary gas or oil fueling. A large use of incineration is for the disposal of liquid industrial hazardous wastes.

Wastes designated as hazardous may be disposed of in incinerators intended specifically for that purpose (e.g., Figure 16-4), but much is disposed of in industrial boilers or furnaces, such as the cement kilns discussed in Section 12.4.4. In fact, over half the hazardous wastes incineration in the United States is done in cement kilns. Incineration of hazardous wastes in the United States is governed by the fairly elaborate EPA Boilers and Industrial Furnace (BIF) regulations, which in summary call for minimum destruction of 99.99% (99.9999% for certain materials like dioxins) destruction of the waste. Notice that this is not a limit on the amount of unburned material that can be released in total, just a limit on the fraction of the input that can be released. There are other restrictions such as on CO, metal, and particle release.

A well-designed combustion chamber can minimize stagnant regions that may not reach proper temperatures and maximize mixing, but operation under less than optimum conditions can defeat this. Because hazardous wastes are likely to be of variable composition (except for burners dedicated to incineration of a specific process stream), maintaining optimum conditions may not be easy. Monitoring release is also a problem because of the large number of possible products involved. Carbon monoxide monitoring can be used to indicate deviations from proper conditions but is not a direct measure of what is being released.

Ocean incineration, carried out aboard special incinerator ships, has had some use. Here, the theory is that the incineration occurs far from any populated area, and any emissions will be absorbed by the ocean before they can be carried to land. This practice would permit incineration of very toxic materials,

Chimney

Chimney

Rotary Kiln Incinerator
FIGURE 16-4 Schematic of a rotary kiln hazardous waste incinerator. From http://www.eur-tits.org/reports/9702.htm. Used by permission of the European Union for Responsible Incineration and Treatment of Special Waste and Indaver.

but if carried out on a large scale raises questions about effects of toxic releases on the ocean food chain if combustion were not complete.

16.6 THE TIRE PROBLEM

About 250 million worn-out automobile tires are discarded annually in the United States, and many more globally. Disposal of these waste tires has long been a problem. Landfilling is difficult because tires tend to work their way to the surface, as mentioned earlier. Open dumps, of which there are many, are breeding grounds for mosquitoes because of the stagnant water that readily collects in the tires, and they are subject to fires that are both extremely smoky and notoriously difficult to put out. These fires release toxic products and leave toxic residues. One tire fire, in Winchester, Virginia, burned for nine months. A number of states have enacted legislation dealing with scrap tire disposal.

However, nearly two-thirds of the tires now being discarded are not disposed of in any productive way.3

Useful rubber products are produced by vulcanization, a process that uses sulfur to cross-link the polymer chains of the raw rubber to give a material with physical and chemical properties that make it tough, stable, and nonbiodegrad-able. Fillers such as carbon black are added to improve properties, while tires also contain steel or fiber reinforcing belts. Depolymerization of the rubber, while chemically feasible, is not economically practical. A number of processes can be used to pyrolyze the scrap tires to produce gaseous and liquid hydrocarbons useful as fuels or as chemical feedstocks, along with carbon black and recovered steel. Although these have promise, at the time of writing no commercial plants are operating, again because such enterprises are not economically viable. A couple of plants, one in Wind Gap, Pennsylvania, and another in Centralia, Washington, have operated intermittently, and a demonstration pilot plant has been built near Leipzig, Germany, in the town of Grimma.

The major use of scrap tires at present is as tire-derived fuel. Whole or shredded tires, which have very high heat values, are used to supplement traditional fuels in cement kilns, pulp and paper mill boilers, and coal-fired power generating facilities. Growing use as fuel is anticipated as it becomes more commonly recognized that controlled combustion does not involve the smoke emissions associated with free burning.

The properties of scrap tires make them directly useful for some purposes. They can be used to construct artificial reefs in which the tires are tied together and anchored in coastal waters, where they are attractive to barnacles and other marine organisms and for many types of fish. Tires can also be used to construct floating breakwaters to protect harbors and beaches. Stacks of tires have been used as highway crash barriers at bridge piers and other obstructions and for the construction of retaining walls for steep-sided areas next to highways. In a number of states including Florida, West Virginia, Ohio, and Pennsylvania, scrap tires are used constructively in landfills, either as a daily landfill cover or as part of the leachate collection system. In some cases, the tires are shredded before use in landfills.

Shredded tires have many other applications. For example, the shreds have been used to make floor mats and sandals and have been used as fill and insulation under road beds. Very small shreds, known as ground rubber, have been used for running tracks on athletic fields, railroad crossing beds, carpet underlay, and as an asphalt additive.

In some cases, rubber strips cut from the tires are used to make dock bumpers, floor mats, conveyor belts, and so on. In 1998, 23% of U.S. tires were recycled, excluding those used for fuel or recovered for retreading.

3K. Reese, Today's Chem. Work, 4(#2), 75 (February 1995), provides a discussion of the 1994 situation.

16.7 RECYCLING

Recycling is an alternative to the waste disposal methods discussed in the preceding sections of this chapter. The present discussion will be mostly about "municipal solid waste," which comes from households, commerce, industry, and government. The approximate composition of this municipal solid waste in 1998 and the percent of the different materials that were recycled were shown in Table 16-1. Some solid waste can be recycled in a variety of ways to make useful objects. Before any of the various materials that make up this solid waste can be recycled, however, they must be separated, at least in part. Separation at the point of origin provides cleaner, higher quality material than that separated from mixed solid waste, and many municipalities now encourage such separation, but even separated waste does not provide pure materials for recycling. Sometimes the separated but not pure solid wastes can be recycled as such, but often further separation is necessary. For example, as will be seen shortly, glass waste and beverage cans must be further separated before the materials can be reused.

It is difficult to treat the economics and cost-effectiveness of recycling in isolation. In general, the improvement of the quality of the environment due to recycling has not been factored into the economics. In many cases, this makes many types of recycling appear to be cost-ineffective on purely economic grounds. In some cases, however, the use of recycled materials can be cost-effective even without considering these intangible factors. Table 16-2 gave some examples of industries in the developed world that used recycled materials as feedstocks in 1989.

16.7.1 Glass

The municipal solid waste stream contains about 10 wt % glass, most of which is container glass, the glass used to make jars and bottles: mayonnaise and pickle jars, beer bottles, wine and liquor bottles, some soft drink bottles, and so on. Container glass is the only glass that is being recycled in large quantities at present. Glass used in such items as light bulbs, drinking glasses, mirrors, and cookware is different in composition from container glass and is not accepted for recycling by the container industry. Glass pieces that can be recycled are called "cullet" and must often be color-sorted before reuse. Much cullet, called primary cullet, is generated during the manufacture of glass. Traditionally, about 15% of the material used in the manufacture of glass containers has been cullet, but a goal of 30-50% has been established within the container industry. It is actually advantageous to use cullet because it liquefies at a lower temperature than the raw materials: sand (SiO2), lime (CaO), and soda ash (Na2CO3) used in the manufacture of glass. This results in energy savings (for every 1% of recycled glass used, there is a 0.5% drop in energy use) and the extension of the useful life of furnace linings, among other things. If the cullet is not color-separated, it is used for the production of glass beads, roadway materials, and building materials such as fiberglass insulation.

16.7.2 Paper

Paper and paper products are made from wood and consist mostly of cellulose, as discussed in Section 7.3.2. Most paper products contain various additives— fillers such as clay or titanium dioxide, resins to improve wet strength, sizings, coloring, and so on. These differ depending on the type of paper, adding to the complications of paper recycling. Separation of paper to be recycled into various grades is required. As shown in Table 16-1, nearly 42% of the paper produced in the United States was recycled in 1998. The types of paper that are recycled are newsprint, corrugated cardboard, magazines, and so-called high-grade and mixed papers. Domestic demand for reclaimed papers has been rather erratic over the years, and a large amount has been exported: 6.5 million tons in 1990, for example, mostly to the Far East. In 1995, about 20% of all paper reclaimed in the United States was exported. Corrugated cardboard is the main reclaimed paper product that is exported.

Recycled paper is used mainly by four different industries. The first is the paper products industry, to make newsprint, various other writing and printing papers, bags, towels, and tissues. To use recycled newsprint for fresh newspapers, it must first be de-inked in special de-inking mills that were in short supply until the mid-1990s. New de-inking plants were built only after a number of states required that a minimum percentage of recycled paper be used in newsprint. De-inking consists of repulping the paper in a solution of sodium hydroxide and hydrogen peroxide, along with surfactants to release the ink, which is separated by flotation. Other contaminants such as staples and glue must also be removed. The process breaks up the fibers to some degree, reducing the quality of the product and limiting the number of times a given sample of paper can be recycled. To maintain quality, some percentage of virgin fibers or higher quality paper (typically 30% coated paper—magazines, etc.) must be added to recycled newsprint. The second industry is the paper-board products industry, to make corrugated boxes, shoe boxes, and file folders. The third is the construction paper industry, to make roofing materials and acoustic tile. The fourth is the molded paper products industry, to make egg cartons and layers for packing fruit. Recycled paper products are also used to a small extent to make cellulose insulation, bedding for animals, mulch, and liquid and solid fuels.

16.7.3 Metal

The metal content of municipal solid waste consists mostly of aluminum or ferrous metals and these are generally recycled separately.

16.7.3.1 Aluminum

Aluminum scrap that is produced during the manufacture of aluminum products (new scrap) is almost completely reused and recycled. Old scrap comes from discarded aluminum products including cans, aluminum siding, lawn furniture, automobiles, pots and pans, and venetian blinds. Recycling of aluminum cans began in the early 1960s, encouraged by the major aluminum companies, and was viewed mostly in terms of public relations. By 1995, 65% of the aluminum from used beverage cans was recycled back to aluminum can manufacturers, who processed it into so-called can sheet to produce new beverage containers. Each of these newly produced aluminum cans contained about 51% recycled metal. Other types of old scrap aluminum often contain other alloyed metals and are thus not used to make beverage containers or aluminum foil but may be sold to aluminum mills for production of other products.

16.7.3.2 Ferrous Metals

There are three types of ferrous metal scrap. Home scrap is produced in steel mills during production of the steel and is recycled internally. Prompt industrial scrap is produced during machining, stamping, and other fabrication processes during the production of iron and steel items. Obsolete scrap consists of iron and steel products that have served a useful purpose and are now being discarded. Railroad cars and tracks, automobiles, and trucks are major sources of obsolete scrap; other sources include manhole covers, old water pipes, lawn mowers, pots and pans, and steel cans. Steel cans can be collected along with aluminum cans because the two types of can can be easily sorted by means of magnetic separation (the steel cans can be magnetized, while the aluminum cans cannot). Steel mills use scrap iron and steel in the manufacture of new steel (Section 12.2.4).

The obsolete scrap most often recycled by consumers consists of steel cans that have been used as containers for food, beverages, paint, or aerosols. To protect the contents of the cans from corrosion, these steel cans are usually lined with a very thin coating of tin, about 3 x 10~5 in. thick. These cans, often called "tin" cans, have the tin recovered by detinning companies during recycling. Detinning companies use either a chemical and electrolytic process or heat treatment (tin melts at about 232°C) to remove the tin. The chemical process consists of dissolving the tin as sodium stannate with a sodium hydroxide-sodium nitrate solution. In the electrolytic process, the scrap steel is made anodic, and the tin electrolytically dissolved and redeposited on the cathode. Some food cans, including tuna cans, are made with tin-free steel, while some others have an aluminum lid and a steel body (bimetal cans) whose parts can be separated magnetically after shredding. In 1995, almost 56% of steel and bimetal cans in the United States were recycled.

16.7.4 Plastics

All plastics are made of polymers, which were discussed in Section 7.3. Although most plastics waste in the United States has been placed in landfills, Switzerland, Japan, and some other countries, have disposed of most of their plastics waste by incineration. Although the incinerators used include excellent antipollution devices, they have not been accepted by the populations of many countries. Most plastics wastes are not being recycled at this time. For example, the plastics used in some durable goods such as automobiles may consist of mixtures of as many as 50 or 60 different plastics that are not currently being separated for recycling. A number of ways are being found to get around this problem:

1. It is possible to use fewer different plastics in the manufacture of automobiles.

2. Some companies—for example, General Electric—have made long-term agreements with scrap dealers who buy junked automobiles to obtain the plastic parts from these automobiles for recycling.

3. Manufacturers may be asked to put identifying bar codes on each polymer-containing component.

4. Uses can be found for the mixed polymers—for example, tiny beads of mixed plastic can be used by the aircraft industry in a process analogous to sand blasting for removing paint from aluminum.

Many manufacturers of plastics now attempt as much recycling of plastics wastes produced during production as possible. In the rest of this section, however, we shall be concerned only with the postconsumer plastics waste that has been labeled with the Society of the Plastics Industry (SPI) system shown in Figure 16-5. The six main polymers now labeled for recycling make up most of the postconsumer plastics wastes (numbers 2 through 6 make up about 85% of these). Some municipalities collect mixed plastics wastes, while others collect only plastics wastes that have been separated by number. In 1993 about 7% of the plastics labeled with numbers 1 through 6 in Figure 16-5 were recycled in the United States. In western Europe, in the same year, about 21% of municipal plastics waste was either reused or incinerated in a way that produced useful energy (see Section 16.5).

Recycling symbol

Polymer

PETE*

Poly(ethylene terephthalate)

— C—(( ))—O-CH2-CH2-O— L ^-' An

Soft drink bottles, carpets, fiberfill, rope, scouring pads, fabrics, Mylar tape(cassette and computer)

HDPE

High density polyethylene

-fCH2-CH2]-n

Milk jugs, detergent bottles, bags, plastic lumber, garden furniture, flowerpots, trash cans, signs

L J n

Cooking oil bottles, drainage and sewer pipes, tile, institutional furniture, credit cards

4CH^CH+n

Bags, squeeze bottles, wrapping films, container lids

—Pch2-ch—I— L 1 -In CH3

Yogurt containers, automobile batteries, bottles, carpets, rope, wrapping films

6

Disposable cups and utensils, toys, lighting and signs, construction, foam containers, and insulation

Other

All other polymers

Various food containers, hand cream, toothpaste, and cosmetic containers

* PETE is used as an abbreviation for poly(ethyleneterephthalate) in recycling codes, but most chemists use the abbreviation PET.

* PETE is used as an abbreviation for poly(ethyleneterephthalate) in recycling codes, but most chemists use the abbreviation PET.

FIGURE 16-5 Recycling codes for plastics.

16.7.4.1 Mixed Postconsumer Plastics Waste

Mixed plastics, after cleaning and shredding, can be melted and extruded under pressure to make "plastic wood." Since most plastics, including those in postconsumer plastics waste, are not miscible with each other, the particles in the "plastic wood" do not adhere well to each other, and the material contains voids and often pieces of newspaper, aluminum foil, and anything else that was not removed from the plastics waste before extrusion. The material is thus not very strong and cannot be used under tension; also, the extruded pieces must have a large cross section, several inches or more. Plastic wood is used in agriculture (fences, animal pens, tree supports), marine engineering (seawalls, boat docks, boardwalks, lobster traps), recreational equipment (park benches, picnic tables, stadium seating), gardening (compost bins, garden furniture), civil engineering and construction (signposts, siding insulation, shutters, roof tiles), and for industrial uses (highway construction, pipe racks, traffic barriers).

It is generally more useful to separate the postconsumer plastic waste into its constituent plastics (Figure 16-5), either at the source or after collection. There are many ways to do this; some of these are separation by hand, various flotation methods that separate the (shredded) plastics by density, and many other methods that are applicable to particular plastic mixtures. For example, bottles made of poly(vinyl chloride), PVC, can generally be separated from other bottles by using various devices based on the infrared absorption or the fluorescence emission of these bottles as contrasted with that of bottles made from other plastics. The bottles are on a conveyor belt; when the sensor detects a PVC bottle, this bottle is ejected from the belt into a special container. Methods based on the different solubility of the various plastics in different solvents or in the same solvent (xylene) at different temperatures are also being considered; in these cases, the polymers must be recovered from the solvents separately.

16.7.4.2 Reuse of Pure Plastics

Most 1- and 2-liter soda bottles are made primarily of poly(ethylene tereph-thalate), PET, and, in states such as New York State, are recycled separately from other plastic waste; this may be done by imposing a bottle deposit, which is returned to the consumer when the empty bottle is turned in at a collection center. These bottles can be used to produce polyester fiber, which is used in carpets (in 1995, it was used in 50% of all polyester carpeting sold in the United States), outdoor clothing, insulation, furniture stuffing, and so on. Waste nylon is also processed into fiber to make carpets, tennis ball felt, and other items.

High-density polyethylene (HDPE), used for milk and other bottles when first produced, has been recycled for use in non food containers for such products as oil, antifreeze, and laundry detergent. It can also be used for drainage pipes, flower pots, trash cans, kitchen drain boards, and so on. Recycled milk jugs have been made into a superior type of plastic wood that is hard to split, easy to saw, colorable while being produced, and stands up better than treated lumber to weather and insects. Although it costs much more than treated lumber, its durability makes it less expensive in the long run.

Tertiary recycling is the production of basic chemicals and fuels from plastic waste, as defined by the American Society for Testing and Materials (ASTM). The polymers may be chemically decomposed by various methods, or they may be pyrolyzed. Tertiary recycling is used even for polymers that may be recycled as such (see Section 16.7.4.2) because food and drug regulations in the United States forbid the use of melt-process recycled plastics for anything that will contact food.

16.7.4.3.1 Chemical Decomposition of Polymers

Many polymers can be decomposed back into their monomers by using thermal or chemical treatments. These include many more plastics than those few used as illustrations in this section.

A number of different chemical companies have treated PET with ethylene glycol, methanol, or water at elevated temperatures under pressure to produce the chemicals from which PET may be resynthesized, dimethyl terephthalate and ethylene glycol. Equation (16-1) shows the methanolysis reaction, which is widely used because it is relatively insensitive to the additives and contaminants that may be included in the recycled polymer.

Polyurethanes, used in foam mattresses, foam insulation, and proprietary elastic fibers such as Spandex, are various polymeric materials that contain urethane, —NHCO—, groups formed from isocyanates, often aromatic ones, and polyols. When polyurethane foam products are mixed with superheated steam, they liquefy and hydrolyze to the amine corresponding to the starting isocyanate, usually 2,5-diaminotoluene, propylene glycol, and carbon dioxide.

16.7.4.3 Tertiary Recycling of Plastics

NHCOCH2CH2CH2O—

NHCOCH2CH2CH2O—

[Simplified polyurethane structure]

Equation (16-2) shows a very simplified structure of a polyurethane. More than one glycol and more than one aromatic diisocyanate may have been used to make the polyurethane and, in addition, parts of the polymer chains may consist of polyesters, polyethers, and polyamides. Therefore, additional low molecular weight compounds are usually produced by hydrolysis. Alcoholysis using a short-chain alcohol has also been used to decompose polyurethanes; in this case, no carbon dioxide is formed. The low molecular weight compounds produced can be mixed with diisocyanates and new foamed polyurethanes can be produced.

Nylon 6 can be depolymerized at high temperature or by using various hydrolysis reactions to form the monomer, which can be purified and repoly-merized:

16.7.4.3.2 Pyrolysis

Pyrolysis is the thermal fragmentation of plastics into small molecules, either in the absence of oxygen or in an oxygen-deficient atmosphere, at temperatures as high as 500-1000°C. Waste items made of a single polymer, a mixture of polymers, or, for that matter, mixed household wastes, may be and have been pyrolyzed. Gases, liquids, and solids are obtained. The gases obtained may include CO, C02, H2, CH4, ethylene, propylene, and others. Some of these gases are used to heat the pyrolysis plant so that no external energy source is needed for this purpose. The liquids that are formed include benzene, toluene, naphthalene (this is dissolved in the other liquids), fuel oil, and kerosene, depending on the plastics that are pyrolyzed. The solids obtained are mostly carbon black and waxes (these are hydrocarbons). Figure 16-6 shows a schematic of a pyrolysis plant using a fluidized bed of sand.

Nylon 6

Caprolactam

Nylon 6

Caprolactam

Plastics waste

Fluidized bed reactor

Fluidized sand 600-900 C

Burner

Fluidized gas (pyrolysis gas)

Heating

Plastics waste

Fluidized bed reactor

Fluidized sand 600-900 C

Burner

Fluidized gas (pyrolysis gas)

Heating

Recycling Oil Plant Syosset

50% for heating the plant and i _

fluidizing gas ^gfcgjjl • (methane, ethylene,;] propylene) i

50% as an end product

Pyrolysis gas separated into benzene, toluene, wax, etc.

FIGURE 16-6 A fluidized-bed reactor for the pyrolysis of plastic waste. From N. Mustafa, ed., Plastic Waste Management: Disposal, Recycling, and Reuse by courtesy of Marcel Dekker, Inc. Copyright © 1993.

Additional Reading

Andrews, G. D., and P. M. Subramanian, eds., Emerging Technologies in Plastics Recycling.

American Chemical Society, Washington, DC, 1992. Bagchi, A., Design, Construction and Monitoring of Landfills, 2nd ed. Wiley, New York, 1994.

Brandrup, J., M. Bittner, W. Michaeli, and G. Menges, eds., Recycling and Recovery of Plastics.

Hanser/Gardner, Cincinnati, OH, 1995. Diaz, L. F., G. M. Savage, L. L. Eggerth, and C. G. Golueke, Composting and Recycling Municipal Solid Waste. Lewis Publishers, Boca Raton, FL, 1993.

Haug, R. T., The Practical Handbook of Compost Engineering. Lewis Publishers, Boca Raton, FL, 1993.

Kreith, F., ed., Handbook of Solid Waste Management. McGraw-Hill, New York, 1994. Lund, H. F., ed., The McGraw-Hill Recycling Handbook. McGraw-Hill, New York, 1993. Manser, A. G. R., and A. A. Keeling, Practical Handbook of Processing and Recycling Municipal

Solid Waste. Lewis Publishers, Boca Raton, FL, 1996. Mustafa, N., ed., Plastics Waste Management: Disposal, Recycling, and Reuse. Dekker, New York, 1993.

Niessen, W. R., Combustion and Incineration Processes, 2nd ed. Dekker, New York, 1995.

Polprasert, C., Organic Waste Recycling, 2nd. ed. Wiley, Chichester, 1996.

Strong, D. L., Recycling in America. ABC-CLIO, Santa Barbara, CA, 1997.

Wilson, D. J., and A. N. Clarke, Hazardous Waste Site Soil Remediation. Dekker, New York, 1994.

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