13 Sources, Types, and Composition of Municipal Solid Wastes Solid wastes include all solid or semisolid materials that the possessor no longer considers of sufficient value to retain. The management of these waste materials is the fundamental concern of all the activities encompassed in solid waste management—whether the planning level is local, regional or subregional, or state and federal. For this reason, it is important to know as much about municipal solid waste (MSW) as possible. The purpose of this chapter is to identify the sources, types, and composition of solid wastes. 13-1 Sources of Solid Wastes Knowledge of the sources and types of solid wastes, along with data on the composition and rates of generation, is basic to the design and operation of the functional elements associated with the management of solid wastes. To avoid confusion, the term refuse, often used interchangeably with the term solid wastes, is not used in this text. Sources of solid wastes in a community are, in general, related to land use and zoning. Although any number of source classifications can be developed, the following categories are useful: (1) residential, (2) commercial, (3) institutional, (4) construction and demolition, (5) municipal services, (6) treatment plant sites, (7) industrial, and (8) agricultural. 13-2 Types of Solid Wastes As a basis for subsequent discussions, it will be helpful to define the various types of solid wastes that are generated. It is important to be aware that the definitions of solid waste terms and the classifications vary greatly in the literature and in the profession. Consequently, the use of published data requires considerable care, judgment, and common sense. The following definitions are intended to serve as a guide and are not meant to be precise in a scientific sense. Residential and Commercial Residential and commercial solid wastes, excluding special and hazardous wastes discussed below, consist of the organic (combustible) and inorganic (noncombustible) solid wastes from residential areas and commercial establishments. Typically, the organic fraction of residential and commercial solid waste consists of materials such as food waste (also called garbage), paper of all types, corrugated cardboard (also known as paperboard and corrugated paper), plastics of all types, textiles, rubber, leather, wood, and yard wastes. The inorganic fraction consists of items such as glass, crockery, tin cans, aluminum, ferrous metals, and dirt. If the waste components are not separated when discarded, then the mixture of these wastes is also known as commingled residential and commercial MSW. Wastes that will decompose rapidly, especially in warm weather, are also known as putrescible waste. The principal source of putrescible wastes is the handling, preparation, cooking, and eating of foods. Often, decomposition will lead to the development of offensive odors and the breeding of flies. In many locations, the putrescible nature of these wastes will influence the design and operation of the solid waste collection system . Although there are more than 50 classifications for paper, the waste paper found in MSW is typically composed of newspaper, books and magazines, commercial printing, office paper, other paperboard, paper packaging, other nonpackaging paper, tissue paper and towels, and corrugated cardboard. The plastic materials found in MSW fall into the following seven categories: ? Polyethylene terephthalate (PETE/I) ? High-density polyethylene (HDPE/2) ? Polyvinyl chloride (PVC/3) ? Low-density polyethylene (LDPE/4) ? Polypropylene (PP/5) ? Polystyrene (PS/6) ? Other multilayered plastic materials (7) The type of plastic container can be identified by number code (1 through 7) molded into the bottom of the container (see Fig. 13-1). Mixed plastic is the term used for the mixture of the individual types of plastic found in MSW. Special Wastes. Special wastes from residential and commercial sources include bulky items, consumer electronics, white goods, yard wastes that are collected separately, batteries, oil, and tires. These wastes are usually handled separately from other residential and commercial wastes. Bulky items are large worn-out or broken household, commercial, and industrial items such as furniture, lamps, bookcases, filing cabinets, and other similar items. Consumer electronics includes worn-out, broken, and other no-longerwanted items such as radios, stereos, and television sets. White goods are large worn-out or broken household, commercial, and industrial appliances such as stoves, refrigerators, dishwashers, and clothes washers and dryers. Collected separately, white goods are usually dismantled for the recovery of specific materials (e.g., copper, aluminum, etc.). The principal sources of batteries are from households and automobile and other vehicle servicing facilities. Household batteries come in a variety of types, including alkaline, mercury, silver, zinc, nickel, and cadmium. The metals found in household batteries can cause groundwater contamination by their presence in leachate; they can also contaminate air emissions and ash from waste combustion facilities. Many states now prohibit the landfilling of household batteries. Automobiles use lead-acid batteries, each of which contains approximately 18 pounds of lead and a gallon of sulfuric acid, both hazardous materials. The principal source of used oil is from the servicing of automobiles and other moving vehicles by their owners. Waste oil, not collected for recycling, is often poured onto the ground; down sanitary, combined, and storm water sewers; or into trash containers. Waste oil discharged onto the ground or into municipal sewers often contaminates surface water and groundwater as well as the soil. Waste oil placed in the same container as other solid waste components tends to contaminate the waste components and thus reduces their value as recycled materials. Somewhere between 230 and 240 million rubber tires are disposed of annually in landfills or in tire stockpiles. Because tires do not compact well, their disposal in landfills is expensive and wasteful of space. Stockpiling of tires also poses serious aesthetic as well as environmental problems. Large, difficult-to-extinguish fires have occurred in a number of stockpiles. In addition, stockpiled tires form an ideal breeding place for mosquitos. Hazardous Wastes. Wastes or combinations of wastes that pose a substantial present or potential hazard to human health or living organisms have been defined as hazardous wastes. The U.S. EPA has defined RCRA hazardous wastes in three general categories: (1) listed wastes, (2) characteristic hazardous wastes, and (3) other hazardous wastes. Institutional Institutional sources of solid waste include government centers, schools, prisons, and hospitals. Excluding manufacturing wastes from prisons and medical wastes from hospitals, the solid wastes generated at these facilities are quite similar commingled MSW. In most hospitals medical wastes are handled and process separately from other solid wastes. Construction and Demolition Wastes from the construction, remodeling, and repairing of individual residences, commercial buildings, and other structures are classified as construction wastes. The quantities produced are difficult to estimate. The composition is variable but may include dirt; stones; concrete; bricks; plaster; lumber; shingles; and plumbing, heating, and electrical parts. Wastes from razed buildings, broken-out streets, sidewalks, bridges, and other structures are classified as demolition wastes. The composition of demolition wastes is similar to construction wastes, but may include broken glass, plastics, and reinforcing steel. Municipal Services Other community wastes, resulting from the operation and maintenance of municipal facilities and the provision of other municipal services, include street sweepings, road side litter, wastes from municipal litter containers, landscape and tree trimmings, catch-basin debris, dead animals, and abandoned vehicles. Because it is impossible to predict where dead animals and abandoned automobiles will be found, these wastes are often identified as originating from nonspecific diffuse sources. Wastes from nonspecific diffuse sources can be contrasted to that of the residential sources, which are also diffuse but specific in that the generation of the wastes is a recurring event. Treatment Plant Wastes and Other Residues The solid and semisolid wastes from water, wastewater, and industrial waste treatment facilities are termed treatment plant wastes. The specific characteristics of these materials vary, depending on the nature of the treatment process. At present their collection is not the charge of most municipal agencies responsible for solid waste management. However, wastewater treatment plant sludges are commonly co-disposed with MSW in municipal landfills. In the future, the disposal of treatment plant sludges will likely become a major factor in any solid waste management plan. Materials remaining from the combustion of wood, coal, coke, and other combustible wastes are categorized as ashes and residues. (Residues from power plants normally are not included in this category because they are handled am processed separately.) These residues are normally composed of fine, powder materials, cinders, clinkers, and small amounts of burned and partially burned materials. Glass, crockery, and various metals are also found in the residues from municipal incinerators. Industrial Solid Waste Excluding Process Wastes Sources and types of solid waste generated at industrial sites, grouped according to their Standard Industrial Classification (SIC), are reported in Table 13-1 . This list excludes industrial process wastes and any hazardous wastes that may be generated. Agricultural Wastes Wastes and residues resulting from diverse agricultural activities—such as the planting and harvesting of row, field, tree and vine crops; the production of milk; the production of animals for slaughter; and the operation of feedlots—are collectively called agricultural wastes. At present, the disposal of these wastes is not the responsibility of most municipal and county solid waste management agencies. However, in many areas the disposal of animal manure has become a critical problem, especially from feedlots and dairies. 13.3 Composition of Solid Wastes Composition is the term used to describe the individual components that make up a solid waste stream and their relative distribution, usually based on percent by weight. Information on the composition of solid wastes is important in evaluating equipment needs, systems, and management programs and plans. For example, if the solid wastes generated at a commercial facility consist of only paper products, the use of special processing equipment, such as shredders and balers, may be appropriate. Separate collection may also be considered if the city or collection agency is involved in a paper-products recycling program. The potential for significant changes in composition in the future is considered at the end of this chapter. Composition of MSW The total solid wastes from a community are composed of the waste materials identified in Table 13-1. Typical data on the distribution of MSW are presented in Table 13-2. As noted in Table 13-2, the residential and commercial portion makes up about 50 to 75 percent of the total MSW generated in a community. The actual percentage distribution will depend on (1) the extent of the construction and demolition activities, (2) the extent of the municipal services provided, and (3) the types of water and wastewater treatment processes that are used. The wide variation in the special wastes category (3 to 12 percent) is due to the fact that in many communities yard wastes are collected separately. The percentage of construction and demolition wastes varies widely depending on the part of the country and the general health of the local, state, and national economy. The percentage of treatment, plant sludges will also vary widely depending on the extent and type of water and wastewater treatment provided. Distribution of Individual Waste Components Information and data on the physical composition of solid wastes are important in the selection and operation of equipment and facilities , in assessing the feasibility of resource and energy recovery , and in the analysis and design of landfill disposal facilities). Published distribution data should be used cautiously because the effects of recycling activities and the use of kitchen food waste grinders are often not reflected in earlier data. Tab 13-2 Residential Portion of MSW in the United States. Components that typically make up the residential portion of MSW, excluding special and hazardous wastes, and their relative distribution are reported in Table13-3. Although any number of components could be selected, those in Table 13-3 have been selected because they are readily identifiable and consistent with component categories reported in the literature and because they have proven adequate for the characterization of solid wastes for most applications. The data in Table 13-3 are derived from both the literature and the authors' experience. For the purpose of comparison, the percentage distribution of the materials used for packaging is reported in Column 3 of Table 13-3. It is estimated that packaging wastes now account for approximately one-third of the residential and commercial MSW. The values given in Table 13-3 for food waste, plastics, and yard wastes are considerably different from the values given in the corresponding table in the predecessor of this text, published in 1977 . The differences are due largely to (1) improved food processing techniques and the increased use of kitchen food waste grinders, (2) the increased use of plastics for food packaging and other packaging, and (3) the fact that burning of yard wastes is no longer allowed in most communities. Effect of Waste Diversions on Distribution of Components in Residential MSW To assess the impact of waste diversions (resulting from the use of food waste grinders and waste recycling programs) on the distribution of waste components, the distribution given in Table13-3 for as collected residential MSW must adjusted. The adjusted component distribution data are reported in Table 13-4. As shown in Table 13-4, the distribution data for the United States do not change significantly. On the other hand, the distribution data for the city of Davis, California, would change quite a bit more because of the higher percentage of recycling. Tab 13-3 Tab 13-4 13-4 Determination of the Composition of MSW in the Field Because of the heterogeneous nature of solid wastes, determination of the composition is not an easy task. Strict statistical procedures are difficult, if not impossible, to implement. For this reason, more generalized field procedures, based on common sense and random sampling techniques, have evolved for determining composition. Residential MSW The procedure for residential MSW involves unloading and analyzing a quantity of residential waste in a controlled area of a disposal site that is isolated from wine and separate from other operations. A representative residential sample might be a truckload resulting from a typical weekday collection route in a residential area. A mixed sample from an incinerator storage pit or the discharge pit of a shredder would also be representative. Common sense is important in selecting the load to be sampled. For example, a load containing the weekly accumulation of yard wastes (leaves) during autumn would not be typical. To ensure that the results obtained are representative, a large enough sample must be examined. It has been found that measurements made on a sample size of about 200 lb vary insignificantly from measurements made on samples of up to 1700 lb taken from the same waste load . The authors have obtained similar results in field studies performed in Hawaii and at Davis, California. To obtain a sample for analysis, the load is first quartered. One part is then selected for additional quartering until a sample size of about 200 lb is obtained. It is important to maintain the integrity of each selected quarter, regardless of the odor or physical decay, and to make sure that all the components are measured. Only in this way can some degree of randomness and unbiased selection be maintained. Commercial and Industrial MSW The field procedure for component identification for commercial and non-process industrial solid wastes involves the analysis of representative waste samples taken directly from the source, not from a mixed waste load in a collection vehicle. Because commercial and industrial sources are so variable, statistically valid sampling is seldom possible. Estimation of the distribution of waste components and quantities for these activities remains an art form. 13-5 Types of Materials Recovered from MSW The purpose of this section is to identify the types of materials that are now separated from MSW for recycling and introduces and discusses briefly the importance of materials specifications in the processing and marketing of recovered materials. Knowledge of the waste materials that are now recovered for reuse and recycling is important in the conduct of waste generation and diversion studies. Materials Commonly Separated from MSW The most common ones from MSW are aluminum, paper, plastics, glass, ferrous metal, nonferrous metal, yard wastes, and construction and demolition wastes. Each is considered briefly in the following discussion. Aluminum. Aluminum recycling is made up of two sectors: aluminum cans and secondary aluminum. Secondary aluminum includes window frames, storm doors, siding, and gutters. Because secondary materials are of different grades, specifications for recycled aluminum should be checked, to recover the maximum value when selling separated materials to brokers. The demand for recycled aluminum cans is high, as it takes 95 percent less energy to produce an aluminum can from an existing can than from ore. Paper. The principal types of waste paper that are recycled are old newspaper cardboard, high-grade paper, and mixed paper. Each of these four grades consist of individual grades, which are denned according to the type of fiber, source, homogeneity, extent of printing, and physical or chemical characteristics. High-grade paper includes office paper, reproduction paper, computer printout, and other grades having a high percentage of long fibers. Mixed grades include paper with high ground-wood content, such as magazines; coated paper; and individual grades containing excessive percentages of "outthrows" (papers of lower grades than the grade specified). Plastics. Plastics can be classified into two general categories: clean commercial grade scrap and post-consumer scrap. The two types of post-consumer plastics that are now most commonly recycled are polyethylene terephthalate (PETE/1), which is used for the manufacture of soft drink bottles, and high-density polyethylene (HDPE/2), used for milk and water containers and detergent bottles. In 1987, more than 150 million pounds of plastic soft drink bottles were recycled. Even so, less than five percent of the available scrap plastic is being recycled. It is anticipated that all of the other types of plastics will be recycled in greater quantities in the future, however, as processing technologies improve. Glass. Glass is also a commonly recycled material. Container glass (for food and beverage packing), flat glass (e.g., window glass), and pressed or amber and green glass are the three principal types of glass found in MSW. Glass to be reprocessed is often separated by color into categories of clear, green, and amber. Ferrous Metals (Iron and Steel). The largest amount of recycled steel has traditionally come from large items such as cars and appliances. Many communities have large scrap metal piles at the local landfill or transfer station. In many cases, the piles are unorganized and different metals are mixed together, making them unattractive to scrap metal buyers. Steel can recycling is also becoming more popular. Steel cans, used as juice, soft drink, and food containers, are easily separated from mixed recyclables or municipal solid waste using large magnets (which also separate other ferrous metals). Nonferrous Metals. Recyclable nonferrous metals are recovered from common household items (outdoor furniture, kitchen cookware and appliances, ladders, tools, hardware); from construction and demolition projects (copper wire, pipe and plumbing supplies, light fixtures, aluminum siding, gutters and downspouts, doors, windows); and from large consumer, commercial, and industrial products (appliances, automobiles, boats, trucks, aircraft, machinery). Virtually all nonferrous metals can be recycled if they are sorted and free of foreign materials such as plastics, fabrics, and rubber. Yard Wastes Collected Separately. In most communities yard wastes are collected separately. The composting of yard wastes has become of great interest as cities and towns seek to find ways in which to achieve mandated diversion goals. Leaves, grass clippings, bush clippings, and brush are the most commonly composted yard wastes. Stumps and wood are also compostable, but only after they have been chipped to produce a smaller more uniform size. Composting of the organic fraction of MSW is also becoming more popular. Construction and Demolition Wastes. In many locations construction and demolition (C&D) wastes are now being processed to recover marketable items such as wood chips for use as a fuel in biomass combustion facilities, aggregate for concrete in construction projects, ferrous and nonferrous metals for remanufacture, and soil for use as fill material. The reprocessing of C&D wastes is gaining in popularity as disposal fees at landfills continue to increase. When disposal fees were below 5 dollars per ton (early 1970s), reprocessing was not economically feasible. Today (1992), with average landfill disposal fees approaching 60 dollars per ton in many parts of the country, the reprocessing of C&D wastes is economically feasible. Specifications for Recovered Materials As the amount of material recovered from MSW continues to increase as communities develop programs to meet waste diversion goals, materials specifications will become an important factor. In general, there is less contamination in source-separated material, but collection is more labor-intensive, and many communities are choosing to sort all materials at a central materials recovery facility (MRF). In many regions, markets for materials are not keeping pace with the volume collected, and it is expected that buyers will tighten specifications; as a result, vendors will no longer have assured markets, and will be competing to sell materials. As the specifications for recovered materials become more restrictive, recovery program managers must consider buyer specifications carefully when choosing collection and sorting systems, especially where large capital expenditures are involved. 13-6 Future Changes in Waste Composition In terms of solid waste management planning, knowledge of future trends in the composition of solid wastes is of great importance. For example, if a paper-recycling program were instituted on the basis of current distribution data and if paper production were subsequently eliminated, such a program would more than likely become a costly white elephant. Although this case is extreme, it nevertheless illustrates the point that future trends must be assessed carefully in long-term planning. Another important question is whether the quantities are actually changing or only the reporting system has improved. Impacts of Waste Diversion Programs As more states adopt legislation mandating the development of source reduction and recycling programs, the composition of the wastes collected will change. The impact of waste diversion programs on the composition of the wastes collected will vary depending on the other types of waste management programs that are in place. For example, a solid waste combustion facility, developed to serve a commercial area, was planned and designed on the basis that the energy content of the waste would be 6500 Btu/lb. Six years later when the facility went on line, the actual energy content of the waste was 4500 Btu/lb, owing to the extensive recycling of cardboard that had developed in the intervening years. To meet a contractual requirement to generate a firm amount of power, the number of truck loads of waste had to be increased, necessitating a change in the facility permit. Future Changes in Waste Components In planning for future waste management systems, it will be important to consider the changes that may occur in the composition of solid waste with time. Four waste components that have an important influence on the composition of the wastes collected are food waste, paper and cardboard, yard waste, and plastics. Stable refuse has disappeared as a waste category in current classification systems, while plastics were nonexistent in the early 1900s. Food Wastes. The quantity of residential food wastes collected has changed significantly over the years as a result of technical advances and changes in public attitude. Two technological advances that have had a significant effect are the development of the food processing and packaging industry and the use of kitchen food waste grinders. The percentage of food waste, by weight, has decreased from about 14 percent in the early 1960s to about 9 percent in 1992. Recently, because the public has become more environmentally aware and concerned, a trend has developed toward the use of more raw, rather than processed, vegetables. While it would appear that such a trend would increase the quantity of food wastes collected no firm data are available on this subject. Paper and Cardboard. The percentage of paper and cardboard (also known as paperboard and corrugated paper) found in solid wastes has increased greatly over the past half century, rising from about 20 percent in the early 1940s to about 40 percent in 1992. It is expected that use of paper and cardboard will remain stable for the next few years. If the U.S. postal rate for bulk mail were increased to that for first class mail, a significant reduction would occur in the amount of paper collected for disposal. Yard Wastes. The percentage of yard wastes in MSW has also increased significantly during the past quarter century, due primarily to passage of laws that prohibit burning of yard wastes. By weight, yard waste currently accounts for about 16 to 24 percent of the waste stream. Environmental conditions such as droughts have also affected the quantities of yard wastes collected in certain locations. For example, in Santa Barbara, CA, many areas that had lawns have been converted permanently to arid type landscaping with a concomitant decrease in the portion of yard wastes. Whether drought conditions in the south-western United States will continue to affect the quantity of yard wastes is unknown. Plastics. The percentage of plastics in solid wastes has also increased significantly during the past 50 years. The use of plastic has increased from almost non-measurable quantities in the early 1940s to between 7 and 8 percent, by weight, in 1992. It is anticipated the use of plastics will continue to increase, but at a slower rate than during the past 25 years.