The waste originates from numerous sources: residential community (i. e., municipal solid waste or MSW), commercial and light-industrial communi­ties, manufacturing activities such as heavy-industrial and chemical indus­tries (generally classified as hazardous waste), agricultural and forestry waste, human and animal waste, paper and pulp industry waste, automobile and other transportation waste, hospital waste (generally considered as infec­tious waste), nuclear waste, and so on. In addition to man-made waste, there are numerous naturally occurring waste materials generally classified under the category of "lignocellulosic materials (LCM)" and certain forms of crop oils (e. g., algae, waterweed, water hyacinth). These are biomasses that do not have a useful purpose for food or a direct use for human or animal purposes.

Table 6.1 lists the heating values of various waste-derived fuels. This table shows the valuable energy content of various types of waste materials. This chapter does not consider special types of waste such as nuclear and infectious waste as well as several types of hazardous and nonhazardous industrial waste such as glass, metals, and other noncombustible waste. The chapter does, however, examine the appropriate conversion processes for all cellulosic-based waste as well as some polymeric waste such as plastic and rubber tires. A typical material distribution of MSW collected in the United States is illustrated in Figures 6.1a and b. In 1988, approximately 80% of 180 million tons of waste generated in the United States was cellulose-based [10]. This percentage has not changed in the 1990s and 2000s [4]. In Europe, MSW is expected to increase up to 300 million tons by 2015 [5].

In processing waste, noncombustible materials such as glass and dirt are removed. The glass is either recycled or sent to glass-melting furnaces. The heavy metals such as ceramics, heavy metals, and aluminum are routed to the landfill for disposal. A significantly important component of MSW is polymeric materials. Although polymer waste only accounts for 8.5% by mass of the total MSW disposed of in the United States, plastic represents

over 28% by volume [6]. Plastic waste is not biodegradable. Thus it is mostly recycled either for reuse or to recover basic monomers. Polymeric waste ranges from packaging materials used in the food industry to various parts in automobiles to high-density polyethylene (HDPE) containers such as pop bottles, laundry detergent bottles, milk jugs, and so on. It is estimated that over 65% of the food packaging in the United States is from plastics. As of 1978, the Ford Motor Company estimated that the average junked car con­tained 80 kg of plastic and nontire rubber. This number is increasing every year because of the increased use of polymers in various automobile parts.

Some types of plastic waste are recycled directly. For example, milk bot­tles, juice containers, laundry detergent bottles, motor oil cans, spring water bottles, and other similar containers are subjected to a thorough cleaning process and are reused. Some rubber tires are retread and put back for reuse. Polymeric materials including rubber are generally not discarded in a land­fill and are subjected to a conversion process either to recover basic chemi­cals and materials or to generate energy.