There are basically two types of municipal waste that offer opportunities for combined waste disposal and energy recovery—municipal solid waste (MSW, urban refuse, garbage) and biosolids (sewage, sludge). Each has its own distinc­tive set of characteristics as a biomass energy resource.

A. Municipal Solid Waste

Abundance

As the populations of urban areas grow, the production of MSW increases, sometimes in a disproportionate way. To illustrate, the generation of MSW in the United States increased from about 80 million tonnes in 1960 to 180 million tonnes in 1990 and shows no sign of reaching a plateau. During this same period, the corresponding per-capita generation of MSW in 10-year increments was 1.23 kg/person-day in 1960, 1.49 in 1970, 1.65 in 1980, and 1.97 in 1990. The associated difficulties of MSW disposal have become serious problems that do not bode well for future generations of city dwellers and areas that have high population densities. Governments often mandate the use of more environmentally acceptable methods of MSW disposal while limiting and sometimes phasing out some of the more traditional disposal methods. The collection and disposal costs increase and proper disposal becomes more difficult to achieve with the passage of time. The average “tipping fees” of MSW in the United States, for example, increased from about $11 per tonne in 1982 to about $32 per tonne in 1992. In some highly populated areas, the tipping fee is over $90 per tonne. At the same time, the loss of natural resources in the MSW occurs if no effort is made to recover them. The opportunities for combined waste disposal and energy recovery are evident.

Table 5.1 is a detailed summary of relevant data on MSW generation, disposal, and recovery in the United States from 1960 to 1993. Several conclu­sions can be reached from examination of this data. In the 1960s and 1970s, combined disposal-energy recovery systems did not exist to any significant extent even though 20 to 30% of the MSW generated was disposed of by burning. No effort was made to recover the heat evolved on combustion of the MSW. Since then, energy recovery systems have been incorporated into some of the disposal processes so that by the mid-1990s, about 15% of the MSW generated and disposed of by combustion includes energy recovery operations. Throughout this period, the bulk of MSW continued to be disposed of by landfilling. This process will be discussed in some detail in later chapters, but suffice it to say at this point that a medium-energy fuel gas containing about 50 mol % methane is emitted by MSW landfills. The recovery of this gas over long periods of time from many landfills is a well-established commer­cial technology.

It is apparent from the compositional data on the raw MSW in Table 5.1 that the combustible materials make up the bulk of the MSW on a weight percentage basis, about 85 wt % in the 1990s. The amount of the individual MSW components recovered since 1960, presumably for sale of marketable components, has increased to about one-fifth of the total amount generated. The largest components by weight in the recovered material include paper and paperboard and noncombustible metals and glass. Much of this material is recycled.