Solid waste degradation involves many different physical, chemical, and biological processes and generates gaseous and dissolved compounds. When the waste is mostly organic, biological processes will dominate the waste degradation process. Solid waste degradation occurs under two conditions: aerobic waste degradation in the presence of oxygen by aerobic microorganisms, and anaerobic waste degradation in the absence of oxygen by anaerobic microorganisms. As shown in Figure 6.1, the aerobic biodegradation of solid waste produces carbon dioxide, leachate, biomass, and decomposed organic matter (compost), while anaerobic biodegradation leads to the generation of biogas, a mixture of approximately 45%–60% methane and 40%– 60% carbon dioxide, leachate, and biomass (Mata-Alvarez 2003a and b).