Composting may be either aerobic or anaerobic. This section discusses only aerobic composting. The term "anaerobic composting" refers to a process for stabilizing solid waste by using the same microorganisms used in anaerobic digestion; however, anaerobic composting does not optimize energy recovery to the degree that anaerobic digestion does (CRSI, 1989).
As part of an MSW management strategy, composting can be applied to mixed MSW or to separately collected leaves and other yard wastes. MSW composting results in a volume reduction of 50% of the original volume composted, and consumes about 50% of the organic mass (on a dry weight basis), which is released mainly as CO(2) and water. Yard waste composting results in a reduction of 3o-50% of the weight of the original yard waste (Deyle and Hanks, 1991).
In all composting systems, the following basic variables should be simultaneously optimized:
Yard Waste Composting
Leaves alone can be composted in 1 to 3 years using a front-end loader for occasional turning (Deyle and Hanks, 1991). Grass and leaves together require weekly turning to maintain aerobic conditions; a windrow turner is typically used. Systems for general yard waste often include shredding to permit composting of brush and tree trimmings, and composting takes 16-18 months (Deyle and Hanks, 1991). If the system is intended to produce compost for sale or commercial use, process conditions must be carefully controlled, and the final compost product must probably be passed through a sizing screen.
Processing MSW to make compost is a much more complex process than composting yard waste alone. MSW requires preprocessing, which includes shredding, air classifying, and screening (CRSI, 1989), to improve the rate of composting and the quality of the product. Preprocessing methods similar to, but less intensive than, those used to make RDF are often employed, and only 40-60% of the MSW remains after preprocessing as feed to the composting operation (Spencer, 1991). A typical process flowsheet is shown in Figure 8.1 and explained in detail in Appendix G. One large, technically successful operation in Wilmington, Delaware, makes about 37,000 tons per year of compost from RDF. The compost operation is part of a larger facility that makes RDF for combustion and recovers ferrous metals, aluminum, and glass from MSW (see Appendix G).
Specialized preprocessing equipment, such as the Dano Drum, developed in Europe is being used in the largest U.S. MSW composting plant (Apotheker, l991a). Figure 8.1 shows a flowsheet for that 600 ton per day plant.
PROCESS FLOWSHEET FOR MSW COMPOSTING
Click here to expand figure.
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