COMPOSTING

8. COMPOSTING

Composting is low-temperature partial oxidation of the easily degradable proteins, fats, simple sugars, and carbohydrates contained in plant cells and animal tissues. It produces little alteration in difficult-to-degrade organic components such as cellulose, leather, and polymers, or in insoluble inorganics such as dirt, glass, ceramics, and metals. However, because composting requires a moist, warm environment under conditions that range from acidic to basic, metals may corrode (oxidize) during the composting time (CRSI, 1989).

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).

TECHNOLOGY DESCRIPTION

Composting is a technically proven biological process. Three basic systems are used:

Static windrows (piles)
Turned windrows
In-vessel composting.

The three systems differ mainly in the manner in which oxygen is transferred into the compost. Windrows are long piles, up to 6 feet high, of the material to be composted. Static windrows are built on a porous deck that allows air to be blown through the piles. The piles are not moved until composting is completed. Turned windrows are aerated by periodic mechanical mixing. For in-vessel composting, the material is placed in a tank, where it is aerated and mixed by tumbling or stirring. Composting in a vessel is a relatively short term process. It is followed by additional open-air composting.

In all composting systems, the following basic variables should be simultaneously optimized:

Substrate (the feed material) and particle size
Bacteria, fungi, and protozoa, which cause the reactions
Additional nutrients (nitrogen, phosphorus, potassium, and trace metals), perhaps from sewage sludge, which is often added to MSW during composting to provide moisture, nitrogen, bacteria, and nutrients (Glaub et al., 1989)
pH, which drops to about 4.5 in the early stages and should not significantly exceed 8.5 during the later stages of the compost cycle
Aeration, which is critical because adequate O(2) is needed to prevent anaerobic conditions
Moisture content, which must be greater than 12% but less than saturation
Process residence time.

The system chosen depends on whether a community decides on separate curbside collection of leaves and/or yard waste or a system to produce a compost product from mixed MSW. All three systems are used for composting mixed MSW. Yard waste composting uses the simpler, less capital-intensive windrow methods (Hammer, 1992). The differences among these two approaches are outlined below.

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.

MSW Composting

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.

Figure 8.1
PROCESS FLOWSHEET FOR MSW COMPOSTING

Click here to expand figure.

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