This subsection presents the results of a life-cycle analysis of emissions. The bases are the same as those used for calculating the energy balance. Data on releases from composting are limited and incomplete. The information in this subsection should therefore be used with caution.

This subsection presents data or releases from composting alone. Air emissions from separate collection are included in the "Integrated Strategy Example" in a later subsection.

Air Emissions

Air releases from composting include emissions from the preparation steps, in the form of dust, and emissions from the compost as it warms and is aerated during composting. If yard wastes for composting are collected separately, emissions resulting from that step should be included.

The primary emissions are water vapor and CO2, but it is reasonable to expect that volatile solvents in the MSW will be vaporized during aeration. If the process is not carefully controlled, the piles of compost can become anaerobic, releasing methane and foul odors. One new, large (150 ton per day), mechanized facility (Agripost) was recently closed because of odor problems (Allen, 1991). The 600 ton per day facility that uses the Dano Drum was reported to have odor problems, at least during startup (Apotheker, l991a). Other air releases include the inevitable odor of MSW (CRSI, 1989); dust from turning, screening, and packaging; and possibly release of bacteria.

Water Emissions

Composting requires the addition of water above the amount normally in the refuse. In the largest U.S. facility, the water content of the incoming MSW is 23%, and the level is doubled for composting. Because of evaporation during composting, the final product is 40% moisture. To prevent water emissions in outdoor composting, the compost must be protected from snow and rain, or the composting must take place on an impervious surface so that the leachate and runoff can be collected or sent to a sewage treatment plant, if the local plant will accept it (CRSI, 1989).

Land Use

Composting requires more space than most waste management options because of the time required for composting and subsequent "stabilization." For complete reaction and aeration, composting windrows, for example, should not exceed 6 feet in height (Apotheker, l991a); that constraint limits how high the MSW or yard waste can be piled.

The 600 ton per day MSW composting facility composts the material in two areas, each of which is 150 feet by 350 feet. The compost is aerated in 6-foot-high piles for 21 days, and then removed to other areas 75 feet wide by 350 feet long for another 21 days. Altogether, the waste is processed and stored for at least 6 weeks, and the facility must be large enough to hold that amount of material for that length of time.

The processing and storage period for yard waste is 1 to 3 years. Thus, significantly larger land areas would be required to process the same volume of compost as is processed in the MSW composting facility described above.

When compost is used, it decomposes and releases materials into the soil. Table 8.4 provides rough estimates of concentrations of trace elements in soils and MSW composts. Controversy over standards reflects uncertainties about the effects of increased metal concentrations in soil.

One concern is whether compost products may contain dioxins. Just as dioxin is found in raw MSW, it is found in compost (Jones, 1991). The levels of dioxin appear to be low (Cannon, 1991), but the releases have not been well characterized. Similarly, polychlorinated biphenyls (PCBs) at concentrations of 10 ppm have been found in some samples of compost derived from MSW (Killam, 1987).

Integrated Strategy Example: Yard Waste Composting with Landfill


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