FIG. 10.14.1 Generalized flow diagram for the composting process. (Reprinted, with permission, from G. Tchobanoglous, H. Theissen, and S. Vigil, 1993, Integrated solid waste management [New York: McGraw-Hill].)
in the integrated solid waste management hierarchy. Nonetheless, composting facilities must be carefully planned and managed for successful operation. The key elements are elucidated by the Solid Waste Composting Council (1991) and include:
1. Recovery and preparation of compostables
4. Good neighbor planting
5. Positive control of litter, dust, odors, noise, and runoff
The first step involves preprocessing (as previously described). This processing results in the preparation of a good feed stock for composting and the recovery of recyclables. The second step is the composting, which must be properly controlled (as described in Section 7.43). Refining involves postcomposting management (e.g., screening) to improve product quality. Good neighbor planting includes a carefully selected site, pleasing appearance, paved access, parking, a secure site, and a clean site. The positive control element includes the treatment of odors and other emissions, pathogen and toxin control, air-borne dust management, noise control, and run-off control.
Compost quality, an important issue, is a function of the physical, chemical, and biological characteristics of the product. In terms of physical aspects, good compost should be dark in color; have uniform particle size; have a pleasant, earthy odor; and be free of clumps and identifiable contaminants, such as glass fragments and pieces of metal and plastic. Chemical characteristics include not only the positive contribution from organic and inorganic nutrients, which are helpful for plant production, but also the detriments associated with heavy metals and toxic organics. Other chemical characteristics include weed seeds, salts, plant pathogens, and possibly human pathogens. Stability and maturity are significant concerns for compost quality and process control.
Quality is a major component of marketing compost, and marketing plays a key role in the effectiveness of any program to compost waste. The primary objective in finding a market for compost is finding an end use of the product. Since composting significantly reduces the volume of MSW, even if the compost is landfilled (as intermediate cover) that use may justify a composting program. However, the value of any program increases when a better end use is secured which further reduces the required landfill space and recovers a resource—a soil conditioner. While compost can be sold, revenue from composting is a secondary objective. While operating a compost facility for profit from compost sales is possible, this situation rarely occurs. Of course, any revenue generated from compost sales can offset the processing cost.
Composting MSW or various portions of the waste stream is an important component of integrated solid waste management. The use of composting is part of the strategy of minimizing incineration and landfilling while promoting source reduction and recycling. At least 50% of the MSW stream is compostable. Composting diverts these materials from less beneficial disposal methods and provides a more environmentally sound MSW program.
A central issue is the tradeoff between collection ease and management concerns. Source separated organics are easier to compost and yield a compost product of higher quality but require separate collection. The use of composting processes and the type of waste to be composted (mixed MSW versus source separation) must be integrated into the overall waste management plan for a given region. In terms of mixed MSW, preprocessing is important to obtain a high-quality product. Regardless of the final compost use or source of feedstock, some degree of preprocessing is necessary to prepare the feedstock for composting. This preprocessing insures proper particle size, moisture content, and nutritional balance.
—Michael S. Switzenbaum
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