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2.5 Topic c: Composting

Upwards of 5% of the MSW stream in North America is now managed through centralized composting programs, which were insignificant prior to the mid-1980s. The compostable portion of MSW can constitute 30-60% of a community's waste stream. Composting programs have been designed for a variety of organic waste streams, including yard wastes (grass trimmings, leaves, or tree prunings), food wastes, agricultural wastes, and wastewater treatment sludge. Another alternative, which has been used only on a limited basis in North America, is mixed waste composting. Mixed waste processing facilities accept unsorted MSW in the same form as it would be received at a landfill or a waste-to-energy facility, and separate recyclable materials. The relatively small community of Guelph, Ontario, has been operating such a facility successfully since early 1996.

There are currently about 20 MSW composting facilities operating in the US, 12 of which are relatively small-scale, processing less than 50 tons per day. In addition, a composting facility can benefit from bans on leaf and yard wastes at landfill sites. Encouraging residents to undertake their own composting activities can reduce the amount of materials requiring centralized composting, thereby reducing the costs of such programs. This can be supplemented by legislative changes, as undertaken by the City of Toronto in 1996, where lawn clippings are no longer allowed to be landfilled or taken for centralized composting. Residents can either compost them or leave them on the lawn. The number of yard waste composting programs in the US now totals over 3,200. This growth was spurred on by yard waste disposal bans that have been enacted by about half of the states. Composting is the primary method of diverting yard trimmings; other management methods include land application and mulching. Many communities have also started programs to collect Christmas trees and make them into wood chips, which residents can collect for their gardens. Canada currently has over 160 composting projects throughout the country.

Like recycling, the success of a composting program is determined in large part by markets for the material. Currently, most communities find that it is possible to give compost away, but not to sell it. In order to market compost commercially, communities need to be able to provide users with a steady supply of a high quality product. This becomes more difficult as composting programs expand to include more materials. With yard waste composting, for example, the ratio of grass to leaves to brush varies with the season. At certain times of the year nutrients are sometimes added to the final product to maintain standards. With careful planning and experimentation, communities can use diverse materials to produce several products. Some states are establishing standards for compost that should help give potential users more confidence in the product.

Composting methods can be as simple as a small pile in the backyard or as complex as a multi-million dollar centralized in-vessel tank system. Low-tech methods can be implemented inexpensively, but take longer to produce a finished product. High-tech methods shorten the compost time, but have higher capital costs for buildings and equipment.

Backyard composting and mulching is a source reduction activity that saves money for both the municipality and the homeowner. Some households compost food scraps in a backyard pile or in a composting bin, typically for use in vegetable or flower gardens. Grass clippings can be left on freshly mowed lawns instead of being bagged. If the clippings are short enough, they will fall through to the ground and be assimilated into the soil. Leaves can be shredded in a special mulching lawn mower. Small branches and brush can be shredded in a wood-chipper and used for landscaping, or if sufficiently reduced in size, added to a compost pile. Many communities in North America have developed programs to encourage backyard composting, by offering educational materials and by distributing composting bins. A number of communities buy the bins in bulk and distribute them free of charge or for a nominal fee. The successful program in Vancouver, Canada differs from this approach; they charge $25 for the bins, judging that the expenditure solidifies a commitment by the resident to actually use the bin.

The City of Seattle has implemented a sophisticated multi-point composting program that uses a combination of curbside pickup, drop-off, and backyard composting elements. The City sponsors four composting demonstration sites, three in urban gardens and one next to an urban market. The City also funds a backyard composting education program run by a local organization of urban gardeners that trains volunteers to be proficient at composting. The volunteers then perform 40 hours of community outreach and education. Toronto operates a similar program through the Recycling Council of Ontario. In 1989, 5% of both Seattle s and Toronto s total waste stream was composted by residents.

Anaerobic digestion most often refers to the production of methane from the organic fraction of solid waste in enclosed, controlled reactors. No controlled, commercial, solid waste anaerobic digestors have yet operated in the US. A large-scale demonstration facility (RefCoM) was operated in Florida from 1978 to 1985, but was shut down when the demonstration period ended. Due to increasing interest in the development of low-cost, environmentally acceptable alternative energy sources, however, methane production through anaerobic digestion has become the subject of research and development efforts throughout the world.

There has been interest in North America in pyrolysis, a process that chemically and physically changes biomass through the addition of heat and in the absence of oxygen (or under partial oxidation). The end products of pyrolysis include a solid char, a liquid char (pyrolysis oil), and a gas (pyrolysis gas, synthesis gas, or syngas), all of which are marketable forms of energy. Although several pyrolysis and related gasification systems have been designed and constructed in the past two decades, most have been demonstration and laboratory-scale systems. The larger-scale demonstration plants in the US experienced technology problems and are no longer operating. There are currently no commercial-scale solid waste pyrolysis or gasification systems operating in North America.

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