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2.3 Topic c: Composting

Separation and handling of compostables

Unlike in North America, collection of compostable materials for transport to centralized composting facilities is a central feature of most Northern European waste management and source separation programs. Collection of so-called "bio-waste" (garden and kitchen organics) from households is generally performed using 120-liter green rolling carts; in some urban areas, collection from smaller, 35-liter pails or from paper bags is also common. In the literature, the compostable stream is referred to as "green waste," "bio-waste," or similar terms. This is contrasted with the remaining stream of non-compostable mixed waste, which is usually referred to as the "rest-fraction," "rest-waste," or "remainder fraction," to indicate that it represents what is left over after the target stream has been source-separated out.

The most frequent model for this collection is to alternate bio-waste collection one week with the rest-fraction the next. In some places, both waste streams are collected weekly during the summer months, when the chance of odors is higher. Alternating collection weeks brings the cost and energy usage of separate collection to within what appears to be acceptable budgetary and environmental levels in many Northern European countries. Because organic materials represent such a high percentage of MSW in Europe, even the modest 35 to 50% capture rates for this stream result in sufficient reduction in volume and putrescibility of the remainder fraction as to make biweekly collection acceptable to most residents. Centralized composting has a long tradition in Western Europe in particular, where some plants still in operation came on-line as early as the 1960s. Most European compost installations are aerobic systems, with the compost having a short residency time in a reactor or pre-composter and a longer time in aerated static piles. Windrow composting is less common, but does exist.

Evolution of centralized composting in Europe

The earliest approach to centralized composting involved attempts to compost mixed solid waste. Installations were designed with complex pre-processing and separation machinery intended to remove the non-compostables before mixing and composting. In theory, these non-compostables could then be recovered for recycling. Plants like this are still operating, and still being built, in countries like Greece and Spain, where early experiments with source separation of organics have shown a relatively low participation and capture rate.

Such mixed waste composting is possible, but its key problem has been that the waste arrives at the facility not only mixed, but compacted, with the result that both compostables and recyclables are highly contaminated. High metals content of the compost, and poor quality, marginally marketable recyclables, have led to a number of modification steps.

The second stage evolution for centralized composting was based on a "green bin" or wet-dry separation approach, tried and largely rejected in Germany and the Netherlands in the early 1980s. Residents were asked to put their dry recyclables and paper in one bin, and the wet waste in the other, which went to composting. This approach produced an improvement in compost quality over the attempts to compost mixed waste, but had some problems. The first was that there was an ambiguity in the distinction between ÒwetÓ and "dry." For example, residents included household chemicals and detergents in the wet waste, while considering soil from potted plants to be dry.

Although wet-dry separation produced an improved quality of recyclables, the compost was not much improved over compost derived from mixed waste, a result which ultimately led to the approach of collecting source-separated compostables as a separate stream. This bio-waste stream usually includes only yard, garden, and kitchen waste, and is collected for transfer to a centralized composting facility. Source separation of organics on this model in Denmark, Germany, and the Netherlands, in particular, and increasingly in other European countries, has proven to be the lasting model, and is described in the "Composting" part of the Sound Practices section. It produces a clean compost that meets increasingly strict European standards; and, by limiting, in most cases, the compostable fraction to kitchen and garden waste, it presents a clear separation protocol for residents.

Although a number of the early installations designed either for mixed waste or wet-dry separation are still in use, and a few are still being built and installed in France, Spain, and the UK, where composting is less well developed, the approach has gradually been abandoned and the facilities redesigned or modified to accommodate separate collection of compostables, with the result that more and more of the front-end separation steps have been bypassed. The composting installation in Heidelberg, Germany, is a great example of this. A hole has been punched in the facility wall to allow the source separated bio-waste to bypass all the high-technology processing steps and to convey the yard and kitchen waste directly to the composting process.

Current composting technology

Technically, centralized composting facilities can be divided into pre-processing stages, including removal of non-compostables by magnets, eddy currents, ballistics separators, and/or vibrating screens; size reduction; mixing and/or pre-composting; composting; curing; post-processing (usually consisting primarily of screening); packaging; and marketing. For the mixing and composting stages, the use of a vessel, usually a large horizontal drum or tunnel reactor, is common in Europe. The earliest and at one time the most common of these types of composters, the Dano drum, has been in use for so long that patent has expired, with the result that this is no longer a proprietary technology. Other technical approaches are proprietary, and cannot be built without a purchase or licensing agreement from the inventor or an authorized vendor.

Centralized composting facilities typically have a design capacity of 50 to 200 tons per day. Because of contamination removal through pre-processing, as well as moisture loss and volume reduction during the process of composting, a 100-ton-per-day facility will produce only 30-50 tons of compost per day.

Marketing considerations

The marketability of this compost product will vary according to the chemical constituency of the input material; for source-separated organics, the resulting compost is accepted for repeated application on agricultural land in most of Europe, and under the rules of the European Union. Efforts to market compost in Europe must take into account that even high quality compost is not usually considered to be a fertilizer; rather, it is a soil conditioner, useful for its water-holding capacity (it limits evaporation and erosion, and functions as a mulch), its slow release of the nutrients nitrogen, phosphorus, and potassium, and its ability to return organic matter to depleted, excessively mineralized soils. It is especially useful on slopes, or for reclaiming land degraded by erosion or through mining, quarrying, or rapid construction and development.

Anaerobic digestion

In the last 10 years, the predominantly aerobic in-vessel composting systems have been joined on the market by several, small- to medium-scale anaerobic composters made in Belgium and France. Reports as to their effectiveness vary.

Anaerobic composting, sometimes referred to as anaerobic digestion or methanization, occurs in the absence of oxygen, and is frequently performed in a pressurized silo or vessel. The processing temperatures seldom reach those of aerobic composting. Unlike aerobic composting, which produces compost and waste heat, anaerobic digestion produces methane gas, which can be captured for power or steam generation, or refined for use as a fuel.

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