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Composting

Composting in solid waste management systems is the decomposition and stabilization of the organic fraction of municipal solid waste (MSW) carried out by a microbial community under controlled, aerobic conditions. Though composting has been practiced by people since they first settled in agricultural communities, it is now emerging as a centralized waste management method that both reduces the volume of waste that must be disposed and creates useful products. Most biogenic matter can be composted; its degradation occurs in four stages and is carried out by an ecological succession of microbial communities. The resulting product (compost) can be used as a soil conditioner, fertilizer, mulch, or a replacement for peat.

Overall Process

Composting is a naturally occurring biological process undertaken by a succession of microbial communities, and it is used by people to manage the organic fraction of waste. The aerobic degradation process can be written as

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Overall, a consortium of microorganisms breaks down organic matter in the presence of oxygen, reducing the volume and mass of waste by approximately 50 percent (on a dry-weight basis); the other 50 percent of the mass is released as carbon dioxide (CO2), water (H2O), and ammonia (NH3). Composting is an exothermic process; it releases heat and raises the temperature of the substances being degraded. The process is considered complete when only stabilized organic matter—matter that has low biologic activity that can be stored without giving rise to health or nuisance problems—is left over.

Objectives and Uses for Compost

There are four main objectives to composting as a waste management method: to reduce the volume of waste, stabilize waste, sterilize waste, and produce a valuable product from the waste. Composting greatly decreases the mass and volume of waste to be managed, which translates into a reduction of costs for managing that waste. Stabilization of waste allows for safe storage of the waste; if not composted, stored organic waste is likely to emit odors and to contain pathogens. The high temperatures reached in the composting process (upward of 65 degrees Celsius) destroy most pathogens and weed seeds contained in the organic waste. Finally, the main objective of composting is to create a valuable resource. Compost can be used as a soil conditioner, fertilizer, mulch, or a replacement for peat. Though compost's nutrient content is lower than that of commercial fertilizer, its nutrient release is slow and sustained.

Most plant matter, animal tissue, and microbial components can be degraded in an aerobic composting process. Cellulose is the most abundant component of plants and is found in most organic wastes; under aerobic conditions, many fungi and microbes are involved in cellulose degradation. Lignin, a structural component of plants and a major component of wood, is degraded much more slowly. Organic matter such as plastic and leather are difficult to break down, while inorganic substances such as glass or metal are relatively inert.

Biology and Chemistry of Composting

Composting involves an ecological succession of microorganisms with a presence depending on environmental conditions. Bacteria, archaea, fungi, protozoa, and worms are all involved in the aerobic degradation of organic waste. Microorganisms use the carbon in organic waste to produce energy and to synthesize cellular components. In addition to carbon, microorganisms need other macronutrients (N, P, and K) to thrive as well as several other micronutrients (Co, Mn, Mg, Cu, and Ca). A ratio of carbon to nitrogen of 20–25:1 is considered ideal for composting. An effective substrate for composting will balance carbonaceous wastes (such as dry leaves, hay, and paper) with nitrogenous wastes (such as grass, food waste, and sludge). To create an environment in which microorganisms are able to degrade organic waste, the moisture content must be between 12 percent (the minimum required for biological activity) and about 65 percent, the level at which oxygen availability becomes too low. The ideal moisture content for composting is 45–50 percent. The pH levels must hover around neutrality; bacteria prefer 6–7.5, and fungi prefer pH levels between 5.5 and 8. Temperature naturally varies throughout the composting process, but fluctuates between the mesophilic (25–40 degrees Celsius) and thermophilic (45–60 degrees Celsius) ranges. These ranges are shown in Table 1.

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