Generalized Composting Process

Composting is the aerobic decomposition of biodegradable organic matter, producing compost. The decomposition is performed primarily by facultative and obligate aerobic bacteria, yeasts, and fungi, and also helped in the cooler initial and ending phases by a number of larger organisms, such as ants, nematodes, and oligochaete worms. Composting recycles organic household and yard waste and manures into a useful humus-like, soil end-product called compost. Ultimately, this permits the return of needed organic matter and nutrients into the foodchain. Composting can significantly reduce the amount of biowaste going into burgeoning landfills. The decomposition process is a result of raised temperatures. The elevated heat results from exothermic processes, and the heat in turn reduces the generational time of microorganisms and thereby speeds the energy and nutrient exchanges taking place.

Although it would be very difficult to find a universally accepted definition of composting, Haug (1993) gives a practical definition, which very well summarizes the main points of the composting process: Composting is the biological decomposition and stabilization of organic substrates, under conditions that allow development of thermophilic temperatures as a result of biologically produced heat, to produce a final product that is stable, free of pathogens, and plant seeds, and can be beneficially applied to land. Thus, composting is a form of waste stabilization, but one that requires special conditions of moisture and aeration to produce thermophilic temperature. The latter are generally considered to be above about 45°C. Maintenance of thermophilic temperatures is the primary mechanism for pathogen inactivation and seed destruction (Fig. 12.1).

The generalized diagram of the composting process is shown in Fig. 12.2. There are two major approaches to composting: active and passive (in Haug 1993): Active (hot) composting is defined as composting at close to ideal conditions, allowing aerobic bacteria to thrive. Aerobic bacteria break down material faster and produce less odor and fewer pathogens and destructive greenhouse gases than anaerobic bacteria. Commercial-grade composting operations actively control the composting conditions, such as the C/N ratio. When the temperature exceeds 55°C for several days only some highly resistant pathogenic bacteria like Clostridium can survive. To achieve the elevated temperatures, the compost bin must be kept warm, insulated, and damp. From the chemical point of view as it produces ultimately only energy in the form of waste heat and CO2 and H2O is aerated composting an efficient form of composting. With aerated composting, fresh air (i.e., oxygen) is introduced throughout the mix of materials using any appropriate mechanism. The air stimulates the microorganisms that are already in the mix, and their by-product is heat. In a properly operated compost system, pile temperatures are sufficient to stabilize the raw material, and the oxygen-rich conditions within the core of the pile eliminate offensive odors. High temperatures also destroy fly larvae and weed seeds, yielding a safe, high-quality finished product.

Phase I

Phase II

Phase III

Phase I

Phase II

Phase III

Temperature

Antagonistic population--

TIME

Fig. 12.1 Composting process. During phase I the initial heating takes place and readily soluble components are degraded. During Phase II, cellulose and hemicellulose are degraded under high temperature (thermophilic) conditions. This is accompanied by the release of water, carbon dioxide, ammonia and heat. Finally, during Phase III, curing and stabilization are accompanied by a drop in temperatures and increased humification of the material. Recolonization of the compost by mesophilic microorganisms occurs during Phase III. Included in these microbial communities are populations of antagonists (in: Smith 1992)

Temperature

Antagonistic population--

TIME

Fig. 12.1 Composting process. During phase I the initial heating takes place and readily soluble components are degraded. During Phase II, cellulose and hemicellulose are degraded under high temperature (thermophilic) conditions. This is accompanied by the release of water, carbon dioxide, ammonia and heat. Finally, during Phase III, curing and stabilization are accompanied by a drop in temperatures and increased humification of the material. Recolonization of the compost by mesophilic microorganisms occurs during Phase III. Included in these microbial communities are populations of antagonists (in: Smith 1992)

Finally, aeration expedites the composting process through the mechanism of heating insofar as the elevated heat will drive biochemical processes faster, so that a finished product can be rendered in 60-120 days. Aerated compost is an excellent source of macro- and micronutrients as well as stable organic matter, which support healthy plant growth. In addition, the microorganisms in compost aid in the suppression of plant pathogens. Compost retains water extremely well resulting in improved drought resistance, a longer growing season, and reduced soil erosion. Passive composting is composting in which the level of physical intervention is kept to a minimum, and often as a result the temperatures never reach much above 30°C. It is slower but is the more common type of composting in most domestic garden compost bins. Such composting systems may be either enclosed (home container composting, industrial in-vessel composting) or in exposed piles (industrial windrow composting). Kitchen scraps are put in the garden compost bin and left untended. This scrap bin cab has a very high water content, which reduces aeration, and so becomes odorous.

Composting systems are often divided into a first stage, high-rate phase and a second stage, curring phase. The first stage may use windrow, aerated static pile, or reactor processes. It is characterized by high oxygen uptake rates, thermophilic temperatures, high biodegradable volatile solids (BVS) reductions, and higher odor potential. The second phase is characterized by lower temperatures, reduced oxygen uptake rates, and lower odor production potential. The curing phase provides the time required for (i) degradation of the more refractory organics, (ii)

Fig. 12.2 Generalized diagram for composting (in: Gray and Biddlestone 1981)

overcoming the "slowing" effects imposed by kinetic rate limitations, and (iii) reestablishing lower temperature microbial populations, which may be beneficial in "maturing" the compost, metabolizing phytotoxic compounds, and suppressing plant diseases.

Organic Gardeners Composting

Organic Gardeners Composting

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