Composting, microbiological aspects

Composting is the conversion of organic material, such as plant material and household foodstuffs, to a material having a soil-like consistency. This material is called compost. The composting process, which is one of decomposition, relies upon living organisms. Insects and earthworms participate. Bacteria and fungi are of fundamental importance.

Composting is a natural process and enables nutrients to be cycled back into an ecosystem. The end products of composition are compost, carbon dioxide, water and heat.

The decomposition process is achieved mainly by bacteria and fungi. Bacteria predominate, making up 80 to 90% of the microorganisms found in compost.

There are several phases to the composting process, which involve different microorganisms. The first phase, which lasts a few days after addition of the raw material to the compost pile, is a moderate temperature (mesophilic) phase. As microbial activity produces decomposition and by-products, including heat, a high-temperature (thermophilic) phase takes over. The dominant microorganisms will become those that are adapted to life at higher temperature, the so-called thermophiles. The high-temperature (thermophilic) phase will last anywhere from a few days to a few months. Finally, as decomposition activity of the microbial population slows and ceases, a cooling-down phase ensues over several months.

Initially, the mesophilic microorganisms break down compounds that readily dissolve in water. This decomposition is rapid, causing the temperature inside the compost pile to rise quickly. The microbes involved at this stage tend to be those that predominate in the soil. One example is Actinomyces , which resemble fungi but which are actually bacteria composed of filaments. They are what give the soil its earthy smell. Enzymes in Actinomyces are capable of degrading grass, bark and even newspaper. Species of fungi and protozoa can also be active at this stage.

As the internal temperature of the pile exceeds 40° C (104° F), the mesophiles die off and are replaced by the thermophilic microbes. A decomposition temperature around 55° C (131° F) is ideal, as microbial activity is pronounced and because that temperature is lethal to most human and animal microbial pathogens. Thus, the composting process is also a sterilizing process, from an infectious point of view. However, temperatures much above this point can kill off the microbes involved in the decomposition. For this reason, compost piles are occasionally agitated or "turned over" to mix the contents, allow oxygen to diffuse throughout the material (efficient decomposition requires the presence of oxygen) and to disperse some of the heat. The ideal blend of microorganisms can be established and maintained by the addition of waste material to the compost pile so as to not let the pile become enriched in carbon or nitrogen. A proper ratio is about 30 parts carbon to one part nitrogen by weight.

Thermophilic bacteria present at this stage of decomposition include Bacillus stearothermophilus and bacteria of the genus Thermus. A variety of thermophilic fungi are present as well. These include Rhizomucor pusills, Chaetomium thermophile, Humicola insolens, Humicola lanuginosus, Thermoascus aurabtiacus, and Aspergillus fumigatus.

Thermophilic activity decomposes protein, fat, and carbohydrates such as the cellulose that makes up plants and grass. As this phase of decomposition ends, the temperature drops and once again the lower-temperature microbes become dominant. The decomposition of the complex materials by the thermophilic organisms provides additional nutrients for the continued decomposition by the mesophilic populations.

Microbiological composting is becoming increasingly important as space for waste disposal becomes limited. Some 30% of yard and household waste in the United States is compostable. An average household can decompose about 700 pounds of material per year. If such waste is added to landfills intact, the subsequent decomposition produces methane gas and acidic run-off, both of which are environmentally undesirable.

See also Chemoautotrophic and chemolithotrophic bacteria; Economic uses and benefits of microorganisms; Soil formation, involvement of microorganisms