Microbiology and Immunology

Composting, microbiological aspects WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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COMPOSTING, MICROBIOLOGICAL

ASPECTSComposting, 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.

Bacteriaand fungiare of fundamental importance.

Composting is a natural process and enables nutrients to

be cycled back into an ecosystem. The end products of com-

position are compost, carbon dioxide, water and heat.

The decomposition process is achieved mainly by bac-

teria and fungi. Bacteria predominate, making up 80 to 90% of

the microorganismsfound 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-prod-

ucts, 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. Enzymesin Actinomyces are capable of degrad-

ing grass, bark and even newspaper. Species of fungi and pro-

tozoacan 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 ther-

mophilic 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 ani-

mal 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,

A compost bin.

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