Compostable materials normally contain a large number of many different types of bacteria, fungi, molds, and other living organisms. More species of bacteria are involved in aerobic decomposition than in anaerobic putrefaction. Many of the same organisms are no doubt as active in anaerobic composting such as sludge digestion. However, since environmental conditions of aerobic compost stacks, particularly moisture and nutritional materials, differs greatly from that of sludge digestion tanks, the biological population would also be expected to differ.

Although many types of organisms are required to decompose different materials, the necessary variety is usually present and organisms thrive when environmental conditions are satisfactory. During decomposition, marked changes take place in the nature and abundance of the biological population. Some of the many species will multiply rapidly at first but will dwindle as the environment changes and other organisms are able to thrive. Temperature and changes in the available food supply probably exert the greatest influence in determining the species of organisms comprising the population at any one time.

Aerobic composting is a dynamic process in which the work is done by combined activities of a wide succession of mixed bacterial, actinomycetes, fungal, and other biological populations. Since each is suited to a particular environment of relatively limited duration and each is most active in decomposition of some particular type of organic matter, the activities of one group complement those of another. The mixed populations parallel the complex environments afforded by the heterogeneous nature of the compostable material. Except for short periods during turning, the temperature increases steadily in proportion to the amount of biological activity until equilibrium (state of balance) with heat losses is reached, or the material becomes well stabilized.

In aerobic composting, bacteria, actinomycetes, and fungi are the most active. Mesophilic (low temperature) bacteria are characteristically predominant in the start of the process, soon giving way to thermophilic (high temperature) bacteria, which inhabit all parts of the stack where the temperature is satisfactory; this is eventually, most of the stack. Thermophilic fungi usually appear after 5 to 10 days and actinomycetes become conspicuous in the final stages when short duration, rapid composting is practiced.

Except in the final stages of the composting period, when the temperature drops, actinomycetes and fungi are confined to a sharply defined outer zone of the stack, 2 to 6 inches in thickness, beginning just under the outer surface. Some molds also grow in this outer zone. Unless very frequent turning is practiced, so that there is adequate time or conditions for growth, the population of fungi and actinomycetes is often great enough to impart a distinctly grayish white appearance to this outer zone. The sharply defined inner and outer limits of the shell (in which actinomycetes and fungi grow during the high temperature active-composting period) are due to the inability of these organisms to grow at the higher temperatures of the interior of the stack. The thermophilic actinomycetes and fungi have been found to grow in the temperature range between 120° and 150° Fahrenheit. Frequent turning -such as is sometimes necessary for fly control- inhibits their growth, since the cooler outer shell is turned into the interior before they can develop in large numbers.

Various investigations show that many different types of thermophilic bacteria apparently play a major part in decomposing protein and other readily broken down organic matter. They appear to be solely responsible for the intense activity characteristic of the first few days, when temperatures reach 150° to 160° Fahrenheit. Major changes in the nature of the compost stack are taking place then: the stack is drastically shrinking and the appearance of the material is undergoing rapid change. They continue to predominate throughout the process in the interior of the piles, where temperatures are inhibitory to actinomycetes and fungi.

Fungi and actinomycetes play an important role in the decomposition of cellulose, lignin, and other more resistant materials, despite being confined primarily to the outer layers and becoming active only during the latter part of the composting period. These tough materials are attacked after more readily decomposed materials have been used. There are many bacteria that attack cellulose. However, in the parts of compost stacks populated chiefly by bacteria, paper hardly breaks down, whereas in the layers or areas inhabited by actinomycetes and fungi it becomes almost unrecognizable.

Considerable cellulose and lignin decomposition by actinomycetes and fungi can occur near the end of the composting period or “curing” when the temperatures have begun to drop and the environment in a larger part of the pile is satisfactory for their growth. Hence, in the interest of their activity, turning should not be more frequent during curing than is necessary for providing aerobic conditions and controlling flies. Among the actinomycetes, streptomyces and micromonospora common in compost, micromonospora are the most prevalent. Compost fungi include Termonmyces sp., Penicillium dupontii, and Aspergilus fumigatus.

Since organisms necessary for composting are usually present and will carry on the process when the environment is suitable, an extensive knowledge of the characteristics of the various organisms is not necessary for operating a compost operation. A more detailed knowledge of the organisms, however, may lead to further improvement and economics in the process.