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.
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.
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.
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.
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.
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.
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.
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
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.