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.
|