Composting without oxygen results in fermentation.
This causes organic compounds to break down
by the action of living anaerobic organisms.
As in the aerobic process, these organisms
use nitrogen, phosphorus, and other nutrients
in developing cell protoplasm. However, unlike
aerobic decomposition, this reduces organic
nitrogen to organic acids and ammonia. Carbon
from organic compounds, is released mainly
as methane gas (CH4). A small portion of
carbon may be respired as CO2.
This anaerobic process takes place in nature.
Examples include decomposing organic mud
at the bottom of marshes and buried organic
materials with no access to oxygen. Marsh
gas is largely methane. Intensive reduction
of organic matter by putrefaction is usually
accompanied by unpleasant odors of hydrogen
sulfide and of reduced organic compounds
that contain sulfur, such as mercaptans (any
sulfur-containing organic compound).
Since anaerobic destruction of organic matter
is a reduction process, the final product,
humus, is subject to some aerobic oxidation.
This oxidation is minor, takes place rapidly,
and is of no consequence in the utilization
of the material.
There is enough heat energy liberated in
the process to raise the temperature of the
putrefying material. In the anaerobic dissolution
of the glucose molecule, only about 26 kcal
of potential energy per gram of glucose molecules
is released compared to 484 to 674 kcal for
aerobic decomposition. The energy of the
carbon is in the released methane (CH4).
The conversion of CH4 to CO2 produces large
amounts of heat. This energy from anaerobic
decomposition of organic matter can be used
in engines for power and burned for heat.
Pathogens could cause problems in anaerobic
composting because there is not enough heat
to destroy them. However, aerobic composting
does create high enough temperatures. Although
heat does not play a part in the destruction
of pathogenic organisms in anaerobic composting,
they do disappear in the organic mass because
of the unfavorable environment and biological
antagonisms. They disappear slowly. The composted
material must be held for periods of six
months to a year to ensure relatively complete
destruction of Ascaris eggs, for example.
Ascaris are nematode worms that can infest
the intestines. They are the most resistant
of the fecal-borne disease parasites in wastes.
Anaerobic composting may be accomplished
in large, well packed stacks or other composting
systems. These should contain 40% to 75%
moisture, into which little oxygen can penetrate,
or 80% to 99% moisture so that the organic
material is a suspension in the liquid. When
materials are composted anaerobically, the
odor nuisance may be quite severe. However,
if the material is kept submerged in water,
gases dissolve in the water and are usually
released slowly into the atmosphere. If the
water is replaced from time to time when
removing some of the material, odor does
not become a serious nuisance.
Both aerobic and anaerobic composting require
bacteria. Some bacteria work better in one
or the other environment. Compost piles under
aerobic conditions may attain a temperature
of 140° to 160° F in one to five days depending
upon the material and the condition of the
composting operation. This temperature can
also be maintained for several days before
further aeration is needed. The heat necessary
to produce and maintain this temperature
must come from aerobic decomposition, which
requires oxygen. After a period of time,
the material will become anaerobic unless
it is aerated. There is probably a period
between the times when the oxygen is depleted
and anaerobic conditions become evident,
during which the process is aerobic.
"Aerobic composting" requires a
considerable amount of oxygen and produces
none of the characteristic features of anaerobic
putrefaction. Aerobic composting can be defined
as a process in which, under suitable environmental
conditions, aerobic organisms utilize considerable
amounts of oxygen in decomposing organic
matter to fairly stable humus.
"Anaerobic composting" describes
the process of putrefactive breakdown of
organic matter by reduction in the absence
of oxygen where end products such as CH4
and hydrogen sulfide (H2S) are released.