Materials & methods to ensure quality
compost)
temperature
In aerobic composting proper temperature
is important. Heat is released in the process.
Since composting material has relatively
good insulation properties, a composting
mass large enough (3’ x 3’) will retain the
heat of the exthermo-biological reaction
and high temperatures will develop.
High temperatures are essential for destruction
of pathogenic organisms and undesirable weed seeds. Also, decomposition
is more rapid in the thermophilic temperature
range. The optimum temperature range is 135°
-160° Fahrenheit. Since few thermophilic
organisms actively carry on decomposition
above 160° F, it is undesirable to have temperatures
above this for extended periods.
Eggs of parasites, cysts and flies have survived in compost stacks for days
when the temperature in the interior of the
stack is around 135° F. Since a higher temperature
can be readily maintained during a large
part of the active composting period, all
the material should be subjected to a temperature
of at least 150° F for safety.
Sometimes compost operators avoid prolonged
high temperatures because the nitrogen loss
is greater at high temperatures because ammonia
vaporizes, which takes place when the C:N ratio is low. But there are other ways of
minimizing nitrogen loss than operating at
a lower temperature. The advantages of destroying
pathogenic organisms and weed seeds, controlling
flies, and providing better decomposition
outweigh any small nitrogen loss due to high
temperatures.
A drop in temperature in the compost pile
before material is stabilized can mean that
the pile is becoming anaerobic and should
be aerated. High temperatures do not persist
when the pile becomes anaerobic. The temperature
curve for different parts of the pile varies
somewhat with the size of the pile, the ambient
(surrounding) temperature, the moisture content,
the degree of aeration, and the character
of the composting material. To maintain high
temperatures during decomposition, compost
must be aerobic. The size of the compost
pile or windrow may be increased to provide
higher temperatures in cold weather or decreased
to keep the temperatures from becoming too
high in warm weather. Experience shows that
turning to release the heat of compost piles,
which have become so hot (170°-180° F.) that
bacterial activity is inhibited, is not very
effective. When the material is actively
decomposing, the temperature, which falls
slightly during turning, will return to the
previous level in two or three hours. Also,
it is impossible to bring about any significant
drop in temperature by watering the material
without water logging the mass.
Variations in moisture content between 30%
and 75% have little effect on the maximum
temperature in the interior of the pile.
The initial temperature rises a little more
rapidly when the moisture content is 30%
to 50% than when it is 70%. Studies show
an important and significant correlation
between the moisture content and the temperature
distribution within the pile. When moisture
content is high, temperatures near the surface
will be higher, and the high temperature
zone will extend nearer to the surface than
when the moisture content is low. For example,
in experiments at University of California
during mild weather when the air temperature
fluctuated between 50° and 80° Fahrenheit,
the zone of maximum temperature in a pile
with a moisture content of 61% extended to
within about one inch of the surface while
the maximum temperature zone in a pile containing
40% moisture began 6 inches below the surface.
Deeper piles caused higher temperatures and
better temperature distribution, and subject
more material to a high temperature at any
one time. Hence, the actual mass of the material
evolving heat is important in providing adequately
high temperatures.
Shredding or pulverizing feedstock also provides
better temperature distribution and less
heat loss.
Materials with a high C:N ratio or containing large amounts of ash
or mineral matter usually attains high temperatures
more slowly in the compost pile.
Aeration to maintain aerobic conditions in
the compost pile is essential for high temperatures.
When the compost pile becomes anaerobic,
temperature drops rapidly. Even small areas
which have become anaerobic will often exhibit
a lower temperature than surrounding aerobic
material.
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