Aeration
Aerobic organisms need to breathe air to
survive. Aeration is necessary in high temperature
aerobic composting for rapid odor-free decomposition.
Aeration is also useful in reducing high
initial moisture content in composting materials.
Several different aeration techniques can
be used. Turning material is the most common
method of aeration when composting is done
in stacks. Hand turning of the compost piles
or in units is most commonly used for small
garden operations. Mechanical turning or
static piles with a forced air system are
most economical in large municipal or commercial
operations.
The most important consideration in turning
compost, apart from aeration, is to ensure
that material on the outside of the pile
of units is turned into the center where
it will be subject to high temperatures.
In hand turning with forks, this can be easily
accomplished. For piles or windrows on top
of the ground, material from the outer layers
can be placed on the inside of the new pile.
For static piles with a forced air system,
finished compost or a physical “cover” can
be placed on the composting material, ensuring
it reaches high temperatures uniformly. Volume
reduces during the compost process. Piles
or windrows can eventually be combined when
turned, particularly if long composting periods
are used.
Frequencies of turning or total number of
turns are governed primarily by moisture
content and type of material. Moisture is
the most important. High moisture content
reduces the pore space available for air
as well as reducing the structural strength
of the material. This permits greater compaction
and less interstitial or void space for air
in the pile. Materials with a high C:N ratio may not have to be aerated as often
as material which decomposes more actively
and rapidly.
Studies at the University of California indicated
that turning at fairly frequent intervals
during the first 10 to 15 days of composting
achieved approximately the same degree of
stabilization as making the same number of
turns over a longer period. Greater aeration
during the initial stages of decomposition
intensifies the activity of the microorganisms,
shortens the period of active decomposition,
and, consequently, reduces time and land
area needed for composting.
Air availability is a function not only of
turning frequency but also moisture content
and structure of the material. Air requirement
for the biological activity depends on the
availability of nutrients in the feedstocks
(e.g., a very high C:N ratio material would not support as large
a biological population). Thus, it is impossible
to specify a minimum frequency of turning
or number of turns for a variety of different
conditions. Studies on composting of mixed
refuse, (lawn and tree trimmings, and considerable
quantities of paper and combustible rubbish)
at the University of California indicated
that the following schedule of turning is
adequate to permit rapid decomposition.
If the initial moisture content is below
70%, the first turn should be made about
the 3rd day. Thereafter, turn approximately
as follows until the 10th or 12th day:
- Moisture 60%-70%: turn at 2 day intervals;
approximate number of turns, 4 to 5
- Moisture 40%-60%: turn at 3-day intervals;
approximate number of turns, 3 to 4
- Moisture below 40%: add water.
If material initially contains much more
than 70% moisture, it should be turned every
day until the moisture content is reduced
to less than 70%. The above schedule should
then be followed.
This turning schedule will permit rapid decomposition
at thermophilic temperatures. Fewer turns
would not produce as rapid composting but
might be sufficient to prevent serious anaerobic
conditions and odor.
When compost is stored before using, moving
it into a stack can sometimes serve as the
last turn. It should be noted that, while
the above schedule was desirable for mixed
refuse, less frequent turning might have
been satisfactory under other conditions.
Experienced operators can estimate turning
and water needs. If foul odors of anaerobic
and putrefactive conditions exist when the
pile is disturbed either by turning or by
digging into it for inspection purposes,
turn the pile daily until odors disappear.
No matter how anaerobic a pile may become,
it will recover under a schedule of daily
turning that reduces moisture and provides
aeration. Sometimes daily turning is necessary
to controlling fly breeding. A temperature
drop during the first 7 or 10 days of composting
is a good indication that turning for aeration
is necessary.
Daily turning inhibits development of fungi
and actinomycetes. In piles turned daily
these organisms only develop sporadically,
whereas in piles allowed to remain undisturbed
for 2 or 3 days, they form a thick continuous
layer, which reaches a maximum thickness
in about 4 days. Some prefer to manage a
hot or thermophilic pile for several weeks,
then stop turning the pile letting mesophilic
organisms take over, which encourages fungi
and actinomycetes development. Fungi and
actinomycetes are the best decomposers of
woody matter, such as sawdust or branches.
Actinomycetes gives compost the earthy smell—like
that of the forest floor.
In summary, avoiding anaerobic conditions,
maintaining high temperatures, and controlling
flies are the important criteria for degree
of aeration.
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