(Materials & methods
to ensure quality compost)
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
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
40%-60%: turn at 3-day intervals; approximate number of turns,
3 to 4
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.
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.