COMPOSTING 187
composted feedstock in order to sustain the microbial attack
under aerobic conditions. At least one investigator recom-
mends 10–30 cubic feet of air per day per pound of volatile
solids initial charge^9 while oxygen utilization was found to
be 0.0244 parts of oxygen per minute per 100 parts of dry
weight.^10
Moisture content of a combined feedstock is determi-
nant of the oxygen supply: If it’s too dry, insufficient mois-
ture exists for metabolism; if it’s too moist, the pore spaces
are filled with water, blocking oxygen from metabolic sites
and causing the process to become anaerobic.^11 The opti-
mal range for moisture content is 40–65% depending on the
feedstock. Composting MSW and yard waste often requires
the addition of water; it can be added at the beginning of
the process (as noted, biosolids are an ideal source of mois-
ture) and/or during biological stabilization. During this later
stage, moisture content must be monitored on an on-going
basis as particle size reduction from microbial and physical
decomposition increases the density of the substrate.^12
For metabolism and growth of microorganisms to occur
in substrate, the macro-nutrients, carbon and nitrogen, should
be present at an optimum ratio (C:N ratio, 30:1). Some
municipal solid waste streams require the addition of nitro-
gen (in the form of ammonium sulfate, urea or protein) to
bring the nutrient levels of the feedstock into balance (see
Compost Microbiology).
The bulk density of a mixed feedstock is important in
large scale windrow operations especially where food waste
is a large fraction of the compost mix. A recent pilot study of
grocery waste composting found that air-filled, macro pore
space volume is the single most important component of the
initial mix.^12 The more dense the mixture, the less oxygen,
the more likely odor-producing anaerobic conditions will
develop. The study concluded that ideal density is less than
900 pounds of SSOW per cubic yard.
The pH of composting material is another important
parameter for microbial growth. While composting will
occur with broad pH limits of 5 to 9, no one pH assures suc-
cess. Rather, the pH changes during the process, reflecting
the sequential decomposition of the substrate. Stated simply,
at first, as carbohydrates present in the raw feedstock are
degraded to organic acids and carbon dioxide, the pH drops;
as the carbohydrate fraction is depleted, and the protein sub-
strate falls to microbial attack, the pH, in turn, rises. The pH
is seldom controlled during the process unless levels remain
depressed due to low moisture content or other process
upsets. In such situations, lime or caustic, carefully added,
since buffers are characteristic of composting materials, is
used to readjust the pH.Biological StabilizationThe heart of the compost process is biological decomposi-
tion and stabilization of the substrate. Whether the feedstock
is heaped into windrows (long haystack-shaped piles) or
“digested” in-vessel (within an enclosed container or system),
decomposition proceeds because of the metabolic activities
of a diverse microbial flora. Compost stability (i.e., the pointat which no further microbial attack will occur unless new
feedstock is introduced) is measured by respirometric meth-
ods focusing on oxygen uptake, heat output and carbon diox-
ide evolution. These measurements give a good indication of
microbial activity and odor potential.^13 Depending on facility
operations and processes (see sections on Batch Composting
and Continuous/Intermittent Composting) and the biological
availability of the substrate, the time needed for stabilization
(retention time) can be a matter of days, several weeks or
several months.
Temperature is another critical parameter of the compost
process. During decomposition, heat is generated by micro-
bial metabolism. Due to the insulating capacity of the com-
posted materials, the heat accumulates and temperatures rise
rapidly to well above ambient. The temperature rise enhances
the succession of microbial attack on the substrate; and the
temperatures attained, typically 55–60C, destroy weed seeds
and fecal pathogens that may be present. Many reports have
shown that 37C to be within the optimal temperature range
for activity of compost microorganisms,^13 while excessively
high temperatures limit growth. Aerating the decomposing
material by mechanized turning and mixing, forced aeration
or some combination of the two methods moderates tempera-
ture and provides oxygen necessary for aerobic metabolism.
Temperatures can rise more than once during the process:
It is possible to induce a temperature cycle de novo if, for
example, additional moisture is added to a substrate drying
out too quickly, or if the materials are remixed and aerated
thereby exposing new surfaces to microbial attack.
Eventually, the biological processes run their course; the
temperature of the pile returns to ambient levels. In theory,
as long as the moisture content is adequate, decomposition
in a compost pile can go on indefinitely, or until no organic
material remains. From an operational standpoint, however,
when the temperature of the compost falls to about 2C over
ambient, the stabilization phase is complete. This final drop
in temperature is probably the most reliable test for the com-
post’s stability.
Biological availability refers to how readily a substrate
can be used as food by microorganisms (i.e., how fast it
degrades). When composting a municipal solid waste stream,
the biological availability of the substrate will be as diverse
as the incoming materials. Because of the variety of MSW
and SSOW, it is difficult to estimate availability of even the
most prevalent macro-nutrient, organic carbon; however, a
first order approximation can be made on the basis of a BOD
test (as used in testing waste waters). In general terms, decom-
position proceeds sequentially with the most readily available
substances: soluble carbohydrates, proteins and fats, attacked
first. Polysaccharides, proteins and materials high in cellulose,
hemi-cellulose and lignin (e.g. paper, cardboard, wood) only
become available after a succession of microbial attacks breaks
down compounds into their constituent units. Enzymes, syn-
thesized by the microorganisms themselves, are the catalysts
that enable these biochemical reactions to occur^14 (see section
on Compost Microbiology).
It should be noted that decomposition can proceed in
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