188 COMPOSTING
conditions. All large-scale facilities go to great lengths to
maintain aerobic conditions throughout the composting
process, for anaerobic conditions in decomposing organic
matter can produce foul-smelling odors. A number of com-
post operations have failed solely because of nuisance prob-
lems with odors. Waste water sludge composting facilities
often include a controlled anaerobic step in the processes,
but further discussion of the anaerobic composting process
is beyond the scope of this entry.Compost MicrobiologyThe microorganisms that decompose MSW, SSOW and yard
waste substrates are generally indigenous to the soil and refuse
environment. Successful composting depends on a variety of
organisms: bacteria, fungi, actinomycetes and to some extent,
protozoa, attacking the substrate in successive waves of meta-
bolic activity. In this succession, one group of organisms paves
the way for a succeeding group with the metabolic end prod-
ucts produced by the first organism serving as starting nutrients
for the second. It is thought that some microorganisms excrete
small amounts of vitamins or amino acids that are essential for
other’s growth.^15 This cycle of microbial attack, characterized
by a synergistic, nutritional and metabolic interaction between
a variety of organisms, continues until the available organic
material is virtually exhausted.
In the first wave, initiated by bacteria, the most available
substrate (soluble carbohydrates, etc.) is consumed. The tem-
perature of the composted material rises quickly, and bacte-
ria and yeasts, multiplying at exponential rates, become the
dominant population. At peak activity, they number as many
as 10^8 –10^9 per gram of compost. As decomposition contin-
ues and the pH rises, spore-forming fungi and actinomycetes,
undergoing a succession of their own, become the predomi-
nant population. During each successive wave of metabolic
activity, microorganisms synthesize and release specific
enzymes, breaking down complex molecules through enzy-
matic reactions.^14 A variety of microbial species needs to be
present in the composting mixture for complete “digestion” of
the substrate as each group of microorganism produces only
specific enzymes; i.e., it takes the combined metabolic activi-
ties of different types of organisms in the feedstock to produce
finished compost.
The majority of microorganisms present in the compost
environment are saprophytic (i.e., capable of living on dead
organic matter), aerobic and facultatively anaerobic. Among
the bacteria are aerobic, spore-forming rods, Bacillus spp.,
as well as Achromobacter, Enterobacter, Pseudomonas and
Xanthomona spp., and numerous small gram-negative rods
(coliforms, etc). Among the fungi are Aspergillus, Fusarium,
Penicillium, Rhizopus and Trichoderma. Among the actino-
mycetes are Steptomyces, Nocardia, Micromonospora and
Streptosporangium. These microorganisms exhibit optimal
growth at different temperature ranges and, indeed, flourish
at different times during the stabilization temperature cycle.
They are mesophilic (thriving in 20C to 45C range) or
thermophilic microorganisms (thriving in the 45C to 70C
range). While mesophilic bacteria are more efficient thanthermophilic groups for organic decomposition, thermophilic
temperatures are necessary to kill pathogenic microorganisms
and weed seeds. Operators strive to maintain temperatures in
the decomposing material at an ideal range of 35C to 55C,
enabling both types of microorganisms to flourish.
As noted, the most important microbial nutrients (macro-
nutrients) are organic carbon and nitrogen. Available carbon
is the major energy source for microorganisms, nitrogen is
essential to microbial growth and reproduction. Phosphorous,
and other minor nutrients (sulfur, iron, calcium) and trace ele-
ments (manganese, zinc, copper, cobalt, and molybdenum) are
normally found in appropriate quantities in municipal refuse
and are not usually determining factors. The optimum ratio
of carbon to nitrogen is 30:1. However, this is an ideal ratio;
the biological processes of composting will occur if the ratio
is somewhat higher or lower. If the ratio of carbon to nitrogen
is too great, reproduction and growth decline; and when nitro-
gen is unavailable, the compost process slows to a halt:^15 it is,
in a sense, one of the limiting factors in composting. With a
C:N ratio lower than 30:1 (e.g., organic grocery debris has an
extremely high nitrogen content), the microorganisms elimi-
nate unused nitrogen as ammonia, producing increased odors,
reducing the available nitrogen content of the finished com-
post product and creating a potential health hazard.
While composting operations have occasionally added
microbial “seed” to a composting unit, special inocula are
not needed for composting. As mentioned above, the micro-
organisms responsible for decomposition are already present
in the raw material: MSW, SSOW, yard wastes, waste water
sludges, etc., and, in general, the microbial population most
suited for substrate attack will develop on its own. On the
other hand, seed recycle (i.e., adding microorganisms grown
previously on the same substrate under the same environ-
mental conditions) can help sustain continuous processes at
peak activity, it can serve to shorten the retention time for
batch processes (see sections on Continuous/Intermittent
Composting and Batch Composting).THE PROCESSTwo methods of large scale composting are in use in the United
States: batch operations and continuous (or semi-continuous)
operations. But methods require some degree of mechaniza-
tion depending on facility siting, volume of incoming material,
recovery operations and desired finished product. Community
yard waste composting is a typical batch composting process;
municipal solid waste composting is a typical continuous flow
composting process. Three general steps are common to both
operations: 1. Feedstock preparation, including particle size
reduction and nutrient balancing; 2. Aeration and moisture
content adjustment to maximize decomposition by microor-
ganisms; 3. Curing and screening.Batch CompostingIn batch composting, solid waste materials are heaped into
windrows. As noted, the feedstock is usually a combinationC003_003_r03.indd 188C003_003_r03.indd 188 11/18/2005 1:00:20 PM11/18/2005 1:00:20 P