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Composting MSW 219
TABLE 8.2
Carbon/Nitrogen Ratios of Various Wastes and Materials
Material C:N
Sawdust 200-500:1
Wheat straw 125-150:1
Grass clippings 12-20:1
Corn stalks 60:1
Humus 10:1
Activated sludge 6:1
Cow manure 18:1
Horse manure 25:1
Poultry manure 15:1
Food scraps 15:1
Mixed MSW 50-60:1
Adapted from Diaz, L. F. et al., Composting and Recycling Municipal Solid
Waste, 1994. Reproduced with kind permission of Lewis Publishers, an imprint
of CRC Press. Boca Raton, FL.
8.5.5 AERATION
The atmosphere within the interstices of the composting mass will vary significantly during decom-
position. When the organic feedstock is delivered to the compost site, the oxygen supply available
to microbes occurs from the diffusion of ambient air, and in air originally trapped within the voids.
However, the rate of diffusion of ambient air into the mass is very limited; hence, interstitial air is
the major source of oxygen.
At the outset, the composition of air in voids is similar to that of ambient air (i.e., approximately
20.9% O and 0.03% CO , v/v). Within a short time (hours to days), however, the heterotrophic pio-
2
2
neer communities become activated and begin decomposition of the raw organic substrates, with the
concurrent increase in CO content and decrease in O level. With a closely monitored composting
2
2
system, the oxygen content can be varied from 15 to 20% and CO from 0.5 to 5% for the process
2
to be successful.
During aerobic respiration, organic chemicals are oxidized to carbon dioxide and water or other
end products using molecular oxygen as the terminal electron acceptor. Aerobic respiration occurs
under highly oxygenated conditions. The reaction for the aerobic oxidation of a glucose molecule is
C H O 6O → 6CO 6H O energy (8.1)
12
6
2
2
2
6
If the O concentration falls below approximately 15%, facultative anaerobic microorganisms are
2
activated and rapidly become dominant. Fermentation and anaerobic respiration reactions take over.
Undesirable products such as acetic acid, ethanol, methane, and ethane will form. These are odor-
iferous and may inactivate beneficial compost microorganisms:
C H O → 2C H OH 6CO 6H O (8.2)
2
2
5
2
6
12
6
glucose ethanol
Additional reactions under anaerobic conditions are described later.
The decomposition of organic materials is significantly faster and more complete in the pres-
ence of oxygen. The energy available in Equation 8.1 is approximately 14 times greater than that
for anaerobic decomposition of glucose (Equation 8.2) (Zubay, 1983).