Page 245 - Materials Chemistry, Second Edition

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The actinomycetes are often considered a middle group between the bacteria and the fungi.
Most genera of actinomycetes produce slender, branched filaments that develop into a mycelium.
Actinomycetes are widely distributed in soil, compost piles, river sediments, and other environ-
ments. They are second to bacteria in terms of abundance. Actinomycetes decompose aromatics,
steroids, phenols, and other complex organic molecules (U.S. EPA, 1983; Eweis, et al., 1998).
Macroorganisms also play a role in composting. Rotifers, nematodes, mites, springtails, sowbugs,
beetles, and earthworms reduce the size of the compost feedstock by foraging, moving
within the pile, or breaking up particles of the feedstock. These actions physically break down the mate-
rials, creating greater surface area and sites for microbes to attach and metabolize (U.S. EPA, 1994).
The bacteria and fungi important in decomposing MSW feedstock can be classified by optimal
temperature regime as mesophilic or thermophilic. Mesophilic microorganisms experience most
rapid growth at temperatures between 25 and 45°C (77 to 113°F). These are dominant within the
pile early in the process when temperatures are near ambient. The mesophiles use oxygen within
the interstices (pores) to oxidize carbon and thus acquire energy. End products of the reactions
include carbon dioxide (CO ) and water. Heat is also generated as chemical bonds in the substrate
2
are broken during metabolism.
If the pile is insulated from the local environment with no aeration or turning, most of the heat
generated is trapped within the pile. In the insulated center, when temperatures of the mass rise to
about 45°C (112°F), the mesophiles die or are inactivated. At this time, thermophilic microorganisms,
i.e., those that prefer temperatures between 45 and 70°C (112 and 158°F), are activated. These mul-
tiply and metabolize substrates and replace the mesophiles in most sections of the pile.
Thermophiles generate even greater quantities of heat than mesophiles. The temperatures reached
during this phase of the process are sufficiently high to kill most pathogens and also weed seeds.
Many composting facilities maintain a temperature of 55°C (131°F) in the interior of the pile for
72 h to ensure pathogen destruction and to inactivate weed seeds.
The thermophiles continue decomposing the feedstock as long as nutrient and energy sources are
available. As the substrates are depleted, the thermophiles die and the pile temperature falls. Mesophiles
are again activated and decompose the remaining substrate until all available energy sources are uti-
lized (U.S. EPA, 1994). Figure 8.2 provides a typical temperature pattern for composting processes,
and Table 8.1 shows the density of microorganisms as a function of temperature during composting.

8.5 FACTORS AFFECTING THE COMPOSTING PROCESS
Composting is strongly influenced by many environmental factors; as a result, much research into
system design and environmental controls has been conducted in attempts to optimize the process.

70
Temperature (°C) 50
60

40

30
0 5 10 15 20 25 30 35 40
Days into compost cycle

FIGURE 8.2 Temperature trends in the early stages of composting (U.S. EPA, 1984).