Page 244 - Materials Chemistry, Second Edition

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Composting MSW 215
Different types of microorganisms are active at different phases in the composting pile. Bacteria
have the most significant effect on decomposition; they are the first to become established in the
pile, processing readily decomposable substrates (e.g., proteins, carbohydrates, and sugars) faster
than any other group. Nitrogen-fixing bacteria are also present in the compost pile, which will fix
atmospheric N for incorporation into cellular mass. Table 8.1 lists some of the major microbial
types involved in composting. Commercial products are available that claim to speed the compost-
ing process via the introduction of selected strains of bacteria, but inoculating compost piles has not
been found to bring about completion any more rapidly (Gray et al., 1971; Haug, 1980; Rynk, 1992;
U.S. EPA, 1994).
Fungi play an important role in composting as the pile dries, since fungi can tolerate low-mois-
ture environments better than bacteria. Some fungi also have lower nitrogen requirements than bac-
teria and are, therefore, able to decompose lignin and cellulose materials, which bacteria cannot.
Because fungi are numerous in composting, concern has arisen over the growth of genera such as
Aspergillus, which pose a potential health hazard.

Cover
material

Organic Aerated Finished
fraction of pile Curing Drying Screening compost
MSW composting covered
storage

Aeration Aeration
Distribution

FIGURE 8.1 Flow chart showing the steps involved in the aerobic composting process.

TABLE 8.1
Microbial Populations During Aerobic Composting
Number per wet gram of compost

Microbe Mesophilic Thermophilic Mesophilic Species
Initial Temperature (40-70°°C) (70°°C to cooler) Identified
(40°°C)
Bacteria
Mesophilic 10 8 10 6 10 11 6
Thermophilic 10 4 10 6 10 7 1
Actinomycetes
Thermophilic 10 4 10 8 10 5 14
Fungi
Mesophilic 10 6 10 3 10 5 18
Thermophilic 10 3 10 7 10 6 16
Source: Haug, R. T., The Practical Handbook of Compost Engineering, Lewis Publishers, Boca Raton, FL,
1993. Reproduced with kind permission. Copyright Lewis Publishers, an imprint of CRC Press. Boca Raton, FL