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90 Waste Management Practices: Municipal, Hazardous, and Industrial

CH 2 OH

H O H

H
OH H

HO OH

H OH
FIGURE 4.9 Structure of a glucose molecule.

4.5.7 CRUDE FIBERS
This category includes natural fibers such as cotton, wool, and leather, which are generally resist-
ant to degradation. The major polymers are cellulose and lignin. Cellulose is a polymer of glu-
cose and due to the nature of its chemical bonds, it is only slowly biodegradable (Figure 4.10).
Lignin is composed of a number of monomers, with benzene being the most common
(Figure 4.11). The benzene ring is resistant to biodegradation. Both cellulose and lignin con-
tribute to the formation of soil humus, also a very resistant organic compound. Natural fibers
found in paper products, food waste, and yard waste are the major source of these polymers.
Cellulose may account for 25 to 30% of the dry weight of MSW, while lignin may comprise 8
to 10% (Pfeffer, 1992).

4.5.8 PROTEINS
All proteins possess a backbone of an amine group (–NH ) and an organic acid (R–COOH) (Figure
2
4.12). Food wastes and yard wastes are sources of proteins, which comprise about 5 to 10% of the
dry solids in MSW. Proteins are important in the biodegradation of MSW, as they are an important
N source for heterotrophic microorganisms. Efficient microbial degradation of carbonaceous wastes
requires a sufficient supply of N. Partial decomposition of proteins can result in the production of
amines, which produce intense odors. Common names for some of these amines include “putrescine”
and “cadaverine” (Pfeffer, 1992).

4.5.9 LIPIDS
Also known as fats, oil, and grease, these may comprise approximately 8 to 10% of MSW on a dry
weight basis. A generalized structure of a lipid molecule in shown in Figure 4.13. The main sources
of lipids are putrescible garbage, fat, and cooking oils. Lipids typically possess a high energy value,
in the range of 35,775 to 38,000 MJ/kg (16,000 to 17,000 Btu/lb) (Pfeffer, 1992). Solid wastes, high
in lipid content, are well suited for energy recovery processes. Lipids become fluid at slightly above
ambient temperatures. This can add to the liquid content of MSW and will change physical prop-
erties due to wetting of paper products. Lipids have a low solubility in water that renders them
slowly biodegradable.

4.5.10 BIODEGRADABILITY OF MSW FRACTIONS
The above compounds serve as a substrate for a wide range of micro- and macroorganisms impor-
tant in composting and other biological processing. All of these organic components can be bio-
logically converted into gases and relatively stable organic and inorganic solids.