Page 243 - Materials Chemistry, Second Edition

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214 Waste Management Practices: Municipal, Hazardous, and Industrial
● Competition for plant nutrients. The most important nutrient to most crop plants is nitro-
gen. With the addition of raw waste to soil, microorganisms attack the carbon (an energy
source), and will also require large quantities of nitrogen for manufacturing cell biomass.
Being opportunistic and fast-growing, microorganisms can incorporate and render
unavailable virtually all plant-available soil N such that plants cannot obtain sufficient
quantities. This situation is termed “nitrogen depression.”
● Leaching. Potentially toxic materials (e.g., salts, metals, acids, microbial cells) are
released from raw waste into soil and water.

In contrast, composting transforms the organic feedstock via:

● Mineralizing the simple, easily assimilable substances, i.e., protein, cellulose, sugars, and
lipids to carbon dioxide and simple N compounds (e.g., nitrate).
● Humifying more complex compounds such as lignin to produce a more homogeneous
and stable organic product.

The final humus-like product is hygienically safer, more aesthetically appealing, and substan-
tially lower in odor than the original MSW. The finished organic product has several potential appli-
cations. A primary application of compost is for agriculture: compost serves as a soil conditioner
(i.e., an organic matter source which improves water-holding capacity, increases aeration, and
improves drainage), and it supplies nutrients, particularly N, P, and S, all of which occur primarily
in the organic form in soils. Compost also provides a number of micronutrients including Cu, Fe,
Zn, and Ni. Many such trace nutrients will occur as organic chelates and complexes which are rel-
atively plant-available. Finally, because composts are often circumneutral in pH they moderate pH
extremes of the recipient soil. Compost is also used in landfill operations as a daily cover material,
for landscaping applications, and for remediation of contaminated sites and mined lands (U.S.
EPA,1997, 1998).

8.3 OVERVIEW OF THE COMPOSTING PROCESS
Composting on the commercial scale occurs in three major phases. Initial processing includes size
reduction to enhance microbial reactions. First, separation of inert materials (glass, plastic, metals,
etc.) from the organic fraction is necessary. Size reduction and chemical or biological conditioning
are extremely important at the outset if the finished product is to be used in agriculture. Next,
microorganisms decompose the raw feedstock into simpler compounds, producing heat as a result
of their metabolic activities. The volume of the compost pile is reduced during this stage and the
heat generated destroys many pathogens. In the final stage, the compost product is “cured.”
Microorganisms deplete the supply of available nutrients in the pile, which, in turn, slows down
their activity. As a result, heat generation diminishes and the compost mass dries. When curing is
complete, the compost is considered “stabilized” or “mature.” Any further microbial decomposition
occurs only very slowly. Figure 8.1 provides the overall steps involved in the aerobic composting
of the organic fraction of MSW.

8.4 THE ROLE OF MICROORGANISMS IN COMPOSTING

Composting is an aerobic biological process; a diverse consortium of microorganisms acting con-
currently controls this process. The most active players in composting are bacteria, actinomycetes,
fungi, and protozoa. These microorganisms are naturally present in most organic materials, includ-
ing food waste, soil, leaves, grass clippings, and other organics.
Composting is also dependent upon a succession of microbial activities, whereby the environ-
ment created by one group of microorganisms ultimately promotes the activity of successor groups.