Page 246 - Materials Chemistry, Second Edition

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Composting MSW 217
8.5.1 PREPROCESSING OF THE FEEDSTOCK
The preparation of the organic component of MSW for composting can be carried out in two ways:

● Mechanical. Composting a mixed waste material, i.e., organics commingled with inerts
such as plastic and metal, is fraught with hazards and complications and should be
avoided. Inerts (nonreactives) may benefit the process by acting as a bulking agent and
promoting aeration of the mass; however, they end up as useless and sometimes haz-
ardous components of the finished material. Mechanical processing involves size reduc-
tion by shredding, followed by separation of inert materials by screening, magnetic
separation, and other unit operations (see Chapter 7). The resulting material possesses a
higher surface area for reaction and a more available substrate for microbes.
● Biological and mechanical. In a combined process, waste is physically processed as
described above, and then transferred to biological reactors (Figure 8.3) for 1 to 3 days.
The reactors are rotating heated cylinders mounted on a slight incline. Biological activ-
ity increases significantly in the reactor and degradation of organics begins along with
size reduction. After the preliminary treatment in the reactor, the feedstock is transferred
to a compost pile.

8.5.2 ENVIRONMENTAL FACTORS
Microorganisms are clearly central to successful composting; therefore, the factors that affect their
proliferation and activity are those which determine the rate and extent of composting. The principal
environmental factors regulating the speed and degree of decomposition include nutrient levels,
nutrient balance (e.g., carbon to nitrogen ratio), aeration, moisture, temperature, pH, and particle size
of the feedstock material. Any shift in these factors are interdependent; a change in one parameter
often results in changes in others. The closer these factors collectively approach optimum levels, the
more rapid will be the rate of composting. The chemical and physical nature of the substrate, and aer-
ation are especially important in process design.

FIGURE 8.3 Photo of a biological reactor. Reproduced with kind permission of Cornell Waste Management
Institute.