6.2.2
Gasification.
This technology is not practical for usage at this
time.
Do not use.
6.2.3 Composting. In the engineering sense, composting is the
biostabilization of organic matter under controlled conditions. The orga
matter in solid waste may be converted to a soil-like product through the
composting process. The purpose of composting solid waste, or fractions
derived therefrom, is to produce a usable product and to thereby reduce t
mass of material requiring landfill disposal.
There are two basic types of composting processes. In anaerobic
composting, decomposition and biostabilization of the organic material oc
in the absence of molecular oxygen. On the other hand, in aerobic
composting, decomposition and biostabilization of the material occur in a
oxygen-rich environment. Aerobic composting has the disadvantage of
requiring regular aeration of all parts of the composting mass. However,
aerobic composting is generally considered preferable to anaerobic
composting because biostabilization is achieved at a faster rate, less
objectionable odors are produced, and the associated heat generation
produces temperatures above the thermal depth point of pathogens generall
associated with solid waste (and sewage sludge, which is often added).
Generally, solid waste is processed before composting in order to rem
most of the inorganic components. Drying, screening, and size reduction
the composted material may be required after completion of the composting
phase in order to meet user requirements. The extent of preprocessing an
postprocessing depends on the composition of the solid waste feedstock, o
the intended use of the produce, and on the economics of the project.
Three factors that generally affect the rate of aerobic composting, a
the rate of aeration, the moisture content of the feedstock, and the rati
of available carbon to nitrogen in the feedstock. The second and third
factors can be beneficially influenced by the addition of sewage sludge t
the solid waste fraction. At the same time, the utilization of solid was
in the composting of sludge is beneficial in terms of providing bulk and
porosity to the mixture, of lowering the moisture content, and of increas
the carbon/nitrogen ratio.
Aeration of the composting mass is affected by regular turning, mixin
and agitation of the mass or by forced aeration of the mass using an arra
of perforated ducts embedded in the compost piles and blowers to supply t
air. Mechanical mixing or forced aeration may be used as the aeration
technique in open systems in which windrows (i.e., elongated piles of
compost) are placed on the ground; or these processes may also be used in
closed or semi-closed systems in which bins, silos, or other enclosures
contain the composting mass.
For the case of composting 40 tons (35 metric tons) per day of organi
materials derived from solid waste, an open windrow composting system wou
require about 4.5 acres (20,000 square meters), exclusive of land
requirements for preprocessing and postprocessing operations and storage
finished compost. An enclosed composting system would require less land
than an open windrow system, but would require a greater expenditure for
equipment. An illustration of a turned windrow composting operation
(utilizing a mechanical turning vehicle) is shown in Figure 12.
5.10-47
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