Source separation may also be instituted at commercial and
institutional establishments. Generally, containers for each recyclable
component are located at each work station, at a central collection area,
both. The removal of stored material occurs at regular intervals dependi
on the type of component, the rate of recovery, and the material sales
arrangements.
6.1.2 Size Reduction. Size reduction is a mechanical process whereby
materials are reduced in size through breaking and tearing as a consequen
of the action of rotating elements. The materials are held within a size
reduction device until their particle sizes are less than those of the
discharge opening(s) of the device. The top size of the size-reduced
product and its size distribution are governed by a number of factors,
including the configuration of the rotating elements, geometry of the
discharge openings, the characteristics of the feedstock, and the feed ra
Hammermills are the most commonly employed pieces of size reduction
equipment for processing refuse. Hammermills utilize high-speed, rotatin
hammers to achieve size reduction. Size reduction is often the first
operation in a refuse processing system because it renders the waste more
compatible with other processing equipment, and it liberates and exposes
materials so that they can be separated downstream by other processing
equipment.
Size reduction is also employed in some instances in landfilling
applications where allowed by regulations, inasmuch as landfills composed
shredded waste (termed "shred fills") require less cover soil than the
landfilling of unprocessed solid waste (i.e., direct from the collection
vehicle).
6.1.3 Air Classification. Air classification is a process employing
principles of aerodynamics to segregate materials into a "light fraction"
and a "heavy fraction." Particles with a high drag-to-weight ratio are
entrained in a stream of air and compose the light fraction. In the case
air classification of shredded solid waste, light fraction is typically
composed of paper, plastic, and very small particles of all material
categories, and is it termed a combustible fraction. Air-classified ligh
fraction is one form of refuse-derived fuel (RDF). The particles not
entrained in the air stream compose the heavy fraction. Metals, rock,
leather, rubber, dense plastics, glass, and wet bulky materials such as
garbage are concentrated in the air classified heavy fraction. The
concentration of recoverable materials in the heavy fraction (such as met
and glass) facilitates their recovery by subsequent processing equipment.
Air classification is also employed to remove contamination (e.g., pa
and plastic) from highly concentrated fractions, such as ferrous scrap
recovered using a magnetic separator.
6.1.4 Screening. Screening is a process whereby particles are segregate
according to size in two dimensions. The desired segregation of particle
sizes is effected by exposing the particles to a surface composed of
openings of the appropriate size. The "oversized fraction" from a screen
operation refers to particles that are either too large to penetrate the
openings or are not exposed to openings due to interference. The
"undersized fraction" refers to the materials that pass through the
openings.
5.10-43
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