Page 235 - Materials Chemistry, Second Edition

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206 Waste Management Practices: Municipal, Hazardous, and Industrial
classification, and a reduction in populations of potentially pathogenic microorganisms. The dryer
is typically installed just before or just after the air classifier.
A drying system is equipped with: (1) a blower to circulate the heat and to force the MSW
charge forward; (2) a combustion chamber to generate heat using RDF, (3) a rotary drum with a
variable speed motor, and (4) a cyclone separator which separates the MSW from the air stream.
The MSW inlet is positioned directly in front of the rotary drum. The waste and hot air stream flow
through the drum in multiple stages, typically passing the length of the drum several times before
exiting to the cyclone separator. As the drum rotates constantly, the waste is agitated and increased
surface area is exposed to the heat (Bendersky, 1982).

7.5 MATERIALS FLOW IN THE MRF
There are a number of designs for unit operations in a MRF. As mentioned earlier, a MRF can be
“clean” or “dirty.” The former processes materials that have already been source-separated into var-
ious fractions. The latter type will accept bags of commingled wastes collected directly from the
curbside. Considerations as to clean vs. dirty modes and what equipment to employ will vary
according to factors such as initial capital costs and funding available, political pressures, and con-
venience to the consumer. A simplified waste separation scheme is depicted in Figure 7.39.
Placement of unit operations in an MRF varies substantially depending on the types of material
desired for separation and the purity desired. For example, the placement of a trommel screen up-
stream of a shredder will result in the removal of stones and other small abrasive debris. This
removal will lengthen the lifetime of the hammermill shredder by reducing hammer wear.
Additionally, trommels will separate a large proportion of glass containers. If not removed, glass
will shatter in the shredder and become embedded in paper and other potentially recyclable mate-
rials. If this paper product is combusted as fuel, there are implications for handling larger volumes
of ash.
In some communities, source-separated wastes may be collected in see-through bags which
are collected in the same truck along with mixed MSW. A typical process flow diagram for a MRF
employing separation, manual combined with mechanical, of materials from commingled MSW
and source-separated wastes is illustrated in Figure 7.39. Commingled MSW from residential and
other sources are discharged in the receiving area. Hazardous items are removed immediately by
hand. Recyclable and oversized materials such as lumber, white goods, and furniture are also
removed in this first-stage operation. Source-separated materials in see-through plastic bags are
also removed from the commingled MSW. Next, the commingled waste is loaded onto an inclined
conveyor. Additional cardboard and large items are removed manually from the conveyor at the
second presorting station as the waste material is transported to the bag-breaking station. The next
step involves breaking open the plastic bags, which can be accomplished either manually or
mechanically. In some facilities, a short enclosed trommel equipped with protruding blades is
used as a bag breaker. Flail mills, shear shredders, and screw augers have also been used as bag
breakers.
After the presorting steps the materials typically removed include paper, cardboard, all types of
plastic, glass, and metals. In some operations, different types of plastic (e.g., PETE and HDPE) are
separated simultaneously. Material remaining on the conveyor is discharged into a trommel or disc
screen for size separation. The oversized material is sorted manually a second time (second-stage
sorting). Mixed source-separated materials are further sorted using the second-stage sorting line.
Source-separated mixed paper and cardboard are processed separately using a flow diagram such
as the one given in Figure 7.40. The undersized material from the trommel screening and the mate-
rial remaining after the second-stage sorting operation are hauled away for landfill disposal,
processed further and combusted, or used to produce compost to be used as daily landfill cover. As
shown in Figure 7.40, further processing of the residential materials usually involves shredding and
magnetic separation (Tchobanoglous et al., 1993).