Page 210 - Materials Chemistry, Second Edition

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Municipal Solid Waste Processing; Materials Recovery Facilities 181

FIGURE 7.10 Hand-sorting along a conveyor belt. Note workers are not wearing eye or hearing protection.

to remove those items that are detrimental either to the workers downstream; to the quality of the
final, separated products; or to system equipment. This could include removing toxic and poten-
tially explosive items. According to Vesilind et al. (2002), the material along the conveyor is rec-
ognized visually (“coding”) by such properties as color, reflectivity, and opacity; verified by noting
its density; and removed (separated) by hand-picking.
Important factors in the design of the manual picking area are the width of the belt, the speed of
the belt, and the average thickness of material placed on the belt for picking. A picking belt usually
measures no more than 60 cm (24 in.) wide for one-sided picking or 120 cm (48 in.) wide for pickers
on both sides of the belt. Belt speeds have varied from approx. 450 to 2700 cm/min (15 to 90 ft/min)
depending on the material to be processed and the amount of any preprocessing which may have
already taken place. The belt should not move faster than about 900 cm/min (30 to 40 ft/min) depend-
ing on the number of pickers (Engdahl, 1969; Vesilind et al., 2002). Belt speeds of 1800 cm/min (60
ft/min) were used in sorting facilities at the turn of the century (Hering and Greeley, 1921). The aver-
age thickness of wastes on the belt for effective picking is about 6 in. (Tchobanoglous et al., 1993).
The picking operation is best performed under natural lighting. Artificial light, for example
from fluorescent bulbs, emits only a narrow band of light that makes identification (coding) of cer-
tain components difficult (Vesilind et al., 2002).
At those facilities where waste is not preprocessed, sorting is inefficient. Pickers can salvage
about 450 kg (1000 lb)/person/h depending on the material density (Vesilind et al., 2002). For
example, a worker removing metallic objects and wood will remove more materials by weight than
a picker removing lightweight plastic containers.
Hand-picking is, obviously, dirty and dangerous work. There are significant quantities of dust
and the wastes being handled are odoriferous. Wastes may be hazardous to the worker by being
sharp-edged, explosive, flammable, or infected with pathogenic microorganisms. Noise from equip-
ment can be extreme in some facilities. Heavy equipment may be routinely moving across the facil-
ity floor and the unit operations themselves are noisy. Appropriate worker safety including
protection of eyes, skin and hearing, as mandated under OSHA statutes, is essential in a MRF.

7.4.2 SCREENING
Screening is a unit operation designed for the separation of waste input into oversize and undersize
fractions (also labeled reject and product, respectively). In many MRFs, the oversize materials