Page 215 - Materials Chemistry, Second Edition

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186 Waste Management Practices: Municipal, Hazardous, and Industrial
found to occur with a solids retention time of 30 to 60 sec, with the material making 5 to 6 revolu-
tions within the drum (Vesilind et al., 2002).
A practical advantage with waste separation in trommels is its avoidance of clogging. Some
material may attach to the interior of the drum, but, with the continued tumbling motion and
impaction by other materials, this attached material will eventually fall out and be removed.

7.4.2.2 Disk Screens

A disk screen is not a “screen” in the conventional sense, rather, it occurs as a series of rounded or
lobed-shaped disks mounted on shafts (Figures 7.15 and 7.16). The disks are parallel and inter-
locked. The shafts rotate in one direction, carrying the waste charge along in a fashion analogous
to a conveyor belt. Due to turbulence and the irregular shape of the disks, however, undersized
materials fall between the spaces in the disks and are collected in one hopper while the larger par-
ticles are carried along the top to be deposited in a second hopper.
The spacing between the outer diameter of the shafts and the spacing of the disks on the shaft
determine the size of separation. Particles having two dimensions less than or equal to these spac-
ings can fall through. Most particles have a tendency to orient such that the two larger dimensions
are situated horizontally. Therefore, the size separation is usually based on these two larger dimen-
sions (Pfeffer, 1992). Varying the spacings of the disks on the drive shaft will change the desired
particle size ranges.
In the event of blockage, an electronic sensor will signal for the shafts to rotate in the opposite
direction to clear any materials.

7.4.2.3 Vibrating Screens

Another variation of screening is the so-called vibrating screen which consists of a mounted flat
screen and undergoes a reciprocating or gyrating motion. Such flat screens are typically not used to
process mixed MSW, however. They are most successful in purifying more concentrated fractions
of waste that have previously been processed into a relatively fine particle size. Examples include
glass, metals, and wood chips. Flat screening may be applied to remove impurities (e.g., broken
glass, ceramics, and stones) from compost feedstock (Rhyner et al., 1995).

FIGURE 7.15 Disk screen.