Page 214 - Materials Chemistry, Second Edition

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Municipal Solid Waste Processing; Materials Recovery Facilities 185
more cycles of rising and dropping, the greater the separation efficiency. At the same time, however,
the rate of throughput must be considered so that there is a limit as to how long a waste charge
should remain in the trommel.
The following equation was developed for calculating trommel throughput (Sullivan et al., 1992):
D [11.36 Qm/(d F K g 0.5 tan a)] 0.4 (7.7)
v
b
where D is the trommel diameter (m), Qm the trommel throughput, (kg/s), d the bulk specific
b
3
weight of MSW (kg/m ), α the angle of trommel from base frame (deg), K the velocity correction
v
factor (K 1.35 when α 3°, and K 1.85 when α 5°), F the fillage factor (a typical range is
v
v
2
between 0.25 and 0.33), and g 9.81 m/s .
Using data such as these shown in Figure 7.14, separation efficiency at a particular screen size
can be estimated. For example, in a trommel with an aperture of 10 cm, about 90% of metals and
glass will fall through and be captured as undersize material. Concurrently, about 30 to 35% of the
paper and plastics will also fall through and thus contaminate the metal or glass fraction.
Many factors influence the separation efficiency of a trommel including:
• Characteristics of the incoming materials (dense, loose, fragile, and wet)
• Quantity of the incoming materials (feed rate)
• Size ranges of the cylinder screen
• Incline angle of the cylinder
• Rotational speed
• Size and number of screen openings

The primary factor influencing separation is, ultimately, retention time of the waste within
the trommel. The average waste retention time in trommels ranges from 25 to 60 sec for raw waste
prior to shredding, to about 10 sec for shredded, air-classified light materials. Long trommels are
desirable because they achieve a more thorough screening. Optimal trommel performance was

FIGURE 7.14 Hypothetical particle size distribution as relates to screening MSW. (Reproduced with kind
permission of Diaz, L.W. et al., Resource Recovery from Municipal Solid Wastes, Vols. 1 and 2, CRC Press,
Boca Raton, FL, 1982.)