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Municipal Solid Waste Processing; Materials Recovery Facilities 171
7.2.1 UNIT OPERATIONS
Unit operations in a centralized facility include screening, magnetic separation, shredding, and air
classification. The unit operations for the separation and processing of wastes are designed to
accomplish the following (Tchobanoglous et al., 1993):
• Modify the physical characteristics of the waste so that the components can be removed
more easily.
• To remove specific, useful components from the waste stream.
• To remove contaminants from the waste stream.
• To process and prepare the separated materials for subsequent uses.

Common unit operations for MSW separation are shown in Table 7.2.
Even though many MRF systems are highly mechanized, human labor is still needed to carry
out a number of duties. For example, the removal of hazardous wastes (batteries, paint cans, and
pesticide containers) from MSW can only be accomplished by manual sorting. The same is true for
2 L soda bottles made from PETE. Additionally, nearly all MRFs that sort glass by color must rely
on the human eye and hand.

TABLE 7.2
Common Unit Operations and Facilities for the Separation and Processing
of Separated and Commingled MSW
Unit Operation Function and Material Processed
Shredding
Hammer mills Size reduction
Flail mills Size reduction, also used as bag breaker
Shear shredder Size reduction, also used as bag breaker
Glass crushers Size reduction, glass
Wood Grinders Size reduction, yard trimmings, and wood wastes
Screening Separation of over and under-sized material; trommel also
used as bag breaker
Cyclone separator Separation of light combustible materials from air stream
Air classification Separation of light combustible materials from air stream
Magnetic separation Separation of ferrous metal from commingled wastes
Densification
Balers Compaction into bales or paper, cardboard, plastics,
textiles, aluminum
Can crushers Compaction and flattening; aluminum and tin cans
Weighing facilities
Platform scales Operational records
Small scales Operational records
Handling moving and storage facilities
Conveyer belts Materials transport; all types of materials
Picking belts Manual separation of waste material; source-separated and
commingled MSW
Movable equipment Materials handling and moving; all types of waste
Storage facilities Materials storage; all types of recovered materials
Source: Tchobanoglous, G., et al., Integrated Solid Waste Management: Engineering Principles and
Management Issues, McGraw-Hill, New York, 1993. Data reproduced with kind permission
of the McGraw-Hill Companies, Inc.