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Waste Pre-treatment – Size Separation

Size separation, or screening, involves the separation of a mixture of materials into two or more portions by means of one or more screening surfaces, which are used as go or no-go gauges. Size separation can be accomplished dry or wet, with the former being most common in solid waste-processing systems. Screens have been used before and after shredding and after air classification in the processing of refuse-derived fuel (RDF). They are also used in the processing od compost and mulch to produce a more uniform product. The types of screens that are used, and the performance characteristic, design criteria, and selection of separation equipment are discussed in this section.


Types of equipment

The most common types of screens used for the separation of solid waste are vibrating screens, trommel screens (also known as rotary drum screens), and disc screens. Each type of screen is the best adapted to specific situations.


Selection of size separation equipment

Factors that should be considered in the selection of size screening equipment are summarized below.

  1. Waste characteristics: particle size, shape, bulk specific weight, moisture content, particle size distribution, clumping tendency etc.
  2. Materials specifications for screened components: performance characteristics of screen should match required product.
  3. Screen design parameters: Size of opening (in), percentage open space, total surface screening area(ft²), oscillation rate for reciprocating screens (times/min), rotational speed for trommels etc.
  4. Separation efficiency: recovery (%), efficiency (%), purity (%).
  5. Operational characteristics: energy requirements (hp), maintenance, complexity of operation, noise, and air and wastewater emissions.
  6. Site factors: floor space and vertical space availability, access.

Because screens are available from a number of manufactures, designers should develop performance-based specifications for recovery, effectiveness, and purity to choose the most cost-effective screening system to meet their needs. Designs made with the methods suggested in this section, from the literature and from manufacturers’ data, should be verified by measurements made at operating plants or by pilot testing.

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