MSW is a complex mixture of inorganic and organic materials (Table 5.1). Efficient separation and economic recovery of RDF and the components that can be recycled is the ultimate challenge to engineers who specialize in design­ing resource recovery equipment for the large-scale processing of solid wastes generated by urban communities. Unfortunately, the number of MSW plants designed to recover recyclables and RDF make up only about 20 to 25% of the total MSW-to-energy facilities in the United States. This is probably caused by the success of mass burn technologies (Chapter 7) and the fluctuating markets for recyclables. Nevertheless, the processing schemes and hardware employed to separate MSW are innovative and justify some elaboration. Liter­ally hundreds of hardware designs and machines have been developed to separate and recover most of the components in MSW. Some resource recovery facilities have even installed equipment for recovering coinage, which is just a small fraction of the total mass of MSW.

One of the first comprehensive resource recovery plants in the world was built in Dade County, Florida (Todd, 1984; Berenyi and Gould, 1988). A brief description of this facility when it was in full-scale operation to re­cover recyclables and RDF is informative. The plant was designed to pro­cess 2720 t/day (3000 ton/day) of MSW, but it frequently processed over 3630 t/day (4000 ton/day), and could process 4540 t/day (5000 ton/day) if only household garbage were received. It was designed to accept, in addition to household garbage, a wide variety of solid wastes including trash, garden clippings, trees, tires, plastics, pathological wastes, white goods (i. e., stoves, refrigerators, air conditioners, etc.), and industrial, commercial, and demolition wastes. RDF and shredded tires, approximately 1000/day, were burned for on­site power generation in a 77-MW power plant, and glass, aluminum, ferrous metals, and other materials including the ash and flyash were recovered and sold. The plant achieved a 97% volumetric reduction compared to as-received MSW. Only 6 wt % of the total incoming MSW remained as unsalable residue; this was disposed of in a landfill. The plant also conformed to all effluent, leachate, emissions, noise, and odor requirements. Impressive results such as these depended on the availability and reliability of efficient separation methods.

A simplified description of the first comprehensive materials recovery facility of its type in the United States illustrates how one plant was designed to accomplish some of these separations (Waste Management, Inc., 1977). The plant, called Recovery 1, was built in New Orleans, Louisiana, to process 590 t/day of MSW. The waste was delivered and unloaded at one of two receiving pit conveyors, and transported by conveyors to the first separation unit, a 13.7-m long by 3-m diameter rotating trommel that contained circular holes 12 cm in diameter. Plastic and paper bags tumbling in the trommel were broken open by lifters. The smaller, heavier objects such as heavy metal and glass bottles that fell through the holes were transported directly to a magnetic ferrous recovery station and an air classifier. The larger and lighter materials such as paper, textiles, and aluminum containers that passed through the trommel were conveyed to a 746-kW primary shredder. The shredded material was then conveyed to the ferrous recovery station and the air classifier. In the air classifier, a high-speed air current blows the light materials out of the top of the classifier. This fraction, RDF, consists of shredded paper, plastic, wood, yard wastes, and food wastes. The heavy fraction is essentially glass, aluminum, other nonferrous metals, and some organic material. It was routed to the recovery building for further processing. A secondary, 746-kW shredder system handled oversized, bulky wastes without passage through the trommel. The output was also conveyed to the air classifier, where RDF was obtained as the overhead, and the heavy fraction was conveyed to the recovery building. Each shredder system was sized to process 590 t of MSW in about 12 h to ensure operating reliability.

Three modules were located in the recovery building. The first module consisted of a vibrating screen to separate the shredded material by particle size, a drum magnet to separate residual ferrous material, an eddy current separator to remove the nonmagnetic aluminum and other nonferrous metals, and a small hammermill to further shred the aluminum fraction to increase its bulk density. The output from the first module consisted of the ferrous fraction, the aluminum fraction, and a fraction that contained primarily glass and some nonferrous metals. The glass fraction containing some residual nonferrous metal was conveyed to the second recovery module, which con­sisted of a crusher, another vibrating screen, a rod mill, and a two-deck, fine — mesh vibrating screen. The glass fraction was crushed and screened in the second module. The smaller fraction was treated with a pulsed water stream that separated the light fraction, which was discarded. The heavier glass frac­tions were pumped as slurries to the bottom deck of the fine-mesh second screen to separate the larger particles for crushing in the rod mill. Recycling of the milled material back to the top deck of the fine-mesh screen yielded a glass cullet fraction for further treatment in the third module, and a nonferrous metal fraction which was removed from the second screen. The third module contained a hydrocyclone, a froth flotation tank, and a glass dryer. The glass cullet fraction from the second module was mixed with clean water in a prefloat tank to remove any remaining organic particles, separated from the slurry through centrifugal separation and froth flotation, dried, and conveyed to the loadout building for shipment. RDF was recovered from the air classifier, and the ferrous, aluminum, and glass fractions were recovered from the “bottoms” of the classifier.

This is a simplified description of how MSW is separated into recyclables and fuel. There are many refinements of these operations.