Materials Recovery Facilities, Part 2
This second installment of a multi-part article covers design considerations. Click here to view the first installment, covering MRF capability and class.
The important issues and design considerations loosely can be divided according to whether the MRF handles mixed waste or source-separated mixtures.
The characteristics of mixed waste and of source-separated mixtures are dramatically different in terms of their contamination. Food wastes, broken glass, and dirt adulterate the former's valued components, which may include newspaper and aluminum cans. Post-consumer materials separated at the source can have low levels of contamination, provided the collection program is properly administered and monitored.
All MRFs must be designed for the size distribution and other characteristics of the materials to be processed. All also must achieve the necessary purity and yield of end product. Obtaining the yield and purity of end products may require a blend of both mechanical and manual processing systems.
All MRFs use a variety of equipment. Shredders, crushers, magnetic separators, screens, and densifiers (for example, balers) find frequent use. Rolling equipment used at MRFs includes front-end loaders, forklifts, and trailers. Conveyors (mainly belt types) are commonplace.
Materials move through the plant by conveyors, usually at variable speeds. In recent years many technical innovations have achieved increasing control over separation processes, allowing different streams to be recovered or eliminated. For example, some plants use a large vibrating belt, which can be tilted. Both belt speed and vibration frequency can be changed. Round, heavy objects are separated along the length of the belt, while less dense items are separated across the width of the belt, flowing away in different directions.
The sort room is the heart of the MRF. Although some MRFs are fully automated, most employ people to work on either side of a conveyer, in a ventilated chamber. In many MRFs, metal cans remain on the conveyer through the sort room. Here cross belt arrangements are provided so that metal cans from all lines can be recovered. Magnets are used for steel cans, and eddy-current separators are used for aluminum cans.
Recyclables picked from materials on the conveyer are dropped through openings into bins beneath the room. When a bin is full, it is replaced with an empty one. When sufficient recyclables have been collected, the bins are emptied into the hopper of a baler that compacts and ties the bales, which go to an interim storage area.
Allowances can be made in the design to segregate corrugated and other marketable wastepaper grades composed predominantly of paper materials that arrive by means of a wheel loader. Once enough paper is deposited on the tipping floor, the materials are transported directly to a baler, by-passing the mixed waste processing equipment.
In addition to the equipment applying to all MRFs, the design of mixed-waste MRFs include systems to open bags (paper and plastic) to liberate their contents for segregation and potential recovery. Mixed waste MRFs sometimes employ trommel screens with knives to puncture and tear open bags.
The process design of a source-separated materials-recovery facility assumes that materials arrive at the facility as both source-separated (such as aluminum cans) and commingled (that is, mixtures of several source-separated material). Source-separated MRFs that accept bagged materials require processes similar to those at mixed-waste facilities.
This feature article was adapted from 1997 Update to the Materials Recycling and Processing Industry in the United States by Eilleen Brettler Berenyi, PhD, Government Advisory Associates, Inc., Westport, CT 06880.