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Recycling
Separating, recovering, and reusing components of solid waste that may still have economic value is called recycling. One type of recycling is the recovery and reuse of heat energy, a practice discussed separately in incineration. Composting can also be considered a recycling process, since it reclaims the organic parts of solid waste for reuse as mulch or soil conditioner. Still other waste materials have potential for reuse. These include paper, metal, glass, plastic, and rubber, and their recovery is discussed here.
Separation
Before any material can be recycled, it must be separated from the raw waste and sorted. Separation can be accomplished at the source of the waste or at a central processing facility. Source separation, also called curbside separation, is done by individual citizens who collect newspapers, bottles, cans, and garbage separately and place them at the curb for collection. Many communities allow “commingling” of nonpaper recyclables (glass, metal, and plastic). In either case, municipal collection of source-separated refuse is more expensive than ordinary refuse collection.
At a typical MRF, commingled recyclables are loaded onto a conveyor. Steel cans (“tin” cans are actually steel with only a thin coating of tin) are removed by an electromagnetic separator, and the remaining material passes over a vibrating screen in order to remove broken glass. Next, the conveyor passes through an air classifier, which separates aluminum and plastic containers from heavier glass containers. Glass is manually sorted by colour, and aluminum cans are separated from plastics by an eddy-current separator, which repels the aluminum from the conveyor belt.
In lieu of source separation, recyclable materials can be separated from garbage at centralized mechanical processing plants. Experience has shown that the quality of recyclables recovered from such facilities is lowered by contamination with moist garbage and broken glass. The best practice, as now recognized, is to have citizens separate refuse into a limited number of categories, including newspaper; magazines and other wastepaper; commingled metals, glass, and plastics; and garbage and other nonrecyclables. The newspaper, other paper wastes, and commingled recyclables are collected separately from the other refuse and are processed at a centralized material recycling facility, or MRF (pronounced “murf” in waste-management jargon). A modern MRF can process about 300 tons of recyclable wastes per day.
Reuse
Recovered broken glass can be crushed and used in asphalt pavement. Colour-sorted glass is crushed and sold to glass manufacturers as cullet, an essential ingredient in glassmaking. Steel cans are baled and shipped to steel mills as scrap, and aluminum is baled or compacted for reuse by smelters. Aluminum is one of the smallest components of municipal solid waste, but it has the highest value as a recyclable material. Recycling of plastic is a challenge, mostly because of the many different polymeric materials used in its production. Mixed thermoplastics can be used only to make lower-quality products, such as “plastic lumber.”
In the paper stream, old newspapers are sorted by hand on a conveyor belt in order to remove corrugated materials and mixed papers. They are then baled or loose-loaded into trailers for shipment to paper mills, where they are reused in the making of more newspaper. Mixed paper is separated from corrugated paper for sale to tissue mills. Although the processes of pulping, de-inking, and screening wastepaper are generally more expensive than making paper from virgin wood fibres, the market for recycled paper has grown with the establishment of more processing plants.
Rubber is sometimes reclaimed from solid waste and shredded, reformed, and remolded in a process called revulcanization, but it is usually not as strong as the original material. Shredded rubber can be used as an additive in asphalt pavements and artificial turf and is also sold directly as an outdoor mulch. Discarded tires may be employed as swings and other recreational structures for use by children in “tire playgrounds.”
In general, the most difficult problem associated with the recycling of any solid-waste material is finding applications and suitable markets. Recycling by itself will not solve the growing problem of solid-waste management and disposal. There will always be some unusable and completely valueless solid residue requiring final disposal.Jerry A. Nathanson
refuse
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Alternate titles: MSW, municipal refuse, municipal solid waste
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refuse, also called municipal solid waste, nonhazardous solid waste that requires collection and transport to a processing or disposal site. Refuse includes garbage and rubbish. Garbage is mostly decomposable food waste or yard waste that is highly putrescible, while rubbish is mostly dry material such as glass, paper, cloth, or wood that does not readily decompose. Some forms of rubbish are recyclable, and some municipalities compost yard waste and other garbage on a large scale. Trash is rubbish that includes bulky items such as old refrigerators, couches, or large tree stumps. Trash, especially electronic waste, may require special collection and handling. The sources of refuse include residential, commercial, institutional, and industrial activities. See also solid waste management.
The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Melissa Petruzzello.
environmental engineering
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Alternate titles: bioenvironmental engineering, environmental health engineering, sanitary engineering
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What is environmental engineering?
What are some structures that environmental engineers have built?
What role can living organisms play in environmental engineering?
environmental engineering, the development of processes and infrastructure for the supply of water, the disposal of waste, and the control of pollution of all kinds. These endeavours protect public health by preventing disease transmission, and they preserve the quality of the environment by averting the contamination and degradation of air, water, and land resources.
Environmental engineering is a field of broad scope that draws on such disciplines as chemistry, ecology, geology, hydraulics, hydrology, microbiology, economics, and mathematics. It was traditionally a specialized field within civil engineering and was called sanitary engineering until the mid-1960s, when the more accurate name environmental engineering was adopted.
Projects in environmental engineering involve the treatment and distribution of drinking water (see water supply system); the collection, treatment, and disposal of wastewater (see wastewater treatment); the control of air pollution and noise pollution; municipal solid-waste management and hazardous-waste management; the cleanup of hazardous-waste sites; and the preparation of environmental assessments, audits, and impact studies. Mathematical modeling and computer analysis are widely used to evaluate and design the systems required for such tasks. Chemical and mechanical engineers may also be involved in the process. Environmental engineering functions include applied research and teaching; project planning and management; the design, construction, and operation of facilities; the sale and marketing of environmental-control equipment; and the enforcement of environmental standards and regulations.
The education of environmental engineers usually involves graduate-level course work, though some colleges and universities allow undergraduates to specialize or take elective courses in the environmental field. Programs offering associate (two-year) degrees are available for training environmental technicians. In the public sector, environmental engineers are employed by national and regional environmental agencies, local health departments, and municipal engineering and public works departments. In the private sector, they are employed by consulting engineering firms, construction contractors, water and sewerage utility companies, and manufacturing industries.
Jerry A. Nathanson
plasma arc gasification
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Alternate titles: PAG
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plasma arc gasification (PAG), waste-treatment technology that uses a combination of electricity and high temperatures to turn municipal waste (garbage or trash) into usable by-products without combustion (burning). Although the technology is sometimes confused with incinerating or burning trash, plasma gasification does not combust the waste as incinerators do. Instead, it converts the organic waste into a gas that still contains all its chemical and heat energy and converts the inorganic waste into an inert vitrified glass called slag. The process can reduce the volume of waste sent to landfills and generate electricity.
Process
In the PAG process an electrical arc gasifier passes a very high voltage electrical current through two electrodes, creating an arc between them. Inert gas, which is under high pressure, then passes through the electrical arc into a sealed container (called a plasma converter) of waste materials. Temperatures in the arc column can reach more than 14,000 °C (25,000 °F), which is hotter than the surface of the Sun. Exposed to such temperatures, most waste is transformed into gas consisting of basic elements, while complex molecules are torn apart into individual atoms.