The Recycling of Household and Commercial Waste
In the past few decades, there has been mounting pressure in developed countries to reduce the amount of material discarded as waste after a single use. The incentives here are to conserve the natural resources, including energy, from which the materials are produced and to reduce the volume of material that must be buried as garbage, incinerated, etc. The four Rs of such waste management philosophies are:
• Reduce the amount of materials used (sometimes called source reduction).
• Reuse materials once they are formulated.
• Recycle materials to recover components that can be refabricated.
• Recover the energy content of the materials if they cannot be used in any other way.
These principles can be and are applied to all types of wastes, including hazardous ones, but in the following discussions we concentrate on their application to domestic materials, particularly in regard to recycling.
A distinction is often made between preconsumer recycling, which involves the use of waste generated during a manufacturing process, and postconsumer recycling, which involves the reuse of materials that have been recovered from domestic and commercial consumers. The postconsumer items most often collected for recycling are
• paper (especially newspapers and cardboard),
• aluminum (especially beverage cans),
• steel (especially food cans), and
• plastic and glass containers.
The labor, energy, and pollution costs associated with collecting the materials, sorting them, and transporting them to facilities where they can be reused must be considered in any analysis of recycling. In addition, historically the demand for recycled materials in these categories has been unstable, with prices swinging wildly in response to changes in supply and demand. For these and similar reasons, the recycling of paper, glass, and plastics usually needs to be justified on noneconomic and nonenergy grounds— such as savings in landfill space. The most economically viable forms of recycling materials usually involve a minimum of chemical reprocessing and correspond more closely to reuse—e.g., using newspapers to make paper-board or insulation and reusing glass and plastic containers. Debates still rage both in the popular press and in the scientific literature as to whether or not recycling is a worthwhile activity.
The Recycling of Metals and Glass
From the viewpoints of both economics and energy conservation, the recycling of metals makes sense. Virgin metal must be obtained by reduction of the oxidized form of the element found in nature. The reduction process requires energy that does not need to be expended again when the metallic form of the element is recycled.
Consider the reduction of aluminum and iron from the oxide ores. By definition, the enthalpies of these processes equal the negative of their enthalpies of formation:
A1203-> 2 A1 + | Oz AH° = -AHf° (Al203)
Recycling aluminum cans saves 95% of the energy that is needed to produce A1 metal from bauxite ore. Since the energy required for the aluminum reduction must come in the form of electricity, and since this energy accounts for about 25% of the cost of its production, it makes good economic sense to recycle this metal. However, the recycling rate for aluminum cans dropped from 65% in 1992 to only 45% in 2005 in the United States. In contrast, Sweden, which requires deposits on beverage containers that are refunded upon their return, collects 85% of aluminum cans. Considerable savings in aluminum have resulted from the reduction by about one-third of the weight of individual cans over the last few decades. Aluminum can be recycled endlessly without loss in quality. Globally, recycled aluminum provides about one-third of the production of the metal. Recycling steel cans saves about two-thirds of the energy required to produce them from iron ore.
The enthalpy of formation of the principal ore of iron, Fe203, is —824 kj/mol. Calculate the enthalpy of the reaction in which 1.00 g of metallic iron is formed from the ore. Given your result, would you expect the price that recycling operators are willing to pay for scrap iron per kilogram to be greater or less than that for scrap aluminum?
In the case of paper, glass, and plastics, there is no significant change in the average oxidation number of the principal materials during their transformation from inexpensive raw material components—wood, sand and lime, and oil, respectively—to finished products; thus there are no great energy savings when they are recycled.
Modern, low-polluting, and energy-efficient electric furnaces cannot handle as high a proportion of used glass as can their more polluting, more energy-consuming counterparts that use fossil fuels. Consequently, the recycling of too much glass can produce more pollution and use more energy than would otherwise be the case! In any event, the use of glass containers for beverages has fallen sharply, and their recycling rate dropped from 31% in 1992 to 20% in 2003 in the United States.
The Recycling of Paper
People in developed countries throw away more paper than any of the other components of municipal solid waste (see Figure 16-1), and it seems an obvious material to recycle. However, the production of virgin paper, for example, uses only about one-quarter more energy than the recycling of old paper. The transportation of waste paper to recycling mills and the de-inking process itself are heavy consumers of energy. Notwithstanding these considerations, tremendous quantities of paper, especially newsprint, are currently recycled. Indeed, corrugated paper products are the most intensely recycled material in North America.
The first step in recycling paper is mechanical dispersal into its component fibers in water. Then it is cleaned to remove nonfibrous contaminants, followed by treatment with sodium hydroxide or sodium carbonate to de-ink it. A detergent is added to help disperse the pigment, and the ink particles are removed by washing or flotation on air bubbles, which rise to the top. The resulting de-inked stock is usually less white than virgin fiber, so the two types are often blended. If necessary, the whiteness of the recycled stock can be improved by bleaching, usually with peroxides and hydrosulfites, The used ink, which is recovered in a sludge with some pulp fibers, is later pressed to remove water and then can be burned to produce steam for use in mill operations, or it can be treated to detoxify it. In general, the use and release into the environment of materials such as chlorine or other bleaching agents, acids, and organic solvents are significantly less with the production of recycled paper than with the creation of the virgin material.
Paper of different types is composed of fibers of very different lengths (long ones in office paper, short ones in newsprint) that cannot be mixed in recycling to produce high-quality paper. In addition, there is a limit to the number of times that paper can be recycled, since with each cycle the pulp fibers become shorter and so lose some of their integrity. Newsprint can be recycled back into newsprint about six to eight times. Food boxes and egg cartons are usually made from recycled pulp fiber.
From 1985 to 2000, the paper industry in the United States spent almost $20 billion on the technology and capital investment required to recycle paper; the ultimate goal was to recycle about half of all paper used in the country. Indeed, by the mid-2000s, close to half the paper and paperboard products in North America was being recycled, compared to two-thirds in many European countries. Recycling rates in the United States in 2003 were 82% for newspapers, 71% for corrugated boxes, 56% for office paper, 33% for magazines, and 16% for phone directories.
The issue of whether it is preferable to recycle paper, rather than bury it in a landfill, is controversial. Since paper is made from wood, which has grown by extracting carbon dioxide from the atmosphere, landfilling paper that will never rot is a form of sequestering C02. Research indicates that about 70% of the carbon in paper—and more than 97% of that in other wood products—remains undecomposed in landfills. However, this argument is negated if any significant fraction of the paper decomposes to emit methane; nor does it take into account the larger amounts of energy and water required to produce virgin paper compared to recycled paper.
Perhaps a more clever use of waste paper in the future will be its conversion to fuel ethanol, as discussed in Chapter 8. Paper can be incinerated directly to recover its energy content, reducing the amount of fossil fuels burned in power stations. According to an analysis by Britain's Centre for Environmental Technology, recycling paper is environmentally superior to landfilling it but is actually inferior to burning it for its fuel value when all factors are taken into consideration.
Continue reading here: The Recycling ofTires
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