Solid Waste Treatment Technologies
Solid Waste Treatment Technologies
Eleven billion tons of solid waste are produced every year in the United States. Most of it is still buried in landfill sites with no prior treatment, other than removal of materials that can be recycled. However, costs of landfill are soaring and the number of suitable sites is decreasing. Therefore, there is increasing interest in treating solid waste to re-use its material or extract energy from it, rather than merely dumping it.
Incineration and composting are traditional approaches to treating solid waste. However, new designs can lead to more widely applicable, safer, and more efficient versions of these treatments. For example, gasification and ash melting technology are being applied to incineration of solid waste to deal with the ash that remains and thereby reduce the potential for dioxin pollution. Meanwhile, treatment of hazardous solid wastes, including toxic wastes, is important to render them harmless and stop widespread environmental contamination.
Historical Background and Scientific Foundations
Domestic, commercial, and industrial human activities produce nearly 11 billion tons of solid waste a year. Around half of this comes from agriculture and includes manure and crop residues, which farmers generally put back into the soil. Mining and metal processing account for another third of solid wastes produced per year. Other industries produce about 400 million metric tons of solid waste, much of which may be dealt with by the company concerned at their own facilities. Around 60 million metric tons of this is hazardous waste, including toxic waste, and this may require special treatments to make it safe. Municipal waste, which is domestic and commercial refuse, amounts to more than 200 million metric tons a year, according to the U.S. Environmental Protection Agency (EPA). Most of this is dealt with by local refuse collection agencies.
Previously, solid wastes were dumped on the ground, wherever space could be found. Dumping has become socially and economically unacceptable in industrialized societies, although vast waste sites can still be seen in some developing countries. Over the last 50 years, it was realized that treating solid waste can lessen our dependence on Earth’s resources. Solid waste can yield value in terms of energy and materials, which provides incentives for increasingly sophisticated technologies to be developed for their extraction.
Most solid waste in the United States is still disposed of by landfill. Solid waste receives minimal treatment before being put into a landfill site, other than removal of items that can be recycled. Once in the site, the waste needs to be monitored for leaks. More advanced landfills are managed for production of methane, which can be used as a source of energy. Landfill is increasingly expensive and appropriate sites are becoming scarce. In Japan, where there is little land available for landfill, solid waste is more likely to be treated by recycling or incineration.
The amount of solid waste dealt with by recycling or incineration has increased in the United States. Recycling begins with separating out recyclable materials from solid waste. Recycled material can be processed to produce another version of the same object. For instance, aluminum cans are made into more aluminum cans. Recycled material can also be processed to make something completely different, such as tires that are made into road surfacing material.
Incineration is used to deal with about 20% of solid waste in the United States. The waste may be sorted first to remove any non-combustible material, particularly plastics that may give off toxic emissions. This sorted material is called refuse-derived fuel. It is particularly useful for energy recovery, where the heat from the
WORDS TO KNOW
BIOREMEDIATION: The use of living organisms to help repair damage such as that caused by oil spills.
COMPOSTING: Breakdown of organic material by microorganisms.
DEMANUFACTURING: The disassembly, sorting, and recovering of valuable or toxic materials from electronic products such as televisions and computers.
ENERGY RECOVERY: Incineration of solid waste to produce energy.
REFUSE-DERIVED FUEL: Solid waste from which unburnable materials have been removed.
incinerator is used either directly or to generate electricity. Incinerator facilities are costly to build, but the investment is offset if energy is produced alongside the waste disposal. There is often public resistance to the building of incinerators in an area because the ash remaining after incineration could contain toxins. Incineration of plastics produces dioxins, which are very toxic to humans, and it tends to remain in fine particulate ash, which could enter the surrounding air. Therefore, pollution controls on incinerators need to be very strict if the facility is to be operated safely. Plastics and batteries, which contain toxic heavy metals, must be removed from solid waste prior to incineration.
Composting is the other main method of treating solid waste. Composting involves using microbial action to turn waste with an organic content, such as kitchen and garden waste, into a nutrient-rich addition to soil. Composting can be accomplished by an individual household or on a larger scale. Compost itself does not have a high market value, but the composting process also produces methane, which can be captured and sold as fuel in an advanced composting facility.
Hazardous and toxic waste requires careful treatment to make it safe. One important area is demanufacturing, which is applied to the increasing number of discarded computers, televisions, cell phones, and refrigerators. This involves retrieval of toxic heavy metals, such as mercury, lead, and gallium, from electronic components, along with plastics from the casing. Hazardous wastes produced by industry may be subjected to chemical treatment such as oxidation or neutralization in order to convert them into harmless substances. Sometimes isolating the toxic component is all that is needed, such as trapping it in a charcoal filter, for instance.
Some toxic materials, such as the polychlorinated biphenyls that were widely used in industry before their health hazards were appreciated, can often be treated by bioremediation. Many microbes will feed on unusual carbon-containing compounds, even if they are toxic to humans, breaking them down into carbon dioxide and water. If applied to land contaminated with toxic waste, they can render if safe in the long term. Plants, such as the mustards, reeds, and the water hyacinth, have the ability to detoxify waste by taking up heavy metals and other dangerous substances in their roots. Phytoremediation is an attractive and cheap option for treating toxic waste, although care has to be taken that the plants do not release the toxins back into the environment.
Hazardous solid waste cannot always be treated to make it safe, and in this instance, safe storage may be the only option. Storage locations include deep ground burial, perhaps in a discarded mine, or in a remote, secure building. Often retrievable storage is used so that if the waste begins to leak, it can be rescued before it causes damage to the environment. Vitrification, where the waste is injected into glass, is sometimes used for long-term safe storage of hazardous material including radioactive waste.
Impacts and Issues
An increasing number of new technologies is being applied to solid waste treatment. For instance, the thermal conversion process applies heat and high pressure to a mixture of manure, tires, plastics, and sewage sludge, converting it by a complex series of chemical reactions into gasoline, oil, and methane. Another approach, being developed in Japan, is gasification and ash melting applied to incineration. This technique uses the energy from the waste to treat the ash and reduce its dioxin content to harmless levels in a system that is readily adaptable to municipal incinerators. If waste is seen as a scientific and technical challenge, then its safe disposal and recovery of valuable materials are more likely.
Cunningham, W.P., and A. Cunningham. Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.
Virtual Centre for Environmental Technology Exchange. “Future Municipal Solid Waste Treatment Technologies.” http://www.apec-vc.or.jp/e/modules/tinyd00/inde.php?id=122&kh_open_cid_00=11 (accessed March 27, 2008).