Treatment And Disposal Technologies
Hazardous wastes that need treatment or disposal may be freshly generated from an industrial operation, they may be old stored chemicals, or they may have been sitting in a dumpsite for many years. At a dumpsite, component chemicals difficult to identify, they are likely to have reacted with one another, and they may have already affected the surrounding soil and water. Land disposal and incineration are two main dumpsite remediation methods. Types of waste treatment include physical, biological, and chemical neutralization or stabilization. Some treated hazardous wastes can even be reclaimed or recycled.
Industries in the United States dispose of about 60% of their hazardous waste using a land disposal method called deep well injection. Liquid wastes are injected into wells located in impervious rock formations that supposedly keep the waste isolated from groundwater and surface water. Unfortunately, hydrogeologists now predict that groundwater flow actually does occur in most previously-designated impervious rock formations, and injected waste often migrates into groundwater reservoirs called aquifers. Other underground burial locations for hazardous wastes include deep mines, natural caverns, and man-made deep pits.
Landfilling is the other primary land disposal method for hazardous waste disposal in the United States. Hazardous waste landfills are similar to regular solid waste landfills, but they must meet much higher standards for safety and environmental protection. The EPA requires that most hazardous wastes be treated before being discarded in properly-designed, approved landfills and burial sites.
Incineration, or burning, is a controversial, but still common, method of handling hazardous wastes. The EPA estimates that five million tons of hazardous wastes are burned each year in the United States. Various incineration technologies exist for a variety of types of waste. For example, volatile chemicals like paint thinners, oils, and solvents are destroyed by combustion at cement plants whose furnaces, called kilns, reach temperatures of 2,700°F (1,500°C). Needless to say, residents living near cement plants and other hazardous waste incinerators often have concerns about air pollution. In 1993, the EPA tightened its regulations on emissions from most hazardous waste incinerators, including cement kilns, after discovering that the emissions contained like dioxins, furans and other substances that cause cancer and other health problems in humans. Another recent EPA study noted that medical waste incinerators that many hospitals use to burn hazardous wastes also emit dioxins.
Some hazardous wastes, including certain tars, drilling muds, and mining sludges, are relatively well-suited for incineration. Some other wastes, however, should not be burned, such as those that contain heavy metals. Burning does not destroy the metals, and they end up in the incinerator ash. Ash from hazardous waste incinerators that contains high concentrations of metals is a dangerous material in its own right, and requires careful disposal.
Stabilization, also called solidification, is a physical treatment method sometimes used on incinerator ash and other hazardous wastes before landfilling or underground burial. In this method, additives are combined with the waste material to make it more solid, or to prevent chemical reactions. Other physical treatment methods include soil washing at hazardous waste dumpsites, filtering hazardous waste solids out of liquids, and distillation.
Various biological treatments utilize microbes to break down wastes through a series of organic chemical reactions. Through these methods, substances that could cause damage to humans or the environment can be rendered harmless. New substances created by microbial reactions may be suitable for reuse or recycling. Research in genetic engineering, though controversial, could lead to breakthroughs in biological treatment. In chemical treatment, materials are added to or removed from the hazardous waste to produce new, less hazardous chemicals. Chemical neutralization, for example, involves mixing a corrosive acid with carbonate lime or another high-pH material until it is no longer acidic.
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