IV.

Hazardous-Waste Control

A.

Source Reduction

The best way to eliminate hazardous wastes is not to generate them in the first place. For example, improvements have been made in the production of integrated circuits: The toxic chlorinated hydrocarbons commonly used in the 1970s were replaced in the 1980s by less toxic glycol ethers and in the 1990s by low-toxicity esters and alcohols.

B.

Recycling

Recycling is the recovery or reuse of usable materials from waste. About 5 percent of hazardous waste in the United States is recycled as solvents; a similar amount is recovered as metals.

For example, approximately 15 percent of sulfuric acid is recycled in chemical manufacturing. In the past, most sulfur used for sulfuric acid production was mined; now the amount of sulfur recovered from smelters (facilities that remove metals from ores), refineries (facilities that purify substances), and manufacturers is more than double that produced by mining.

In the United States, the practice of using industrial wastes, which often contain hazardous wastes, as ingredients in commercial fertilizers is encouraged as a means of recycling hazardous wastes. The safety of this practice has recently been called into question, however, and some states are starting to regulate it.

C.

Treatment

Wastes may be made less hazardous by physical, chemical, or biological treatment. Nearly 10 percent of hazardous waste in the United States is treated with water; another 11 percent undergoes other treatment. For example, sodium hydroxide has been used to treat acid wastes at integrated-circuit plants. Some newer plants now treat hydrofluoric acid wastes with lime, producing relatively harmless calcium fluoride, the mineral fluorite. Sulfuric acid wastes, if not recycled, can be treated with ammonia wastes from the same plant, forming ammonium sulfate, a fertilizer.

Incineration has been used since human beings learned to control fire. It is the preferred method of handling infectious medical wastes. However, it should not be used for wastes that contain toxic heavy metals or chlorinated hydrocarbons: When burned, old painted surfaces can release lead or arsenic into the air, whereas chlorinated hydrocarbons produce hydrochloric acid and dioxins. Solids left over from incineration may have to be disposed of as hazardous waste. About 6 percent of hazardous waste in the United States is incinerated, and another 11 percent is burned along with fuel.

Solidification of wastes involves melting them and mixing them with a binder, a substance that eventually hardens the mix into an impenetrable mass. One suggested treatment of radioactive waste involves turning it into a glass through a process known as vitrification.

Approximately 8 percent of hazardous waste in the United States is stabilized—kept from moving through groundwater and air. Sometimes waste can be stabilized on-site; simple remedies such as covering the waste may be sufficient. Other stabilization methods involve building a barrier around the waste. This barrier can be of plastic, steel, concrete, clay, or even glass.

D.

Disposal

Surface impoundment (placing liquid or semiliquid wastes in unlined pits) keeps waste in long-term storage, but it is not considered a method of final disposal. About 8 percent of hazardous waste is injected into deep wells; 21 percent enters landfills (large, unlined pits into which solid wastes are placed) as its ultimate resting place.

Abandoned and particularly serious waste sites may qualify as “Superfund” sites, eligible for cleanup with government funding under legislation passed in 1980. In 1993, of about 38,000 hazardous-waste sites inventoried by the Environmental Protection Agency (EPA), 1407 sites were listed on or proposed for the National Priority List (NPL) for waste cleanup. In 1995 the EPA estimated that 73 million people lived within 4 miles of a Superfund site in the United States. Before 1995, 3300 emergency removals—urgent cleanups of hazardous wastes because of the immediate hazard they present—were conducted.

The serious problem of underground plumes of hazardous materials leaving the original disposal sites has only partial solutions at this time. The typical method of handling this problem is the drilling of wells around a plume's perimeter. Hazardous materials are then removed from some wells, and water may be injected into other wells to produce a barrier to the plume's motion. Drilling wells and monitoring holes near a toxic site carries risks; a plume originally confined between strata (horizontal layers of rock) may penetrate vertically through a drilled hole and escape confinement.

A recent method of treatment for shallow plumes of chlorinated solvents depends on their chemical reactivity. A trench is dug around the leaking waste site and filled with a mixture of soil and powdered iron. The iron then reacts with the chlorinated solvents, turning them into simple hydrocarbons, which are less hazardous.