solid waste discarded materials other than fluids. In the United States in 1996, nearly 210 million tons—about 4.3 lb. (2 kg) per person daily (up from 2.7 lb./1.2 kg in 1960)—were collected and disposed of by municipalities. In that year, municipal garbage included 12.4 million tons of glass and about 80 million tons of paper and paperboard (by far the largest constituent); in addition enormous tonnages of food residues, yard trimmings, textiles, plastics, and sludge formed in sewage treatment were produced. Although the amount of the increase has been slowed somewhat by recycling and composting programs and improvements in packaging, the amount of solid waste continues to increase annually. Moreover, the most common disposal methods pollute land, water, or air to some degree (see pollution ). Management of solid waste therefore presents an increasingly acute problem.

See also environmentalism ; radioactive waste .

Landfills

Approximately 62% of municipal waste is placed in landfills. If the waste is dumped untreated, it can promote the proliferation of rats, flies, and other vermin, encourage growth of disease-carrying organisms, contaminate surface and underground water, scar the land, and preempt open space. An alternative method of solid waste disposal is the sanitary landfill, first employed in Fresno, Calif., in 1937: waste is spread in thin layers, each tamped compactly and covered by a layer of earth. While more expensive than open dumping, the sanitary landfill eliminates health hazards and permits reclamation of the site for construction, recreation, or other purposes. The chief drawbacks are that feasible locations are relatively rare and costly and that sites must be insulated from water resources to avoid polluting them (see water pollution ). Both open dump and sanitary landfill disposal depend on the natural degradability of wastes for an ultimate return to normal earth conditions. Decay, however, takes time; buried paper, for example, can persist as long as 60 years. Many plastics and synthetic textiles do not degrade at all.

Incineration

To reduce the bulk of solid waste burning of paper, plastic, and other components is often resorted to, either in open dumps or incinerators. Fly ash, noxious gases, and chemical contaminants can thus be released into the air (see air pollution ). However, new techniques for "scrubbing" pollutants from incinerator stacks are being developed. Incineration of typical garbage reduces its weight and volume by as much as 80%. Approximately 15.9% of municipal solid waste is combusted.

Recycling

Recycling of solid wastes is an option that many municipalities have explored in recent years. It not only facilitates disposal but conserves energy, cuts pollution, and preserves natural resources. To make cans from recovered aluminum, for example, requires 10% of the energy needed to make them from virgin ore. At the same time ore is saved, and the pollution resulting from mining and processing are avoided. Making steel bars from scrap requires 74% less energy and 50% less water, while reducing air-polluting emissions by 85% and mining wastes by 95%.

Similarly, sludge from treated sewage can be used for fertilizer, but it has been less costly to dump it at sea or on open land (see sewerage ). Dumped sludge has killed marine life and threatened beaches along the Eastern seaboard; elsewhere in the United States it is a growing nuisance. Between 1975 and 1985 the amount of sludge dumped in U.S. coastal waters increased by 60%; the effects of dumping and illegal dumping are still felt despite the fact that it has been illegal since the beginning of 1992. Recycling and composting take care of approximately 2.7% of municipal solid waste.

New Techniques

The federal government now provides assistance to localities in developing new means of recovering materials and energy from solid waste, and encourages private industry to seek similar goals. One technique being tried involves intensified combustion of wastes to produce heat for generating power. A second promising approach is pyrolysis, the thermal decomposition of wastes in controlled amounts of oxygen to produce valuable petrochemicals; the residue is an inert char of little bulk. Another method of reducing solid wastes is to replace polystyrene packaging with less bulky wrapping made largely of paper. Wider application of such processes is being advocated not only to diminish pollution of the environment by solid waste, but also to conserve natural resources.

waste disposal methods Methods used for the disposal of wastes are designed to minimize their perceived hazard to the environment. Wastes can be divided into four broad categories: municipal, medical, industrial, and hazardous. Municipal wastes include basic household wastes such as papers, cans, bottles, food scraps, and other debris typically generated by households. Medical wastes result from treatment of patients in hospitals, clinics, and surgeries. Industrial wastes result from the demolition of buildings, roads, and factories, and include other construction debris that is non-hazardous. Hazardous wastes include solids and liquids that are corrosive, flammable, reactive, or toxic. Hazardous wastes also include radioactive materials, which are not discussed here. Waste disposal methods include landfarming, landfilling, incineration, and injection wells.

Landfarming

Oilfield waste, such as petroleum-contaminated soils, can be disposed of in industrial landfills or landfarmed if contaminant levels are within certain limits. Landfarming entails spreading waste over the ground surface in shallow layers. The ground is periodically ploughed to aerate the waste and encourage biodegradation by soil microbes.

Landfilling

Municipal wastes are disposed of in landfills. In the past, landfills have consisted of little more than open dumps located in places of little economic value, such as wetlands, ravines, and gravel pits. In 1993, about 20 per cent of the 1270 hazardous waste sites on the United States Federal Superfund Priority Cleanup list were closed municipal waste sites. No comparable list is available in the European Union. Minimum standards were adopted in 1991 in the USA. regulating the location, design, construction, and operation of municipal landfills to prevent contamination of the groundwater below the landfills. The primary goal of current government regulations in the Western world is to protect groundwater. To achieve this requires extensive monitoring and corrective action in the event of groundwater contamination.

Municipal wastes are covered daily in the landfill, either with soil, to aid in biodegradation, or with synthetic liners, to control odours. A collection system gathers and removes gases (usually a mixture of methane and carbon dioxide produced by biodegration) from the landfill. Gas monitoring is performed at the landfill perimeter. Collection and monitoring of gas are critical to prevent its accumulation in explosive concentrations during the processes of decomposition. Some municipal landfills harness this gas for conversion to electricity.

Solid wastes containing metals or small amounts of organic compounds are also disposed of in landfills. These wastes must be treated according to types of contaminants, before landfilling. Landfills in the USA can no longer accept free liquids. Landfills that are built to accept hazardous waste must undergo a lengthy and strict permitting process that involves public, federal, and state agency interaction. Because of this lengthy process, only a few landfills gain permits each year in the USA.

The bottom and sides of hazardous-waste landfills are constructed of a double-liner system of highly impermeable clay and thick plastic liners. Construction and placement of these liners have to conform to strict engineering specifications to prevent the movement of liquid waste into the groundwater below. Trenches containing gravel and sand surrounding a non-corrosive piping system must be placed in the bottom of the landfill to collect and transport any leachate to a storage area for treatment and discharge. (Leachate is water that percolates through the solid waste and accumulates on top of the liner at the base of the landfill.)

Waste is placed into the landfill in layers or lifts to prevent puncturing of the plastic liner. The location of a particular waste stream within the landfill is recorded to ensure compatibility with other wastes and to identify wastes if future groundwater problems occur. Permits in the USA require groundwater monitoring systems around the landfill to detect any leakage.

After the landfill reaches capacity, the surface is graded and then covered with the same double-liner system. The final clay liner is covered with graded topsoil. The leachate collection system continues to function as before in monitoring leakage through the surface cap.

When a landfill reaches capacity, US federal regulations require that owners and operators have financial assurance to care for the landfill after its closure. Closure requirements mandate continued environmental management of landfills for 30 years after closure.

Industrial wastes are usually landfilled in minimally regulated facilities. Asbestos is an industrial waste exception that must be disposed of according to special regulations and in specially permitted landfills. Because of the airborne fibre problem, asbestos is bagged in specially marked plastic containers, landfilled, and then covered with soil.

Incineration

Solid or liquid industrial wastes with a high organic content or containing chemicals such as polychlorinated biphenyls or other chlorinated hydrocarbons must be incinerated. Medical wastes are also incinerated in specially permitted facilities or commercial hazardous waste incinerators.

Incinerators are designed to destroy gases, liquids, solids, sludges, and slurries at high temperatures. One incineration process, the rotary kiln process, uses a cylindrical shell mounted on its side at a slight angle to the horizontal. As the kiln rotates, the waste moves down the slope, and the organic compounds burn. The kiln or burner is designed to withstand and maintain extremely high temperatures. Wastes are fed into the incinerator in batches or in a continuous stream. Liquid wastes are generally pumped into incinerators through a nozzle that breaks the liquid into fine droplets that burn more easily. Solid wastes may be fed into the incinerator in bulk or in containers using a conveyor or gravity system.

As the wastes are heated, they are converted from solids or liquids into gases mixed with air, and they are then passed through a very hot flame. As the temperature of the gases rises, the organic contaminants begin to break down and recombine with oxygen from the air, forming carbon dioxide and water. Other inorganic compounds may also be formed, according to the waste feed composition.

Temperatures in the combustion chamber generally range from 1000 to 1400°C. The wastes are usually maintained at these elevated temperatures for less than a second to several seconds. Some incinerators may have a second combustion chamber that maintains higher temperatures than the first chamber. Secondary combustion is used to process chlorinated hydrocarbons such as polychlorinated biphenyls because they are difficult to burn.

Combustion yields two residual products: solid ash and gases. During combustion, most ash collects at the bottom of the combustion chamber but some is carried along with the gases as particulate matter. The composition of the ash depends on the composition of the waste feed. Incineration of highly liquid wastes produces little ash, while the amount of ash produced by combustion of solid wastes can be 10–30 per cent of the original amount of waste. Ash must usually be disposed of in a hazardous waste landfill, regardless of whether the original waste was hazardous or not.

After combustion, the gases produced move through various devices that cool and cleanse them before release to the atmosphere through the incinerator stack. Gases flow from the secondary combustion chamber through the quench chamber, and then through air pollution control devices to remove acid gases and particulates. Fans maintain a predetermined gas pressure and draw rate. The exact number and types of devices used depend on the incinerator and the types of wastes burned.

Complete combustion is not possible. Three factors determine the completeness of combustion in an incinerator: the temperature in the combustion chamber, the length of time the wastes are maintained at the high temperatures, and the degree of mixing of the waste with oxygen to ensure maximum burning. US permit requirements allow for ranges in incinerator operating conditions and mandate continuous monitoring of these conditions.

As part of the energy recovery process to conserve diminishing fossil fuels, hazardous wastes are being disposed of in cement kilns, another type of incineration process. Solvents containing waste are processed to produce a hazardous-waste-derived fuel (HWDF) that is then burned in the cement kiln. The blended HWDF must have a high energy value to qualify for valid energy recovery. Cement kilns are usually of the rotary type and burn at the same temperatures as commercial incinerators. Powdered limestone (90 per cent of the raw material used in cement kilns) chemically scrubs the hot gases as they pass through the preheater of the cement kiln, neutralizing acid gases. The permit process for cement kilns is less strict than for commercial incinerators.

Mobile incinerators were designed for use on sites when it is not economical to transport wastes to an off-site incinerator. Mobile incinerators can be mounted on flat-bed trucks and on ships. Another type of incinerator is the fluidized-bed incinerator, which burns finely divided solids, sludges, slurries, and liquids. The bed consists of an inert granular material, usually sand, that is suspended by pressurized air in a highly turbulent or fluidized state above the combustion chamber floor. Waste in the fluidized bed has direct contact with the bed material, thereby improving the transfer of heat. Combustion gases move out of the combustion chamber for cooling and further treatment. Ash caught in the bed material is eventually removed when the bed material is replaced.

Injection wells

In the USA, industrial wastes that are primarily liquid are usually disposed of in injection wells. Injection wells receiving aqueous wastes can be placed in highly permeable, underground geological formations. These formations are well below 1000 m underground, which is lower than the depth of most aquifers used as sources of drinking water. Before injection, liquid wastes are filtered to remove suspended solids and skimmed for phased organic compounds. Filtration prevents the plugging of the injection formation. If the waste is reactive, it is converted to less reactive compounds before injection. The pH value of the waste may also be adjusted to conform with that of the injection formation.

J. H. Baker