Solid Waste Disposal

It is the elimination of useless solid or semi-solid materials generated by human and animal activities. They are separated into four categories: agricultural, industrial, commercial, and domestic waste. Commercial and household waste is often organic materials, either combustible, such as paper, wood, and cloth, or non-combustible, such as metals, glass, and ceramics. Industrial waste can be ashes from solid fuels, debris from building demolition, chemicals, paints, and slag; agricultural residues are usually animal manure and crop residues.

Removal Methods

Waste disposal by controlled dumping is the most widely used method. The rest of the waste is incinerated and a small part is used as organic fertilizer. The selection of one disposal method or another is based primarily on economic criteria, reflecting local circumstances. As for recycling, it is expected that by the year 2000 half of household waste will be recycled.

Controlled Discharge

Controlled dumping is the cheapest way to dispose of waste, but it depends on the existence of suitable sites. In general, the collection and transport of waste account for 75% of the total cost of the process. This method consists of storing waste in layers in excavated places. Each layer is pressed with machines until it reaches a height of 3 meters; then it is covered with a layer of earth and pressed again. It is essential to choose the right land so that there is no contamination either on the surface or in groundwater. To do this, the soil is leveled and cultivated on top of the waste, drainage is diverted from higher areas, soils with few seepages are selected, and areas exposed to flooding or near underground springs are avoided. The anaerobic decomposition of organic residues generates gases. If a considerable amount of methane is concentrated, explosions can occur, so the landfill must have good ventilation. More recent techniques rely on the use of these gases from decomposition as an energy resource.

Incineration

Conventional incinerators are refractory ovens or chambers in which waste is burned; combustion gases and solids that remain are burned in a second stage. Combustible materials burn by 90%. In addition to generating heat, usable as an energy source, incineration generates carbon dioxide, sulfur and nitrogen oxides, and other gaseous pollutants, fly ash, and unburned solid waste. The emission of fly ash and other particles is controlled with filters, scrubbers, and electrostatic precipitators.

Preparation of Fertilizers

The elaboration of fertilizers or fertilizers from solid residues consists of the degradation of organic matter by aerobic microorganisms. First, the waste is classified to separate materials with some other use and those that cannot be degraded, and the rest is buried to favor the decomposition process. The resulting humus contains 1 to 3% nitrogen, phosphorus, and potassium, depending on the materials used. After three weeks the product is ready to be mixed with additives, packaged, and sold.

Recovery of Energy Resources

It is possible to recover energy from some waste disposal processes. In general two groups can be made: combustion processes and pyrolysis processes. Some incinerators are used to generate steam. Boiler tubes are placed on the walls of the combustion chamber; the water that circulates through the tubes absorbs the heat generated by the combustion of the waste and produces steam.

Pyrolysis or destructive distillation is a process of chemical decomposition of solid residues using heat in an atmosphere with little oxygen. This generates a gas stream composed of hydrogen, methane, carbon monoxide, carbon dioxide, inert ash, and other gases, depending on the organic characteristics of the pyrolyzed material.

Recycling

The practice of recycling solid waste is very old. Metal utensils have been melted down and reshaped since prehistoric times. Currently, recyclable materials are recovered in many ways, such as shredding, magnetic separation of metals, separation of light and heavy materials, screening, and washing. Another recovery method is pulping. The waste is mixed with water and turned into a pasty slurry when passed through a grinder. Metal bits and other solids are removed with magnetic devices and the pulp is fed into a centrifuge. Here the heavier materials, such as pieces of glass, are separated and sent to recycling systems; other lighter materials are sent to paper and fiber recycling plants, and the remaining waste is incinerated or landfilled.

Local authorities in many countries ask consumers to deposit bottles, cans, paper, and cardboard in separate containers from other garbage. Special trucks collect the containers and send these materials to recycling facilities, reducing work in incinerators and waste in landfills.

Dangerous Residues

Some biological substances, toxic and flammable chemical compounds, and radioactive waste are hazardous. These substances can be gaseous, liquid, semisolid, solid, or gaseous.

Radioactive substances are dangerous because prolonged exposure to their radiation damages living organisms (see Biological Effects of Radiation), and because the substances retain radioactivity for a long time. This type of waste is not eliminated, it is stored inside containers in protected places. They have been stored in marine trenches, but this method does not allow recovery of what has been deposited or control of the condition of the containers. Other more suitable methods are stored in concrete silos or deep geological formations, although neither is entirely reliable in the long term.